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Alhena, star γ of the Gemini constellation of magnitude 1.9, spectral class A, 44 years distance light. Alhena, also called Almeisan, is the third brightest star in the constellation of Gemini, Castor and Pollux after. It shines at magnitude 1.95, making it the forty-brightest star in the entire sky. Alhena the name derives from ‘Arabic الهنعه At Han’ah, that means branding, and an alias Almeisan comes from المیسان At Maisan, which means shining. It is a star 160 times more luminous than the sun. Alhena is located in the south-west of the Twins, at the foot of Pollux. It also lies just north of one of the most well known asterisms, the Winter Triangle. Having declination + 16 °, that is, being located close enough to the celestial equator, although it is a star of the northern hemisphere is also visible in all the inhabited regions of the Southern Hemisphere. It becomes circumpolar only in the far north of the northern hemisphere, as well as the parallel 74º, that in more northern parts of Russia, Canada and Greenland.
The abundance of metals of Alhena turns out to be comparable to sunlight, while its rotation speed is 32 km / s, which allows her to complete a rotation on itself in about 80 days. The expansion in which the star is going to encounter is slowly slowing its speed of rotation because of the law of conservation of angular momentum. Alhena is actually a double star, having a main partner. This was discovered in 1905. It is not possible to solve the two components using a telescope given the distance of the system, that between the two components and their great difference in brightness. It was therefore resorted to spectroscopic methods, measuring the radial velocity of the main differences. However, January 13, 1991, the asteroid 381 Myrrha concealed Alhena and it was possible to directly observe the secondary for the short amount of time in which the principal was still eclipsed by asteroid, while the secondary was not already over.
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