A guide to star Alhena (Gamma Geminorum)
Alhena is Gemini’s 3rd-brightest star and Pollux’s left foot.
Alhena (Gamma (γ) Geminorum) is a bright star in Gemini marking the position of the twin Pollux’s left foot. Its name is derived from the Arabic word for a brand applied to the neck of a camel.
Alhena is the third-brightest star in Gemini and stands out in a region with few other stars to rival it, roughly midway between the Twin stars Castor (Alpha (α) Geminorum) and Pollux (Beta (β) Geminorum), and Betelgeuse (Alpha (α) Orionis).
Compared to mag. +1.6 Castor, at mag. +1.9 Alhena is not far behind in brightness.
Alhena lies 109 lightyears away. At that distance, to appear as bright as it does to us it must be 123 times more luminous than our Sun.
Its spectral classification is A1 IV + G, which means it is a white (A1) subgiant (IV).
The ‘+ G’ refers to a G-type companion that, with the primary, forms a spectroscopic binary system with a period of 12.6 years.
The primary has a mass 2.8 times larger than our Sun, while the secondary’s mass is 1.1 times that of our Sun.
The orbit of both stars around their common centre of mass is known to be highly elliptical: with an average separation of 8.5 AU, it brings the pair as close as the Earth–Sun distance and as far apart as Uranus is from the Sun.
Alhena was occulted by the asteroid 381 Myrrha in 1991, an event which revealed that Myrrha has a diameter of 140km and that the spectroscopic binary companion is 200 times dimmer that Alhena.
This guide originally appeared in the January 2023 issue of BBC Sky at Night Magazine.