Gamma Andromedae

In 1778, German physicist Johann Tobias Mayer discovered that γ Andromedae is a double star. When examined in a small telescope, it appears to be a bright, golden-yellow star next to a dimmer, indigo-blue star, separated by approximately 10 arcseconds. The pair is often considered by stargazers to be a beautiful double star with a striking contrast of color.^[11]

The brighter member, γ¹ Andromedae, is the primary of the system, and is thus designated component γ Andromedae A. It has the official proper name Almach /ˈælmæk/, which was used as the traditional name of the naked eye star, and thus the system as a whole.^[12][13] The fainter secondary is γ² Andromedae or γ Andromedae B. It was later discovered that γ² Andromedae is itself a triple star system. What appears as a single star to the naked eye is thus a quadruple star system.^[11][14]

γ Andromedae (Latinised to Gamma Andromedae) is the system's Bayer designation; γ¹ and γ² Andromedae those of its two constituents. The designations of those constituents as Gamma Andromedae A and B derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the International Astronomical Union (IAU).^[15] In 2016, the IAU organized a Working Group on Star Names (WGSN)^[16] to catalog and standardize proper names for stars. The WGSN approved the name Almach for the component Gamma Andromedae A on 20 July 2016 and it is now so included in the List of IAU-approved Star Names.^[13]

Almach was the traditional name (also spelt as Almaach, Almaack, Almak, Almaak, or Alamak), derived from the Arabic العناق (al-‘anāq),^[17] "the caracal" (desert lynx).^[18] Another term for this star used by medieval astronomers writing in Arabic was رجل المسلسلة (Rijl al Musalsalah), "Foot of The [Chained] Woman".^[17] In the catalogue of stars in the Calendarium of Al Achsasi al Mouakket, this star was designated الخامس النعامة (Al Khamis al Na'amah), which was translated into Latin as Quinta Struthionum, meaning the fifth ostrich.^[19][20]

In Chinese, 天大將軍 (Tiān Dà Jiāng Jūn), meaning Heaven's Great General, refers to an asterism consisting of γ Andromedae, φ Persei, 51 Andromedae, 49 Andromedae, χ Andromedae, υ Andromedae, τ Andromedae, 56 Andromedae, β Trianguli, γ Trianguli and δ Trianguli. Consequently, the Chinese name for γ Andromedae itself is 天大將軍一 (Tiān Dà Jiāng Jūn yī, English: the First Star of Heaven's Great General).^[21]

In the Babylonian star catalogues, γ Andromedae, together with Triangulum, formed the constellation known as ^MULAPIN (𒀯𒀳) "The Plough".^[22] Astrologically, this star was considered "honourable and eminent".^[23]

γ¹ Andromedae A is a massive bright giant star with a spectral classification of K2+IIb.^[4] The star has an apparent visual magnitude of approximately 2.26.^[2] It has been classified as a candidate post-asymptotic giant branch (AGB) star.^[3] However, it does not display a chemical enhancement of s-process elements typical of an AGB star.^[30]

The γ² Andromedae BC sub-system, with an overall apparent visual magnitude of 4.84,^[25] is 9.6 arcseconds away from γ¹ Andromedae at a position angle of 63 degrees.^[10]

γ¹ and γ² have an orbital period of approximately 5,000 years.^[6]

In October 1842, Wilhelm Struve found that γ² Andromedae was itself a double star whose components were separated by less than an arcsecond.^[17] The components are an object of apparent visual magnitude 5.5, γ Andromedae B, and an A-type main-sequence star with apparent visual magnitude 6.3, γ Andromedae C.^[25] They have an orbital period of about 64 years and a high eccentricity of 0.927.^[28] Spectrograms taken from 1957 to 1959 revealed that γ Andromedae B was itself a spectroscopic binary, composed of two B-type main-sequence stars orbiting each other with a period of 2.67 days.^[31] Photometric observations in 2026 revealed that B's components eclipse each other, making it an eclipsing binary.^[32] The two orbits may be coplanar.^[28] As of 2019, the angular distance between the B and C stars was 0.16 arcsecond.^[33]

USS Almaack (AKA-10) was the name of a United States navy ship.

• NGC 752