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.