Beta Capricorni (β Capricorni, abbreviated Beta Cap, β Cap) is a multiple star system in the constellation of Capricornus and located 328 light years from the Sun. Because it is near the ecliptic, Beta Capricorni can be occulted by the Moon, and also (rarely) by planets.
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With binoculars or a small telescope, Beta Capricorni can be resolved into a binary system. The brighter of the two components is designated Beta¹ Capricorni (also named Dabih); the dimmer, Beta² Capricorni. Both of these components are themselves made up of multiple stars. Beta¹ Capricorni has at least three components. It is dominated by a pair of stars, designated Beta Capricorni Aa and Beta Capricorni Ab. It is thought that the Aa component is itself multiple and the Ab component has another, unseen companion, Beta Capricorni Ac. Beta² Capricorni is a binary star, with components designated Beta Capricorni B and Beta Capricorni C.
Two other nearby stars were discovered by John Herschel. Sometimes referred to as Beta Capricorni D and E, it is unclear whether they are simply optical doubles or part of the Beta Capricorni system.
Nomenclature
β Capricorni (Latinised to Beta Capricorni) is the system's Bayer designation; β¹ and β² Capricorni those of its two components. The designations of the sub-components - Beta Capricorni Aa, Ab, Ac, B, C, D and E - derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the International Astronomical Union (IAU).
Beta Capricorni bore the traditional name Dabih, deriving from the Arabic الذابح al-dhābiḥ "the butcher", with Beta¹ and Beta² subsequently named Dabih Major and Dabih Minor, respectively. In 2016, the IAU organized a Working Group on Star Names (WGSN) to catalogue and standardize proper names for stars. The WGSN approved the name Dabih for Beta¹ 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 Beta Capricorni, Alpha² Capricorni, Xi² Capricorni, Pi Capricorni, Omicron Capricorni and Rho Capricorni. Consequently, Beta Capricorni itself is known as 牛宿一 (Niú Su yī, English: the First Star of Ox.)
Properties
Beta¹ Capricorni is the brighter of the two components with an apparent magnitude of +3.05, while the dimmer Beta² Capricorni has an apparent magnitude of +6.09. The two components are separated by 3.5 arcminutes on the sky, putting them at least 21,000 AU (0.34 light years) apart. They take approximately 700,000 years to complete one orbit.
Beta¹ Capricorni
Beta¹ Capricorni is the more complex of the pair and has a spectrum that is difficult to interpret. Its dominant pair of stars are the orange K-type bright giant Beta Capricorni Aa, with an apparent magnitude of +3.08, and the blue-white B-Type main sequence dwarf Beta Capricorni Ab with an apparent magnitude of +7.20. They are separated by 0.05 arcseconds (5 AU) and have an orbital period of 3.77 years.
The Aa component has a surface temperature of 4900 kelvins, a diameter of 35 times that of the Sun, and a luminosity 600 times that of the Sun. The Ab component's unseen companion, Beta Capricorni Ac, orbits Ab with an orbital period of 8.7 days.
Beta² Capricorni
Beta² Capricorni is simpler and more studied. Its brighter component, Beta Capricorni B, has a magnitude of 6.1 and is an A0-giant with 40 times the luminosity of the Sun. The companion, Beta Capricorni C, is approximately 3 arcseconds from B. B is unusual for having large amounts of mercury and manganese in its atmosphere.
Beta Capricorni D and E
These lie 112 arcseconds away from Beta¹ Capricorni.