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{{About|the scientific study of celestial objects}} {{Short description|Scientific study of celestial objects}}
{{Hatnote group|{{About-distinguish-text|the scientific study of celestial objects|], a divinatory pseudoscience}}{{Other uses}}}}
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] of ] shooting a ] to the ]]]


'''Astronomy''' is a ] that studies ] and the ] that occur in the cosmos. It uses ], ], and ] in order to explain their origin and their overall ]. Objects of interest include ], ], ]s, ], ], ]s, ]s, and ]s. Relevant phenomena include ] explosions, ]s, ]s, ]s, ]s, and ]. More generally, astronomy studies everything that originates beyond ]. ] is a branch of astronomy that studies the ] as a whole.
] mosaic of the ], a ]]]


Astronomy is one of the oldest natural sciences. The early civilizations in ] made methodical observations of the ]. These include the ], ], ], ], ], ], and many ancient ]. In the past, astronomy included disciplines as diverse as ], ], ], and the making of ]s.
'''Astronomy''' is a ] that deals with the study of ] (such as ]s, ]s, ]s, ]e, ]s and ]) and ] that originate outside the ] (such as the ]). It is concerned with the evolution, ], ], ], and ] of celestial objects, as well as the ].


Professional astronomy is split into ] and ] branches. Observational astronomy is focused on acquiring data from observations of astronomical objects. This data is then analyzed using basic principles of physics. Theoretical astronomy is oriented toward the development of computer or analytical models to describe astronomical objects and phenomena. These two fields complement each other. Theoretical astronomy seeks to explain observational results and observations are used to confirm theoretical results.
Astronomy is one of the oldest sciences. Prehistoric cultures left behind astronomical artifacts such as the ] and ], and early civilizations such as the ], ], ], and ] performed methodical observations of the night sky. However, the invention of the ] was required before astronomy was able to develop into a modern science. Historically, astronomy has included disciplines as diverse as ], ], observational astronomy, the making of ]s, and even ], but professional astronomy is nowadays often considered to be synonymous with ].


Astronomy is one of the few sciences in which amateurs play an ]. This is especially true for the discovery and observation of ]. ] have helped with many important discoveries, such as finding new comets.
During the 20th century, the field of professional astronomy split into ] and theoretical branches. Observational astronomy is focused on acquiring data from observations of celestial objects, which is then analyzed using basic principles of physics. Theoretical astronomy is oriented towards the development of computer or analytical models to describe astronomical objects and phenomena. The two fields complement each other, with theoretical astronomy seeking to explain the observational results, and observations being used to confirm theoretical results.


== Etymology ==
] have contributed to many important astronomical discoveries, and astronomy is one of the few sciences where amateurs can still play an active role, especially in the discovery and observation of transient ].
]
''Astronomy'' (from the ] ] from ] ''astron'', "star" and -νομία '']'' from ] ''nomos'', "law" or "culture") means "law of the stars" (or "culture of the stars" depending on the translation). Astronomy should not be confused with ], the belief system which claims that human affairs are correlated with the positions of celestial objects.<ref>{{Cite journal |bibcode = 2012JAHH...15...42L|title = 'Astronomy' or 'astrology': A brief history of an apparent confusion|last1 = Losev|first1 = Alexandre|journal = ]|volume = 15|issue = 1|pages = 42–46|year = 2012| doi=10.3724/SP.J.1440-2807.2012.01.05 |arxiv = 1006.5209| s2cid=51802196 |issn=1440-2807}}</ref> Although the ] share a common origin, they are now entirely distinct.<ref name="new cosmos">{{cite book|first=Albrecht |last=Unsöld|author2=Baschek, Bodo|others=Translated by Brewer, W.D.|title=The New Cosmos: An Introduction to Astronomy and Astrophysics|date=2001| location=Berlin, New York|publisher=Springer|isbn =978-3-540-67877-9}}</ref>


=== Use of terms "astronomy" and "astrophysics" ===
Ancient astronomy is not to be confused with ], the belief system which claims that human affairs are correlated with the positions of celestial objects. Although the ] share a common origin and a part of their methods (namely, the use of ]), they are distinct.<ref name="new cosmos">{{cite book|first=Albrecht |last=Unsöld|coauthors=Baschek, Bodo; Brewer, W.D. (translator)|title=The New Cosmos: An Introduction to Astronomy and Astrophysics|year=2001| location=Berlin, New York|publisher=Springer|isbn =3-540-67877-8 }}</ref>
"Astronomy" and "]" are synonyms.<ref name="scharrinhausen">{{cite web|url=http://curious.astro.cornell.edu/question.php?number=30|title= What is the difference between astronomy and astrophysics? |website=Curious About Astronomy |date=January 2002 |last=Scharringhausen|first=B.|access-date=17 November 2016|archive-url=https://web.archive.org/web/20070609102139/http://curious.astro.cornell.edu/question.php?number=30|archive-date=9 June 2007 }}</ref><ref name="odenwald">{{cite web|url=http://www.astronomycafe.net/qadir/q449.html|title=Archive of Astronomy Questions and Answers: What is the difference between astronomy and astrophysics?|last=Odenwald|first=Sten |publisher=The Astronomy Cafe|access-date=20 June 2007|archive-url=https://web.archive.org/web/20070708092148/http://www.astronomycafe.net/qadir/q449.html|archive-date=8 July 2007 |url-status=dead }}</ref><ref name="pennstateerie">{{cite web

|title=School of Science-Astronomy and Astrophysics
==Lexicology==
|website=Penn State Erie

|date=July 18, 2005
The word ''astronomy'' (from the ] words ''astron'' ('']''), "star" and ] from ''nomos'' ('']''), "law" or "culture") literally means "law of the stars" (or "culture of the stars" depending on the translation).
|url=http://www.erie.psu.edu/academic/science/degrees/astronomy/astrophysics.htm

|access-date=20 June 2007
===Use of terms "astronomy" and "astrophysics"===
|archive-url=https://web.archive.org/web/20071101100832/http://www.erie.psu.edu/academic/science/degrees/astronomy/astrophysics.htm

|archive-date=1 November 2007
Generally, either the term "astronomy" or "astrophysics" may be used to refer to this subject.<ref name="scharrinhausen">{{cite web
}}</ref> Based on strict dictionary definitions, "astronomy" refers to "the study of objects and matter outside the Earth's atmosphere and of their physical and chemical properties",<ref name="mw-astronomy">{{cite web
|title=Curious About Astronomy: What is the difference between astronomy and astrophysics?
|title=astronomy
|url=http://curious.astro.cornell.edu/question.php?number=30
|work=Merriam-Webster Online
|first=B. |last=Scharringhausen
|accessdate=2007-06-20}}</ref><ref name="odenwald">{{cite web
|title=Archive of Astronomy Questions and Answers: What is the difference between astronomy and astrophysics?
|url=http://www.astronomycafe.net/qadir/q449.html
|first=S. |last=Odenwald
|accessdate=2007-06-20}}</ref><ref name="pennstateerie">{{cite web
|title=Penn State Erie-School of Science-Astronomy and Astrophysics
|url=http://www.erie.psu.edu/academic/science/degrees/astronomy/astrophysics.htm
|accessdate=2007-06-20}}</ref> Based on strict dictionary definitions, "astronomy" refers to "the study of objects and matter outside the Earth's atmosphere and of their physical and chemical properties"<ref name="mw-astronomy">{{cite web
|title=Merriam-Webster Online
|work=Results for "astronomy"
|url=http://www.m-w.com/dictionary/astronomy |url=http://www.m-w.com/dictionary/astronomy
|accessdate=2007-06-20}}</ref> and "astrophysics" refers to the branch of astronomy dealing with "the behavior, physical properties, and dynamic processes of celestial objects and phenomena".<ref name="mw-astrophysics">{{cite web |access-date=20 June 2007| archive-url= https://web.archive.org/web/20070617131203/http://www.m-w.com/dictionary/astronomy| archive-date= 17 June 2007 | url-status= live}}</ref> while "astrophysics" refers to the branch of astronomy dealing with "the behavior, physical properties, and dynamic processes of celestial objects and phenomena".<ref name="mw-astrophysics">{{cite web
|title=Merriam-Webster Online |title=astrophysics
|work=Merriam-Webster Online
|work=Results for "astrophysics"
|url=http://www.m-w.com/dictionary/astrophysics |url=http://www.m-w.com/dictionary/astrophysics
|access-date=20 June 2007
|accessdate=2007-06-20}}</ref> In some cases, as in the introduction of the introductory textbook ''The Physical Universe'' by ], "astronomy" may be used to describe the qualitative study of the subject, whereas "astrophysics" is used to describe the physics-oriented version of the subject.<ref name="shu1982">{{cite book
|archive-date=21 September 2012
|first = F. H. |last=Shu
|archive-url=https://archive.today/20120921/http://www.m-w.com/dictionary/astrophysics
|url-status=live
}}</ref> In some cases, as in the introduction of the introductory textbook ''The Physical Universe'' by ], "astronomy" may be used to describe the qualitative study of the subject, whereas "astrophysics" is used to describe the physics-oriented version of the subject.<ref name="shu1982">{{cite book
|first = F.H.
|last = Shu
|title = The Physical Universe |title = The Physical Universe
|publisher = University Science Books |publisher = University Science Books
|year = 1982 |date = 1983
|location = Mill Valley, California |location = Mill Valley, California
|isbn = 978-0-935702-05-7
|isbn = 0-935702-05-9}}</ref> However, since most modern astronomical research deals with subjects related to physics, modern astronomy could actually be called astrophysics.<ref name="scharrinhausen"/> Various departments that research this subject may use "astronomy" and "astrophysics", partly depending on whether the department is historically affiliated with a physics department,<ref name="odenwald"/> and many professional astronomers actually have physics degrees.<ref name="pennstateerie"/> One of the leading scientific journals in the field is named ].
|url-access = registration
|url = https://archive.org/details/physicaluniverse00shuf
}}</ref> However, since most modern astronomical research deals with subjects related to physics, modern astronomy could actually be called astrophysics.<ref name="scharrinhausen"/> Some fields, such as ], are purely astronomy rather than also astrophysics. Various departments in which scientists carry out research on this subject may use "astronomy" and "astrophysics", partly depending on whether the department is historically affiliated with a physics department,<ref name="odenwald"/> and many professional ]s have physics rather than astronomy degrees.<ref name="pennstateerie"/> Some titles of the leading scientific journals in this field include '']'', '']'', and '']''.


==History== == History ==
{{Main|History of astronomy}} {{Main|History of astronomy}}
{{For timeline}}
{{See|Archaeoastronomy}}
{{Further|Archaeoastronomy|List of astronomers}}


=== Pre-historic astronomy ===
].]]
] ({{circa|1800–1600 BCE}}), found near a possibly ], most likely depicting the Sun or full Moon, the Moon as a crescent, the ] and the summer and winter solstices as strips of gold on the side of the disc,<ref name="Meller 2021">{{cite book|url=https://www.academia.edu/80363367|title=Time is power. Who makes time?: 13th Archaeological Conference of Central Germany|chapter=The Nebra Sky Disc – astronomy and time determination as a source of power|last=Meller|first=Harald|date=2021|publisher=Landesmuseum für Vorgeschichte Halle (Saale).|isbn=978-3-948618-22-3}}</ref><ref>{{cite AV media |url=https://www.youtube.com/watch?v=0dlijsmVJ9c&t=760s |title=Concepts of cosmos in the world of Stonehenge |website=British Museum |date=2022}}</ref> with the top representing the ]<ref name=":03">{{Cite book |last1=Bohan |first1=Elise |url=https://www.worldcat.org/oclc/940282526 |title=Big History |last2=Dinwiddie |first2=Robert |last3=Challoner |first3=Jack |last4=Stuart |first4=Colin |last5=Harvey |first5=Derek |last6=Wragg-Sykes |first6=Rebecca |last7=Chrisp |first7=Peter |last8=Hubbard |first8=Ben |last9=Parker |first9=Phillip |collaboration=Writers |date=February 2016 |publisher=] |others=Foreword by ] |isbn=978-1-4654-5443-0 |edition=1st American |location=] |page=20 |oclc=940282526}}</ref> and ].]]
In early times, astronomy only comprised the observation and predictions of the motions of objects visible to the naked eye. In some locations, such as ], early cultures assembled massive artifacts that likely had some astronomical purpose. In addition to their ceremonial uses, these ] could be employed to determine the seasons, an important factor in knowing when to plant crops, as well as in understanding the length of the year.<ref name="history">Forbes, 1909</ref>


In early historic times, astronomy only consisted of the observation and predictions of the motions of objects visible to the naked eye. In some locations, early cultures assembled massive artifacts that may have had some astronomical purpose. In addition to their ceremonial uses, these ] could be employed to determine the seasons, an important factor in knowing when to plant crops and in understanding the length of the year.<ref name="history">{{cite book | first=George | last=Forbes | title=History of Astronomy | publisher=Plain Label Books | location=London | date=1909 | isbn=978-1-60303-159-2 | url=http://www.gutenberg.org/ebooks/8172 | access-date=7 April 2019 | archive-date=28 August 2018 | archive-url=https://web.archive.org/web/20180828185512/http://www.gutenberg.org/ebooks/8172 | url-status=live }}</ref>
Before tools such as the telescope were invented early study of the stars had to be conducted from the only vantage points available, namely tall buildings and high ground using the naked eye. As civilizations developed, most notably in ], ], ], ], ], and ], astronomical observatories were assembled, and ideas on the nature of the universe began to be explored. Most of early astronomy actually consisted of mapping the positions of the stars and planets, a science now referred to as ]. From these observations, early ideas about the motions of the planets were formed, and the nature of the Sun, Moon and the Earth in the universe were explored philosophically. The Earth was believed to be the center of the universe with the Sun, the Moon and the stars rotating around it. This is known as the geocentric model of the universe.


===Classical astronomy===
A particularly important early development was the beginning of mathematical and scientific astronomy, which began among ], who laid the foundations for the later astronomical traditions that developed in many other civilizations.<ref>{{cite journal|title=Scientific Astronomy in Antiquity|author=Aaboe, A. |journal=]|volume=276|issue=1257|year=1974|pages=21–42|url=http://www.jstor.org/stable/74272|accessdate=2010-03-09|doi=10.1098/rsta.1974.0007|ref=harv}}</ref> The ] discovered that ] recurred in a repeating cycle known as a ].<ref>{{cite web|url=http://sunearth.gsfc.nasa.gov/eclipse/SEsaros/SEsaros.html|title=Eclipses and the Saros|publisher=NASA|accessdate=2007-10-28}}</ref>
] (7th century BCE). ] made early advances in astronomy. Its use of ]s (e.g. 12, 24, 60, 360) is still being used today through having been broadly adopted for ] and ].<ref name="x754">{{cite web | last=Gent | first=R.H. van | title=Bibliography of Babylonian Astronomy & Astrology | website=science.uu.nl project csg | url=https://webspace.science.uu.nl/~gent0113/babylon/babybibl.htm | access-date=2024-11-22}}</ref>]]
], ], present-day ] 3rd-2nd century BCE.]]
As civilizations developed, most notably in ], ], ], ], ], ], and ], astronomical observatories were assembled and ideas on the nature of the Universe began to develop. Most early astronomy consisted of mapping the positions of the stars and planets, a science now referred to as ]. From these observations, early ideas about the motions of the planets were formed, and the nature of the Sun, Moon and the Earth in the Universe were explored philosophically. The Earth was believed to be the center of the Universe with the Sun, the Moon and the stars rotating around it. This is known as the ] of the Universe, or the ], named after ].<ref>{{cite book|last=DeWitt|first=Richard|title=Worldviews: An Introduction to the History and Philosophy of Science|date=2010|publisher=Wiley|location=Chichester, England|isbn=978-1-4051-9563-8|page=113|chapter=The Ptolemaic System}}</ref>
Following the Babylonians, significant advances in astronomy were made in ] and the ] world. ] is characterized from the start by seeking a rational, physical explanation for celestial phenomena.<ref>{{Cite book| last = Krafft| first = Fritz| year = 2009| contribution = Astronomy| editor-last = Cancik| editor-first = Hubert| editor2-last = Schneider| editor2-first = Helmuth| title = ]| ref = harv| postscript = <!--None-->}}</ref> In the 3rd century BC, ] calculated the size of the Earth, and measured the ], and was the first to propose a ] model of the solar system. In the 2nd century BC, ] discovered ], calculated the size and distance of the Moon and invented the earliest known astronomical devices such as the ].<ref>{{cite web|url=http://www-groups.dcs.st-and.ac.uk/~history/Biographies/Hipparchus.html|title=Hipparchus of Rhodes|publisher=School of Mathematics and Statistics, University of St Andrews, Scotland|accessdate=2007-10-28}}</ref> Hipparchus also created a comprehensive catalog of 1020 stars, and most of the constellations of the northern hemisphere derive are taken from Greek astronomy.<ref>Thurston, H., ''Early Astronomy.'' Springer, 1994. p.2</ref> The ] (c. 150–80 BC) was an early ] designed to calculating the location of the ], ], and ] for a given date. Technological artifacts of similar complexity did not reappear until the 14th century, when mechanical ]s appeared in ].<ref name=insearchoflosttime>In search of lost time, Jo Marchant, ''Nature'' '''444''', #7119 (November 30, 2006), pp. 534–538, {{doi|10.1038/444534a}}.</ref>


A particularly important early development was the beginning of mathematical and scientific astronomy, which began among ], who laid the foundations for the later astronomical traditions that developed in many other civilizations.<ref>{{cite journal|title=Scientific Astronomy in Antiquity|author=Aaboe, A. |journal=]|volume=276|issue=1257|date=1974|pages=21–42|jstor=74272|doi=10.1098/rsta.1974.0007|bibcode = 1974RSPTA.276...21A |s2cid=122508567 }}</ref> The ] discovered that ] recurred in a repeating cycle known as a ].<ref>{{cite web|url=http://sunearth.gsfc.nasa.gov/eclipse/SEsaros/SEsaros.html |title=Eclipses and the Saros |publisher=NASA |access-date=28 October 2007 |archive-url=https://web.archive.org/web/20071030225501/http://sunearth.gsfc.nasa.gov/eclipse/SEsaros/SEsaros.html |archive-date=30 October 2007 }}</ref>
During the Middle Ages, astronomy was mostly stagnant in ] Europe, at least until the 13th century. However, ] and other parts of the world. This led to the emergence of the first astronomical ] in the ] by the early 9th century.<ref name=Kennedy-1962>{{Cite journal |last=Kennedy |first=Edward S. |year=1962 |title=Review: ''The Observatory in Islam and Its Place in the General History of the Observatory'' by Aydin Sayili |journal=] |volume=53 |issue=2 |pages=237–239 |doi=10.1086/349558 |ref=harv |postscript=<!--None--> }}</ref><ref name = "Micheau-992-3">{{Cite journal|last=Micheau|first=Francoise|contribution=The Scientific Institutions in the Medieval Near East|pages=992–3|ref=harv|postscript=<!--None-->}}, in {{Harv|Rashed|Morelon|1996|pp=985–1007}}</ref><ref>{{cite book |last=Nas |first=Peter J |authorlink= |coauthors= |editor= |others= |title=Urban Symbolism |origdate= |origyear= |origmonth= |url= |format= |accessdate= |edition= |series= |date= |year=1993 |month= |publisher=Brill Academic Publishers |location= |language= |isbn=9-0040-9855-0 |oclc= |doi= |id= |pages=350 |chapter= |chapterurl= |quote= }}</ref> In 964, the ], the nearest ] to the ], was discovered by the Persian astronomer ] and first described in his '']''.<ref name="NSOG">{{cite book |last= Kepple |first= George Robert |coauthors= Glen W. Sanner |title= The Night Sky Observer's Guide, Volume 1 |publisher= Willmann-Bell, Inc |year= 1998 |isbn= 0-943396-58-1 |page=18}}</ref> The ] ], the brightest ] stellar event in recorded history, was observed by the Egyptian Arabic astronomer ] and the ] in 1006. Some of the prominent Islamic (mostly Persian and Arab) astronomers who made significant contributions to the science include ], ], ], ], ], ], ], and the astronomers of the ] and ] observatories. Astronomers during that time introduced many ].<ref name="short history">{{cite book|first=Arthur|last=Berry|title=A Short History of Astronomy From Earliest Times Through the Nineteenth Century|publisher=Dover Publications, Inc.|location=New York|year=1961 }}</ref><ref name="Cambridge history">{{cite book|editor=Hoskin, Michael|title=The Cambridge Concise History of Astronomy|publisher=Cambridge University Press|year=1999|isbn = 0-521-57600-8 }}</ref> It is also believed that the ruins at ] and ]<ref>{{cite book|url=http://books.google.com/?id=Pk-bZMS_KdUC&pg=PA103&lpg=PA103&dq=astronomy+in+medieval+Africa|title= The royal kingdoms of Ghana, Mali, and Songhay: life in medieval Africa|first=Pat|last= McKissack|coauthors= McKissack, Frederick|year=1995|publisher=H. Holt|isbn=9780805042597}}</ref> may have housed an astronomical observatory.<ref>{{cite journal|url=http://www.newscientist.com/article/dn3137-eclipse-brings-claim-of-medieval-african-observatory.html|title=Eclipse brings claim of medieval African observatory|year=2002|journal=New Scientist|accessdate=2010-02-03|last=Clark|first=Stuart|coauthors=Carrington, Damian|ref=harv}}</ref> Europeans had previously believed that there had been no astronomical observation in pre-colonial Middle Ages sub-Saharan Africa but modern discoveries show otherwise.<ref>{{cite web|url=http://www.scienceinafrica.co.za/2003/november/cosmic.htm|title=Cosmic Africa explores Africa's astronomy|accessdate=2002-02-03|publisher=Science in Africa}}</ref><ref>{{cite book|url=http://books.google.com/?id=4DJpDW6IAukC&pg=PA180&lpg=PA180&dq=astronomy+in+medieval+Africa|title=African Cultural Astronomy|first=Jarita C. |last= Holbrook|coauthors=Medupe, R. Thebe; Urama, Johnson O.|publisher=Springer|year=2008|isbn=9781402066382}}</ref><ref>{{cite web|url=http://web.archive.org/web/20080609112829/http://royalsociety.org/news.asp?year=&id=4117|title=Africans studied astronomy in medieval times|date=30 January 2006|publisher=The Royal Society|accessdate=2010-02-03}}</ref>


Following the Babylonians, significant advances in astronomy were made in ] and the ] world. ] is characterized from the start by seeking a rational, physical explanation for celestial phenomena.<ref>{{Cite book| last = Krafft| first = Fritz| date = 2009| contribution = Astronomy| editor-last = Cancik| editor-first = Hubert| editor2-last = Schneider| editor2-first = Helmuth| title = Brill's New Pauly| title-link = Brill's New Pauly}}</ref> In the 3rd century BC, ] estimated the ], and he proposed a model of the ] where the Earth and planets rotated around the Sun, now called the ] model.<ref>{{cite journal | title = Aristarchus's On the Sizes and Distances of the Sun and the Moon: Greek and Arabic Texts | journal = Archive for History of Exact Sciences | date = May 2007 | first1 = J.L. | last1 = Berrgren |first2= Nathan |last2= Sidoli | volume = 61 | issue = 3 | pages = 213–54 | doi = 10.1007/s00407-006-0118-4| s2cid = 121872685 }}</ref> In the 2nd century BC, ] discovered ], calculated the size and distance of the Moon and invented the earliest known astronomical devices such as the ].<ref>{{cite web|url=http://www-groups.dcs.st-and.ac.uk/~history/Biographies/Hipparchus.html|title=Hipparchus of Rhodes|publisher=School of Mathematics and Statistics, ], Scotland|access-date=28 October 2007|archive-url=https://web.archive.org/web/20071023062202/http://www-groups.dcs.st-and.ac.uk/~history/Biographies/Hipparchus.html|archive-date=23 October 2007 |url-status=live}}</ref> Hipparchus also created a comprehensive catalog of 1020 stars, and most of the ]s of the northern hemisphere derive from Greek astronomy.<ref>{{cite book|last=Thurston|first=H.|title=Early Astronomy|url=https://books.google.com/books?id=rNpHjqxQQ9oC&pg=PA2|year=1996|publisher=Springer Science & Business Media|isbn=978-0-387-94822-5|page=2|access-date=20 June 2015|archive-date=3 February 2021|archive-url=https://web.archive.org/web/20210203012120/https://books.google.com/books?id=rNpHjqxQQ9oC&pg=PA2|url-status=live}}</ref> The ] ({{circa|150}}–80 BC) was an early ] designed to calculate the location of the ], ], and ] for a given date. Technological artifacts of similar complexity did not reappear until the 14th century, when mechanical ]s appeared in Europe.<ref name=insearchoflosttime>{{cite journal|last1=Marchant|first1=Jo|title=In search of lost time|journal=Nature|volume=444|issue=7119|pages=534–38|date=2006|pmid=17136067|doi=10.1038/444534a|bibcode = 2006Natur.444..534M |doi-access=free}}</ref>
===Scientific revolution===


=== Post-classical astronomy ===
]'s sketches and observations of the ] revealed that the surface was mountainous.]]
] in the ''Compilatio astronomica'', 1493. ] began just before the 9th century to collect and translate ], ] and ] astronomical texts, adding their own astronomy and enabling later, particularly European astronomy to build on.<ref name="n063">{{cite web | last=Akerman | first=Iain | title=The language of the stars | website=WIRED Middle East | date=2023-05-17 | url=https://wired.me/culture/arab-astronomy-the-language-of-stars/ | access-date=2024-11-23}}</ref>]]


] and other parts of the world. This led to the emergence of the first astronomical ] in the ] by the early 9th century.<ref name="Kennedy-1962">{{Cite journal |last=Kennedy |first=Edward S. |date=1962 |title=Review: ''The Observatory in Islam and Its Place in the General History of the Observatory'' by Aydin Sayili |journal=] |volume=53 |issue=2 |pages=237–39 |doi=10.1086/349558 }}</ref><ref name="Micheau-992-3">{{Cite journal|last=Micheau|first=Françoise|editor-last=Rashed|editor-first=Roshdi|editor2-last=Morelon|editor2-first=Régis|title=The Scientific Institutions in the Medieval Near East|journal=Encyclopedia of the History of Arabic Science|volume=3|pages=992–93}}</ref><ref>{{cite book |last=Nas |first=Peter J|title=Urban Symbolism|date=1993 |publisher=Brill Academic Publishers |isbn=978-90-04-09855-8|page=350}}</ref> In 964, the ], the largest ] in the ], was described by the Persian Muslim astronomer ] in his '']''.<ref name="NSOG">{{cite book |last1= Kepple |first1= George Robert |first2=Glen W. |last2=Sanner |title= The Night Sky Observer's Guide |volume= 1 |publisher= Willmann-Bell, Inc. |date= 1998 |isbn= 978-0-943396-58-3 |page=18}}</ref> The ] ], the brightest ] stellar event in recorded history, was observed by the Egyptian Arabic astronomer ] and ] in 1006. Iranian scholar ] observed that, contrary to ], the Sun's ] (highest point in the heavens) was mobile, not fixed.<ref>{{cite news |last1=Covington |first1=Richard |title=Rediscovering Arabic Science |url=http://archive.aramcoworld.com/issue/200703/rediscovering.arabic.science.htm |access-date=6 March 2023 |work=] |issue=3 |volume=58 |date=2007 |archive-date=1 March 2021 |archive-url=https://web.archive.org/web/20210301151438/https://archive.aramcoworld.com/issue/200703/rediscovering.arabic.science.htm |url-status=live }}</ref> Some of the prominent Islamic (mostly Persian and Arab) astronomers who made significant contributions to the science include ], ], ], ], ], ], and the astronomers of the ] and ] observatories. Astronomers during that time introduced many ].<ref name="short history">{{cite book|first=Arthur|last=Berry|title=A Short History of Astronomy From Earliest Times Through the 19th Century|publisher=Dover Publications, Inc.|location=New York|date=1961|isbn=978-0-486-20210-5|url-access=registration|url=https://archive.org/details/shorthistoryofas0000berr}}</ref><ref name="Cambridge history">{{cite book|editor=Hoskin, Michael|title=The Cambridge Concise History of Astronomy|publisher=Cambridge University Press|date=1999|isbn = 978-0-521-57600-0}}</ref>
During the ], ] proposed a ] of the ]. His work was defended, expanded upon, and corrected by ] and ]. Galileo innovated by using telescopes to enhance his observations.<ref name=f58-64/>


It is also believed that the ruins at ] and ]<ref>{{cite book|url=https://archive.org/details/royalkingdomsofg00patr|url-access=registration|page=|title= The royal kingdoms of Ghana, Mali, and Songhay: life in medieval Africa|first=Pat|last= McKissack|author2=McKissack, Frederick|date=1995|publisher=H. Holt|isbn=978-0-8050-4259-7}}</ref> may have housed astronomical observatories.<ref>{{cite journal|url=https://www.newscientist.com/article/dn3137-eclipse-brings-claim-of-medieval-african-observatory.html|title=Eclipse brings claim of medieval African observatory|date=2002|journal=New Scientist|access-date=3 February 2010|last=Clark|first=Stuart|author2=Carrington, Damian|archive-date=30 April 2015|archive-url=https://web.archive.org/web/20150430173144/http://www.newscientist.com/article/dn3137-eclipse-brings-claim-of-medieval-african-observatory.html|url-status=live}}</ref> In ] ], Astronomers studied the movement of stars and relation to seasons, crafting charts of the heavens as well as precise diagrams of orbits of the other planets based on complex mathematical calculations. ] historian ] documented a ] in August 1583.<ref>{{Cite book|last=Hammer|first=Joshua|title=The Bad-Ass Librarians of Timbuktu And Their Race to Save the World's Most Precious Manuscripts|publisher=Simon & Schuster|year=2016|isbn=978-1-4767-7743-6|location=New York|pages=26–27}}</ref><ref>{{cite book |last=Holbrook |first=Jarita C. |url=https://books.google.com/books?id=4DJpDW6IAukC&pg=PA182 |title=African Cultural Astronomy |author2=Medupe, R. Thebe |author3=] |date=2008 |publisher=Springer |isbn=978-1-4020-6638-2 |access-date=19 October 2020 |archive-url=https://web.archive.org/web/20210817020340/https://books.google.com/books?id=4DJpDW6IAukC&pg=PA182 |archive-date=17 August 2021 |url-status=live}}</ref>
Kepler was the first to devise a system that described correctly the details of the motion of the planets with the Sun at the center. However, Kepler did not succeed in formulating a theory behind the laws he wrote down.<ref>Forbes, 1909, pp. 49–58</ref> It was left to ] invention of ] and his ] to finally explain the motions of the planets. Newton also developed the ].<ref name=f58-64>Forbes, 1909, pp. 58–64</ref>
Europeans had previously believed that there had been no astronomical observation in ] during the pre-colonial Middle Ages, but modern discoveries show otherwise.<ref>{{cite web|url=http://www.scienceinafrica.co.za/2003/november/cosmic.htm |title=Cosmic Africa explores Africa's astronomy |access-date=3 February 2002 |publisher=Science in Africa |archive-url=https://web.archive.org/web/20031203055223/http://www.scienceinafrica.co.za/2003/november/cosmic.htm |archive-date=3 December 2003 }}</ref><ref>{{cite book|url=https://books.google.com/books?id=4DJpDW6IAukC&pg=PA180|title=African Cultural Astronomy|first=Jarita C.|last=Holbrook|author2=Medupe, R. Thebe|author3=Urama, Johnson O.|publisher=Springer|date=2008|isbn=978-1-4020-6638-2|access-date=26 August 2020|archive-date=26 August 2016|archive-url=https://web.archive.org/web/20160826084847/https://books.google.com/books?id=4DJpDW6IAukC&pg=PA180|url-status=live}}</ref><ref>{{cite web|url=http://royalsociety.org/news.asp?year=&id=4117 |title=Africans studied astronomy in medieval times|date=30 January 2006|publisher=The Royal Society|access-date=3 February 2010 |archive-url = https://web.archive.org/web/20080609112829/http://royalsociety.org/news.asp?year=&id=4117 |archive-date = 9 June 2008}}</ref><ref>Stenger, Richard {{cite news|url=http://articles.cnn.com/2002-12-05/tech/zimbabwe.observatory_1_supernova-forecast-eclipses-star |title=Star sheds light on African 'Stonehenge' |work=CNN |date=5 December 2002 |archive-url=https://web.archive.org/web/20110512162930/http://articles.cnn.com/2002-12-05/tech/zimbabwe.observatory_1_supernova-forecast-eclipses-star?_s=PM%3ATECH |archive-date=12 May 2011 }}. CNN. 5 December 2002. Retrieved on 30 December 2011.</ref>


For over six centuries (from the recovery of ancient learning during the late Middle Ages into the Enlightenment), the ] gave more financial and social support to the study of astronomy than probably all other institutions. Among the Church's motives was finding the ].<ref>J.L. Heilbron, ''The Sun in the Church: Cathedrals as Solar Observatories'' (1999), p. 3</ref>
Further discoveries paralleled the improvements in the size and quality of the telescope. More extensive star catalogues were produced by ]. The astronomer ] made a detailed catalog of nebulosity and clusters, and in 1781 discovered the planet ], the first new planet found.<ref>Forbes, 1909, pp. 79–81</ref> The distance to a star was first announced in 1838 when the ] of ] was measured by ].<ref>Forbes, 1909, pp. 147–150</ref>


Medieval Europe housed a number of important astronomers. ] (1292–1336) made major contributions to astronomy and ], including the invention of the first astronomical clock, the ] which allowed for the measurement of angles between planets and other astronomical bodies, as well as an ] called the ''Albion'' which could be used for astronomical calculations such as ], ] and ]ary ]s and could predict ]s. ] (1320–1382) and ] (1300–1361) first discussed evidence for the rotation of the Earth, furthermore, Buridan also developed the theory of impetus (predecessor of the modern scientific theory of ]) which was able to show planets were capable of motion without the intervention of angels.<ref>Hannam, James. ''God's philosophers: how the medieval world laid the foundations of modern science''. Icon Books Ltd, 2009, 180</ref> ] (1423–1461) and ] (1436–1476) helped make astronomical progress instrumental to Copernicus's development of the heliocentric model decades later.
During the 18–19th centuries, attention to the ] by ], ], and ] led to more accurate predictions about the motions of the Moon and planets. This work was further refined by ] and ], allowing the masses of the planets and moons to be estimated from their perturbations.<ref>Forbes, 1909, pp. 74–76</ref>


=== Early telescopic astronomy ===
Significant advances in astronomy came about with the introduction of new technology, including the ] and ]. ] discovered about 600 bands in the spectrum of the Sun in 1814–15, which, in 1859, ] ascribed to the presence of different elements. Stars were proven to be similar to the Earth's own Sun, but with a wide range of ]s, ]es, and sizes.<ref name="short history" />
]'s ground-breaking '']'' (1610), publishing his findings from the first telescopic astronomical observations.]]


During the ], ] proposed a heliocentric model of the solar system. His work was defended by ] and expanded upon by ]. Kepler was the first to devise a system that correctly described the details of the motion of the planets around the Sun. However, Kepler did not succeed in formulating a theory behind the laws he wrote down.<ref>{{harvnb|Forbes|1909|pp=49–58}}</ref> It was ], with his invention of ] and his ], who finally explained the motions of the planets. Newton also developed the ].<ref name="f58-64">{{harvnb|Forbes|1909|pp=58–64}}</ref>
The existence of the Earth's galaxy, the ], as a separate group of stars, was only proved in the 20th century, along with the existence of "external" galaxies, and soon after, the expansion of the ], seen in the recession of most galaxies from us.<ref name=Belkora2003>{{cite book|author=Belkora, Leila|title=Minding the heavens: the story of our discovery of the Milky Way|isbn=9780750307307|url=http://books.google.com/?id=qBM-wez94WwC&printsec=frontcover|publisher=]|year=2003|pages=1–14}}</ref> Modern astronomy has also discovered many exotic objects such as ]s, ]s, ]s, and ], and has used these observations to develop physical theories which describe some of these objects in terms of equally exotic objects such as ]s and ]s. ] made huge advances during the 20th century, with the model of the ] heavily supported by the evidence provided by astronomy and physics, such as the ], ], and ].


Improvements in the size and quality of the telescope led to further discoveries. The English astronomer ] catalogued over 3000 stars.<ref>Chambers, Robert (1864) '']''</ref> More extensive star catalogues were produced by ]. The astronomer ] made a detailed catalog of nebulosity and clusters, and in 1781 discovered the planet ], the first new planet found.<ref>{{harvnb|Forbes|1909|pp=79–81}}</ref>
==Observational astronomy==


During the 18–19th centuries, the study of the ] by ], ], and ] led to more accurate predictions about the motions of the Moon and planets. This work was further refined by ] and ], allowing the masses of the planets and moons to be estimated from their perturbations.<ref>{{harvnb|Forbes|1909|pp=74–76}}</ref>
] in ], an example of a ]]]


Significant advances in astronomy came about with the introduction of new technology, including the ] and ]. ] discovered about 600 bands in the spectrum of the Sun in 1814–15, which, in 1859, ] ascribed to the presence of different elements. Stars were proven to be similar to the Earth's own Sun, but with a wide range of ]s, ]es, and sizes.<ref name="short history" />
{{Main|Observational astronomy}}


=== Deep space astronomy ===
In astronomy, the main source of information about ] and other objects is the visible ] or more generally ].<ref>{{cite web|url=http://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.html|title = Electromagnetic Spectrum|publisher = NASA|accessdate = 2006-09-08 }}</ref> Observational astronomy may be divided according to the observed region of the ]. Some parts of the spectrum can be observed from the ]'s surface, while other parts are only observable from either high altitudes or space. Specific information on these subfields is given below.
], by ] from 29 December 1888. With the calculation of its distance in 1923 ] was proven, allowing the calculation of the age and expanse of the ].]]
The existence of the Earth's galaxy, the ], as its own group of stars was only proven in the 20th century, along with the existence of "external" galaxies. The observed recession of those galaxies led to the discovery of the expansion of the ].<ref name=Belkora2003>{{cite book|author=Belkora, Leila|title=Minding the heavens: the story of our discovery of the Milky Way|isbn=978-0-7503-0730-7|url=https://books.google.com/books?id=qBM-wez94WwC|publisher=]|date=2003|pages=1–14|access-date=26 August 2020|archive-date=27 October 2020|archive-url=https://web.archive.org/web/20201027093857/https://books.google.com/books?id=qBM-wez94WwC|url-status=live}}</ref> In 1919, when the ] was completed, the prevailing view was that the universe consisted entirely of the Milky Way Galaxy. Using the Hooker Telescope, ] identified ]s in several spiral nebulae and in 1922–1923 proved conclusively that ] and ] among others, were entire galaxies outside our own, thus proving that the universe consists of a multitude of galaxies.<ref name="SharovNovikov1993">{{cite book|last1=Sharov|first1=Aleksandr Sergeevich|last2=Novikov|first2=Igor Dmitrievich|title=Edwin Hubble, the discoverer of the big bang universe|url=https://books.google.com/books?id=ttEwkEdPc70C&pg=PA34|access-date=December 31, 2011|date=1993|publisher=Cambridge University Press|isbn=978-0-521-41617-7|page=34|archive-date=June 23, 2013|archive-url=https://web.archive.org/web/20130623075250/http://books.google.com/books?id=ttEwkEdPc70C&pg=PA34|url-status=live}}</ref> With this Hubble formulated the ], which allowed for the first time a calculation of the age of the Universe and size of the Observable Universe, which became increasingly precise with better meassurements, starting at 2 billion years and 280 million light-years, until 2006 when data of the ] allowed a very accurate calculation of the age of the Universe and size of the Observable Universe.<ref name="p537">{{cite web | title=Cosmic Times | website=Imagine the Universe! | date=December 8, 2017 | url=https://imagine.gsfc.nasa.gov/educators/programs/cosmictimes/educators/guide/age_size.html | access-date=October 31, 2024}}</ref>

]) ], taken 2019 ], located at the core of ].]]

Theoretical astronomy led to speculations on the existence of objects such as ]s and ]s, which have been used to explain such observed phenomena as ]s, ]s, ]s, and ]. ] made huge advances during the 20th century. In the early 1900s the model of the ] theory was formulated, heavily evidenced by ], ], and the ]. ]s have enabled measurements in parts of the electromagnetic spectrum normally blocked or blurred by the atmosphere.<ref>{{cite book | chapter=Beating the atmosphere | first=Ian S. | last=McLean | title=Electronic Imaging in Astronomy | series=Springer Praxis Books | date=2008 | isbn=978-3-540-76582-0 | pages=39–75 | publisher=Springer | location=Berlin, Heidelberg | doi=10.1007/978-3-540-76583-7_2 }}</ref> In February 2016, it was revealed that the ] project had ] of ] in the previous September.<ref name="Discovery 2016">{{cite journal |title=Einstein's gravitational waves found at last |journal=Nature News |url=http://www.nature.com/news/einstein-s-gravitational-waves-found-at-last-1.19361 |date=11 February 2016 |last1=Castelvecchi |first1=Davide |last2=Witze |first2=Witze |doi=10.1038/nature.2016.19361 |s2cid=182916902 |access-date=11 February 2016 |archive-date=12 February 2016 |archive-url=https://web.archive.org/web/20160212082216/http://www.nature.com/news/einstein-s-gravitational-waves-found-at-last-1.19361 |url-status=live }}</ref><ref name='Abbot'>{{cite journal |title=Observation of Gravitational Waves from a Binary Black Hole Merger| author=B.P. Abbott |collaboration=LIGO Scientific Collaboration and Virgo Collaboration| journal=Physical Review Letters| year=2016| volume=116|issue=6| pages=061102| doi=10.1103/PhysRevLett.116.061102| pmid=26918975| bibcode=2016PhRvL.116f1102A|arxiv = 1602.03837 | s2cid=124959784}}</ref>

== Observational astronomy ==
{{Main|Observational astronomy}}
]
The main source of information about ] and other objects is ], or more generally ].<ref>{{cite web|url=http://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.html|title=Electromagnetic Spectrum|publisher=NASA|access-date=17 November 2016|archive-url=https://web.archive.org/web/20060905131651/http://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.html|archive-date=5 September 2006 }}</ref> Observational astronomy may be categorized according to the corresponding region of the ] on which the observations are made. Some parts of the spectrum can be observed from the Earth's surface, while other parts are only observable from either high altitudes or outside the Earth's atmosphere. Specific information on these subfields is given below.


===Radio astronomy=== ===Radio astronomy===
] in ], an example of a ]]]
{{Main|Radio astronomy}} {{Main|Radio astronomy}}
Radio astronomy uses radiation with ]s greater than approximately one millimeter, outside the visible range.<ref name="cox2000">{{cite book

|editor=Cox, A.N.
Radio astronomy studies radiation with ]s greater than approximately one millimeter.<ref name="cox2000">{{cite book
|editor=Cox, A. N.
|title=Allen's Astrophysical Quantities |title=Allen's Astrophysical Quantities
|year=2000 |date=2000
|url=http://books.google.com/?id=w8PK2XFLLH8C&pg=PA124 |url=https://books.google.com/books?id=w8PK2XFLLH8C&pg=PA124
|publisher=Springer-Verlag |publisher=Springer-Verlag
|page=124 |page=124
|location=New York |location=New York
|isbn=978-0-387-98746-0
|isbn=0-387-98746-0}}</ref> Radio astronomy is different from most other forms of observational astronomy in that the observed ]s can be treated as ]s rather than as discrete ]s. Hence, it is relatively easier to measure both the ] and ] of radio waves, whereas this is not as easily done at shorter wavelengths.<ref name="cox2000"/>
|access-date=26 August 2020
|archive-date=19 November 2020
|archive-url=https://web.archive.org/web/20201119200822/https://books.google.com/books?id=w8PK2XFLLH8C&pg=PA124
|url-status=live
}}</ref> Radio astronomy is different from most other forms of observational astronomy in that the observed ]s can be treated as ]s rather than as discrete ]s. Hence, it is relatively easier to measure both the ] and ] of radio waves, whereas this is not as easily done at shorter wavelengths.<ref name="cox2000"/>


Although some ]s are produced by astronomical objects in the form of ], most of the radio emission that is observed from Earth is seen in the form of ], which is produced when ]s oscillate around ]s.<ref name="cox2000"/> Additionally, a number of ]s produced by ], notably the ] spectral line at 21&nbsp;cm, are observable at radio wavelengths.<ref name="shu1982"/><ref name="cox2000"/> Although some ]s are emitted directly by astronomical objects, a product of ], most of the radio emission that is observed is the result of ], which is produced when ]s orbit ]s.<ref name="cox2000"/> Additionally, a number of ]s produced by ], notably the ] spectral line at 21&nbsp;cm, are observable at radio wavelengths.<ref name="shu1982"/><ref name="cox2000"/>


A wide variety of objects are observable at radio wavelengths, including ]e, interstellar gas, ]s, and ].<ref name="shu1982"/><ref name="cox2000"/> A wide variety of other objects are observable at radio wavelengths, including ]e, interstellar gas, ]s, and ].<ref name="shu1982"/><ref name="cox2000"/>


===Infrared astronomy=== === Infrared astronomy ===
] Observatory is one of the highest observatory sites on Earth. Atacama, Chile.<ref>{{cite news|title=In Search of Space|url=http://www.eso.org/public/images/potw1431a/|access-date=5 August 2014|work=Picture of the Week|agency=European Southern Observatory|archive-date=13 August 2020|archive-url=https://web.archive.org/web/20200813090738/https://www.eso.org/public/images/potw1431a/|url-status=live}}</ref>]]
{{Main|Infrared astronomy}} {{Main|Infrared astronomy}}
Infrared astronomy is founded on the detection and analysis of ] radiation, wavelengths longer than red light and outside the range of our vision. The infrared spectrum is useful for studying objects that are too cold to radiate visible light, such as planets, ]s or nebulae whose light is blocked by dust. The longer wavelengths of infrared can penetrate clouds of dust that block visible light, allowing the observation of young stars embedded in ]s and the cores of galaxies. Observations from the ] (WISE) have been particularly effective at unveiling numerous galactic ]s and their host ].<ref name="wright">{{cite web|url=http://wise.ssl.berkeley.edu/|title=Wide-field Infrared Survey Explorer Mission|date=30 September 2014|publisher=] ]|access-date=17 November 2016|archive-url=https://web.archive.org/web/20100112144939/http://wise.ssl.berkeley.edu/|archive-date=12 January 2010}}</ref><ref name=ma2013>{{Cite journal |bibcode = 2013Ap&SS.344..175M|title = Discovering protostars and their host clusters via WISE|last1 = Majaess|first1 = D.|journal = Astrophysics and Space Science|volume = 344|issue = 1|pages = 175–186|year = 2013|arxiv = 1211.4032|doi = 10.1007/s10509-012-1308-y|s2cid = 118455708}}</ref>

Infrared astronomy deals with the detection and analysis of ] radiation (wavelengths longer than red light). Except at ] close to visible light, infrared radiation is heavily absorbed by the atmosphere, and the atmosphere produces significant infrared emission. Consequently, infrared observatories have to be located in high, dry places or in space. The infrared spectrum is useful for studying objects that are too cold to radiate visible light, such as planets and ]s. Longer infrared wavelengths can also penetrate clouds of dust that block visible light, allowing observation of young stars in ]s and the cores of galaxies.<ref>{{cite news With the exception of infrared ] close to visible light, such radiation is heavily absorbed by the atmosphere, or masked, as the atmosphere itself produces significant infrared emission. Consequently, infrared observatories have to be located in high, dry places on Earth or in space.<ref>{{cite news
|author=Staff|date=2003-09-11 |author=Staff
|date=11 September 2003
|title=Why infrared astronomy is a hot topic |title=Why infrared astronomy is a hot topic
|publisher=ESA |publisher=ESA
|url=http://www.esa.int/esaCP/SEMX9PZO4HD_FeatureWeek_0.html |url=http://www.esa.int/esaCP/SEMX9PZO4HD_FeatureWeek_0.html
|access-date=11 August 2008
|accessdate=2008-08-11 }}</ref> Some molecules radiate strongly in the infrared. This can be used to study chemistry in space; more specifically it can detect water in comets.<ref>{{cite news
|archive-date=30 July 2012
|title=Infrared Spectroscopy – An Overview
|archive-url=https://archive.today/20120730/http://www.esa.int/esaCP/SEMX9PZO4HD_FeatureWeek_0.html
|publisher=NASA/IPAC
|url-status=live
|url=http://www.ipac.caltech.edu/Outreach/Edu/Spectra/irspec.html
}}</ref> Some molecules radiate strongly in the infrared. This allows the study of the chemistry of space; more specifically it can detect water in comets.<ref>{{cite news|url=http://www.ipac.caltech.edu/Outreach/Edu/Spectra/irspec.html|title=Infrared Spectroscopy – An Overview|publisher=] ]|access-date=11 August 2008|archive-url=https://web.archive.org/web/20081005031543/http://www.ipac.caltech.edu/Outreach/Edu/Spectra/irspec.html|archive-date=5 October 2008}}</ref>
|accessdate=2008-08-11 }}</ref>

===Optical astronomy===

] (left) and ] (center) on ], both examples of an observatory that operates at near-infrared and visible wavelengths. The ] (right) is an example of a telescope that operates only at near-infrared wavelengths.]]


=== Optical astronomy ===
] (left) and ] (center) on ], both examples of an observatory that operates at near-infrared and visible wavelengths. The ] (right) is an example of a telescope that operates only at near-infrared wavelengths.]]
{{Main|Optical astronomy}} {{Main|Optical astronomy}}
Historically, optical astronomy, which has been also called visible light astronomy, is the oldest form of astronomy.<ref name="moore1997">{{cite book

Historically, optical astronomy, also called visible light astronomy, is the oldest form of astronomy.<ref name="moore1997">{{cite book
|author=Moore, P. |author=Moore, P.
|title=Philip's Atlas of the Universe |title=Philip's Atlas of the Universe
|year=1997 |date=1997
|publisher=George Philis Limited |publisher=George Philis Limited
|location=Great Britain |location=Great Britain
|isbn=0-540-07465-9}}</ref> Optical images were originally drawn by hand. In the late 19th century and most of the 20th century, images were made using photographic equipment. Modern images are made using digital detectors, particularly detectors using ] (CCDs). Although visible light itself extends from approximately 4000 ] to 7000 Å (400 ] to 700&nbsp;nm),<ref name="moore1997"/> the same equipment used at these wavelengths is also used to observe some ] and ] radiation. |isbn=978-0-540-07465-5}}</ref> Images of observations were originally drawn by hand. In the late 19th century and most of the 20th century, images were made using photographic equipment. Modern images are made using digital detectors, particularly using ]s (CCDs) and recorded on modern medium. Although visible light itself extends from approximately 4000 ] to 7000 Å (400 ] to 700&nbsp;nm),<ref name="moore1997"/> that same equipment can be used to observe some ] and ] radiation.


===Ultraviolet astronomy=== === Ultraviolet astronomy ===
{{Main|Ultraviolet astronomy}} {{Main|Ultraviolet astronomy}}


Ultraviolet astronomy is generally used to refer to observations at ] wavelengths between approximately 100 and 3200 Å (10 to 320&nbsp;nm).<ref name="cox2000"/> Light at these wavelengths is absorbed by the Earth's atmosphere, so observations at these wavelengths must be performed from the upper atmosphere or from space. Ultraviolet astronomy is best suited to the study of thermal radiation and spectral emission lines from hot blue ]s (]s) that are very bright in this wave band. This includes the blue stars in other galaxies, which have been the targets of several ultraviolet surveys. Other objects commonly observed in ultraviolet light include ]e, ]s, and active galactic nuclei.<ref name="cox2000"/> However, as ultraviolet light is easily absorbed by ], an appropriate adjustment of ultraviolet measurements is necessary.<ref name="cox2000"/> Ultraviolet astronomy employs ] wavelengths between approximately 100 and 3200&nbsp;Å (10 to 320&nbsp;nm).<ref name="cox2000"/> Light at those wavelengths is absorbed by the Earth's atmosphere, requiring observations at these wavelengths to be performed from the upper atmosphere or from space. Ultraviolet astronomy is best suited to the study of thermal radiation and spectral emission lines from hot blue ]s (]s) that are very bright in this wave band. This includes the blue stars in other galaxies, which have been the targets of several ultraviolet surveys. Other objects commonly observed in ultraviolet light include ]e, ]s, and active galactic nuclei.<ref name="cox2000"/> However, as ultraviolet light is easily absorbed by ], an adjustment of ultraviolet measurements is necessary.<ref name="cox2000"/>


===X-ray astronomy=== === X-ray astronomy ===
{{Main|X-ray astronomy}} {{Main|X-ray astronomy}}
]
X-ray astronomy uses ]. Typically, X-ray radiation is produced by ] (the result of electrons orbiting magnetic field lines), ] above 10<sup>7</sup> (10&nbsp;million) ]s, and ] above 10<sup>7</sup> Kelvin.<ref name="cox2000"/> Since X-rays are absorbed by the ], all X-ray observations must be performed from ]s, ]s, or ]s. Notable ] include ], ]s, ]s, ], ], and ].<ref name="cox2000"/>


=== Gamma-ray astronomy ===
X-ray astronomy is the study of ]s at ]s. Typically, objects emit X-ray radiation as ] (produced by electrons oscillating around magnetic field lines), ] above 10<sup>7</sup> (10 million) ]s, and ] above 10<sup>7</sup> Kelvin.<ref name="cox2000"/> Since X-rays are absorbed by the ], all X-ray observations must be performed from ], ]s, or ]. Notable ]s include ], ]s, ]s, ], ], and ].<ref name="cox2000"/>

According to NASA's official website, X-rays were first observed and documented in 1895 by ], a German scientist who found them quite by accident when experimenting with vacuum tubes. Through a series of experiments, including the infamous X-ray photograph he took of his wife's hand with a wedding ring on it, Röntgen was able to discover the beginning elements of radiation. The "X", in fact, holds its own significance, as it represents Röntgen's inability to identify exactly what type of radiation it was.

Furthermore, according to the website, in some German speaking countries, X-rays are still sometimes referred to as Röntgen rays, in honor of the man who discovered them.

===Gamma-ray astronomy===
{{Main|Gamma ray astronomy}} {{Main|Gamma ray astronomy}}
Gamma ray astronomy observes astronomical objects at the shortest wavelengths of the electromagnetic spectrum. Gamma rays may be observed directly by satellites such as the ] or by specialized telescopes called ]s.<ref name="cox2000"/> The Cherenkov telescopes do not detect the gamma rays directly but instead detect the flashes of visible light produced when gamma rays are absorbed by the Earth's atmosphere.<ref name="spectrum">{{cite web|url=http://www.pparc.ac.uk/frontiers/latest/feature.asp?article=14F1&style=feature|title=The electromagnetic spectrum|last=Penston|first=Margaret J.|date=14 August 2002|publisher=Particle Physics and Astronomy Research Council|archive-url=https://archive.today/20120908014227/http://www.pparc.ac.uk/frontiers/latest/feature.asp?article=14F1&style=feature|archive-date=8 September 2012|access-date=17 November 2016}}</ref>


Most ] emitting sources are actually ]s, objects which only produce gamma radiation for a few milliseconds to thousands of seconds before fading away. Only 10% of gamma-ray sources are non-transient sources. These steady gamma-ray emitters include pulsars, ]s, and ] candidates such as active galactic nuclei.<ref name="cox2000"/>
Gamma ray astronomy is the study of astronomical objects at the shortest wavelengths of the electromagnetic spectrum. Gamma rays may be observed directly by satellites such as the ] or by specialized telescopes called ]s.<ref name="cox2000"/> The Cherenkov telescopes do not actually detect the gamma rays directly but instead detect the flashes of visible light produced when gamma rays are absorbed by the Earth's atmosphere.<ref name="spectrum">{{cite web|last = Penston|first = Margaret J.|date = 2002-08-14|url=http://www.pparc.ac.uk/frontiers/latest/feature.asp?article=14F1&style=feature|title = The electromagnetic spectrum|publisher = Particle Physics and Astronomy Research Council|accessdate = 2006-08-17 }}</ref>


=== Fields not based on the electromagnetic spectrum ===
Most ] emitting sources are actually ]s, objects which only produce gamma radiation for a few milliseconds to thousands of seconds before fading away. Only 10% of gamma-ray sources are non-transient sources. These steady gamma-ray emitters include pulsars, ]s, and ] candidates such as active galactic nuclei.<ref name="cox2000"/>
In addition to electromagnetic radiation, a few other events originating from great distances may be observed from the Earth.


In ], astronomers use heavily shielded ] such as ], ], and ] for the detection of ]s. The vast majority of the neutrinos streaming through the Earth originate from the ], but 24 neutrinos were also detected from ].<ref name="cox2000"/> ]s, which consist of very high energy particles (atomic nuclei) that can decay or be absorbed when they enter the Earth's atmosphere, result in a cascade of secondary particles which can be detected by current observatories.<ref>{{cite book
===Fields not based on the electromagnetic spectrum===
|first=Thomas K.|last=Gaisser|date=1990
|title=Cosmic Rays and Particle Physics|url=https://archive.org/details/cosmicrayspartic0000gais|url-access=registration|pages=
|publisher=Cambridge University Press|isbn=978-0-521-33931-5}}</ref> Some future ]s may also be sensitive to the particles produced when cosmic rays hit the Earth's atmosphere.<ref name="cox2000"/>


] is an emerging field of astronomy that employs ]s to collect observational data about distant massive objects. A few observatories have been constructed, such as the ''Laser Interferometer Gravitational Observatory'' ]. LIGO made its ] on 14 September 2015, observing gravitational waves from a ].<ref name="PRL-20160211">{{cite journal |collaboration=LIGO Scientific Collaboration and Virgo Collaboration |last1=Abbott |first1=Benjamin P. |title=Observation of Gravitational Waves from a Binary Black Hole Merger |journal=] |volume=116 |issue=6 |pages=061102 |year=2016 |doi=10.1103/PhysRevLett.116.061102 |arxiv=1602.03837 |bibcode=2016PhRvL.116f1102A |pmid=26918975 |s2cid=124959784 }}</ref> A second ] was detected on 26 December 2015 and additional observations should continue but ]s require extremely sensitive instruments.<ref>{{cite web |url=http://www.europhysicsnews.org/index.php?option=article&access=standard&Itemid=129&url=/articles/epn/abs/2003/02/epn03208/epn03208.html |title=Opening new windows in observing the Universe |last1=Tammann |first1=Gustav-Andreas <!-- Gustav Alfred Andreas -->|author-link=Gustav Andreas Tammann |first2=Friedrich-Karl |last2=Thielemann |author-link2=Friedrich-Karl Thielemann |first3=Dirk |last3=Trautmann |date=2003 |publisher=Europhysics News |archive-url=https://archive.today/20120906192257/http://www.europhysicsnews.org/index.php?option=com_article&access=standard&Itemid=129&url=/articles/epn/abs/2003/02/epn03208/epn03208.html |archive-date=6 September 2012 |access-date=17 November 2016 }}</ref><ref>{{Cite journal |author1=LIGO Scientific Collaboration and Virgo Collaboration |last2=Abbott |first2=B. P. |last3=Abbott |first3=R. |last4=Abbott |first4=T. D.|last5=Abernathy |first5=M. R. |last6=Acernese |first6=F. |last7=Ackley |first7=K. |last8=Adams |first8=C. |last9=Adams |first9=T. |date=15 June 2016 |title=GW151226: Observation of Gravitational Waves from a 22-Solar-Mass Binary Black Hole Coalescence |journal=Physical Review Letters |volume=116 |issue=24 |pages=241103 |doi=10.1103/PhysRevLett.116.241103 |pmid=27367379 |arxiv=1606.04855 |bibcode=2016PhRvL.116x1103A |s2cid=118651851 }}</ref>
In addition to electromagnetic radiation, a few other events originating from great distances may be observed from the Earth.


The combination of observations made using electromagnetic radiation, neutrinos or gravitational waves and other complementary information, is known as ].<ref>{{cite web|title=Planning for a bright tomorrow: Prospects for gravitational-wave astronomy with Advanced LIGO and Advanced Virgo|url=http://www.ligo.org/science/Publication-ObservingScenario/index.php|publisher=]|access-date=31 December 2015|archive-date=23 April 2016|archive-url=https://web.archive.org/web/20160423031110/http://www.ligo.org/science/Publication-ObservingScenario/index.php|url-status=live}}</ref><ref>{{cite book |title=Neutrinos in Particle Physics, Astronomy and Cosmology |first1=Zhizhong |last1=Xing |first2=Shun |last2=Zhou |publisher=Springer |date=2011 |isbn=978-3-642-17560-2 |page=313 |url=https://books.google.com/books?id=6QXqlCHLjJkC&pg=PA313 |access-date=20 June 2015 |archive-date=3 February 2021 |archive-url=https://web.archive.org/web/20210203012300/https://books.google.com/books?id=6QXqlCHLjJkC&pg=PA313 |url-status=live }}</ref>
In ], astronomers use special ] such as ], ], and ] for detecting ]s. These neutrinos originate primarily from the ] but also from ]e.<ref name="cox2000"/> ]s, which consist of very high energy particles that can decay or be absorbed when they enter the Earth's atmosphere, result in a cascade of particles which can be detected by current observatories.<ref>{{cite book
|first=Thomas K.|last=Gaisser|year=1990
|title=Cosmic Rays and Particle Physics|pages=1–2
|publisher=Cambridge University Press|isbn=0521339316 }}</ref> Additionally, some future ]s may also be sensitive to the particles produced when cosmic rays hit the Earth's atmosphere.<ref name="cox2000"/> ] is an emerging new field of astronomy which aims to use ]s to collect observational data about compact objects. A few observatories have been constructed, such as the ''Laser Interferometer Gravitational Observatory'' ], but ]s are extremely difficult to detect.<ref>{{cite web|author =Tammann, G. A.; Thielemann, F. K.; Trautmann, D.|year = 2003|url=http://www.europhysicsnews.org/index.php?option=article&access=standard&Itemid=129&url=/articles/epn/abs/2003/02/epn03208/epn03208.html|title = Opening new windows in observing the Universe|publisher = Europhysics News|accessdate = 2010-02-03 }}</ref>


=== Astrometry and celestial mechanics ===
Planetary astronomers have directly observed many of these phenomena through spacecraft and sample return missions. These observations include fly-by missions with remote sensors, landing vehicles that can perform experiments on the surface materials, impactors that allow remote sensing of buried material, and sample return missions that allow direct laboratory examination.

===Astrometry and celestial mechanics===
{{Main|Astrometry|Celestial mechanics}} {{Main|Astrometry|Celestial mechanics}}
] with a nebula]]
One of the oldest fields in astronomy, and in all of science, is the measurement of the positions of celestial objects. Historically, accurate knowledge of the positions of the Sun, Moon, planets and stars has been essential in ] (the use of celestial objects to guide navigation) and in the making of ]s.<ref name=":0">{{Cite book |last=Fraknoi |first=Andrew |url=https://openstax.org/details/books/astronomy-2e |title=Astronomy 2e |date=2022 |display-authors=etal |publisher=OpenStax |isbn=978-1-951693-50-3 |edition=2e |oclc=1322188620 |access-date=16 March 2023 |archive-date=23 February 2023 |archive-url=https://web.archive.org/web/20230223211041/https://openstax.org/details/books/astronomy-2e |url-status=live }}</ref>{{rp|39}}


Careful measurement of the positions of the planets has led to a solid understanding of gravitational ], and an ability to determine past and future positions of the planets with great accuracy, a field known as ]. More recently the tracking of ]s will allow for predictions of close encounters or potential collisions of the Earth with those objects.<ref>{{cite web|last = Calvert|first = James B.|date = 28 March 2003|url = http://www.du.edu/~jcalvert/phys/orbits.htm|title = Celestial Mechanics|publisher = University of Denver|access-date = 21 August 2006|archive-url = https://web.archive.org/web/20060907120741/http://www.du.edu/~jcalvert/phys/orbits.htm|archive-date = 7 September 2006}}</ref>
One of the oldest fields in astronomy, and in all of science, is the measurement of the positions of celestial objects. Historically, accurate knowledge of the positions of the Sun, Moon, planets and stars has been essential in ] and in the making of ]s.

Careful measurement of the positions of the planets has led to a solid understanding of gravitational ], and an ability to determine past and future positions of the planets with great accuracy, a field known as ]. More recently the tracking of ]s will allow for predictions of close encounters, and potential collisions, with the Earth.<ref>{{cite web|last = Calvert|first = James B.|date = 2003-03-28|url=http://www.du.edu/~jcalvert/phys/orbits.htm|title = Celestial Mechanics|publisher = University of Denver|accessdate = 2006-08-21 }}</ref>


The measurement of ] of nearby stars provides a fundamental baseline in the ] that is used to measure the scale of the universe. Parallax measurements of nearby stars provide an absolute baseline for the properties of more distant stars, because their properties can be compared. Measurements of ] and ] show the kinematics of these systems through the Milky Way galaxy. Astrometric results are also used to measure the distribution of ] in the galaxy.<ref>{{cite web|url=http://www.astro.virginia.edu/~rjp0i/museum/engines.html|title = Hall of Precision Astrometry|publisher = University of Virginia Department of Astronomy|accessdate = 2006-08-10 }}</ref> The measurement of ] of nearby stars provides a fundamental baseline in the ] that is used to measure the scale of the Universe. Parallax measurements of nearby stars provide an absolute baseline for the properties of more distant stars, as their properties can be compared. Measurements of the ] and ] of stars allow astronomers to plot the movement of these systems through the Milky Way galaxy. Astrometric results are the basis used to calculate the distribution of speculated ] in the galaxy.<ref>{{cite web|url=http://www.astro.virginia.edu/~rjp0i/museum/engines.html|title=Hall of Precision Astrometry|publisher=] Department of Astronomy|access-date=17 November 2016|archive-url=https://web.archive.org/web/20060826104509/http://www.astro.virginia.edu/~rjp0i/museum/engines.html|archive-date=26 August 2006 }}</ref>


During the 1990s, the astrometric technique of measuring the ] was ] large ]s orbiting nearby stars.<ref name="Wolszczan">{{cite journal| author=Wolszczan, A.; Frail, D. A.| title=A planetary system around the millisecond pulsar PSR1257+12| journal=Nature| year=1992| volume=355| issue=| pages=145–147| doi= 10.1038/355145a0| ref=harv}}</ref> During the 1990s, the measurement of the ] of nearby stars was ] large ]s orbiting those stars.<ref name="Wolszczan">{{cite journal| author=Wolszczan, A.| author2=Frail, D. A.| title=A planetary system around the millisecond pulsar PSR1257+12| journal=Nature| date=1992| volume=355| issue=6356|pages=145–47| doi= 10.1038/355145a0| bibcode=1992Natur.355..145W| s2cid=4260368}}</ref>


==Theoretical astronomy== == Theoretical astronomy ==
{{Nucleosynthesis}} {{Nucleosynthesis}}
{{Main|Theoretical astronomy}} {{Main|Theoretical astronomy}}
Theoretical astronomers use a wide variety of tools which include ] (for example, ]s to approximate the behaviors of a ]) and ]al ]. Each has some advantages. Analytical models of a process are generally better for giving insight into the heart of what is going on. Numerical models can reveal the existence of phenomena and effects that would otherwise not be seen.<ref>{{cite journal|first=H.|last=Roth|title=A Slowly Contracting or Expanding Fluid Sphere and its Stability|journal=Physical Review |volume=39|pages=525–529|year=1932|doi=10.1103/PhysRev.39.525|ref=harv}}</ref><ref>{{cite book|first=A.S.|last=Eddington|title=Internal Constitution of the Stars|publisher=Cambridge University Press|year=1926|url=http://books.google.com/?id=hJW3JbhnFQMC&pg=PA182|isbn=9780521337083}}</ref> Theoretical astronomers use several tools including ] and ]al ]; each has its particular advantages. Analytical models of a process are better for giving broader insight into the heart of what is going on. Numerical models reveal the existence of phenomena and effects otherwise unobserved.<ref>{{cite journal|first=H.|last=Roth|title=A Slowly Contracting or Expanding Fluid Sphere and its Stability|journal=Physical Review |volume=39|issue=3|pages=525–29|date=1932|doi=10.1103/PhysRev.39.525|bibcode = 1932PhRv...39..525R }}</ref><ref>{{cite journal |first=A.S.|last=Eddington|title=Internal Constitution of the Stars|journal=Science|publisher=Cambridge University Press|date=1926|volume=52|issue=1341|pages=233–40|doi=10.1126/science.52.1341.233|url=https://books.google.com/books?id=hJW3JbhnFQMC&pg=PA182|isbn=978-0-521-33708-3|pmid=17747682|bibcode=1920Sci....52..233E |bibcode-access=free |access-date=4 November 2020|archive-date=17 August 2021|archive-url=https://web.archive.org/web/20210817020341/https://books.google.com/books?id=hJW3JbhnFQMC&pg=PA182|url-status=live}}</ref>


Theorists in astronomy endeavor to create theoretical models and figure out the observational consequences of those models. This helps observers look for data that can refute a model or help in choosing between several alternate or conflicting models. Theorists in astronomy endeavor to create theoretical models that are based on existing observations and known physics, and to predict observational consequences of those models. The observation of phenomena predicted by a model allows astronomers to select between several alternative or conflicting models. Theorists also modify existing models to take into account new observations. In some cases, a large amount of observational data that is inconsistent with a model may lead to abandoning it largely or completely, as for ], the existence of ], and the ] of cosmic evolution.


Phenomena modeled by theoretical astronomers include:
Theorists also try to generate or modify models to take into account new data. In the case of an inconsistency, the general tendency is to try to make minimal modifications to the model to fit the data. In some cases, a large amount of inconsistent data over time may lead to total abandonment of a model.
* ] and ]
* ]
* ] of matter in the ]
* the origin of ]s
* ] and ], including ] and ].


Modern theoretical astronomy reflects dramatic advances in observation since the 1990s, including studies of the ], distant ] and ], which have led to the development of a ]. This model requires the universe to contain large amounts of ] and ] whose nature is currently not well understood, but the model gives detailed predictions that are in excellent agreement with many diverse observations.<ref name="PDG">{{cite journal | url=http://pdg.ge.infn.it/2011/reviews/rpp2011-rev-cosmological-parameters.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://pdg.ge.infn.it/2011/reviews/rpp2011-rev-cosmological-parameters.pdf |archive-date=2022-10-09 |url-status=live | title=2013 Review of Particle Physics | author=Beringer, J. | author2=et al. (Particle Data Group) | journal=Phys. Rev. D | date=2012 | volume=86 | issue=1 | page=010001|doi=10.1103/PhysRevD.86.010001|bibcode = 2012PhRvD..86a0001B | doi-access=free }}</ref>
Topics studied by theoretical astronomers include: ] and ]; ]; ] of ] in the ]; origin of ]s; ] and ], including ] cosmology and ]. Astrophysical relativity serves as a tool to gauge the properties of large scale structures for which gravitation plays a significant role in physical phenomena investigated and as the basis for ] (''astro'')] and the study of ].


== Specific subfields ==
Some widely accepted and studied theories and models in astronomy, now included in the ] are the ], ], ], and fundamental theories of ].


=== Astrophysics ===
A few examples of this process:
{{main |Astrophysics}}
] and ] to understand the measurements made by astronomy. Representation of the Observable Universe that includes images from ] and other ].]]


Astrophysics is the branch of astronomy that employs the principles of physics and ] "to ascertain the nature of the ]s, rather than their positions or motions in space".<ref>{{Cite journal | last = Keeler | first = James E. | author-link = James E. Keeler | title = The Importance of Astrophysical Research and the Relation of Astrophysics to the Other Physical Sciences | journal = The Astrophysical Journal | volume = 6 | issue = 4 | pages = 271–88 | date = November 1897 | bibcode = 1897ApJ.....6..271K |doi = 10.1086/140401| pmid = 17796068 | quote = is closely allied on the one hand to astronomy, of which it may properly be classed as a branch, and on the other hand to chemistry and physics.… It seeks to ascertain the nature of the heavenly bodies, rather than their positions or motions in space—''what'' they are, rather than ''where'' they are.… That which is perhaps most characteristic of astrophysics is the special prominence which it gives to the study of radiation.| doi-access = free }}</ref><ref>{{cite web | title=astrophysics | publisher=Merriam-Webster, Incorporated | url=http://www.merriam-webster.com/dictionary/astrophysics | access-date=22 May 2011 | archive-url= https://web.archive.org/web/20110610085146/http://www.merriam-webster.com/dictionary/astrophysics| archive-date= 10 June 2011 | url-status= live}}</ref> Among the objects studied are the ], other ]s, ], ]s, the ] and the ].<ref name="nasa.gov">{{cite web|url=https://science.nasa.gov/astrophysics/focus-areas/|title=Focus Areas – NASA Science|work=nasa.gov|access-date=12 November 2018|archive-date=16 May 2017|archive-url=https://web.archive.org/web/20170516154030/https://science.nasa.gov/astrophysics/focus-areas|url-status=live}}</ref><ref>{{cite encyclopedia|url=https://www.britannica.com/EBchecked/topic/40047/astronomy|title=astronomy|encyclopedia=Encyclopædia Britannica|access-date=12 November 2018|archive-date=10 May 2015|archive-url=https://web.archive.org/web/20150510024116/https://www.britannica.com/EBchecked/topic/40047/astronomy|url-status=live}}</ref> Their emissions are examined across all parts of the ], and the properties examined include ], ], ], and ] composition. Because astrophysics is a very broad subject, ''astrophysicists'' typically apply many disciplines of physics, including ], ], ], ], ], ], ] and ], and ].
{| class="wikitable" border="1"
|-
| |<!-- A --> '''Physical process'''
| |<!-- B --> '''Experimental tool'''
| |<!-- C --> '''Theoretical model'''
| |<!-- D --> '''Explains/predicts'''
|-
| |<!-- A -->]
| |<!-- B -->]s
| |<!-- C -->]
| |<!-- D -->Emergence of a ]
|-
| |<!-- A --> ]
| |<!-- B --> ]
| |<!-- C --> ]
| |<!-- D --> How the stars shine and how ]
|-
| |<!-- A -->]
| |<!-- B -->], ]
| |<!-- C --> ]
| |<!-- D --> ]
|-
| |<!-- A --> ]s
| |<!-- B -->
| |<!-- C --> ]
| |<!-- D --> ]
|-
| |<!-- A --> ]
| |<!-- B --> ]
| |<!-- C --> ]
| |<!-- D --> ]s at the center of ]
|-
| |<!-- A --> ] in ]s
| |<!-- B -->
| |<!-- C -->
| |<!-- D -->
|-
|}


In practice, modern astronomical research often involves a substantial amount of work in the realms of ] and observational physics. Some areas of study for astrophysicists include their attempts to determine the properties of ], ], and ]; whether or not ] is possible, ]s can form, or the ] exists; and the ] and ].<ref name="nasa.gov"/> Topics also studied by theoretical astrophysicists include ]; ] and ]; ]; ]; ] of ] in the universe; origin of ]s; ] and ], including ] cosmology and ].
] and ] are the current leading topics in astronomy,<ref>{{cite web|url=http://imagine.gsfc.nasa.gov/docs/science/know_l1/dark_matter.html|quote=third paragraph, "There is currently much ongoing research by scientists attempting to discover exactly what this dark matter is"|accessdate=2009-11-02|publisher=NASA|title=Dark matter|year=2010}}</ref> as their discovery and controversy originated during the study of the galaxies.


=== Astrochemistry ===
==Specific subfields==
{{main|Astrochemistry}}
===Solar astronomy===
Astrochemistry is the study of the abundance and reactions of ]s in the ], and their interaction with ]. The discipline is an overlap of astronomy and ]. The word "astrochemistry" may be applied to both the ] and the ]. The study of the abundance of elements and ] ratios in Solar System objects, such as ]s, is also called ], while the study of interstellar atoms and molecules and their interaction with radiation is sometimes called molecular astrophysics. The formation, atomic and chemical composition, evolution and fate of ] is of special interest, because it is from these clouds that solar systems form. Studies in this field contribute to the understanding of the ], Earth's origin and geology, ], and the origin of climate and oceans.<ref>{{Cite news|url=https://www.cfa.harvard.edu/research/amp-rg/astrochemistry|title=Astrochemistry|date=15 July 2013|newspaper=www.cfa.harvard.edu/|access-date=20 November 2016|archive-url=https://web.archive.org/web/20161120211934/https://www.cfa.harvard.edu/research/amp-rg/astrochemistry|archive-date=20 November 2016}}</ref>
] image of the Sun's active ] as viewed by the ] space telescope. ''] photo'']]
{{Main|Sun}}


=== Astrobiology ===
At a distance of about eight light-minutes, the most frequently studied star is the Sun, a typical main-sequence ] of ] G2 V, and about 4.6 Gyr in age. The Sun is not considered a ], but it does undergo periodic changes in activity known as the ]. This is an 11-year fluctuation in ] numbers. Sunspots are regions of lower-than- average temperatures that are associated with intense magnetic activity.<ref name="solar FAQ">{{cite web|last = Johansson|first = Sverker|date = 2003-07-27|url=http://www.talkorigins.org/faqs/faq-solar.html|title = The Solar FAQ|publisher = Talk.Origins Archive|accessdate = 2006-08-11 }}</ref>
{{main|Astrobiology}}
Astrobiology is an interdisciplinary scientific field concerned with the ], ], distribution, and future of ] in the ]. Astrobiology considers the question of whether ] exists, and how humans can detect it if it does.<ref name="about">{{cite web| url=http://astrobiology.nasa.gov/about-astrobiology/ |title=About Astrobiology |access-date=20 October 2008 |date=21 January 2008 |work=NASA Astrobiology Institute |publisher=NASA | archive-url= https://web.archive.org/web/20081011192341/http://astrobiology.nasa.gov/about-astrobiology/| archive-date= 11 October 2008}}</ref> The term exobiology is similar.<ref> {{Webarchive|url=https://web.archive.org/web/20180904084642/https://www.merriam-webster.com/dictionary/exobiology |date=4 September 2018 }} (accessed 11 April 2013)</ref>


Astrobiology makes use of ], ], ], ], astronomy, ], ] and ] to investigate the possibility of life on other worlds and help recognize ]s that might be different from that on Earth.<ref>{{cite book |title=The life and death of planet Earth |last1=Ward |first1=P.D. |author2=Brownlee, D. |date=2004 |publisher=Owl Books |location=New York |isbn=978-0-8050-7512-0 }}</ref> ] and early evolution of life is an inseparable part of the discipline of astrobiology.<ref>{{cite web |url=https://link.springer.com/journal/11084 |title=Origins of Life and Evolution of Biospheres |work=Journal: Origins of Life and Evolution of Biospheres |access-date=6 April 2015 |archive-date=8 February 2020 |archive-url=https://web.archive.org/web/20200208140912/https://link.springer.com/journal/11084 |url-status=live }}</ref> Astrobiology concerns itself with interpretation of existing ], and although speculation is entertained to give context, astrobiology concerns itself primarily with ] that fit firmly into existing ].
The Sun has steadily increased in luminosity over the course of its life, increasing by 40% since it first became a main-sequence star. The Sun has also undergone periodic changes in luminosity that can have a significant impact on the Earth.<ref name="Environmental issues : essential primary sources.">{{cite web|first = K. Lee|last=Lerner|coauthors = Lerner, Brenda Wilmoth.|year = 2006|url=http://catalog.loc.gov/cgi-bin/Pwebrecon.cgi?v3=1&DB=local&CMD=010a+2006000857&CNT=10+records+per+page|title = Environmental issues : essential primary sources."|publisher = Thomson Gale|accessdate = 2006-09-11 }}</ref> The ], for example, is believed to have caused the ] phenomenon during the ].<ref name="future-sun">{{cite web|author=Pogge, Richard W.|year=1997|url=http://www.astronomy.ohio-state.edu/~pogge/Lectures/vistas97.html|title=The Once & Future Sun|format=lecture notes|work=New Vistas in Astronomy|accessdate=2010-02-03}}</ref>


This ] field encompasses research on the origin of ]s, origins of ], rock-water-carbon interactions, ] on Earth, ], research on ]s for life detection, and studies on the potential for ] on Earth and in ].<ref name="Goals2016">{{cite news |url=http://astrobiology.com/2016/03/release-of-the-first-roadmap-for-european-astrobiology.html |title=Release of the First Roadmap for European Astrobiology |work=European Science Foundation |publisher=Astrobiology Web |date=29 March 2016 |access-date=2 April 2016 |archive-date=10 June 2020 |archive-url=https://web.archive.org/web/20200610010327/http://astrobiology.com/2016/03/release-of-the-first-roadmap-for-european-astrobiology.html |url-status=live }}</ref><ref name="NYT-20151218-jc">{{cite news |last=Corum |first=Jonathan |title=Mapping Saturn's Moons |url=https://www.nytimes.com/interactive/2015/12/18/science/space/nasa-cassini-maps-saturns-moons.html |date=18 December 2015 |work=] |access-date=18 December 2015 |archive-date=20 May 2020 |archive-url=https://web.archive.org/web/20200520124847/https://www.nytimes.com/interactive/2015/12/18/science/space/nasa-cassini-maps-saturns-moons.html |url-status=live }}</ref><ref>{{cite news | last = Cockell | first = Charles S. | title = How the search for aliens can help sustain life on Earth | date = 4 October 2012 | url = http://edition.cnn.com/2012/10/02/world/europe/astrobiology-aliens-environment-opinion/index.html?hpt=hp_c4 | work = CNN News | access-date = 8 October 2012 | archive-date = 10 September 2016 | archive-url = https://web.archive.org/web/20160910182606/http://edition.cnn.com/2012/10/02/world/europe/astrobiology-aliens-environment-opinion/index.html?hpt=hp_c4 | url-status = live }}</ref>
The visible outer surface of the Sun is called the ]. Above this layer is a thin region known as the ]. This is surrounded by a transition region of rapidly increasing temperatures, then by the super-heated ].


=== Physical cosmology ===
At the center of the Sun is the core region, a volume of sufficient temperature and pressure for ] to occur. Above the core is the ], where the plasma conveys the energy flux by means of radiation. The outer layers form a ] where the gas material transports energy primarily through physical displacement of the gas. It is believed that this convection zone creates the magnetic activity that generates sun spots.<ref name="solar FAQ" />
{{Nature timeline}}
{{Main|Physical cosmology}}


] (from the Greek {{lang|grc|κόσμος}} ({{transliteration|grc|kosmos}}) "world, universe" and {{lang|grc|λόγος}} ({{transliteration|grc|logos}}) "word, study" or literally "logic") could be considered the study of the Universe as a whole.
A solar wind of plasma particles constantly streams outward from the Sun until it reaches the ]. This solar wind interacts with the ] of the Earth to create the ]s, as well as the ] where the lines of the ] descend into the ].<ref>{{cite web|author = Stern, D. P.; Peredo, M.|date = 2004-09-28|url=http://www-istp.gsfc.nasa.gov/Education/Intro.html|title = The Exploration of the Earth's Magnetosphere|publisher = NASA|accessdate = 2006-08-22 }}</ref>


]]]
===Planetary science===
{{Main|Planetary science|Planetary geology}}


Observations of the ], a branch known as ], have provided a deep understanding of the formation and evolution of the cosmos. Fundamental to modern cosmology is the well-accepted theory of the ], wherein our Universe began at a single ], and thereafter ] over the course of 13.8 billion years<ref>{{cite web
This astronomical field examines the assemblage of ]s, ], ]s, ]s, ]s, and other bodies orbiting the Sun, as well as extrasolar planets. The ] has been relatively well-studied, initially through telescopes and then later by spacecraft. This has provided a good overall understanding of the formation and evolution of this planetary system, although many new discoveries are still being made.<ref name="geology">{{cite book|author=Bell III, J. F.; Campbell, B. A.; Robinson, M. S.|title=Remote Sensing for the Earth Sciences: Manual of Remote Sensing|publisher=John Wiley & Sons|edition = 3rd|year=2004|url=http://marswatch.tn.cornell.edu/rsm.html|accessdate = 2006-08-23 }}</ref>
|title = Cosmic Detectives
|url = http://www.esa.int/Our_Activities/Space_Science/Cosmic_detectives
|publisher = The European Space Agency (ESA)
|date = 2 April 2013
|access-date = 15 April 2013
|archive-date = 11 February 2019
|archive-url = https://web.archive.org/web/20190211204726/http://www.esa.int/Our_Activities/Space_Science/Cosmic_detectives
|url-status = live
}}</ref> to its present condition.<ref name=Dodelson2003/> The concept of the Big Bang can be traced back to the discovery of the ] in 1965.<ref name=Dodelson2003>{{cite book|last=Dodelson|first=Scott|title=Modern cosmology|date=2003|isbn=978-0-12-219141-1|publisher=]|pages=1–22}}</ref>


In the course of this expansion, the Universe underwent several evolutionary stages. In the very early moments, it is theorized that the Universe experienced a very rapid ], which homogenized the starting conditions. Thereafter, ] produced the elemental abundance of the early Universe.<ref name=Dodelson2003/> (See also ].)
] climbing a crater wall on ]. This moving, swirling column of ] (comparable to a terrestrial ]) created the long, dark streak. ''] image''.]]


When the first neutral ]s formed from a sea of primordial ions, space became transparent to radiation, releasing the energy viewed today as the microwave background radiation. The expanding Universe then underwent a Dark Age due to the lack of stellar energy sources.<ref name="cosmology 101">{{cite web|last = Hinshaw|first = Gary|date = 13 July 2006|url=http://map.gsfc.nasa.gov/m_uni.html|title = Cosmology 101: The Study of the Universe|publisher = NASA WMAP|access-date =10 August 2006| archive-url= https://web.archive.org/web/20060813053535/http://map.gsfc.nasa.gov/m_uni.html| archive-date= 13 August 2006 | url-status= live}}</ref>
The solar system is subdivided into the inner planets, the ], and the outer planets. The inner ]s consist of ], ], ], and ]. The outer ] planets are ], ], ], and ].<ref name="planets">{{cite web|author = Grayzeck, E.; Williams, D. R.| date = 2006-05-11|url=http://nssdc.gsfc.nasa.gov/planetary/|title = Lunar and Planetary Science|publisher = NASA|accessdate = 2006-08-21 }}</ref> Beyond Neptune lies the ], and finally the ], which may extend as far as a light-year.


A hierarchical structure of matter began to form from minute variations in the mass density of space. Matter accumulated in the densest regions, forming clouds of gas and the earliest stars, the ]. These massive stars triggered the ] process and are believed to have created many of the heavy elements in the early Universe, which, through nuclear decay, create lighter elements, allowing the cycle of nucleosynthesis to continue longer.<ref>Dodelson, 2003, pp. 216–61</ref>
The planets were formed in the ] that surrounded the early Sun. Through a process that included gravitational attraction, collision, and accretion, the disk formed clumps of matter that, with time, became protoplanets. The ] of the ] then expelled most of the unaccreted matter, and only those planets with sufficient mass retained their gaseous atmosphere. The planets continued to sweep up, or eject, the remaining matter during a period of intense bombardment, evidenced by the many ]s on the Moon. During this period, some of the protoplanets may have collided, the ] for how the Moon was formed.<ref name=Montmerle2006>{{cite journal|last=Montmerle|first=Thierry|coauthors=Augereau, Jean-Charles; Chaussidon, Marc et al.|title=Solar System Formation and Early Evolution: the First 100 Million Years|journal=Earth, Moon, and Planets|volume=98|publisher=Spinger|pages=39&ndash;95|year=2006|doi=10.1007/s11038-006-9087-5| url=http://adsabs.harvard.edu/abs/2006EM%26P...98...39M|ref=harv}}</ref>


Gravitational aggregations clustered into filaments, leaving voids in the gaps. Gradually, organizations of gas and dust merged to form the first primitive galaxies. Over time, these pulled in more matter, and were often organized into ] of galaxies, then into larger-scale superclusters.<ref>{{cite web|url=http://www.damtp.cam.ac.uk/user/gr/public/gal_lss.html|title = Galaxy Clusters and Large-Scale Structure|publisher = University of Cambridge|access-date =8 September 2006| archive-url= https://web.archive.org/web/20061010041120/http://www.damtp.cam.ac.uk/user/gr/public/gal_lss.html| archive-date= 10 October 2006 | url-status= live}}</ref>
Once a planet reaches sufficient mass, the materials with different densities segregate within, during ]. This process can form a stony or metallic core, surrounded by a mantle and an outer surface. The core may include solid and liquid regions, and some planetary cores generate their own ], which can protect their atmospheres from solar wind stripping.<ref>Montmerle, 2006, pp. 87–90</ref>


Fundamental to the structure of the Universe is the existence of ] and ]. These are now thought to be its dominant components, forming 96% of the mass of the Universe. For this reason, much effort is expended in trying to understand the physics of these components.<ref>{{cite web|last = Preuss|first = Paul|url=http://www.lbl.gov/Science-Articles/Archive/dark-energy.html|title = Dark Energy Fills the Cosmos|publisher = U.S. Department of Energy, Berkeley Lab|access-date =8 September 2006| archive-url= https://web.archive.org/web/20060811215815/http://www.lbl.gov/Science-Articles/Archive/dark-energy.html| archive-date= 11 August 2006 | url-status= live}}</ref>
A planet or moon's interior heat is produced from the collisions that created the body, radioactive materials (''e.g.'' ], ], and ]), or ]. Some planets and moons accumulate enough heat to drive geologic processes such as ] and tectonics. Those that accumulate or retain an ] can also undergo surface ] from wind or water. Smaller bodies, without tidal heating, cool more quickly; and their geological activity ceases with the exception of impact cratering.<ref name="new solar system">{{cite book| editor=Beatty, J.K.; Petersen, C.C.; Chaikin, A.|title=The New Solar System|publisher=Cambridge press|url=http://books.google.com/?id=iOezyHMVAMcC&pg=PA70|page=70edition = 4th|year=1999|isbn =0-521-64587-5 }}</ref>


===Stellar astronomy=== === Extragalactic astronomy ===


] effect of the cluster of yellow galaxies near the middle of the photograph. The lens is produced by the cluster's gravitational field that bends light to magnify and distort the image of a more distant object.]]
]. Ejecting gas from the dying central star shows symmetrical patterns unlike the chaotic patterns of ordinary explosions.]]
{{Main|Extragalactic astronomy}}
The study of objects outside our galaxy is a branch of astronomy concerned with the ], their morphology (description) and ], the observation of ], and at a larger scale, the ]. Finally, the latter is important for the understanding of the ].<ref name=":0" />


Most ] are organized into distinct shapes that allow for classification schemes. They are commonly divided into ], ] and ] galaxies.<ref>{{cite web|last = Keel|first = Bill|date = 1 August 2006|url=http://www.astr.ua.edu/keel/galaxies/classify.html|title = Galaxy Classification|publisher = University of Alabama|access-date =8 September 2006| archive-url= https://web.archive.org/web/20060901074027/http://www.astr.ua.edu/keel/galaxies/classify.html| archive-date= 1 September 2006 | url-status= live}}</ref>
{{Main|Star}}


As the name suggests, an elliptical galaxy has the cross-sectional shape of an ]. The stars move along ] orbits with no preferred direction. These galaxies contain little or no interstellar dust, few star-forming regions, and older stars.<ref name=":0" />{{Rp|pages=877–878}} Elliptical galaxies may have been formed by other galaxies merging.<ref name=":0" />{{Rp|page=939}}
The study of ]s and ] is fundamental to our understanding of the universe. The astrophysics of stars has been determined through observation and theoretical understanding; and from computer simulations of the interior.<ref name=Amos7>Harpaz, 1994, pp. 7–18</ref>


A spiral galaxy is organized into a flat, rotating disk, usually with a prominent bulge or bar at the center, and trailing bright arms that spiral outward. The arms are dusty regions of star formation within which massive young stars produce a blue tint. Spiral galaxies are typically surrounded by a halo of older stars. Both the ] and one of our nearest galaxy neighbors, the ], are spiral galaxies.<ref name=":0" />{{Rp|page=875}}
] occurs in dense regions of dust and gas, known as ]. When destabilized, cloud fragments can collapse under the influence of gravity, to form a ]. A sufficiently dense, and hot, core region will trigger ], thus creating a ].<ref name=Smith2004/>


Irregular galaxies are chaotic in appearance, and are neither spiral nor elliptical.<ref name=":0" />{{Rp|page=879}} About a quarter of all galaxies are irregular, and the peculiar shapes of such galaxies may be the result of gravitational interaction.<ref>{{Cite web |date=2016-08-08 |title=A lopsided lynx |url=https://esahubble.org/images/potw1632a/ |access-date=2023-03-17 |website=esahubble.org |publisher=] |language=en |archive-date=9 July 2021 |archive-url=https://web.archive.org/web/20210709183618/https://esahubble.org/images/potw1632a/ |url-status=live }}</ref>
Almost all elements heavier than ] and ] were ] inside the cores of stars.<ref name=Amos7/>


An active galaxy is a formation that emits a significant amount of its energy from a source other than its stars, dust and gas. It is powered by a compact region at the core, thought to be a supermassive black hole that is emitting radiation from in-falling material.<ref name=":0" />{{Rp|page=907}} A ] is an active galaxy that is very luminous in the radio portion of the spectrum, and is emitting immense plumes or lobes of gas. Active galaxies that emit shorter frequency, high-energy radiation include ], ]s, and ]s. Quasars are believed to be the most consistently luminous objects in the known universe.<ref>{{cite web|url=http://imagine.gsfc.nasa.gov/docs/science/know_l1/active_galaxies.html|title=Active Galaxies and Quasars|publisher=NASA|access-date=17 November 2016|archive-url=https://web.archive.org/web/20060831033713/http://imagine.gsfc.nasa.gov/docs/science/know_l1/active_galaxies.html|archive-date=31 August 2006 }}</ref>
The characteristics of the resulting star depend primarily upon its starting mass. The more massive the star, the greater its luminosity, and the more rapidly it expends the hydrogen fuel in its core. Over time, this hydrogen fuel is completely converted into helium, and the star begins to ]. The fusion of helium requires a higher core temperature, so that the star both expands in size, and increases in core density. The resulting ] enjoys a brief life span, before the helium fuel is in turn consumed. Very massive stars can also undergo a series of decreasing evolutionary phases, as they fuse increasingly heavier elements.<ref name=Amos>Harpaz, 1994</ref>


The ] is represented by groups and clusters of galaxies. This structure is organized into a hierarchy of groupings, with the largest being the ]s. The collective matter is formed into ] and walls, leaving large ] between.<ref name="evolving universe">{{cite book|author=]|title=Astronomy: The Evolving Universe|edition=8th|publisher=Wiley|date=2002|isbn=978-0-521-80090-7}}</ref>
The final fate of the star depends on its mass, with stars of mass greater than about eight times the Sun becoming core collapse ]e;<ref>Harpaz, 1994, pp. 173–178</ref> while smaller stars form ]e, and evolve into ]s.<ref>Harpaz, 1994, pp. 111–118</ref> The remnant of a supernova is a dense ], or, if the stellar mass was at least three times that of the Sun, a ].<ref name="Cambridge atlas">{{cite book|editor= Audouze, Jean; Israel, Guy|title=The Cambridge Atlas of Astronomy|edition=3rd|publisher=Cambridge University Press|year=1994|isbn=0-521-43438-6 }}</ref> Close binary stars can follow more complex evolutionary paths, such as mass transfer onto a white dwarf companion that can potentially cause a supernova.<ref>Harpaz, 1994, pp. 189–210</ref> Planetary nebulae and supernovae are necessary for the distribution of ] to the interstellar medium; without them, all new stars (and their planetary systems) would be formed from hydrogen and helium alone.<ref>Harpaz, 1994, pp. 245–256</ref>


===Galactic astronomy=== === Galactic astronomy ===
{{Main|Galactic astronomy}}


]'s spiral arms]] ]'s spiral arms]]
{{Main|Galactic astronomy}}
Our ] orbits within the ], a ] that is a prominent member of the ] of galaxies. It is a rotating mass of gas, dust, stars and other objects, held together by mutual gravitational attraction. As the Earth is located within the dusty outer arms, there are large portions of the Milky Way that are obscured from view.
The ] orbits within the ], a ] that is a prominent member of the ] of galaxies. It is a rotating mass of gas, dust, stars and other objects, held together by mutual gravitational attraction. As the Earth is located within the dusty outer arms, there are large portions of the Milky Way that are obscured from view.<ref name=":0" />{{Rp|pages=837–842,944}}


In the center of the Milky Way is the core, a bar-shaped bulge with what is believed to be a ] at the center. This is surrounded by four primary arms that spiral from the core. This is a region of active star formation that contains many younger, ] stars. The disk is surrounded by a ] of older, ] stars, as well as relatively dense concentrations of stars known as ]s.<ref>{{cite web|last = Ott|first = Thomas|date = 2006-08-24|url=http://www.mpe.mpg.de/ir/GC/index.php|title = The Galactic Centre|publisher = Max-Planck-Institut für extraterrestrische Physik|accessdate = 2006-09-08 }}</ref><ref>{{cite journal|last = Faulkner|first = Danny R.|title=The Role Of Stellar Population Types In The Discussion Of Stellar Evolution|journal=CRS Quarterly|year=1993|volume=30|issue=1|pages=174–180|url=http://www.creationresearch.org/crsq/articles/30/30_1/StellarPop.html|accessdate=2006-09-08|ref = harv }}</ref> In the center of the Milky Way is the core, a bar-shaped bulge with what is believed to be a ] at its center. This is surrounded by four primary arms that spiral from the core. This is a region of active star formation that contains many younger, ] stars. The disk is surrounded by a ] of older, ] stars, as well as relatively dense concentrations of stars known as ]s.<ref>{{cite web|url=http://www.mpe.mpg.de/ir/GC/index.php|title=The Galactic Centre|last=Ott|first=Thomas|date=24 August 2006|publisher=Max-Planck-Institut für extraterrestrische Physik|access-date=17 November 2016|archive-url=https://web.archive.org/web/20060904140550/http://www.mpe.mpg.de/ir/GC/index.php|archive-date=4 September 2006 }}</ref>


Between the stars lies the ], a region of sparse matter. In the densest regions, ]s of ] and other elements create star-forming regions. These begin as a compact pre-stellar core or ]e, which concentrate and collapse (in volumes determined by the ]) to form compact protostars.<ref name=Smith2004>{{cite book|first=Michael David|last=Smith|year=2004| pages=53–86|title=The Origin of Stars|chapter=Cloud formation, Evolution and Destruction| publisher=Imperial College Press|isbn=1860945015 |url=http://books.google.com/?id=UVgBoqZg8a4C&dq}}</ref> Between the stars lies the ], a region of sparse matter. In the densest regions, ]s of ] and other elements create star-forming regions. These begin as a compact ] or ]e, which concentrate and collapse (in volumes determined by the ]) to form compact protostars.<ref name=Smith2004>{{cite book|first=Michael David|last=Smith|date=2004|pages=53–86|title=The Origin of Stars|chapter=Cloud formation, Evolution and Destruction|publisher=Imperial College Press|isbn=978-1-86094-501-4|chapter-url=https://books.google.com/books?id=UVgBoqZg8a4C|access-date=26 August 2020|archive-date=13 August 2021|archive-url=https://web.archive.org/web/20210813210429/https://books.google.com/books?id=UVgBoqZg8a4C|url-status=live}}</ref>


As the more massive stars appear, they transform the cloud into an ] of glowing gas and plasma. The ] and supernova explosions from these stars eventually serve to disperse the cloud, often leaving behind one or more young ]s of stars. These clusters gradually disperse, and the stars join the population of the Milky Way.<ref>{{cite book|first=Michael David|last=Smith|year=2004| pages=185–199|title=The Origin of Stars|chapter=Massive stars As the more massive stars appear, they transform the cloud into an ] (ionized atomic hydrogen) of glowing gas and plasma. The ] and supernova explosions from these stars eventually cause the cloud to disperse, often leaving behind one or more young ]s of stars. These clusters gradually disperse, and the stars join the population of the Milky Way.<ref>{{cite book|first=Michael David|last=Smith|date=2004|pages=185–99|title=The Origin of Stars|chapter=Massive stars|publisher=Imperial College Press|isbn=978-1-86094-501-4|chapter-url=https://books.google.com/books?id=UVgBoqZg8a4C|access-date=26 August 2020|archive-date=13 August 2021|archive-url=https://web.archive.org/web/20210813210429/https://books.google.com/books?id=UVgBoqZg8a4C|url-status=live}}</ref>
|publisher=Imperial College Press|isbn=1860945015 |url=http://books.google.com/?id=UVgBoqZg8a4C&dq}}</ref>


Kinematic studies of matter in the Milky Way and other galaxies have demonstrated that there is more mass than can be accounted for by visible matter. A ] appears to dominate the mass, although the nature of this dark matter remains undetermined.<ref>{{cite journal|author=Van den Bergh, Sidney|title=The Early History of Dark Matter|journal=Publications of the Astronomy Society of the Pacific|year=1999|volume=111|pages=657–660|url=http://www.journals.uchicago.edu/doi/full/10.1086/316369|doi=10.1086/316369|ref=harv }}</ref> Kinematic studies of matter in the Milky Way and other galaxies have demonstrated that there is more mass than can be accounted for by visible matter. A ] appears to dominate the mass, although the nature of this dark matter remains undetermined.<ref>{{cite journal|author=Van den Bergh, Sidney|title=The Early History of Dark Matter|journal=Publications of the Astronomical Society of the Pacific|date=1999|volume=111|issue=760|pages=657–60|doi=10.1086/316369|arxiv = astro-ph/9904251 |bibcode = 1999PASP..111..657V |s2cid=5640064}}</ref>


===Extragalactic astronomy=== === Stellar astronomy ===
], often referred to as the Ant planetary nebula. Ejecting gas from the dying central star shows symmetrical patterns unlike the chaotic patterns of ordinary explosions.]]
] effect of the cluster of yellow galaxies near the middle of the photograph. The lens is produced by the cluster's gravitational field that bends light to magnify and distort the image of a more distant object.]]
{{Main|Extragalactic astronomy}} {{Main|Star}}
{{see also|Solar astronomy}}
The study of stars and ] is fundamental to our understanding of the Universe. The astrophysics of stars has been determined through observation and theoretical understanding; and from computer simulations of the interior.<ref name=Amos7>Harpaz, 1994, pp. 7–18</ref> ] occurs in dense regions of dust and gas, known as ]. When destabilized, cloud fragments can collapse under the influence of gravity, to form a ]. A sufficiently dense, and hot, core region will trigger ], thus creating a ].<ref name=Smith2004/>


Almost all elements heavier than ] and ] were ] inside the cores of stars.<ref name=Amos7/>
The study of objects outside our galaxy is a branch of astronomy concerned with the ]; their morphology and ]; and the examination of ], and the ]. The latter is important for the understanding of the ].


The characteristics of the resulting star depend primarily upon its starting mass. The more massive the star, the greater its luminosity, and the more rapidly it fuses its hydrogen fuel into helium in its core. Over time, this hydrogen fuel is completely converted into helium, and the star begins to ]. The fusion of helium requires a higher core temperature. A star with a high enough core temperature will push its outer layers outward while increasing its core density. The resulting ] formed by the expanding outer layers enjoys a brief life span, before the helium fuel in the core is in turn consumed. Very massive stars can also undergo a series of evolutionary phases, as they fuse increasingly heavier elements.<ref name=Amos>Harpaz, 1994</ref>
Most ] are organized into distinct shapes that allow for classification schemes. They are commonly divided into ], ] and ] galaxies.<ref>{{cite web|last = Keel|first = Bill|date = 2006-08-01|url=http://www.astr.ua.edu/keel/galaxies/classify.html|title = Galaxy Classification|publisher = University of Alabama|accessdate = 2006-09-08 }}</ref>


The final fate of the star depends on its mass, with stars of mass greater than about eight times the Sun becoming core collapse ]e;<ref>Harpaz, 1994, pp. 173–78</ref> while smaller stars blow off their outer layers and leave behind the inert core in the form of a ]. The ejection of the outer layers forms a ].<ref>Harpaz, 1994, pp. 111–18</ref> The remnant of a supernova is a dense ], or, if the stellar mass was at least three times that of the Sun, a ].<ref name="Cambridge atlas">{{cite book|editor= Audouze, Jean|editor2= Israel, Guy|title=The Cambridge Atlas of Astronomy|edition=3rd|publisher=Cambridge University Press|date=1994|isbn=978-0-521-43438-6}}</ref> Closely orbiting binary stars can follow more complex evolutionary paths, such as mass transfer onto a white dwarf companion that can potentially cause a supernova.<ref>Harpaz, 1994, pp. 189–210</ref> Planetary nebulae and supernovae distribute the "]" produced in the star by fusion to the interstellar medium; without them, all new stars (and their planetary systems) would be formed from hydrogen and helium alone.<ref>Harpaz, 1994, pp. 245–56</ref>
As the name suggests, an elliptical galaxy has the cross-sectional shape of an ]. The stars move along ] orbits with no preferred direction. These galaxies contain little or no interstellar dust; few star-forming regions; and generally older stars. Elliptical galaxies are more commonly found at the core of galactic clusters, and may be formed through mergers of large galaxies.


=== Solar astronomy ===
A spiral galaxy is organized into a flat, rotating disk, usually with a prominent bulge or bar at the center, and trailing bright arms that spiral outward. The arms are dusty regions of star formation where massive young stars produce a blue tint. Spiral galaxies are typically surrounded by a halo of older stars. Both the ] and the ] are spiral galaxies.
] image of the Sun's active ] as viewed by the NASA's ] space telescope.]]
] (]) built in 1962]]
{{See also|Solar telescope}}
At a distance of about eight light-minutes, the most frequently studied star is the ], a typical main-sequence ] of ] G2 V, and about 4.6 billion years (Gyr) old. The Sun is not considered a ], but it does undergo periodic changes in activity known as the ]. This is an 11-year oscillation in ]. Sunspots are regions of lower-than-average temperatures that are associated with intense magnetic activity.<ref name="solar FAQ">{{cite web|url=http://www.talkorigins.org/faqs/faq-solar.html|title=The Solar FAQ|last=Johansson|first=Sverker|author-link=Sverker Johansson|date=27 July 2003|publisher=Talk.Origins Archive|access-date=11 August 2006|archive-url=https://web.archive.org/web/20060907235636/http://www.talkorigins.org/faqs/faq-solar.html|archive-date=7 September 2006 |url-status=live}}</ref>


The Sun has steadily increased in luminosity by 40% since it first became a main-sequence star. The Sun has also undergone periodic changes in luminosity that can have a significant impact on the Earth.<ref name="Environmental issues : essential primary sources.">{{cite web|url=http://catalog.loc.gov/cgi-bin/Pwebrecon.cgi?v3=1&DB=local&CMD=010a+2006000857&CNT=10+records+per+page|title=Environmental issues: essential primary sources|last1=Lerner|first1=K. Lee|first2=Brenda Wilmoth|date=2006|publisher=Thomson Gale|archive-url=https://archive.today/20120710152134/http://catalog.loc.gov/cgi-bin/Pwebrecon.cgi?v3=1&DB=local&CMD=010a+2006000857&CNT=10+records+per+page|archive-date=10 July 2012|last2=Lerner|access-date=17 November 2016}}</ref> The ], for example, is believed to have caused the ] phenomenon during the ].<ref name="future-sun">{{cite web|author=Pogge, Richard W. |date=1997 |url=http://www.astronomy.ohio-state.edu/~pogge/Lectures/vistas97.html |title=The Once & Future Sun |format=lecture notes |work=New Vistas in Astronomy |access-date=3 February 2010 |archive-url=https://web.archive.org/web/20050527094435/http://www-astronomy.mps.ohio-state.edu/Vistas/ |archive-date=27 May 2005 }}</ref>
Irregular galaxies are chaotic in appearance, and are neither spiral nor elliptical. About a quarter of all galaxies are irregular, and the peculiar shapes of such galaxies may be the result of gravitational interaction.


At the center of the Sun is the core region, a volume of sufficient temperature and pressure for ] to occur. Above the core is the ], where the plasma conveys the energy flux by means of radiation. Above that is the ] where the gas material transports energy primarily through physical displacement of the gas known as convection. It is believed that the movement of mass within the convection zone creates the magnetic activity that generates sunspots.<ref name="solar FAQ" /> The visible outer surface of the Sun is called the ]. Above this layer is a thin region known as the ]. This is surrounded by a transition region of rapidly increasing temperatures, and finally by the super-heated ].<ref name=":0" />{{Rp|pages=498–502}}
An active galaxy is a formation that is emitting a significant amount of its energy from a source other than stars, dust and gas; and is powered by a compact region at the core, usually thought to be a super-massive black hole that is emitting radiation from in-falling material.


A solar wind of plasma particles constantly streams outward from the Sun until, at the outermost limit of the Solar System, it reaches the ]. As the solar wind passes the Earth, it interacts with the ] (]) and deflects the solar wind, but traps some creating the ]s that envelop the Earth. The ] are created when solar wind particles are guided by the magnetic flux lines into the Earth's polar regions where the lines then descend into the ].<ref>{{cite web|author = Stern, D.P.|author2 = Peredo, M.|date = 28 September 2004|url=http://www-istp.gsfc.nasa.gov/Education/Intro.html|title = The Exploration of the Earth's Magnetosphere|publisher = NASA|access-date =22 August 2006| archive-url= https://web.archive.org/web/20060824003619/http://www-istp.gsfc.nasa.gov/Education/Intro.html| archive-date= 24 August 2006 | url-status= live}}</ref>
A ] is an active galaxy that is very luminous in the ] portion of the spectrum, and is emitting immense plumes or lobes of gas. Active galaxies that emit high-energy radiation include ], ]s, and ]s. Quasars are believed to be the most consistently luminous objects in the known universe.<ref>{{cite web|url=http://imagine.gsfc.nasa.gov/docs/science/know_l1/active_galaxies.html|title = Active Galaxies and Quasars|publisher = NASA|accessdate = 2006-09-08 }}</ref>


=== Planetary science ===
The ] is represented by groups and clusters of galaxies. This structure is organized in a hierarchy of groupings, with the largest being the ]s. The collective matter is formed into ] and walls, leaving large ] in between.<ref name="evolving universe">{{cite book|first=Michael|last=Zeilik|title=Astronomy: The Evolving Universe|edition=8th|publisher=Wiley|year=2002|isbn=0-521-80090-0 }}</ref>
] climbing a crater wall on ]. This moving, swirling column of ] (comparable to a terrestrial ]) created the long, dark streak.]]

{{Main|Planetary science|Planetary geology}}
===Cosmology===
Planetary science is the study of the assemblage of ]s, ], ]s, ]s, ]s, and other bodies orbiting the Sun, as well as extrasolar planets. The ] has been relatively well-studied, initially through telescopes and then later by spacecraft. This has provided a good overall understanding of the formation and evolution of the Sun's planetary system, although many new discoveries are still being made.<ref name="geology">{{cite book|url=http://marswatch.tn.cornell.edu/rsm.html|title=Remote Sensing for the Earth Sciences: Manual of Remote Sensing|date=2004|publisher=John Wiley & Sons|edition=3rd|author=Bell III, J. F.|author2=Campbell, B.A.|author3=Robinson, M.S.|access-date=17 November 2016|archive-url=https://web.archive.org/web/20060811220029/http://marswatch.tn.cornell.edu/rsm.html|archive-date=11 August 2006 }}</ref>
{{Main|Physical cosmology}}

Cosmology (from the Greek κόσμος "world, universe" and λόγος "word, study") could be considered the study of the universe as a whole.

Observations of the ] of the ], a branch known as ], have provided a deep understanding of the formation and evolution of the cosmos. Fundamental to modern cosmology is the well-accepted theory of the ], wherein our universe began at a single point in time, and thereafter ] over the course of 13.7 Gyr to its present condition.<ref name=Dodelson2003/> The concept of the big bang can be traced back to the discovery of the ] in 1965.<ref name=Dodelson2003>{{cite book|last=Dodelson|first=Scott|title=Modern cosmology|year=2003|isbn=9780122191411|publisher=]|pages=1–22}}</ref>

In the course of this expansion, the universe underwent several evolutionary stages. In the very early moments, it is theorized that the universe experienced a very rapid ], which homogenized the starting conditions. Thereafter, ] produced the elemental abundance of the early universe.<ref name=Dodelson2003/> (See also ].)

When the first atoms formed, space became transparent to radiation, releasing the energy viewed today as the microwave background radiation. The expanding universe then underwent a Dark Age due to the lack of stellar energy sources.<ref name="cosmology 101">{{cite web|last = Hinshaw|first = Gary|date = 2006-07-13|url=http://map.gsfc.nasa.gov/m_uni.html|title = Cosmology 101: The Study of the Universe|publisher = NASA WMAP|accessdate = 2006-08-10 }}</ref>


The Solar System is divided into the ] (subdivided into the inner planets and the ]), the ] (subdivided into the outer planets and ]), comets, the trans-Neptunian region (subdivided into the ], and the ]) and the farthest regions (e.g., boundaries of the ], and the ], which may extend as far as a light-year). The inner ]s consist of ], ], Earth, and ]. The outer ]s are the ]s (] and ]) and the ]s (] and ]).<ref name="planets">{{cite web|author = Grayzeck, E.|author2 = Williams, D.R.| date = 11 May 2006|url=http://nssdc.gsfc.nasa.gov/planetary/|title = Lunar and Planetary Science|publisher = NASA|access-date =21 August 2006| archive-url= https://web.archive.org/web/20060820173205/http://nssdc.gsfc.nasa.gov/planetary/| archive-date= 20 August 2006 | url-status= live}}</ref>
A hierarchical structure of matter began to form from minute variations in the mass density. Matter accumulated in the densest regions, forming clouds of gas and the ]. These massive stars triggered the ] process and are believed to have created many of the heavy elements in the early universe which tend to decay back to the lighter elements extending the cycle.<ref>Dodelson, 2003, pp. 216–261</ref>


The planets were formed 4.6 billion years ago in the ] that surrounded the early Sun. Through a process that included gravitational attraction, collision, and accretion, the disk formed clumps of matter that, with time, became protoplanets. The ] of the ] then expelled most of the unaccreted matter, and only those planets with sufficient mass retained their gaseous atmosphere. The planets continued to sweep up, or eject, the remaining matter during a period of intense bombardment, evidenced by the many ]s on the Moon. During this period, some of the protoplanets may have collided and one such collision may have ].<ref name=Montmerle2006>{{cite journal|last=Montmerle|first=Thierry|author2=Augereau, Jean-Charles|author3= Chaussidon, Marc|title=Solar System Formation and Early Evolution: the First 100 Million Years|journal=Earth, Moon, and Planets|volume=98|issue=1–4|pages=39–95|date=2006|doi=10.1007/s11038-006-9087-5| bibcode=2006EM&P...98...39M|s2cid=120504344|display-authors=etal}}</ref>
Gravitational aggregations clustered into filaments, leaving voids in the gaps. Gradually, organizations of gas and dust merged to form the first primitive galaxies. Over time, these pulled in more matter, and were often organized into ] of galaxies, then into larger-scale superclusters.<ref>{{cite web|url=http://www.damtp.cam.ac.uk/user/gr/public/gal_lss.html|title = Galaxy Clusters and Large-Scale Structure|publisher = University of Cambridge|accessdate = 2006-09-08 }}</ref>


Once a planet reaches sufficient mass, the materials of different densities segregate within, during ]. This process can form a stony or metallic core, surrounded by a mantle and an outer crust. The core may include solid and liquid regions, and some planetary cores generate their own ], which can protect their atmospheres from solar wind stripping.<ref>Montmerle, 2006, pp. 87–90</ref>
Fundamental to the structure of the universe is the existence of ] and ]. These are now thought to be the dominant components, forming 96% of the mass of the universe. For this reason, much effort is expended in trying to understand the physics of these components.<ref>{{cite web|last = Preuss|first = Paul|url=http://www.lbl.gov/Science-Articles/Archive/dark-energy.html|title = Dark Energy Fills the Cosmos|publisher = U.S. Department of Energy, Berkeley Lab|accessdate = 2006-09-08 }}</ref>


A planet or moon's interior heat is produced from the collisions that created the body, by the decay of radioactive materials (''e.g.'' ], ], and ]), or ] caused by interactions with other bodies. Some planets and moons accumulate enough heat to drive geologic processes such as ] and tectonics. Those that accumulate or retain an ] can also undergo surface ] from wind or water. Smaller bodies, without tidal heating, cool more quickly; and their geological activity ceases with the exception of impact cratering.<ref name="new solar system">{{cite book|editor=Beatty, J.K.|editor2=Petersen, C.C.|editor3=Chaikin, A.|title=The New Solar System|publisher=Cambridge press|url=https://books.google.com/books?id=iOezyHMVAMcC&pg=PA70|page=70edition = 4th|date=1999|isbn=978-0-521-64587-4|access-date=26 August 2020|archive-date=30 March 2015|archive-url=https://web.archive.org/web/20150330114739/http://books.google.com/books?id=iOezyHMVAMcC&pg=PA70|url-status=live}}</ref>
==Interdisciplinary studies==


== Interdisciplinary studies ==
Astronomy and astrophysics have developed significant interdisciplinary links with other major scientific fields. ] is the study of ancient or traditional astronomies in their cultural context, utilizing ] and ] evidence. ] is the study of the advent and evolution of biological systems in the universe, with particular emphasis on the possibility of non-terrestrial life.
Astronomy and astrophysics have developed significant interdisciplinary links with other major scientific fields. ] is the study of ancient or traditional astronomies in their cultural context, utilizing ] and ] evidence. ] is the study of the advent and evolution of biological systems in the Universe, with particular emphasis on the possibility of non-terrestrial life. ] is the application of statistics to astrophysics to the analysis of a vast amount of observational astrophysical data.<ref name="Hilbe 2017">{{citation | last=Hilbe | first=Joseph M. | title=Wiley Stats ''Ref'': Statistics Reference Online | chapter=Astrostatistics | publisher=Wiley | date=2017 | doi=10.1002/9781118445112.stat07961 | pages=1–5| isbn=9781118445112 }}</ref>


The study of ]s found in space, including their formation, interaction and destruction, is called ]. These substances are usually found in ]s, although they may also appear in low temperature stars, brown dwarfs and planets. ] is the study of the chemicals found within the ], including the origins of the elements and variations in the ] ratios. Both of these fields represent an overlap of the disciplines of astronomy and chemistry. As "]", finally, methods from astronomy have been used to solve problems of law and history. The study of ]s found in space, including their formation, interaction and destruction, is called ]. These substances are usually found in ]s, although they may also appear in low-temperature stars, brown dwarfs and planets. ] is the study of the chemicals found within the Solar System, including the origins of the elements and variations in the ] ratios. Both of these fields represent an overlap of the disciplines of astronomy and chemistry. As "]", finally, methods from astronomy have been used to solve problems of art history<ref>{{cite web |url=https://gizmodo.com/scientists-used-the-stars-to-confirm-when-a-famous-sapp-1776569251 |title=Scientists Used the Stars to Confirm When a Famous Sapphic Poem Was Written |website=Gizmodo |first=Jennifer |last=Ouellette |date=2016-05-13 |access-date=2023-03-24 |archive-date=24 March 2023 |archive-url=https://web.archive.org/web/20230324165949/https://gizmodo.com/scientists-used-the-stars-to-confirm-when-a-famous-sapp-1776569251 |url-status=live }}</ref><ref>{{cite web |url=https://www.scientificamerican.com/article/forensic-astronomy-reveals-the-secrets-of-an-iconic-ansel-adams-photo/ |title='Forensic Astronomy' Reveals the Secrets of an Iconic Ansel Adams Photo |first=Summer |last=Ash |website=Scientific American |date=2018-04-17 |access-date=2023-03-24 |archive-date=24 March 2023 |archive-url=https://web.archive.org/web/20230324165949/https://www.scientificamerican.com/article/forensic-astronomy-reveals-the-secrets-of-an-iconic-ansel-adams-photo/ |url-status=live }}</ref> and occasionally of law.<ref>{{cite book|first=Jordan D. |last=Marché |chapter=Epilogue |title=Theaters of Time and Space: American Planetaria, 1930–1970 |year=2005 |pages=170–178 |chapter-url=https://www.jstor.org/stable/j.ctt5hjd29.14 |publisher=Rutgers University Press |jstor=j.ctt5hjd29.14 |isbn=0-813-53576-X}}</ref>


==Amateur astronomy== == Amateur astronomy ==
].]]
{{Main|Amateur astronomy}} {{Main|Amateur astronomy}}


].]]
Astronomy is one of the sciences to which amateurs can contribute the most.<ref>{{cite journal Astronomy is one of the sciences to which amateurs can contribute the most.<ref>{{cite journal
|last = Mims III|first = Forrest M. |last = Mims III|first = Forrest M.
|title=Amateur Science—Strong Tradition, Bright Future |title=Amateur Science—Strong Tradition, Bright Future
|journal=Science|year=1999|volume=284|issue=5411 |journal=Science|date=1999|volume=284|issue=5411
|pages=55–56 |pages=55–56
|url=http://www.sciencemag.org/cgi/content/full/284/5411/55
|accessdate=2008-12-06
|doi=10.1126/science.284.5411.55 |doi=10.1126/science.284.5411.55
|quote=Astronomy has traditionally been among the most fertile fields for serious amateurs |quote=Astronomy has traditionally been among the most fertile fields for serious amateurs
|bibcode = 1999Sci...284...55M |s2cid = 162370774
|ref = harv}}</ref>
}}</ref>


Collectively, amateur astronomers observe a variety of celestial objects and phenomena sometimes with ]. Common targets of amateur astronomers include the Moon, planets, stars, comets, meteor showers, and a variety of ]s such as star clusters, galaxies, and nebulae. One branch of amateur astronomy, amateur ], involves the taking of photos of the night sky. Many amateurs like to specialize in the observation of particular objects, types of objects, or types of events which interest them.<ref>{{cite web|url=http://www.amsmeteors.org/|title = The Americal Meteor Society|accessdate = 2006-08-24 }}</ref><ref>{{cite web|first=Jerry|last=Lodriguss|url=http://www.astropix.com/|title = Catching the Light: Astrophotography|accessdate = 2006-08-24 }}</ref> Collectively, amateur astronomers observe a variety of celestial objects and phenomena sometimes with consumer-level equipment or ]. Common targets of amateur astronomers include the Sun, the Moon, planets, stars, comets, ]s, and a variety of ]s such as star clusters, galaxies, and nebulae. Astronomy clubs are located throughout the world and many have programs to help their members set up and complete observational programs including those to observe all the objects in the Messier (110 objects) or Herschel 400 catalogues of points of interest in the night sky. One branch of amateur astronomy, ], involves the taking of photos of the night sky. Many amateurs like to specialize in the observation of particular objects, types of objects, or types of events that interest them.<ref>{{cite web|url=http://www.amsmeteors.org/|title = The American Meteor Society|access-date =24 August 2006| archive-url= https://web.archive.org/web/20060822135040/http://www.amsmeteors.org/| archive-date= 22 August 2006 | url-status= live}}</ref><ref>{{cite web|first=Jerry|last=Lodriguss|url=http://www.astropix.com/|title = Catching the Light: Astrophotography|access-date =24 August 2006| archive-url= https://web.archive.org/web/20060901185541/http://www.astropix.com/| archive-date= 1 September 2006 | url-status= live}}</ref>


Most amateurs work at visible wavelengths, but a small minority experiment with wavelengths outside the visible spectrum. This includes the use of infrared filters on conventional telescopes, and also the use of radio telescopes. The pioneer of amateur radio astronomy was ], who started observing the sky at radio wavelengths in the 1930s. A number of amateur astronomers use either homemade telescopes or use radio telescopes which were originally built for astronomy research but which are now available to amateurs (''e.g.'' the ]).<ref>{{cite web|author=Ghigo, F.|date = 2006-02-07|url=http://www.nrao.edu/whatisra/hist_jansky.shtml|title = Karl Jansky and the Discovery of Cosmic Radio Waves|publisher = National Radio Astronomy Observatory|accessdate = 2006-08-24 }}</ref><ref>{{cite web|url=http://www.users.globalnet.co.uk/~arcus/cara/|title = Cambridge Amateur Radio Astronomers|accessdate = 2006-08-24 }}</ref> Most amateurs work at visible wavelengths, but many experiment with wavelengths outside the visible spectrum. This includes the use of infrared filters on conventional telescopes, and also the use of radio telescopes. The pioneer of amateur radio astronomy was ], who started observing the sky at radio wavelengths in the 1930s. A number of amateur astronomers use either homemade telescopes or use radio telescopes which were originally built for astronomy research but which are now available to amateurs (''e.g.'' the ]).<ref>{{cite web|author=Ghigo, F.|date = 7 February 2006|url=http://www.nrao.edu/whatisra/hist_jansky.shtml|title = Karl Jansky and the Discovery of Cosmic Radio Waves|publisher = National Radio Astronomy Observatory|access-date =24 August 2006| archive-url= https://web.archive.org/web/20060831105945/http://www.nrao.edu/whatisra/hist_jansky.shtml| archive-date= 31 August 2006 | url-status= live}}</ref><ref>{{cite web|url=http://www.users.globalnet.co.uk/~arcus/cara/|title=Cambridge Amateur Radio Astronomers|access-date=24 August 2006|archive-date=24 May 2012|archive-url=https://archive.today/20120524/http://www.users.globalnet.co.uk/~arcus/cara/|url-status=live}}</ref>


Amateur astronomers continue to make scientific contributions to the field of astronomy. Indeed, it is one of the few scientific disciplines where amateurs can still make significant contributions. Amateurs can make occultation measurements that are used to refine the orbits of minor planets. They can also discover comets, and perform regular observations of variable stars. Improvements in digital technology have allowed amateurs to make impressive advances in the field of astrophotography.<ref>{{cite web|url=http://www.lunar-occultations.com/iota/iotandx.htm|title = The International Occultation Timing Association|accessdate = 2006-08-24 }}</ref><ref>{{cite web|url=http://www.cfa.harvard.edu/iau/special/EdgarWilson.html|title = Edgar Wilson Award|publisher = IAU Central Bureau for Astronomical Telegrams|accessdate = 2010-08-24 }}</ref><ref>{{cite web|url=http://www.aavso.org/|title = American Association of Variable Star Observers|publisher = AAVSO|accessdate = 2010-02-03 }}</ref> Amateur astronomers continue to make scientific contributions to the field of astronomy and it is one of the few scientific disciplines where amateurs can still make significant contributions. Amateurs can make occultation measurements that are used to refine the orbits of minor planets. They can also discover comets, and perform regular observations of variable stars. Improvements in digital technology have allowed amateurs to make impressive advances in the field of astrophotography.<ref>{{cite web| url= http://www.lunar-occultations.com/iota/iotandx.htm| title= The International Occultation Timing Association| access-date= 24 August 2006| archive-url= https://web.archive.org/web/20060821180723/http://www.lunar-occultations.com/iota/iotandx.htm| archive-date= 21 August 2006}}</ref><ref>{{cite web|url=http://cbat.eps.harvard.edu/special/EdgarWilson.html |title=Edgar Wilson Award |publisher=IAU Central Bureau for Astronomical Telegrams |access-date=24 October 2010 |archive-url=https://web.archive.org/web/20101024202325/http://www.cbat.eps.harvard.edu/special/EdgarWilson.html |archive-date=24 October 2010 }}</ref><ref>{{cite web|url=http://www.aavso.org/|title = American Association of Variable Star Observers|publisher = AAVSO|access-date =3 February 2010| archive-url= https://web.archive.org/web/20100202050715/http://www.aavso.org/| archive-date= 2 February 2010 | url-status= live}}</ref>


==Major problems== == Unsolved problems in astronomy ==
{{See also|Unsolved problems in physics}} {{Main|List of unsolved problems in astronomy}}
In the 21st century there remain important unanswered questions in astronomy. Some are cosmic in scope: for example, what are ] and ]? These dominate the evolution and fate of the cosmos, yet their true nature remains unknown.<ref name="physics questions">{{cite web|url=http://www.pnl.gov/energyscience/01-02/11-questions/11questions.htm

|title = 11 Physics Questions for the New Century
Although the scientific discipline of astronomy has made tremendous strides in understanding the nature of the universe and its contents, there remain some important unanswered questions. Answers to these may require the construction of new ground- and space-based instruments, and possibly new developments in theoretical and experimental physics.
|publisher = Pacific Northwest National Laboratory
* What is the origin of the stellar mass spectrum? That is, why do astronomers observe the same distribution of stellar masses&nbsp;– the ]&nbsp;– apparently regardless of the initial conditions?<ref>{{cite journal
|access-date =12 August 2006 |archive-url = https://web.archive.org/web/20060203152634/http://www.pnl.gov/energyscience/01-02/11-questions/11questions.htm |archive-date = 3 February 2006}}</ref> What will be the ]?<ref>{{cite web
|last = Hinshaw|first = Gary|date = 15 December 2005
|url = http://map.gsfc.nasa.gov/m_uni/uni_101fate.html
|title = What is the Ultimate Fate of the Universe?
|publisher = NASA WMAP|access-date =28 May 2007| archive-url= https://web.archive.org/web/20070529145436/http://map.gsfc.nasa.gov/m_uni/uni_101fate.html| archive-date= 29 May 2007 | url-status= live}}</ref> Why is the abundance of ] in the cosmos four times lower than predicted by the standard ] model?<ref>{{Cite journal|last1=Howk|first1=J. Christopher|last2=Lehner|first2=Nicolas|last3=Fields|first3=Brian D.|last4=Mathews|first4=Grant J.|date=6 September 2012|title=Observation of interstellar lithium in the low-metallicity Small Magellanic Cloud|journal=Nature|language=en|volume=489|issue=7414|pages=121–23|doi=10.1038/nature11407|pmid=22955622|arxiv = 1207.3081 |bibcode = 2012Natur.489..121H |s2cid=205230254}}</ref> Others pertain to more specific classes of phenomena. For example, is the ] normal or atypical?<ref>{{cite journal | title=How special is the Solar system? | last1=Beer | first1=M. E. | last2=King | first2=A. R. | last3=Livio | first3=M. | last4=Pringle | first4=J. E. | journal=Monthly Notices of the Royal Astronomical Society | volume=354 | issue=3 | pages=763–768 | date=November 2004 | doi=10.1111/j.1365-2966.2004.08237.x | doi-access=free | arxiv=astro-ph/0407476 | bibcode=2004MNRAS.354..763B | s2cid=119552423 }}</ref> What is the origin of the stellar mass spectrum? That is, why do astronomers observe the same distribution of stellar masses—the ]—apparently regardless of the initial conditions?<ref>{{cite journal
|last = Kroupa|first = Pavel |last = Kroupa|first = Pavel
|title=The Initial Mass Function of Stars: Evidence for Uniformity in Variable Systems |title=The Initial Mass Function of Stars: Evidence for Uniformity in Variable Systems
|journal=Science|year=2002|volume=295|issue=5552 |journal=Science|date=2002|volume=295|issue=5552
|pages=82–91 |pages=82–91
|url=http://www.sciencemag.org/cgi/content/full/295/5552/82?ijkey=3Dzzwlrn9nK7LUM&keytype=3Dref&siteid=3Dsci
|accessdate=2007-05-28
|doi=10.1126/science.1067524 |doi=10.1126/science.1067524
|pmid=11778039 |pmid=11778039
|arxiv = astro-ph/0201098 |bibcode = 2002Sci...295...82K
|ref = harv }}</ref> A deeper understanding of the formation of stars and planets is needed.
|s2cid = 14084249
* Is there other ]? Especially, is there other intelligent life? If so, what is the explanation for the ]? The existence of life elsewhere has important scientific and philosophical implications.<ref>{{cite web
}}</ref> Likewise, questions remain about the formation of the ],<ref>{{cite web|title=FAQ – How did galaxies form?|url=http://origins.stsci.edu/faq/galaxies.html|publisher=NASA|access-date=28 July 2015|archive-url=https://web.archive.org/web/20150628054952/http://origins.stsci.edu/faq/galaxies.html|archive-date=28 June 2015}}</ref> the origin of ]s,<ref>{{cite web|title=Supermassive Black Hole|url=http://astronomy.swin.edu.au/cosmos/S/Supermassive+Black+Hole|publisher=Swinburne University|access-date=28 July 2015|archive-date=14 August 2020|archive-url=https://web.archive.org/web/20200814110807/https://astronomy.swin.edu.au/cosmos/S/Supermassive+Black+Hole|url-status=live}}</ref> the source of ]s,<ref>{{cite journal|journal=Annual Review of Astronomy and Astrophysics|title=The Origin of Ultra-High-Energy Cosmic Rays|last=Hillas|first=A.M.|volume=22|date=September 1984|doi=10.1146/annurev.aa.22.090184.002233|pages=425–44|quote=This poses a challenge to these models, because |bibcode = 1984ARA&A..22..425H }}</ref> and more.
|url=http://www.astrobio.net/news/article236.html
|title = Complex Life Elsewhere in the Universe?
|publisher = Astrobiology Magazine|accessdate = 2006-08-12 }}</ref><ref>{{cite web
|url=http://www.bigear.org/vol1no2/sagan.htm
|title = The Quest for Extraterrestrial Intelligence
|publisher = Cosmic Search Magazine
|accessdate = 2006-08-12 }}</ref> Is the Solar System normal or atypical?
* What caused the Universe to form? Is the premise of the ] hypothesis correct? If so, could this be the result of ]? What caused the ] that produced our homogeneous universe? Why is there a ]?
* What is the nature of ] and ]? These dominate the evolution and fate of the cosmos, yet their true nature remains unknown.<ref name="physics questions">{{cite web|url=http://web.archive.org/web/20060203152634/http://www.pnl.gov/energyscience/01-02/11-questions/11questions.htm
|title = 11 Physics Questions for the New Century
|publisher = Pacific Northwest National Laboratory
|accessdate = 2006-08-12 }}</ref> What will be the ]?<ref>{{cite web
|last = Hinshaw|first = Gary|date = 2005-12-15
|url = http://map.gsfc.nasa.gov/m_uni/uni_101fate.html
|title = What is the Ultimate Fate of the Universe?
|publisher = NASA WMAP|accessdate = 2007-05-28 }}</ref>
* How did the first galaxies form? How did supermassive black holes form?
* What is creating the ]s?


Is there other ]? Especially, is there ]? If so, what is the explanation for the ]? The existence of life elsewhere has important scientific and philosophical implications.<ref>{{cite web
==International Year of Astronomy 2009==
|url=http://www.astrobio.net/debate/236/complex-life-elsewhere-in-the-universe
{{Main|International Year of Astronomy}}
|archive-url=https://web.archive.org/web/20110628214416/http://www.astrobio.net/debate/236/complex-life-elsewhere-in-the-universe
|archive-date=28 June 2011
|url-status=dead
|title = Rare Earth: Complex Life Elsewhere in the Universe?
|work = Astrobiology Magazine|access-date =12 August 2006|date=15 July 2002
}}</ref><ref>{{cite web|url=http://www.bigear.org/vol1no2/sagan.htm|title=The Quest for Extraterrestrial Intelligence|last=Sagan|first=Carl|work=Cosmic Search Magazine|access-date=12 August 2006|archive-url=https://web.archive.org/web/20060818144558/http://www.bigear.org/vol1no2/sagan.htm|archive-date=18 August 2006 |url-status=live}}</ref>


== See also ==
During the 62nd General Assembly of the ], ] was declared to be the ] (IYA2009), with the resolution being made official on 20 December 2008. A global scheme laid out by the ] (IAU), it was also endorsed by ]&nbsp;– the ] body responsible for Educational, Scientific and Cultural matters. IYA2009 was intended to be a global celebration of astronomy and its contributions to society and culture, stimulating worldwide interest not only in astronomy but science in general, with a particular slant towards young people.
* {{Annotated link|Cosmogony}}
* {{Annotated link|Outline of astronomy}}
* {{Annotated link|Outline of space science}}
* {{Annotated link|Space exploration}}


==See also== === Lists ===
{{Main|Outline of astronomy}} * {{Annotated link|Glossary of astronomy}}
* {{Annotated link|List of astronomical instruments}}
* {{Annotated link|List of astronomical observatories}}
* {{Annotated link|List of astronomy acronyms}}
* {{Annotated link|List of software for astronomy research and education}}


== References ==
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==References== == Bibliography ==
* {{Cite EB1911|wstitle= Astronomy | volume= 2 |last1= Newcomb |first1= Simon |author1-link= Simon Newcomb ||last2= Clerke |first2= Agnes Mary |author2-link= Agnes Mary Clerke | pages = 800–819 |short=1}}
{{Reflist|2}}
* {{cite book

|last1 = Harpaz|first1 = Amos
==Bibliography==
*{{cite book| first=George |last=Forbes|title=History of Astronomy|publisher=Plain Label Books|location=London| year=1909|isbn=1603031596}} Available at ,
*{{cite book
|last1 = Harpaz
|first1 = Amos
|title = Stellar Evolution |title = Stellar Evolution
|date= 1994|isbn = 978-1-56881-012-6|url = https://books.google.com/books?id=kd4VEZv8oo0C|publisher = A K Peters, Ltd}}
|year = 1994
* {{cite book|last=Unsöld|first=A.|title=The New Cosmos: An Introduction to Astronomy and Astrophysics|date=2001|publisher=Springer|isbn=978-3-540-67877-9|author2=Baschek, B.}}
|isbn = 9781568810126
* {{cite book|last=James|first=C. Renée|author-link=C. Renée James|title=Things That Go Bump in the Universe: How Astronomers Decode Cosmic Chaos |date=2023|publisher=Johns Hopkins University Press|isbn=978-1421446936}}
|chapter =
|url = http://books.google.com/?id=kd4VEZv8oo0C&dq
|publisher = A K Peters, Ltd
}}


== External links ==
<!-- Wish to suggest The Collapsing Universe The Story of Black Holes by Isaac Asimov ISBN 0-671-49886-X WFPM] (]) 04:45, 19 May 2008 (UTC) -->
{{Commons}}

{{Wikibooks}}
==External links==
{{Sisterlinks|Astronomy}}

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* ()
* IYA2009 Main website
* and in Astronomy, from the Smithsonian/NASA ]
* from the American Institute of Physics
*
* Educational site for Astronomical journeys through space
*
* , Astrophysical Chemistry Lecture Series. 8 Freeview Lectures provided by the Vega Science Trust.
* and in Astronomy, from the Smithsonian/NASA ]


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Latest revision as of 20:57, 8 December 2024

Scientific study of celestial objects This article is about the scientific study of celestial objects. Not to be confused with Astrology, a divinatory pseudoscience. For other uses, see Astronomy (disambiguation).

The Paranal Observatory of European Southern Observatory shooting a laser guide star to the Galactic Center

Astronomy is a natural science that studies celestial objects and the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry in order to explain their origin and their overall evolution. Objects of interest include planets, moons, stars, nebulae, galaxies, meteoroids, asteroids, and comets. Relevant phenomena include supernova explosions, gamma ray bursts, quasars, blazars, pulsars, and cosmic microwave background radiation. More generally, astronomy studies everything that originates beyond Earth's atmosphere. Cosmology is a branch of astronomy that studies the universe as a whole.

Astronomy is one of the oldest natural sciences. The early civilizations in recorded history made methodical observations of the night sky. These include the Egyptians, Babylonians, Greeks, Indians, Chinese, Maya, and many ancient indigenous peoples of the Americas. In the past, astronomy included disciplines as diverse as astrometry, celestial navigation, observational astronomy, and the making of calendars.

Professional astronomy is split into observational and theoretical branches. Observational astronomy is focused on acquiring data from observations of astronomical objects. This data is then analyzed using basic principles of physics. Theoretical astronomy is oriented toward the development of computer or analytical models to describe astronomical objects and phenomena. These two fields complement each other. Theoretical astronomy seeks to explain observational results and observations are used to confirm theoretical results.

Astronomy is one of the few sciences in which amateurs play an active role. This is especially true for the discovery and observation of transient events. Amateur astronomers have helped with many important discoveries, such as finding new comets.

Etymology

Astronomical Observatory, New South Wales, Australia 1873

Astronomy (from the Greek ἀστρονομία from ἄστρον astron, "star" and -νομία -nomia from νόμος nomos, "law" or "culture") means "law of the stars" (or "culture of the stars" depending on the translation). Astronomy should not be confused with astrology, the belief system which claims that human affairs are correlated with the positions of celestial objects. Although the two fields share a common origin, they are now entirely distinct.

Use of terms "astronomy" and "astrophysics"

"Astronomy" and "astrophysics" are synonyms. Based on strict dictionary definitions, "astronomy" refers to "the study of objects and matter outside the Earth's atmosphere and of their physical and chemical properties", while "astrophysics" refers to the branch of astronomy dealing with "the behavior, physical properties, and dynamic processes of celestial objects and phenomena". In some cases, as in the introduction of the introductory textbook The Physical Universe by Frank Shu, "astronomy" may be used to describe the qualitative study of the subject, whereas "astrophysics" is used to describe the physics-oriented version of the subject. However, since most modern astronomical research deals with subjects related to physics, modern astronomy could actually be called astrophysics. Some fields, such as astrometry, are purely astronomy rather than also astrophysics. Various departments in which scientists carry out research on this subject may use "astronomy" and "astrophysics", partly depending on whether the department is historically affiliated with a physics department, and many professional astronomers have physics rather than astronomy degrees. Some titles of the leading scientific journals in this field include The Astronomical Journal, The Astrophysical Journal, and Astronomy & Astrophysics.

History

Main article: History of astronomy For a chronological guide, see Timeline of astronomy. Further information: Archaeoastronomy and List of astronomers

Pre-historic astronomy

The Nebra sky disc (c. 1800–1600 BCE), found near a possibly astronomical complex, most likely depicting the Sun or full Moon, the Moon as a crescent, the Pleiades and the summer and winter solstices as strips of gold on the side of the disc, with the top representing the horizon and north.

In early historic times, astronomy only consisted of the observation and predictions of the motions of objects visible to the naked eye. In some locations, early cultures assembled massive artifacts that may have had some astronomical purpose. In addition to their ceremonial uses, these observatories could be employed to determine the seasons, an important factor in knowing when to plant crops and in understanding the length of the year.

Classical astronomy

A Babylonian planisphere (7th century BCE). Babylonian astronomy made early advances in astronomy. Its use of sexagesimals (e.g. 12, 24, 60, 360) is still being used today through having been broadly adopted for timekeeping and astrometry.

As civilizations developed, most notably in Egypt, Mesopotamia, Greece, Persia, India, China, and Central America, astronomical observatories were assembled and ideas on the nature of the Universe began to develop. Most early astronomy consisted of mapping the positions of the stars and planets, a science now referred to as astrometry. From these observations, early ideas about the motions of the planets were formed, and the nature of the Sun, Moon and the Earth in the Universe were explored philosophically. The Earth was believed to be the center of the Universe with the Sun, the Moon and the stars rotating around it. This is known as the geocentric model of the Universe, or the Ptolemaic system, named after Ptolemy.

A particularly important early development was the beginning of mathematical and scientific astronomy, which began among the Babylonians, who laid the foundations for the later astronomical traditions that developed in many other civilizations. The Babylonians discovered that lunar eclipses recurred in a repeating cycle known as a saros.

Following the Babylonians, significant advances in astronomy were made in ancient Greece and the Hellenistic world. Greek astronomy is characterized from the start by seeking a rational, physical explanation for celestial phenomena. In the 3rd century BC, Aristarchus of Samos estimated the size and distance of the Moon and Sun, and he proposed a model of the Solar System where the Earth and planets rotated around the Sun, now called the heliocentric model. In the 2nd century BC, Hipparchus discovered precession, calculated the size and distance of the Moon and invented the earliest known astronomical devices such as the astrolabe. Hipparchus also created a comprehensive catalog of 1020 stars, and most of the constellations of the northern hemisphere derive from Greek astronomy. The Antikythera mechanism (c. 150–80 BC) was an early analog computer designed to calculate the location of the Sun, Moon, and planets for a given date. Technological artifacts of similar complexity did not reappear until the 14th century, when mechanical astronomical clocks appeared in Europe.

Post-classical astronomy

Portrait of Alfraganus in the Compilatio astronomica, 1493. Islamic astronomers began just before the 9th century to collect and translate Indian, Persian and Greek astronomical texts, adding their own astronomy and enabling later, particularly European astronomy to build on.

Astronomy flourished in the Islamic world and other parts of the world. This led to the emergence of the first astronomical observatories in the Muslim world by the early 9th century. In 964, the Andromeda Galaxy, the largest galaxy in the Local Group, was described by the Persian Muslim astronomer Abd al-Rahman al-Sufi in his Book of Fixed Stars. The SN 1006 supernova, the brightest apparent magnitude stellar event in recorded history, was observed by the Egyptian Arabic astronomer Ali ibn Ridwan and Chinese astronomers in 1006. Iranian scholar Al-Biruni observed that, contrary to Ptolemy, the Sun's apogee (highest point in the heavens) was mobile, not fixed. Some of the prominent Islamic (mostly Persian and Arab) astronomers who made significant contributions to the science include Al-Battani, Thebit, Abd al-Rahman al-Sufi, Biruni, Abū Ishāq Ibrāhīm al-Zarqālī, Al-Birjandi, and the astronomers of the Maragheh and Samarkand observatories. Astronomers during that time introduced many Arabic names now used for individual stars.

It is also believed that the ruins at Great Zimbabwe and Timbuktu may have housed astronomical observatories. In Post-classical West Africa, Astronomers studied the movement of stars and relation to seasons, crafting charts of the heavens as well as precise diagrams of orbits of the other planets based on complex mathematical calculations. Songhai historian Mahmud Kati documented a meteor shower in August 1583. Europeans had previously believed that there had been no astronomical observation in sub-Saharan Africa during the pre-colonial Middle Ages, but modern discoveries show otherwise.

For over six centuries (from the recovery of ancient learning during the late Middle Ages into the Enlightenment), the Roman Catholic Church gave more financial and social support to the study of astronomy than probably all other institutions. Among the Church's motives was finding the date for Easter.

Medieval Europe housed a number of important astronomers. Richard of Wallingford (1292–1336) made major contributions to astronomy and horology, including the invention of the first astronomical clock, the Rectangulus which allowed for the measurement of angles between planets and other astronomical bodies, as well as an equatorium called the Albion which could be used for astronomical calculations such as lunar, solar and planetary longitudes and could predict eclipses. Nicole Oresme (1320–1382) and Jean Buridan (1300–1361) first discussed evidence for the rotation of the Earth, furthermore, Buridan also developed the theory of impetus (predecessor of the modern scientific theory of inertia) which was able to show planets were capable of motion without the intervention of angels. Georg von Peuerbach (1423–1461) and Regiomontanus (1436–1476) helped make astronomical progress instrumental to Copernicus's development of the heliocentric model decades later.

Early telescopic astronomy

The first sketches of the Moon's topography, from Galileo's ground-breaking Sidereus Nuncius (1610), publishing his findings from the first telescopic astronomical observations.

During the Renaissance, Nicolaus Copernicus proposed a heliocentric model of the solar system. His work was defended by Galileo Galilei and expanded upon by Johannes Kepler. Kepler was the first to devise a system that correctly described the details of the motion of the planets around the Sun. However, Kepler did not succeed in formulating a theory behind the laws he wrote down. It was Isaac Newton, with his invention of celestial dynamics and his law of gravitation, who finally explained the motions of the planets. Newton also developed the reflecting telescope.

Improvements in the size and quality of the telescope led to further discoveries. The English astronomer John Flamsteed catalogued over 3000 stars. More extensive star catalogues were produced by Nicolas Louis de Lacaille. The astronomer William Herschel made a detailed catalog of nebulosity and clusters, and in 1781 discovered the planet Uranus, the first new planet found.

During the 18–19th centuries, the study of the three-body problem by Leonhard Euler, Alexis Claude Clairaut, and Jean le Rond d'Alembert led to more accurate predictions about the motions of the Moon and planets. This work was further refined by Joseph-Louis Lagrange and Pierre Simon Laplace, allowing the masses of the planets and moons to be estimated from their perturbations.

Significant advances in astronomy came about with the introduction of new technology, including the spectroscope and photography. Joseph von Fraunhofer discovered about 600 bands in the spectrum of the Sun in 1814–15, which, in 1859, Gustav Kirchhoff ascribed to the presence of different elements. Stars were proven to be similar to the Earth's own Sun, but with a wide range of temperatures, masses, and sizes.

Deep space astronomy

The earliest known photograph of the Great Andromeda "Nebula", by Isaac Roberts from 29 December 1888. With the calculation of its distance in 1923 intergalactic space was proven, allowing the calculation of the age and expanse of the Universe.

The existence of the Earth's galaxy, the Milky Way, as its own group of stars was only proven in the 20th century, along with the existence of "external" galaxies. The observed recession of those galaxies led to the discovery of the expansion of the Universe. In 1919, when the Hooker Telescope was completed, the prevailing view was that the universe consisted entirely of the Milky Way Galaxy. Using the Hooker Telescope, Edwin Hubble identified Cepheid variables in several spiral nebulae and in 1922–1923 proved conclusively that Andromeda Nebula and Triangulum among others, were entire galaxies outside our own, thus proving that the universe consists of a multitude of galaxies. With this Hubble formulated the Hubble constant, which allowed for the first time a calculation of the age of the Universe and size of the Observable Universe, which became increasingly precise with better meassurements, starting at 2 billion years and 280 million light-years, until 2006 when data of the Hubble Space Telescope allowed a very accurate calculation of the age of the Universe and size of the Observable Universe.

First ever direct image of a (supermassive) black hole, taken 2019 in radio wavelength, located at the core of Messier 87.

Theoretical astronomy led to speculations on the existence of objects such as black holes and neutron stars, which have been used to explain such observed phenomena as quasars, pulsars, blazars, and radio galaxies. Physical cosmology made huge advances during the 20th century. In the early 1900s the model of the Big Bang theory was formulated, heavily evidenced by cosmic microwave background radiation, Hubble's law, and the cosmological abundances of elements. Space telescopes have enabled measurements in parts of the electromagnetic spectrum normally blocked or blurred by the atmosphere. In February 2016, it was revealed that the LIGO project had detected evidence of gravitational waves in the previous September.

Observational astronomy

Main article: Observational astronomy
Overview of types of observational astronomy by observed wavelengths and their observability

The main source of information about celestial bodies and other objects is visible light, or more generally electromagnetic radiation. Observational astronomy may be categorized according to the corresponding region of the electromagnetic spectrum on which the observations are made. Some parts of the spectrum can be observed from the Earth's surface, while other parts are only observable from either high altitudes or outside the Earth's atmosphere. Specific information on these subfields is given below.

Radio astronomy

The Very Large Array in New Mexico, an example of a radio telescope
Main article: Radio astronomy

Radio astronomy uses radiation with wavelengths greater than approximately one millimeter, outside the visible range. Radio astronomy is different from most other forms of observational astronomy in that the observed radio waves can be treated as waves rather than as discrete photons. Hence, it is relatively easier to measure both the amplitude and phase of radio waves, whereas this is not as easily done at shorter wavelengths.

Although some radio waves are emitted directly by astronomical objects, a product of thermal emission, most of the radio emission that is observed is the result of synchrotron radiation, which is produced when electrons orbit magnetic fields. Additionally, a number of spectral lines produced by interstellar gas, notably the hydrogen spectral line at 21 cm, are observable at radio wavelengths.

A wide variety of other objects are observable at radio wavelengths, including supernovae, interstellar gas, pulsars, and active galactic nuclei.

Infrared astronomy

ALMA Observatory is one of the highest observatory sites on Earth. Atacama, Chile.
Main article: Infrared astronomy

Infrared astronomy is founded on the detection and analysis of infrared radiation, wavelengths longer than red light and outside the range of our vision. The infrared spectrum is useful for studying objects that are too cold to radiate visible light, such as planets, circumstellar disks or nebulae whose light is blocked by dust. The longer wavelengths of infrared can penetrate clouds of dust that block visible light, allowing the observation of young stars embedded in molecular clouds and the cores of galaxies. Observations from the Wide-field Infrared Survey Explorer (WISE) have been particularly effective at unveiling numerous galactic protostars and their host star clusters. With the exception of infrared wavelengths close to visible light, such radiation is heavily absorbed by the atmosphere, or masked, as the atmosphere itself produces significant infrared emission. Consequently, infrared observatories have to be located in high, dry places on Earth or in space. Some molecules radiate strongly in the infrared. This allows the study of the chemistry of space; more specifically it can detect water in comets.

Optical astronomy

The Subaru Telescope (left) and Keck Observatory (center) on Mauna Kea, both examples of an observatory that operates at near-infrared and visible wavelengths. The NASA Infrared Telescope Facility (right) is an example of a telescope that operates only at near-infrared wavelengths.
Main article: Optical astronomy

Historically, optical astronomy, which has been also called visible light astronomy, is the oldest form of astronomy. Images of observations were originally drawn by hand. In the late 19th century and most of the 20th century, images were made using photographic equipment. Modern images are made using digital detectors, particularly using charge-coupled devices (CCDs) and recorded on modern medium. Although visible light itself extends from approximately 4000 Å to 7000 Å (400 nm to 700 nm), that same equipment can be used to observe some near-ultraviolet and near-infrared radiation.

Ultraviolet astronomy

Main article: Ultraviolet astronomy

Ultraviolet astronomy employs ultraviolet wavelengths between approximately 100 and 3200 Å (10 to 320 nm). Light at those wavelengths is absorbed by the Earth's atmosphere, requiring observations at these wavelengths to be performed from the upper atmosphere or from space. Ultraviolet astronomy is best suited to the study of thermal radiation and spectral emission lines from hot blue stars (OB stars) that are very bright in this wave band. This includes the blue stars in other galaxies, which have been the targets of several ultraviolet surveys. Other objects commonly observed in ultraviolet light include planetary nebulae, supernova remnants, and active galactic nuclei. However, as ultraviolet light is easily absorbed by interstellar dust, an adjustment of ultraviolet measurements is necessary.

X-ray astronomy

Main article: X-ray astronomy
X-ray jet made from a supermassive black hole found by NASA's Chandra X-ray Observatory, made visible by light from the early Universe

X-ray astronomy uses X-ray wavelengths. Typically, X-ray radiation is produced by synchrotron emission (the result of electrons orbiting magnetic field lines), thermal emission from thin gases above 10 (10 million) kelvins, and thermal emission from thick gases above 10 Kelvin. Since X-rays are absorbed by the Earth's atmosphere, all X-ray observations must be performed from high-altitude balloons, rockets, or X-ray astronomy satellites. Notable X-ray sources include X-ray binaries, pulsars, supernova remnants, elliptical galaxies, clusters of galaxies, and active galactic nuclei.

Gamma-ray astronomy

Main article: Gamma ray astronomy

Gamma ray astronomy observes astronomical objects at the shortest wavelengths of the electromagnetic spectrum. Gamma rays may be observed directly by satellites such as the Compton Gamma Ray Observatory or by specialized telescopes called atmospheric Cherenkov telescopes. The Cherenkov telescopes do not detect the gamma rays directly but instead detect the flashes of visible light produced when gamma rays are absorbed by the Earth's atmosphere.

Most gamma-ray emitting sources are actually gamma-ray bursts, objects which only produce gamma radiation for a few milliseconds to thousands of seconds before fading away. Only 10% of gamma-ray sources are non-transient sources. These steady gamma-ray emitters include pulsars, neutron stars, and black hole candidates such as active galactic nuclei.

Fields not based on the electromagnetic spectrum

In addition to electromagnetic radiation, a few other events originating from great distances may be observed from the Earth.

In neutrino astronomy, astronomers use heavily shielded underground facilities such as SAGE, GALLEX, and Kamioka II/III for the detection of neutrinos. The vast majority of the neutrinos streaming through the Earth originate from the Sun, but 24 neutrinos were also detected from supernova 1987A. Cosmic rays, which consist of very high energy particles (atomic nuclei) that can decay or be absorbed when they enter the Earth's atmosphere, result in a cascade of secondary particles which can be detected by current observatories. Some future neutrino detectors may also be sensitive to the particles produced when cosmic rays hit the Earth's atmosphere.

Gravitational-wave astronomy is an emerging field of astronomy that employs gravitational-wave detectors to collect observational data about distant massive objects. A few observatories have been constructed, such as the Laser Interferometer Gravitational Observatory LIGO. LIGO made its first detection on 14 September 2015, observing gravitational waves from a binary black hole. A second gravitational wave was detected on 26 December 2015 and additional observations should continue but gravitational waves require extremely sensitive instruments.

The combination of observations made using electromagnetic radiation, neutrinos or gravitational waves and other complementary information, is known as multi-messenger astronomy.

Astrometry and celestial mechanics

Main articles: Astrometry and Celestial mechanics
Star cluster Pismis 24 with a nebula

One of the oldest fields in astronomy, and in all of science, is the measurement of the positions of celestial objects. Historically, accurate knowledge of the positions of the Sun, Moon, planets and stars has been essential in celestial navigation (the use of celestial objects to guide navigation) and in the making of calendars.

Careful measurement of the positions of the planets has led to a solid understanding of gravitational perturbations, and an ability to determine past and future positions of the planets with great accuracy, a field known as celestial mechanics. More recently the tracking of near-Earth objects will allow for predictions of close encounters or potential collisions of the Earth with those objects.

The measurement of stellar parallax of nearby stars provides a fundamental baseline in the cosmic distance ladder that is used to measure the scale of the Universe. Parallax measurements of nearby stars provide an absolute baseline for the properties of more distant stars, as their properties can be compared. Measurements of the radial velocity and proper motion of stars allow astronomers to plot the movement of these systems through the Milky Way galaxy. Astrometric results are the basis used to calculate the distribution of speculated dark matter in the galaxy.

During the 1990s, the measurement of the stellar wobble of nearby stars was used to detect large extrasolar planets orbiting those stars.

Theoretical astronomy

Nucleosynthesis
Related topics
Main article: Theoretical astronomy

Theoretical astronomers use several tools including analytical models and computational numerical simulations; each has its particular advantages. Analytical models of a process are better for giving broader insight into the heart of what is going on. Numerical models reveal the existence of phenomena and effects otherwise unobserved.

Theorists in astronomy endeavor to create theoretical models that are based on existing observations and known physics, and to predict observational consequences of those models. The observation of phenomena predicted by a model allows astronomers to select between several alternative or conflicting models. Theorists also modify existing models to take into account new observations. In some cases, a large amount of observational data that is inconsistent with a model may lead to abandoning it largely or completely, as for geocentric theory, the existence of luminiferous aether, and the steady-state model of cosmic evolution.

Phenomena modeled by theoretical astronomers include:

Modern theoretical astronomy reflects dramatic advances in observation since the 1990s, including studies of the cosmic microwave background, distant supernovae and galaxy redshifts, which have led to the development of a standard model of cosmology. This model requires the universe to contain large amounts of dark matter and dark energy whose nature is currently not well understood, but the model gives detailed predictions that are in excellent agreement with many diverse observations.

Specific subfields

Astrophysics

Main article: Astrophysics
Astrophysics applies physics and chemistry to understand the measurements made by astronomy. Representation of the Observable Universe that includes images from Hubble and other telescopes.

Astrophysics is the branch of astronomy that employs the principles of physics and chemistry "to ascertain the nature of the astronomical objects, rather than their positions or motions in space". Among the objects studied are the Sun, other stars, galaxies, extrasolar planets, the interstellar medium and the cosmic microwave background. Their emissions are examined across all parts of the electromagnetic spectrum, and the properties examined include luminosity, density, temperature, and chemical composition. Because astrophysics is a very broad subject, astrophysicists typically apply many disciplines of physics, including mechanics, electromagnetism, statistical mechanics, thermodynamics, quantum mechanics, relativity, nuclear and particle physics, and atomic and molecular physics.

In practice, modern astronomical research often involves a substantial amount of work in the realms of theoretical and observational physics. Some areas of study for astrophysicists include their attempts to determine the properties of dark matter, dark energy, and black holes; whether or not time travel is possible, wormholes can form, or the multiverse exists; and the origin and ultimate fate of the universe. Topics also studied by theoretical astrophysicists include Solar System formation and evolution; stellar dynamics and evolution; galaxy formation and evolution; magnetohydrodynamics; large-scale structure of matter in the universe; origin of cosmic rays; general relativity and physical cosmology, including string cosmology and astroparticle physics.

Astrochemistry

Main article: Astrochemistry

Astrochemistry is the study of the abundance and reactions of molecules in the Universe, and their interaction with radiation. The discipline is an overlap of astronomy and chemistry. The word "astrochemistry" may be applied to both the Solar System and the interstellar medium. The study of the abundance of elements and isotope ratios in Solar System objects, such as meteorites, is also called cosmochemistry, while the study of interstellar atoms and molecules and their interaction with radiation is sometimes called molecular astrophysics. The formation, atomic and chemical composition, evolution and fate of molecular gas clouds is of special interest, because it is from these clouds that solar systems form. Studies in this field contribute to the understanding of the formation of the Solar System, Earth's origin and geology, abiogenesis, and the origin of climate and oceans.

Astrobiology

Main article: Astrobiology

Astrobiology is an interdisciplinary scientific field concerned with the origins, early evolution, distribution, and future of life in the universe. Astrobiology considers the question of whether extraterrestrial life exists, and how humans can detect it if it does. The term exobiology is similar.

Astrobiology makes use of molecular biology, biophysics, biochemistry, chemistry, astronomy, physical cosmology, exoplanetology and geology to investigate the possibility of life on other worlds and help recognize biospheres that might be different from that on Earth. The origin and early evolution of life is an inseparable part of the discipline of astrobiology. Astrobiology concerns itself with interpretation of existing scientific data, and although speculation is entertained to give context, astrobiology concerns itself primarily with hypotheses that fit firmly into existing scientific theories.

This interdisciplinary field encompasses research on the origin of planetary systems, origins of organic compounds in space, rock-water-carbon interactions, abiogenesis on Earth, planetary habitability, research on biosignatures for life detection, and studies on the potential for life to adapt to challenges on Earth and in outer space.

Physical cosmology

Nature timeline
This box:
−13 —–−12 —–−11 —–−10 —–−9 —–−8 —–−7 —–−6 —–−5 —–−4 —–−3 —–−2 —–−1 —–0 —Dark AgesReionizationMatter-dominated
era
Accelerated expansionWater on EarthSingle-celled lifePhotosynthesisMulticellular
life
Vertebrates
Earliest Universe
Earliest stars
Earliest galaxy
Earliest quasar / black hole
Omega Centauri
Andromeda Galaxy
Milky Way spirals
NGC 188 star cluster
Alpha Centauri
Earth / Solar System
Earliest known life
Earliest oxygen
Atmospheric oxygen
Sexual reproduction
Earliest fungi
Earliest animals / plants
Cambrian explosion
Earliest mammals
Earliest apes / humans
L
i
f
e
(billion years ago)
Main article: Physical cosmology

Cosmology (from the Greek κόσμος (kosmos) "world, universe" and λόγος (logos) "word, study" or literally "logic") could be considered the study of the Universe as a whole.

Hubble Extreme Deep Field

Observations of the large-scale structure of the Universe, a branch known as physical cosmology, have provided a deep understanding of the formation and evolution of the cosmos. Fundamental to modern cosmology is the well-accepted theory of the Big Bang, wherein our Universe began at a single point in time, and thereafter expanded over the course of 13.8 billion years to its present condition. The concept of the Big Bang can be traced back to the discovery of the microwave background radiation in 1965.

In the course of this expansion, the Universe underwent several evolutionary stages. In the very early moments, it is theorized that the Universe experienced a very rapid cosmic inflation, which homogenized the starting conditions. Thereafter, nucleosynthesis produced the elemental abundance of the early Universe. (See also nucleocosmochronology.)

When the first neutral atoms formed from a sea of primordial ions, space became transparent to radiation, releasing the energy viewed today as the microwave background radiation. The expanding Universe then underwent a Dark Age due to the lack of stellar energy sources.

A hierarchical structure of matter began to form from minute variations in the mass density of space. Matter accumulated in the densest regions, forming clouds of gas and the earliest stars, the Population III stars. These massive stars triggered the reionization process and are believed to have created many of the heavy elements in the early Universe, which, through nuclear decay, create lighter elements, allowing the cycle of nucleosynthesis to continue longer.

Gravitational aggregations clustered into filaments, leaving voids in the gaps. Gradually, organizations of gas and dust merged to form the first primitive galaxies. Over time, these pulled in more matter, and were often organized into groups and clusters of galaxies, then into larger-scale superclusters.

Fundamental to the structure of the Universe is the existence of dark matter and dark energy. These are now thought to be its dominant components, forming 96% of the mass of the Universe. For this reason, much effort is expended in trying to understand the physics of these components.

Extragalactic astronomy

This image shows several blue, loop-shaped objects that are multiple images of the same galaxy, duplicated by the gravitational lens effect of the cluster of yellow galaxies near the middle of the photograph. The lens is produced by the cluster's gravitational field that bends light to magnify and distort the image of a more distant object.
Main article: Extragalactic astronomy

The study of objects outside our galaxy is a branch of astronomy concerned with the formation and evolution of galaxies, their morphology (description) and classification, the observation of active galaxies, and at a larger scale, the groups and clusters of galaxies. Finally, the latter is important for the understanding of the large-scale structure of the cosmos.

Most galaxies are organized into distinct shapes that allow for classification schemes. They are commonly divided into spiral, elliptical and Irregular galaxies.

As the name suggests, an elliptical galaxy has the cross-sectional shape of an ellipse. The stars move along random orbits with no preferred direction. These galaxies contain little or no interstellar dust, few star-forming regions, and older stars. Elliptical galaxies may have been formed by other galaxies merging.

A spiral galaxy is organized into a flat, rotating disk, usually with a prominent bulge or bar at the center, and trailing bright arms that spiral outward. The arms are dusty regions of star formation within which massive young stars produce a blue tint. Spiral galaxies are typically surrounded by a halo of older stars. Both the Milky Way and one of our nearest galaxy neighbors, the Andromeda Galaxy, are spiral galaxies.

Irregular galaxies are chaotic in appearance, and are neither spiral nor elliptical. About a quarter of all galaxies are irregular, and the peculiar shapes of such galaxies may be the result of gravitational interaction.

An active galaxy is a formation that emits a significant amount of its energy from a source other than its stars, dust and gas. It is powered by a compact region at the core, thought to be a supermassive black hole that is emitting radiation from in-falling material. A radio galaxy is an active galaxy that is very luminous in the radio portion of the spectrum, and is emitting immense plumes or lobes of gas. Active galaxies that emit shorter frequency, high-energy radiation include Seyfert galaxies, quasars, and blazars. Quasars are believed to be the most consistently luminous objects in the known universe.

The large-scale structure of the cosmos is represented by groups and clusters of galaxies. This structure is organized into a hierarchy of groupings, with the largest being the superclusters. The collective matter is formed into filaments and walls, leaving large voids between.

Galactic astronomy

Observed structure of the Milky Way's spiral arms
Main article: Galactic astronomy

The Solar System orbits within the Milky Way, a barred spiral galaxy that is a prominent member of the Local Group of galaxies. It is a rotating mass of gas, dust, stars and other objects, held together by mutual gravitational attraction. As the Earth is located within the dusty outer arms, there are large portions of the Milky Way that are obscured from view.

In the center of the Milky Way is the core, a bar-shaped bulge with what is believed to be a supermassive black hole at its center. This is surrounded by four primary arms that spiral from the core. This is a region of active star formation that contains many younger, population I stars. The disk is surrounded by a spheroid halo of older, population II stars, as well as relatively dense concentrations of stars known as globular clusters.

Between the stars lies the interstellar medium, a region of sparse matter. In the densest regions, molecular clouds of molecular hydrogen and other elements create star-forming regions. These begin as a compact pre-stellar core or dark nebulae, which concentrate and collapse (in volumes determined by the Jeans length) to form compact protostars.

As the more massive stars appear, they transform the cloud into an H II region (ionized atomic hydrogen) of glowing gas and plasma. The stellar wind and supernova explosions from these stars eventually cause the cloud to disperse, often leaving behind one or more young open clusters of stars. These clusters gradually disperse, and the stars join the population of the Milky Way.

Kinematic studies of matter in the Milky Way and other galaxies have demonstrated that there is more mass than can be accounted for by visible matter. A dark matter halo appears to dominate the mass, although the nature of this dark matter remains undetermined.

Stellar astronomy

Mz 3, often referred to as the Ant planetary nebula. Ejecting gas from the dying central star shows symmetrical patterns unlike the chaotic patterns of ordinary explosions.
Main article: Star See also: Solar astronomy

The study of stars and stellar evolution is fundamental to our understanding of the Universe. The astrophysics of stars has been determined through observation and theoretical understanding; and from computer simulations of the interior. Star formation occurs in dense regions of dust and gas, known as giant molecular clouds. When destabilized, cloud fragments can collapse under the influence of gravity, to form a protostar. A sufficiently dense, and hot, core region will trigger nuclear fusion, thus creating a main-sequence star.

Almost all elements heavier than hydrogen and helium were created inside the cores of stars.

The characteristics of the resulting star depend primarily upon its starting mass. The more massive the star, the greater its luminosity, and the more rapidly it fuses its hydrogen fuel into helium in its core. Over time, this hydrogen fuel is completely converted into helium, and the star begins to evolve. The fusion of helium requires a higher core temperature. A star with a high enough core temperature will push its outer layers outward while increasing its core density. The resulting red giant formed by the expanding outer layers enjoys a brief life span, before the helium fuel in the core is in turn consumed. Very massive stars can also undergo a series of evolutionary phases, as they fuse increasingly heavier elements.

The final fate of the star depends on its mass, with stars of mass greater than about eight times the Sun becoming core collapse supernovae; while smaller stars blow off their outer layers and leave behind the inert core in the form of a white dwarf. The ejection of the outer layers forms a planetary nebula. The remnant of a supernova is a dense neutron star, or, if the stellar mass was at least three times that of the Sun, a black hole. Closely orbiting binary stars can follow more complex evolutionary paths, such as mass transfer onto a white dwarf companion that can potentially cause a supernova. Planetary nebulae and supernovae distribute the "metals" produced in the star by fusion to the interstellar medium; without them, all new stars (and their planetary systems) would be formed from hydrogen and helium alone.

Solar astronomy

An ultraviolet image of the Sun's active photosphere as viewed by the NASA's TRACE space telescope.
Solar observatory Lomnický štít (Slovakia) built in 1962
See also: Solar telescope

At a distance of about eight light-minutes, the most frequently studied star is the Sun, a typical main-sequence dwarf star of stellar class G2 V, and about 4.6 billion years (Gyr) old. The Sun is not considered a variable star, but it does undergo periodic changes in activity known as the sunspot cycle. This is an 11-year oscillation in sunspot number. Sunspots are regions of lower-than-average temperatures that are associated with intense magnetic activity.

The Sun has steadily increased in luminosity by 40% since it first became a main-sequence star. The Sun has also undergone periodic changes in luminosity that can have a significant impact on the Earth. The Maunder minimum, for example, is believed to have caused the Little Ice Age phenomenon during the Middle Ages.

At the center of the Sun is the core region, a volume of sufficient temperature and pressure for nuclear fusion to occur. Above the core is the radiation zone, where the plasma conveys the energy flux by means of radiation. Above that is the convection zone where the gas material transports energy primarily through physical displacement of the gas known as convection. It is believed that the movement of mass within the convection zone creates the magnetic activity that generates sunspots. The visible outer surface of the Sun is called the photosphere. Above this layer is a thin region known as the chromosphere. This is surrounded by a transition region of rapidly increasing temperatures, and finally by the super-heated corona.

A solar wind of plasma particles constantly streams outward from the Sun until, at the outermost limit of the Solar System, it reaches the heliopause. As the solar wind passes the Earth, it interacts with the Earth's magnetic field (magnetosphere) and deflects the solar wind, but traps some creating the Van Allen radiation belts that envelop the Earth. The aurora are created when solar wind particles are guided by the magnetic flux lines into the Earth's polar regions where the lines then descend into the atmosphere.

Planetary science

The black spot at the top is a dust devil climbing a crater wall on Mars. This moving, swirling column of Martian atmosphere (comparable to a terrestrial tornado) created the long, dark streak.
Main articles: Planetary science and Planetary geology

Planetary science is the study of the assemblage of planets, moons, dwarf planets, comets, asteroids, and other bodies orbiting the Sun, as well as extrasolar planets. The Solar System has been relatively well-studied, initially through telescopes and then later by spacecraft. This has provided a good overall understanding of the formation and evolution of the Sun's planetary system, although many new discoveries are still being made.

The Solar System is divided into the inner Solar System (subdivided into the inner planets and the asteroid belt), the outer Solar System (subdivided into the outer planets and centaurs), comets, the trans-Neptunian region (subdivided into the Kuiper belt, and the scattered disc) and the farthest regions (e.g., boundaries of the heliosphere, and the Oort Cloud, which may extend as far as a light-year). The inner terrestrial planets consist of Mercury, Venus, Earth, and Mars. The outer giant planets are the gas giants (Jupiter and Saturn) and the ice giants (Uranus and Neptune).

The planets were formed 4.6 billion years ago in the protoplanetary disk that surrounded the early Sun. Through a process that included gravitational attraction, collision, and accretion, the disk formed clumps of matter that, with time, became protoplanets. The radiation pressure of the solar wind then expelled most of the unaccreted matter, and only those planets with sufficient mass retained their gaseous atmosphere. The planets continued to sweep up, or eject, the remaining matter during a period of intense bombardment, evidenced by the many impact craters on the Moon. During this period, some of the protoplanets may have collided and one such collision may have formed the Moon.

Once a planet reaches sufficient mass, the materials of different densities segregate within, during planetary differentiation. This process can form a stony or metallic core, surrounded by a mantle and an outer crust. The core may include solid and liquid regions, and some planetary cores generate their own magnetic field, which can protect their atmospheres from solar wind stripping.

A planet or moon's interior heat is produced from the collisions that created the body, by the decay of radioactive materials (e.g. uranium, thorium, and Al), or tidal heating caused by interactions with other bodies. Some planets and moons accumulate enough heat to drive geologic processes such as volcanism and tectonics. Those that accumulate or retain an atmosphere can also undergo surface erosion from wind or water. Smaller bodies, without tidal heating, cool more quickly; and their geological activity ceases with the exception of impact cratering.

Interdisciplinary studies

Astronomy and astrophysics have developed significant interdisciplinary links with other major scientific fields. Archaeoastronomy is the study of ancient or traditional astronomies in their cultural context, utilizing archaeological and anthropological evidence. Astrobiology is the study of the advent and evolution of biological systems in the Universe, with particular emphasis on the possibility of non-terrestrial life. Astrostatistics is the application of statistics to astrophysics to the analysis of a vast amount of observational astrophysical data.

The study of chemicals found in space, including their formation, interaction and destruction, is called astrochemistry. These substances are usually found in molecular clouds, although they may also appear in low-temperature stars, brown dwarfs and planets. Cosmochemistry is the study of the chemicals found within the Solar System, including the origins of the elements and variations in the isotope ratios. Both of these fields represent an overlap of the disciplines of astronomy and chemistry. As "forensic astronomy", finally, methods from astronomy have been used to solve problems of art history and occasionally of law.

Amateur astronomy

Amateur astronomers can build their own equipment, and hold star parties and gatherings, such as Stellafane.
Main article: Amateur astronomy

Astronomy is one of the sciences to which amateurs can contribute the most.

Collectively, amateur astronomers observe a variety of celestial objects and phenomena sometimes with consumer-level equipment or equipment that they build themselves. Common targets of amateur astronomers include the Sun, the Moon, planets, stars, comets, meteor showers, and a variety of deep-sky objects such as star clusters, galaxies, and nebulae. Astronomy clubs are located throughout the world and many have programs to help their members set up and complete observational programs including those to observe all the objects in the Messier (110 objects) or Herschel 400 catalogues of points of interest in the night sky. One branch of amateur astronomy, astrophotography, involves the taking of photos of the night sky. Many amateurs like to specialize in the observation of particular objects, types of objects, or types of events that interest them.

Most amateurs work at visible wavelengths, but many experiment with wavelengths outside the visible spectrum. This includes the use of infrared filters on conventional telescopes, and also the use of radio telescopes. The pioneer of amateur radio astronomy was Karl Jansky, who started observing the sky at radio wavelengths in the 1930s. A number of amateur astronomers use either homemade telescopes or use radio telescopes which were originally built for astronomy research but which are now available to amateurs (e.g. the One-Mile Telescope).

Amateur astronomers continue to make scientific contributions to the field of astronomy and it is one of the few scientific disciplines where amateurs can still make significant contributions. Amateurs can make occultation measurements that are used to refine the orbits of minor planets. They can also discover comets, and perform regular observations of variable stars. Improvements in digital technology have allowed amateurs to make impressive advances in the field of astrophotography.

Unsolved problems in astronomy

Main article: List of unsolved problems in astronomy

In the 21st century there remain important unanswered questions in astronomy. Some are cosmic in scope: for example, what are dark matter and dark energy? These dominate the evolution and fate of the cosmos, yet their true nature remains unknown. What will be the ultimate fate of the universe? Why is the abundance of lithium in the cosmos four times lower than predicted by the standard Big Bang model? Others pertain to more specific classes of phenomena. For example, is the Solar System normal or atypical? What is the origin of the stellar mass spectrum? That is, why do astronomers observe the same distribution of stellar masses—the initial mass function—apparently regardless of the initial conditions? Likewise, questions remain about the formation of the first galaxies, the origin of supermassive black holes, the source of ultra-high-energy cosmic rays, and more.

Is there other life in the Universe? Especially, is there other intelligent life? If so, what is the explanation for the Fermi paradox? The existence of life elsewhere has important scientific and philosophical implications.

See also

Lists

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