ALPHA MIC (Alpha Microscopii). The Alpha stars of constellations are expected to be important, to dominate their patterns. Not so in dim modern Microscopium, the Microscope, whose faint stars are hard to find to the south of Capricornus. The Microscope's stars are in fact not ordered by brightness, as is at least expected to be the case, but like those of the Big Dipper in Ursa Major, are in order of position from west to east. A northern tier is lettered Alpha through Epsilon, while a southern tier was given Zeta through Theta. Yet more southern stars got Iota and Nu, leaving out Kappa and Lambda. As a result, fifth magnitude (4.90) Alpha Mic ranks third after the somewhat brighter luminary, fifth magnitude Gamma (4.67) and even Epsilon (4.71). With stars like these, it's no wonder the figure -- one of the faintest in the sky -- is so obscure. As such, Microscopium ranks right up there with Mensa (the Table), whose brightest star (Alpha Men) is even fainter. Consistently, Alpha Mic is as obscure as its parent constellation. Over the past century, a typical bright star will have had hundreds of references in the scientific literature. Alpha Mic has scored a miserable 14. The star itself, however, a class G (G7) giant, exceeds its press. At a distance of 380 light years, it shines with the light of 163 Suns from a coolish 4920 Kelvin surface. Combination of temperature and luminosity reveal a radius of 17.5 times that of the Sun, all typical of a common helium-fusing giant that is quietly developing its carbon-oxygen core. Theory shows that this three solar mass star began life 420 million years ago as a class B8 hydrogen-fusing dwarf, and for the past 70 million years has been developing toward the giant it is today. Twenty seconds of arc away is a much fainter 10th magnitude star (Alpha Mic B), close enough that it at first appears to be a real companion. However, the observations say otherwise. Since 1834, it has moved seven seconds of arc closer to Alpha Mic proper, far too great for any kind of orbital motion, revealing the "companion" to be simply yet another line-of-sight coincidence, and about which nothing at all is known.
Written by Jim Kaler 9/21/07. Return to STARS.