Beta Capricorni

Beta Capricorni is a multiple star system in the constellation of Capricornus. Its name is a Bayer designation that is Latinized from β Capricorni, and abbreviated Beta Cap or β Cap. Based on Parallax measurements, it is located at a distance of approximately 390 light years from the Sun. The system is drifting closer with a line of sight velocity of −19 km/s.^[5] Because it is positioned near the ecliptic, Beta Capricorni can be occulted by the Moon,^[8] and also (rarely) by planets.

β¹/β² Capricorni

Beta Capricorni circled on a chart of Capricorn; the map is to be held up as with most sky maps so that the right of page is west.
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Capricornus[1]
β1 Cap
Right ascension	20h 21m 0.6667s[2]
Declination	14° 46′ 53.067″[2]
Apparent magnitude (V)	+3.05[1]
β2 Cap
Right ascension	20h 20m 46.5479s[3]
Declination	14° 47′ 05.604″[3]
Apparent magnitude (V)	+6.09[4]
Characteristics
Spectral type	K0II+B8V/A0III
U−B color index	0.28/−0.11
B−V color index	0.79[1]/−0.016[4]
Astrometry
Radial velocity (Rv)	−19.0±0.6[5] km/s
Proper motion (μ)	RA: +44.133 mas/yr[2] Dec.: +0.360 mas/yr[2]
Parallax (π)	8.3966±0.6348 mas[2]
Distance	390 ± 30 ly (119 ± 9 pc)
Absolute magnitude (MV)	−2.03[1]/+1.03[4]
Details
β1 Cap Aa
Mass	4[6] M☉
Radius	35[6] R☉
Luminosity	600[6] L☉
Temperature	4,900[6] K
β Cap Ab1
Mass	3.28[7] M☉
β Cap Ab2
Mass	0.94[7] M☉
β Cap Ba
Mass	2.53[7] M☉
β Cap Bb
Mass	1.23[7] M☉
Other designations
Dabih, Dabikh, Dikhabda, 9 Capricorni, FK5 762, WDS J20210-1447AB
β1 Cap: BD−15°5629, HD 193495, HIP 100345, HR 7776, WDS J20210-1447A
β2 Cap: BD−15°5626, HD 193452, HIP 100325, HR 7775, WDS J20210-1447B
Database references
SIMBAD	β Cap
	β1 Cap
	β2 Cap

The system is believed to consist of five stars.^[9] With binoculars or a small telescope, Beta Capricorni can be resolved into a binary pair. The brighter of the two is designated Beta¹ Capricorni or Beta Capricorni A; the dimmer, Beta² Capricorni or Beta Capricorni B. Both are themselves made up of multiple stars. Beta¹ Capricorni has three components: a single star designated Beta Capricorni Aa (formally named Dabih /ˈdeɪbiː/, the traditional name of the system)^[10][11] and a binary pair, Beta Capricorni Ab (whose two components are designated Beta Capricorni Ab1 and Ab2). Beta² Capricorni is also a binary pair, with components designated Beta Capricorni Ba and Bb.

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.^{[citation needed]}

Nomenclature

β Capricorni (Latinised to Beta Capricorni) is the system's Bayer designation; β¹ and β² Capricorni those of its two constituents. The designations of the two constituents as Beta Capricorni A and B, and those of the sub-components - Beta Capricorni Aa, Ab, Ab1, Ab2, Ba and Bb - derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the International Astronomical Union (IAU).^[12]

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.^[13] In 2016, the IAU organized a Working Group on Star Names (WGSN)^[14] to catalogue and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems.^[15] It approved the name Dabih for the component Beta Capricorni Aa on 21 August 2016 and it is now so included in the List of IAU-approved Star Names.^[11]

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.^[16] Consequently, the Chinese name for Beta Capricorni itself is 牛宿一 (Niú Su yī, English: the First Star of Ox).^[17]

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.

Aa

Sep. = 0.05″

Ab1

Period = 8.7d

Ab2

Ba

Sep. = 3″

Bb

Hierarchy of orbits in the β Capricorni system

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 close binary system Beta Capricorni Ab1 with an apparent magnitude of +7.20.^{[citation needed]} They are separated by 0.04 arcseconds (5 AU^[6]) and have an orbital period of 3.77 years.^[7] Beta¹ Capricorni is sufficiently close to the ecliptic to be occulted by the Moon.^[6]

The Aa component has a surface temperature of 4,900 K, a radius 35 times that of the Sun, and a luminosity 600 times that of the Sun.^[6] The Ab1 and Ab2 components are separated by about 0.1 au^[6] and complete an orbit around each other every 8.68 days. The Ab1 component is a B-type main-sequence star, while Ab2 does not has a spectral class, but is estimated to have 0.94 times the mass of the Sun.^[7]

Beta² Capricorni

Beta² Capricorni is simpler and more studied. Its brighter component, Beta Capricorni Ba, has a magnitude of 6.1 and is an A0-giant with 40 times the luminosity of the Sun. The companion, Beta Capricorni Bb, is approximately 3 arcseconds from Ba. Ba is unusual for having large amounts of mercury and manganese in its atmosphere.