MIRACH (Beta Andromedae). Andromeda's three brightest stars, Alpheratz, Mirach, and Almach, lie along a graceful curve that extends to the northeast of the Great Square of Pegasus (Alpheratz part of the Square itself). Since the three line up in order of brightness, Bayer did not have to concern himself with how to letter them, whether by brightness or direction, assigning them in order Alpha, Beta, and Gamma. They are in order except for the odd coincidence that Alpheratz and Mirach (Alpha and Beta) have identical magnitudes (2.06). But their constellation and apparent brightness is about all they have in common. Alpheratz is a hot, blue (class B) star, whereas Mirach is a cool red class M (M0) giant (the color difference notable to the eye), and quite a bright giant at that. Mirach's name is something of a mistake. It was originally meant to be "Mizar," meaning "loin," which for us today refers to the famed star in the Big Dipper's handle, but a variety of misspellings made the name take on its present form. As a giant it is truly luminous and large. From its distance of 200 light years and its temperature of 3800 degrees Kelvin (which tells how much invisible infrared is radiated), it shines 1900 times more brightly than the Sun. From temperature and luminosity, as well as from the direct measure of angular diameter (a mere 0.012 seconds of arc), it is 0.8 Astronomical Units 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 the state, it will soon (in astronomical terms of course) die as a dense white dwarf rather like the companion to Sirius. Mirach, like many cool class M giants, seems to be slightly variable, though the variations are not well documented. It also has a quite-marvelous companion, a very dim hydrogen fusing (like the Sun) star of very low mass, and from the Earth only 14th magnitude, over 60,000 times fainter than Mirach proper. At minimum, the two are 1700 Astronomical Units apart, over 40 times Pluto's average distance from the Sun. From Mirach, the companion would shine like a bright red Venus, whereas from the companion, Mirach would sear the sky 120 times more brightly than our full Moon.
Written by Jim Kaler. Return to STARS.