Mirach (Beta (β) Andromedae) is in the constellation Andromeda and is found halfway along Andromeda’s body, between Alpheratz (Alpha (α) Andromedae) and Almach (Gamma (γ) Andromedae).
The name Mirach probably results from a corruption of mi’zar, meaning girdle or waist-cloth.
Mirach and Alpheratz have the same average brightness of mag. +2.06, but look markedly different because Mirach is a red giant and Alpheratz a hot blue star.
Mirach is 197 lightyears away and 1,900 times more luminous than the Sun.
Its radius is estimated at around 0.4 AU, or 85 times larger than the Sun, giving it a circumference equivalent to the orbit of Mercury.
As the star is approaching the end of its life its core probably consists of helium or carbon.
As is common with old red giants, Mirach’s light is inconstant, and it’s classed as a semi-regular variable star.
The changes in brightness are small, ranging between +2.01 to +2.10.
Like many stars, Mirach is not alone and has a 14th-magnitude main-sequence companion that is 60,000 times dimmer than Mirach itself.
Their orbits bring them physically to within 40 times the Sun-Pluto distance of one another.
From Mirach the companion would appear as bright as Venus appears from Earth.
Mirach would appear 120 times brighter than the full Moon from the companion.
Through the eyepiece Mirach has an additional deep sky companion in the form of the isolated dwarf lenticular galaxy, NGC 404.
The apparent separation between star and galaxy is just 7 arcminutes giving NGC 404 the appearance of a lens flare caused by Mirach.
Known informally as Mirach’s Ghost, one of the biggest hurdles to seeing it is the brightness of Mirach itself.
This guide originally appeared in the September 2017 issue of BBC Sky at Night Magazine.
