Gamma Andromedae, the Celebrated Double

Almach is Gamma Andromedae, apparent magnitude about 2.1, at roughly 350 light years, and it is one of the finest coloured double stars in the sky for a small telescope: a golden-orange giant primary beside a noticeably blue-green companion, the contrast striking enough that observers have written about it for two centuries. The companion is itself multiple, so the system resolves into at least four stars on closer inspection. It marks the foot of Andromeda, at the end of the chain of stars running out from the Great Square.

The blue-green companion's colour is a well-attested case of a contrast effect: the human eye judges a star's hue partly against its neighbour, and observers have long debated how much of the companion's apparent blue-green tint is intrinsic to the star and how much is a perceptual consequence of sitting beside Almach's much brighter gold-orange primary, a question later spectroscopy has only partly settled. Almach's system was first resolved into a double by William Herschel in 1778, and the fainter component was itself found to be a spectroscopic binary only with instruments unavailable in Herschel's own lifetime, so the four-star structure now known was established across more than a century of steadily improving observation.

Sources AstronomyAndrew Fraknoi, David Morrison, and Sidney C. Wolff (OpenStax) with Almagest (Mathematike Syntaxis)Claudius Ptolemy