# Pi Geminorum

π Geminorum
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Gemini
Right ascension  07h 47m 30.32300s[1]
Declination +33° 24′ 56.5034″[1]
5.14[2]
Characteristics
Spectral type M1 IIIa[3]
U−B color index +1.90[2]
B−V color index +1.59[2]
Astrometry
Radial velocity (Rv)−13.36±0.34[4] km/s
Proper motion (μ) RA: -19.59[1] mas/yr
Dec.: -29.33[1] mas/yr
Parallax (π)4.93 ± 0.32[1] mas
Distance660 ± 40 ly
(200 ± 10 pc)
Absolute magnitude (MV)−1.04[5]
Details
Luminosity1,007[7] L
Temperature3,900[7] K
Other designations
π Gem, 80 Geminorum, BD+33° 1585, FK5 296, HD 62898, HIP 38016, HR 3013, SAO 60340.[8]
Database references

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,[2] it is faintly visible to the naked eye on a dark night. Based upon an annual parallax|shift of 4.93 mas,[1] 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.[5]

This is a evolved red giant star with a stellar classification of M1 IIIa.[3] The measured angular diameter of this star is 2.58±0.20 mas.[9] At the estimated distance of this star, this yields a physical size of about 56 times the radius of the Sun.[6] It is radiating roughly a thousand times the luminosity of the Sun from its outer atmosphere at an effective temperature of 3,900 K.[7]

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°.[10] This star is suspected to be an astrometric companion of the primary component.[3]

## 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.
2. ^ a b c d Mermilliod, J.-C. (1986), "Compilation of Eggen's UBV data, transformed to UBV (unpublished)", Catalogue of Eggen's UBV Data, SIMBAD, Bibcode:1986EgUBV........0M.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. ^ a b c 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.
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.
5. ^ a b 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.
6. ^ a b Lang, Kenneth R. (2006), Astrophysical formulae, Astronomy and astrophysics library, 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. ^ a b c 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.
8. ^ "pi. Gem". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2016-12-07.
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.