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ALHENA

A great many stars were named by the Arabs according to their positions within the Greek constellations. An equal number, however, have to do with their own culture, our star, Alhena, alluding to a "brand," the "mark on the camel's neck." Though one star now carries the name, the term really refers to one or more of those that occupy the southwestern end of our Gemini, those that complete the long rectangle started by Castor and Pollux. With a mid-second magnitude of 1.9 ("second" runs from 1.5 to 2.5), Alhena is third-ranked in the constellation not far below Castor (at 1.6), and is thus Bayer's Gamma star. As are so many of the naked eye stars, Alhena is class A, denoting one with strong hydrogen absorptions and a temperature near 9000 Kelvin (this one 9200). The magnitude and an accurate distance of 105 light years reveal a luminosity 160 times that of the Sun, consistent with a slightly overluminous and evolved "subgiant" star, one 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 angular diameter lead to a diameter about five times that of the Sun, the star having more-or-less solar chemical abundances. Other than the possibility of a strong magnetic field, the star seems rather ordinary, if bright. Its fame (beyond its apparent brightness) rests on other factors. It is a "spectroscopic" double, one whose duplicity was first known because of the shifting of the star's line-of-sight velocity caused by a companion that orbits with a 12.6 year period. Alhena is also the brightest star ever observed to be occulted (crossed over, eclipsed) by an asteroid, the minor planet 381 Myrrha in 1991, the crossing time leading to an asteroidal diameter of 140 kilometers. The occultation revealed the fainter companion to be almost 200 times fainter than Alhena proper and to be a main sequence (hydrogen-fusing) G star like the Sun. From the accumulated observations, the companion, of about one solar mass, orbits the 2.8 solar mass primary (Alhena) at an average separation of about 8.5 astronomical units (Earth-Sun distances), about the size of the orbit of Saturn. The orbit, however, is very elliptical, taking the little one from about as close as Earth is to the Sun to what would be close to the orbit of Uranus. Planets would seem quite impossible! (Thanks to Jason Pero, who helped research this star.)