Epoch J2000.0 Equinox J2000.0 (ICRS)
|Right ascension||20h 18m 03.25595s|
|Declination||−12° 32′ 41.4684″|
|Apparent magnitude (V)||+3.57|
|U−B color index||+0.69|
|B−V color index||+0.94|
|Radial velocity (Rv)||±0.47−0.47 km/s|
|Proper motion (μ)||RA: +62.63 mas/yr
Dec.: +2.66 mas/yr
|Parallax (π)||30.82 ± 0.19 mas|
|Distance||105.8 ± 0.7 ly
(32.4 ± 0.2 pc)
|Absolute magnitude (MV)||+0.07
|α2 Cap A|
|Surface gravity (log g)||3.0 cgs|
|Metallicity [Fe/H]||±0.10 −0.15dex|
|Rotational velocity (v sin i)||2.7 km/s|
Alpha2 Capricorni (α2 Capricorni), or Algedi, is a triple star system in the southern constellation of Capricornus. It is visible to the naked eye with an apparent visual magnitude of +3.57, forming part of the double star Alpha Capricorni along with α¹ Capricorni. Based upon an annual parallax shift of 30.82 mas as seen from the Earth, the star is located 106 light years from the Sun.
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. The star 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°.
α² Capricorni (Latinised to Alpha² Capricorni) is the star's Bayer designation. 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 Su), 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 Su èr, English: the Second Star of Ox.)
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