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π Geminorum (Latinised as Pi Geminorum, abbreviated to π Gem or pi Gem) is a star located in the constellation Gemini, to the north of Castor. With an apparent visual magnitude of 5.14, it is faintly visible to the naked eye on a dark night. Based upon an annual parallax|shift of 4.93 mas, Pi Geminorum is located roughly 660 light years from the Sun. At that distance, the visual magnitude of the star is diminished by an interstellar absorption factor of 0.033 due to interstellar dust.

This is an evolved red giant star with a stellar classification of M1 IIIa. The measured angular diameter of this star is 2.58±0.20 mas. At the estimated distance of this star, this yields a physical size of about 56 times the radius of the Sun. It is radiating roughly a thousand times the luminosity of the Sun from its outer atmosphere at an effective temperature of 3,900 K.

Unexpectedly for a red giant, Pi Geminorum was found to be an X-ray source during the ROSAT all-sky survey. The most likely source for this emission is an 11.4 magnitude companion star located at an angular separation of 21 arcseconds along a position angle of 214°. This star is suspected to be an astrometric companion of the primary component.

References

1. ^ van Leeuwen, F. (2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv:0708.1752, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357, S2CID 18759600.
2. ^ Mermilliod, J.-C. (1986), "Compilation of Eggen's UBV data, transformed to UBV (unpublished)", Catalogue of Eggen's UBV Data, SIMBAD, Bibcode:1986EgUBV........0M.
3. ^ Eggleton, P. P.; Tokovinin, A. A. (September 2008), "A catalogue of multiplicity among bright stellar systems", Monthly Notices of the Royal Astronomical Society, 389 (2): 869–879, arXiv:0806.2878, Bibcode:2008MNRAS.389..869E, doi:10.1111/j.1365-2966.2008.13596.x, S2CID 14878976.
4. Famaey, B.; et al. (2009), "Spectroscopic binaries among Hipparcos M giants. I. Data, orbits, and intrinsic variations", Astronomy and Astrophysics, 498 (2): 627–640, arXiv:0901.0934, Bibcode:2009A&A...498..627F, doi:10.1051/0004-6361/200810698, S2CID 18739721.
5. ^ Ryon, Jenna; et al. (August 2009), "Comparing the Ca ii H and K Emission Lines in Red Giant Stars", Publications of the Astronomical Society of the Pacific, 121 (882): 842–856, arXiv:0907.3346, Bibcode:2009PASP..121..842R, doi:10.1086/605456, S2CID 17821279.
6. ^ Lang, Kenneth R. (2006), Astrophysical formulae, Astronomy and astrophysics library, vol. 1 (3rd ed.), Birkhäuser, ISBN 3-540-29692-1.. The radius (R_*) is given by:

   ${\displaystyle {\begin{aligned}2\cdot R_{*}&={\frac {(203\cdot 2.58\cdot 10^{-3})\ {\text{AU}}}{0.0046491\ {\text{AU}}/R_{\bigodot }}}\\&\approx 112.6\cdot R_{\bigodot }\end{aligned}}}$

7. ^ McDonald, I.; et al. (2012), "Fundamental Parameters and Infrared Excesses of Hipparcos Stars", Monthly Notices of the Royal Astronomical Society, 427 (1): 343–57, arXiv:1208.2037, Bibcode:2012MNRAS.427..343M, doi:10.1111/j.1365-2966.2012.21873.x, S2CID 118665352.
8. "pi. Gem". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2016-12-07.{{cite web}}: CS1 maint: postscript (link)
9. Richichi, A.; et al. (February 2005), "CHARM2: An updated Catalog of High Angular Resolution Measurements", Astronomy and Astrophysics, 431 (2): 773–777, Bibcode:2005A&A...431..773R, doi:10.1051/0004-6361:20042039.
10. Hunsch, Matthias; et al. (February 1998), "On the X-ray emission from M-type giants", Astronomy and Astrophysics, 330: 225–231, Bibcode:1998A&A...330..225H.

v t e Constellation of Gemini
List of stars in Gemini Gemini in Chinese astronomy
Stars	Bayer α (Castor) β (Pollux) γ (Alhena) δ (Wasat) ε (Mebsuta) ζ (Mekbuda) η (Propus) θ ι κ λ μ (Tejat) ν ξ (Alzirr) ο (Jishui) π ρ σ τ υ φ χ ω Flamsteed 1 3 6 25 30 37 38 (e) 57 (A) 64 (b) 65 (b) 74 (f) 81 (g) Variable R U TV BG DM DN OU WY V429 HR 2877 HD 44780 50554 63433 260655 Other 2MASS 0727+1710 CXOU J061705.3+222127 Geminga Gliese 251 HAT-P-20 HAT-P-24 HAT-P-33 LB-1 PSR J0659+1414 WD J0651+2844 WISEP J060738.65+242953.4
Exoplanets HAT-P-24b HAT-P-50b HD 50554 b HD 59686 Ab Pollux b τ Geminorum b
Star clusters	NGC 2129 2158 2266 2277 2355 2420 Other Koposov 2 Messier 35
Nebulae IC 443 IC 444 Medusa Nebula NGC 2371-2 NGC 2392
Galaxies	NGC 2291 2294 2357 Other 2MASX J07322028+3138009 IC 485 PKS 0735+178
Astronomical events GRB 070125
Category

• M-type giants
• Suspected variables
• Gemini (constellation)
• Bayer objects
• Durchmusterung objects
• Flamsteed objects
• Henry Draper Catalogue objects
• Hipparcos objects
• Bright Star Catalogue objects