At magnitude 1.9, Gamma Geminorum is the third brightest member of the constellation Gemini. It has traditional names Alhena and Almeisan. The name Alhena is from the Arabic "Al Han'ah", meaning "the brand" (on the neck of the camel), while the alternate name Almeisan is from the Arabic "Al Maisan", meaning "the shining one". Alhena originally referred to one or more stars of an Arabian constellation at the southwestern end of our Gemini, and complete the long rectangle started by Castor and Pollux. It is known as the Third Star of the Well in Chinese.
Properties
Alhena is a class A0 III subgiant, with a temperature near 9200 K. Its distance of 105 light years gives it a luminosity 160 times the Sun's, consistent with its slightly overluminous and evolved "subgiant" stage, whose core hydrogen fusion has shut down in the first stage of stellar death. The temperature and luminosity, the strength of the star's gravity as revealed by its spectrum, and direct measures of its angular size, reveal a diameter about five times the Sun's. The star has more-or-less solar chemical abundances. Other than the possibility of a strong magnetic field, the star seems rather ordinary.
Companion
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 km) to be determined, and also the fact that the dimmer companion 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 AU - 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. Planets would seem quite impossible
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]