 U Sagittarii (center) is the brightest star in the open cluster Messier 25. | |
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Sagittarius |
| Right ascension | 18 31 53.332 |
| Declination | −19° 07′ 30.26″ |
| Apparent magnitude (V) | 6.28 to 7.15 |
| Characteristics | |
| Evolutionary stage | Supergiant |
| Spectral type | G1Ib |
| U−B color index | +0.85 |
| B−V color index | +1.06 |
| Variable type | δ Cep |
| Astrometry | |
| Radial velocity (Rv) | 2.2±0.3 km/s |
| Proper motion (μ) | RA: −1.795 mas/yr Dec.: −6.127 mas/yr |
| Parallax (π) | 1.5693 ± 0.0224 mas |
| Distance | 2,080 ± 30 ly (637 ± 9 pc) |
| Absolute magnitude (MV) | −3.86 to −3.10 |
| Details | |
| Mass | 6.54 to 6.64 M☉ |
| Radius | 55.5 to 56.3 R☉ |
| Luminosity | 4,370 to 4,650 L☉ |
| Surface gravity (log g) | 0.99 cgs |
| Temperature | 5,802 K |
| Metallicity | 0.17 dex |
| Rotational velocity (v sin i) | 15.4 km/s |
| Age | 91 Myr |
| Other designations | |
| U Sgr, BD−19°5047, GC 25287, HD 170764, HIP 90836, HR 6947, SAO 161571, WDS J18319-1908A | |
| Database references | |
| SIMBAD | data |
U Sagittarii is a variable star in the southern constellation of Sagittarius, abbreviated U Sgr. It is a classical Cepheid variable that ranges in brightness from an apparent visual magnitude of 6.28 down to 7.15, with a pulsation period of 6.745226 days. At its brightest, this star is dimly visible to the naked eye. The distance to this star is approximately 2,080 light years based on parallax measurements, and it is drifting further away with a radial velocity of 2 km/s.
The variability of this star was announced by J. Schmidt in 1866, who found a preliminary period of 6.74784 days. It was later determined to be a variable of the Cepheid type. In 1925, P. Doig assumed that the star is a member of the open cluster Messier 25 (M25), but actual evidence of its membership would not be available until 1932 when P. Hayford made radial velocity measurements of the cluster. Membership in this cluster is now reasonably established, and as such this Cepheid serves as one of the anchors for the cosmic distance scale since the distance to the cluster can be determined independently from the star. Indeed, new research indicates U Sgr's host cluster (M25) may constitute a ternary (triple) star cluster together with NGC 6716 and Collinder 394.
This is an evolved G-type supergiant star with a typical stellar classification of G1Ib. It appears to be making its third traversal of the instability strip with its period changing at the rate of +0.073±0.010 s·yr. Elemental abundances are similar to those in the Sun. It has an estimated 6.6 times the mass of the Sun and 56 times the Sun's radius. The star is radiating over 4,000 times the Sun's luminosity from its photosphere at an effective temperature of 5,802 K.
References
- Luginbuhl, Christian B.; Skiff, Brian A. (1998), Observing Handbook and Catalogue of Deep-Sky Objects, Cambridge University Press, p. 213, ISBN 9780521625562.
- ^ Brown, A. G. A.; et al. (Gaia collaboration) (2021). "Gaia Early Data Release 3: Summary of the contents and survey properties". Astronomy & Astrophysics. 649: A1. arXiv:2012.01533. Bibcode:2021A&A...649A...1G. doi:10.1051/0004-6361/202039657. S2CID 227254300. (Erratum: doi:10.1051/0004-6361/202039657e). Gaia EDR3 record for this source at VizieR.
- ^ Samus, N. N.; et al. (2017), "General Catalogue of Variable Stars", Astronomy Reports, 5.1, 61 (1): 80–88, Bibcode:2017ARep...61...80S, doi:10.1134/S1063772917010085, S2CID 125853869.
- ^ Houk, N.; Smith-Moore, M. (1988), Michigan Catalogue of Two-dimensional Spectral Types for the HD Stars, vol. 4, Bibcode:1988mcts.book.....H.
- ^ Sandage, Allan (May 1960), "Cepheids in Galactic Clusters.VI. U SGR in M25", Astrophysical Journal, 131: 610, Bibcode:1960ApJ...131..610S, doi:10.1086/146873.
- ^ Breger, M. (1967), "Velocity, radius and light variations of the cepheid U Sgr", Monthly Notices of the Royal Astronomical Society, 136: 61, Bibcode:1967MNRAS.136...61B, doi:10.1093/mnras/136.1.61.
- ^ Fadeyev, Yu. A. (August 2018), Shustov, B. M.; Wiebe, D. S. (eds.), "Secular period change in the cepheid U Sgr and the theory of stellar evolution", Stars and Satellites, Proceedings of the Memorial Conference Devoted to A.G. Masevich 100th Anniversary, held in Moscow, Russia, in Oct 15-16, 2018, INASAN Science Proceedings (in Russian), Moscow: Yanus-K, pp. 152–156, Bibcode:2018sas..conf..152F, doi:10.26087/INASAN.2018.2.2.024.
- ^ Prugniel, Ph.; et al. (July 2011), "The atmospheric parameters and spectral interpolator for the MILES stars", Astronomy & Astrophysics, 531: A165, arXiv:1104.4952, Bibcode:2011A&A...531A.165P, doi:10.1051/0004-6361/201116769, S2CID 54940439, A165.
- Jönsson, Henrik; et al. (2020), "APOGEE Data and Spectral Analysis from SDSS Data Release 16: Seven Years of Observations Including First Results from APOGEE-South", The Astronomical Journal, 160 (3): 120, arXiv:2007.05537, Bibcode:2020AJ....160..120J, doi:10.3847/1538-3881/aba592, S2CID 220496397.
- Skowron, Dorota M.; et al. (2019), "A three-dimensional map of the Milky Way using classical Cepheid variable stars", Science, 365 (6452): 478–482, arXiv:1806.10653, Bibcode:2019Sci...365..478S, doi:10.1126/science.aau3181, PMID 31371611, S2CID 199064609.
- "U Sgr". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2022-12-03.
- "ASAS All Star Catalogue". The All Sky Automated Survey. Retrieved 5 December 2022.
- Schmidt, Johann Friedrich Julius (March 1868), "Über die Lichtphasen von A, γ und U Sagittarii in den Jahren 1866 und 1867", Astronomische Nachrichten (in German), 71: 139, Bibcode:1868AN.....71..139S.
- Shapley, H. (December 1918), "Studies based on the colors and magnitudes in stellar clusters. VIII. The luminosities and distances of 139 Cepheid variables", Astrophysical Journal, 48: 279–294, Bibcode:1918ApJ....48..279S, doi:10.1086/142435.
- Wallerstein, George (April 1957), "The Absolute Magnitude of U Sagittarii and Its Membership in M 25", Publications of the Astronomical Society of the Pacific, 69 (407): 172, Bibcode:1957PASP...69..172W, doi:10.1086/127041, S2CID 121527741.
- Chen, Xiaodian; et al. (January 2015), "A search for open cluster Cepheids in the Galactic plane", Monthly Notices of the Royal Astronomical Society, 446 (2): 1268–1282, arXiv:1410.4489, Bibcode:2015MNRAS.446.1268C, doi:10.1093/mnras/stu2165.
- ^ Majaess, D.; et al. (December 2013), "Anchors for the cosmic distance scale: the Cepheids U Sagittarii, CF Cassiopeiae, and CEab Cassiopeiae", Astronomy & Astrophysics, 560: A22, arXiv:1311.0865, Bibcode:2013A&A...560A..22M, doi:10.1051/0004-6361/201322670, S2CID 55934597, A22.
- Majaess, D.; et al. (August 2024), "A Rare Cepheid-hosting Open Cluster Triad in Sagittarius", Research Notes of the AAS, 8 (8): 205, doi:10.3847/2515-5172/ad7139.
Further reading
- Berdnikov, L. N.; Majaess, D. (April 2014), "Search for evolutionary changes in the periods of Cepheids: U Sgr", Astronomy Letters, 40 (4): 193–200, Bibcode:2014AstL...40..193B, doi:10.1134/S1063773714030025, S2CID 255203281.
- Simon, Norman R. (March 1989), "Application of the Baade-Wesselink inversion technique to SW Dra, X Ari and U Sgr", Monthly Notices of the Royal Astronomical Society, 237: 163–185, Bibcode:1989MNRAS.237..163S, doi:10.1093/mnras/237.1.163.
- Burnham, Robert (1978), Burnham's Celestial Handbook, An Observer's Guide to the Universe Beyond the Solar System, vol. 3, Dover Publications, ISBN 9780486236735.
- Jacobsen, T. S. (February 1970), "A High-Dispersion Velocity Curve of U Sagittarii", Astrophysical Journal, 159: 569, Bibcode:1970ApJ...159..569J, doi:10.1086/150331.
- Wampler, J.; et al. (May 1961), "Cepheids in Galactic Clusters. VIII. a Reinvestigation of U SGR in M25 (= IC 4725).", Astrophysical Journal, 133: 895, Bibcode:1961ApJ...133..895W, doi:10.1086/147092.
- Mitchell, Richard I.; et al. (May 1961), "U, B, V, Observations of U SGR", Astrophysical Journal, 133: 1083, Bibcode:1961ApJ...133.1083M, doi:10.1086/147114.
- Voûte, J.; ten Bruggencate, P. (1927), "U Sagittarii", Annals of the Bosscha Observatory Lembang (Java) Indonesia, 2: B28 – B33, Bibcode:1927AnBos...2B..28V.
- Yendell, Paul S. (1895), "On variable stars of short period. IV--The Sagittarius Group", Popular Astronomy, 2: 362–366, Bibcode:1895PA......2..362Y.
