Alhena (Gamma Geminorum)


Alhena (center) with Mu Geminorum at upper right. Image: © Akira Fujii.

Alhena (Gamma Geminorum) is a subgiant A star and the third brightest member of the constellation Gemini. Its Arabic name (alternatively given as Almeisan) refers to a brand on a horse or a camel. Alhena is a spectroscopic binary with a period of 12.6 years and is the brightest star ever observed to be occulted by an asteroid. In 1991, 381 Myrrha passed in front of Alhena enabling not only the asteroid's diameter (140 kilometers) to be determined but also the fact that the dimmer companion star is a Sun-like G star almost 200 times fainter than Alhena proper. Accumulated observations have shown that the companion, of about one solar mass, orbits the 2.8-solar-mass primary at an average separation of about 8.5 astronomical units – about the size of Saturn's orbit – but ranges from about as close as Earth is to the Sun to about the distance of Uranus.


visual magnitude 1.93
absolute magnitude -0.61
spectral type A0IV
surface temperature 9,750 K
luminosity 160 Lsun
distance 105 light-years (32 pc)
position RA 06h 37m 42.7s,
Dec +16° 23' 57"
other designations Almeisan, 24 Geminorum,
HR 2421, BD +16°1223,
HD 47105, GCTP1539.00,
SAO 95912, FK5 251, HIP 31681