SJAA Ephemeris December 2010 | SJAA Home | Contents | Previous | Next

Starry, Starry Night


Paul Kohlmiller


The star Alhena lies at the “foot” of Pollux in Gemini. It is the third brightest star in that constellation after Castor and Pollux. Hence its Bayer designation is Gamma Geminorum. Its magnitude is 1.9. The name Alhena has to do with a brand or mark on the neck of a camel. This is a class A star with a surface temperature of 9200 Kelvin (Sol is 5800 Kelvin). It is 105 light years away and its absolute magnitude is -0.64 meaning its intrinsic brightness exceeds the twins themselves. Alhena has a companion which was discovered using a technique similar to the wobble method that Geoff Marcy and colleagues use to detect planets. The companion star has the size of our Sun and it orbits Alhena once every 12.6 years. The semi-major axis of the elliptical orbit is 8.5 AU but the distance between the two stars varies from 1 to 20 AU. It seems unlikely that any planets could be present in that configuration.

Alhena is the brightest star ever used to study an occultation of an asteroid. This was done in 1991 with the asteroid 381 Myrrha and this gave an estimated diameter of 140 kilometers. These observations confirmed that Alhena’s companion star is a class G star (like Sol).

Alhena crosses the meridian around midnight by the end of December. It is near the moon on the winter solstice.


Previous | Contents | Next