Alpha Microscopii

At magnitude 4.90, Alpha Microscopii ranks third brightest in Microscopium. This constellation is one of the faintest in the sky; consistently, α Mic is as obscure as its parent constellation.

Properties

The star itself is a class G7 III giant. At a distance of 380 light years, it shines with the luminosity of 163 suns from a cool, 4920 K surface. Temperature and luminosity imply a radius 17.5 times the Sun's, typical of a helium-fusing giant that is developing a carbon-oxygen core. This three-solar-mass star began life 420 million years ago as a class B8 V star, and for the past 70 million years it has been developing into the giant it is today.

Companion

A much fainter, 10th magnitude star (α Mic B) lies 20" away, close enough that it at first appears to be a real companion. However, observations of its motion indicate otherwise. Since 1834, it has moved 7" arc closer to α Mic A, far too great for any kind of orbital motion - and thus revealing the "companion" to be simply a line-of-sight coincidence.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]