Alpha2 Capricorni
Star in the constellation Capricornus
title: "Alpha2 Capricorni" type: doc version: 1 created: 2026-02-28 author: "Wikipedia contributors" status: active scope: public tags: ["g-type-giants", "triple-star-systems", "capricornus", "bayer-objects", "bright-star-catalogue-objects", "durchmusterung-objects", "flamsteed-objects", "henry-draper-catalogue-objects", "hipparcos-objects", "stars-with-proper-names"] description: "Star in the constellation Capricornus" topic_path: "science/astronomy" source: "https://en.wikipedia.org/wiki/Alpha2_Capricorni" license: "CC BY-SA 4.0" wikipedia_page_id: 0 wikipedia_revision_id: 0
::summary Star in the constellation Capricornus ::
| name = Alpha2 Capricorni | image= |image=Capricornus constellation map.svg|alt=|float=center|width=280 |label=|position=right |mark=Red circle.svg|mark_width=10|mark_link=Alpha1 |x=818|y=334 | caption = Location of the α Capricorni pair in the red circle | epoch = J2000.0 | equinox = J2000.0 (ICRS) | constell = Capricornus | ra = | dec = | appmag_v = +3.57 | class = G8.5III-IV | type = | b-v = +0.94 | u-b = +0.69 | r-i = | variable = | radial_v = | prop_mo_ra = +62.63 | prop_mo_dec = +2.66 | pm_footnote = | parallax = 31.9795 | p_error = 0.3485 | parallax_footnote = | absmag_v = | source = | component1 = α2 Cap A | mass = | radius = | luminosity = | temperature = | gravity = 3.0 | metal_fe = | rotational_velocity = 2.7 | age_gyr = | names = | Simbad = alf02+Cap
Alpha2 Capricorni is a triple star system in the southern constellation of Capricornus. It has the proper name Algedi, pronounced ; Alpha2 Capricorni is its Bayer designation. This system is visible to the naked eye as a point of light with an apparent visual magnitude of +3.57. It is separated from the fainter α¹ Capricorni by 0.11° of the sky, a gap just resolvable with the naked eye, similar to Mizar and Alcor. Based on parallax measurements, the star is approximately 101 ly away from the Sun.
Properties
The primary, component A, is an evolved G-type star with a stellar classification of G8.5III-IV, indicating that the spectrum displays mixed traits of a giant and subgiant star. At the age of 1.3 billion years, is currently on the red giant branch and is generating energy through hydrogen fusion along a shell surrounding an inert helium core. The star has around double the mass of the Sun and has expanded to more than eight times the Sun's radius. It is radiating 40 times the solar luminosity from its photosphere at an effective temperature of 5,030 K.
The secondary components B and C form a binary system that orbit each other with a period of about 244 years. Both stars have masses about half that of the Sun. They orbit the primary with an estimated period of around 1,500 years. As of 2010, the pair lies at an angular separation of 6.6 arc seconds from the primary along a position angle of 196°.
Nomenclature
α² Capricorni (Latinised to Alpha² Capricorni) is the star's Bayer designation, which is abbreviated Alpha2 Cap or α2 Cap. It bore the traditional names Secunda Giedi or Algiedi Secunda and shared the name Algedi (from the Arabic الجدي al-jadii 'the goat') with α¹ Capricorni. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalogue and standardize proper names for stars. The WGSN approved the name Algedi for α² Capricorni on 21 August 2016 and it is now so entered in the IAU Catalog of Star Names.
In Chinese, 牛宿 (Niú Xiù), meaning Ox (asterism), refers to an asterism consisting of α² Capricorni, Beta Capricorni, ξ² Capricorni, Pi Capricorni, Omicron Capricorni and Rho Capricorni. Consequently, α² Capricorni itself is known as 牛宿二 (Niú Xiù èr, the Second Star of Ox.)
References
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References
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