3 Geminorum

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3 Geminorum
Gemini constellation map.svg
Red circle.svg
Location of 3 Gem (circled)
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Gemini
Right ascension 06h 09m 43.98481s[1]
Declination +23° 06′ 48.4677″[1]
Apparent magnitude (V) 5.75[2]
Spectral type B2.5 Ib[3]
U−B color index −0.63[2]
B−V color index +0.21[2]
Variable type α Cyg[4]
Radial velocity (Rv)16.00 ± 4.3[5] km/s
Proper motion (μ) RA: −0.92 ± 0.75[1] mas/yr
Dec.: −2.65 ± 0.49[1] mas/yr
Parallax (π)0.23 ± 0.88[1] mas
Absolute magnitude (MV)−6.26[6]
Mass21 M
Radius36.6 R
Diameter36.6 D
Luminosity129,000 L
Surface gravity (log g)2.50 cgs
Temperature18,000 K
Rotational velocity (v sin i)71 km/s
Other designations
3 Geminorum, PU Gem, HR 2173, HIP 29225, HD 42087, BD+23°1226, AAVSO 0603+23
Database references

3 Geminorum is a blue supergiant star in the constellation Gemini. It is a small amplitude pulsating variable and a close double star, with a mean combined apparent visual magnitude of 5.75.

3 Geminorum was found to be an α Cygni variable in 1998 and given the designation PU Geminorum. It varies by a few tenths of a magnitude[4] with a main period of 3.81 days.[7]

3 Geminorum is also a close double star. The brighter component is the variable blue supergiant. The companion is 2.5 magnitudes fainter. The separation is about 0.6 arc-seconds.[8] There is also a much fainter, approximately 14th magnitude, star 14" away.[9]

Faint emission lines have been detected in the spectrum of 3 Geminorum,[10] but this is not usually expressed in published spectral classifications.[3] An "e" is only occasionally appended to the spectral type to reflect the emission lines.[11][4] MK spectral types consistently classify 3 Geminorum as a normal supergiant (luminosity class Ib), although spectral classes derived in other ways often result in a bright supergiant (Ia) luminosity class.[12]

3 Geminorum can be occulted by the moon. Observations of these occulations can give information about the angular diameter of a star, or about close companions. Occultations of 3 Geminorum have been observed, but no double or diameter information has been published.[13]


  1. 1.0 1.1 1.2 1.3 1.4 Van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics 474 (2): 653. arXiv:0708.1752. Bibcode 2007A&A...474..653V. doi:10.1051/0004-6361:20078357.
  2. 2.0 2.1 2.2 Ducati, J. R. (2002). "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system". CDS/ADC Collection of Electronic Catalogues 2237. Bibcode 2002yCat.2237....0D.
  3. 3.0 3.1 Prinja, R. K.; Massa, D. L. (2010). "Signature of wide-spread clumping in B supergiant winds". Astronomy and Astrophysics 521: L55. arXiv:1007.2744. Bibcode 2010A&A...521L..55P. doi:10.1051/0004-6361/201015252.
  4. 4.0 4.1 4.2 Samus, N. N.Expression error: Unrecognized word "etal". (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/gcvs. Originally published in: 2009yCat....102025S 1. Bibcode 2009yCat....102025S.
  5. Gontcharov, G. A. (2006). "Pulkovo Compilation of Radial Velocities for 35 495 Hipparcos stars in a common system". Astronomy Letters 32 (11): 759. arXiv:1606.08053. Bibcode 2006AstL...32..759G. doi:10.1134/S1063773706110065.
  6. 6.0 6.1 Searle, S. C.; Prinja, R. K.; Massa, D.; Ryans, R. (2008). "Quantitative studies of the optical and UV spectra of Galactic early B supergiants. I. Fundamental parameters". Astronomy and Astrophysics 481 (3): 777. arXiv:0801.4289. Bibcode 2008A&A...481..777S. doi:10.1051/0004-6361:20077125.
  7. Waelkens, C.; Aerts, C.; Kestens, E.; Grenon, M.; Eyer, L. (1998). "Study of an unbiased sample of B stars observed with Hipparcos: the discovery of a large amount of new slowly pulsating B star". Astronomy and Astrophysics 330: 215–21. Bibcode 1998A&A...330..215W.
  8. Roberts, Lewis C.; Turner, Nils H.; Bradford, L. William; Ten Brummelaar, Theo A.; Oppenheimer, Ben R.; Kuhn, Jeff R.; Whitman, Kathryn; Perrin, Marshall D. et al. (2005). "Adaptive Optics Photometry and Astrometry of Binary Stars". The Astronomical Journal 130 (5): 2262. Bibcode 2005AJ....130.2262R. doi:10.1086/491586.
  9. Mason, Brian D.; Wycoff, Gary L.; Hartkopf, William I.; Douglass, Geoffrey G.; Worley, Charles E. (2001). "The 2001 US Naval Observatory Double Star CD-ROM. I. The Washington Double Star Catalog". The Astronomical Journal 122 (6): 3466. Bibcode 2001AJ....122.3466M. doi:10.1086/323920.
  10. Rosendhal, J. D. (1973). "A survey of H-alpha emission in early-type high-luminosity stars". Astrophysical Journal 186: 909. Bibcode 1973ApJ...186..909R. doi:10.1086/152555.
  11. Kohoutek, L.; Wehmeyer, R. (1999). "Catalogue of H-alpha emission stars in the Northern Milky Way". Astronomy and Astrophysics Supplement 134 (2): 255. Bibcode 1999A&AS..134..255K. doi:10.1051/aas:1999101.
  12. Skiff, B. A. (2014). "VizieR Online Data Catalog: Catalogue of Stellar Spectral Classifications (Skiff, 2009-2016)". VizieR On-line Data Catalog: B/mk. Originally published in: Lowell Observatory (October 2014) 1. Bibcode 2014yCat....1.2023S.
  13. Meyer, C.; Rabbia, Y.; Froeschle, M.; Helmer, G.; Amieux, G. (1995). "Observations of lunar occultations at Observatoire de la Cote d'Azur". Astronomy and Astrophysics Supplement 110: 107. Bibcode 1995A&AS..110..107M.