Almach - Gamma Andromedae - γ And - Double Star

Almach (γ And) is one of the finest double stars in the sky. To the naked eye, it appears as a single bright star of magnitude +2.10, but small scopes reveal an outstanding double, made up of a bright yellow or slightly orange primary and a fainter deep blue secondary. It's widely regarded as the second best colour contrast double in the sky, surpassed only by Albireo in Cygnus.

Almach is positioned in the eastern part of Andromeda. It's the third brightest star in the constellation and is comparable in brightness to Polaris and the main stars of Ursa Major. To locate Almach, a useful starting point is the Great Square of Pegasus and in particular its north-eastern star, Alpheratz (α And). With a mag. of +2.07, Alpheratz is marginally brighter than Almach. From Alpheratz, imagine a line moving north-easterly towards Perseus. This line passes through delta Andromedae (δ And - mag. +3.3) then Mirach (β And - mag. +2.1) followed by Almach.

Any small scope will split Almach. For example, an 80mm (3.1 inch) refractor at 75x magnification will easily transform it into two colourful stars, separated by 9.4 arc seconds. The colour contrast is stark, the brighter member is a type K3 orange yellow giant, while its companion is a type B9 hot blue main-sequence star. Depending on your eyes, and seeing conditions, you may perceive slightly different colours than described. This star was first identified as a double by German physicist and astronomer Johann Tobias Mayer in 1778.

The blue secondary is actually a triple star and with large amateur scopes on good nights, can be split into components of magnitudes +5.1 and +6.3. However, the separation is a measly 0.3 arc seconds, therefore difficult and a reflector of at least 250mm (10-inch) aperture is recommended. German astronomer Wilhelm Struve first resolved this pair in October 1842. In addition, the brighter member is also a spectroscopic double, making this a four star system in total.

Almach is a wonderful double star for all types of telescopes. The system is located 358 light-years from Earth and is best seen from northern latitudes during the months of October, November and December.

Finder Chart for Almach - pdf format (credit:- freestarcharts)

Almach Data Table