Beta Andromedae

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β Andromedae, also known as Mirach, is a bright star in the constellation of Andromeda located halfway between α Andromedae and γ Andromedae.

Naming and mythology

Mirach is known under a variety of different spellings and was originally named Mi'zar by Arab astronomers, indicating the loins of Andromeda. In 1521 the name Mirat was introduced from which the present form, meaning the sides of Andromeda, was derived.[1]

Observational attributes

Mirach's brightness is equal to that of Alpheratz at magnitude +2.06. As opposed to the blue star Alpheratz however, Mirach is decidedly ruddy. It is located at coordinates R.A. 1h 9m 43.9s and Dec. +35° 37ˈ 14ˈˈ.[2] From the measured parallax of 0.049ˈˈ[2] a distance of roughly 66.5 light years can be deducted although the true value seems to be closer to 200 light years.[3]

Together with the star μ Andromedae, Mirach can be used as a guide to locate the Andromeda Galaxy as it is located on the extension of the line joining these two stars.

Physical characteristics

β Andromedae is a red giant star of spectral class M0+IIIa and a total luminosity of around 1.900 solar luminosities[3] and an absolute magnitude of -1.8. Like all red giants, Mirach is relatively cool with a surface temperature of about 3.800 kelvins. The diameter of the star is estimated to be 120 million km or 0.8 AU.[3] β And. exhibits slight variations in brightness in the range of magnitudes +2.0 - +2.1 with no regular period apparent.[4]

Mirach is a multiple star system with at least 4 components in addition to the primary. The brightest of these, β Andromedae D, is of magnitude +12.1.[5]


  1. Star Names: Their Lore and Meaning, Richard H. Allen, Dover Publications 1963 (revised edition)
  2. 2.0 2.1 Bright Star Catalog 5th revised edition, 1991
  3. 3.0 3.1 3.2 Jim Kaler's Stars, online at
  4. Combined General Catalogue of Variable Stars, 1998
  5. The Washington Visual Double Star Catalog, 1996