Beta Andromedae - Mirach

Mirach, or β Andromedae, has a magnitude of 2.06, identical to Alpheratz, or α Andromedae. Mirach's name is something of a mistake. It was originally meant to be "Mizar," meaning "loin," which today refers to the middle in the Big Dipper's handle, but a variety of misspellings gave the name its present form.

Properties

Mirach is a cool, red, class M0 giant - and quite a bright giant at that. From its distance of 200 light years, and its temperature of 3800 K, it shines 1900 times more brightly than the Sun. From temperature and luminosity, as well as from the direct measure of its angular diameter (about 0.012"), it is 0.8 AU across - about the size of Mercury's orbit. It is difficult to say just what state the star is in. It is clearly massive, having three or four times the mass of the Sun, but it may have a core made of helium or one made of carbon. Whatever its state, it will soon become a dense white dwarf rather like the companion of Sirius. Mirach, like many cool class M giants, seems to be slightly variable, though its variations are not well documented.

Mirach also has a companion, a very dim hydrogen-fusing main sequence star of very low mass. From the Earth the companion appears at only 14th magnitude - over 60,000 times fainter than Mirach proper. At minimum, the two are 1700 AU apart, over 40 times Pluto's average distance from the Sun.

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