Beta Capricorni
Star system in the constellation Capricornus
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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.[9] Because it is positioned near the ecliptic, Beta Capricorni can be occulted by the Moon,[11] and also (rarely) by planets.
 | |
| Observation data Epoch J2000.0 Equinox J2000.0 | |
|---|---|
| Constellation | Capricornus[1] |
| β1 Cap | |
| Right ascension | 20h 21m 00.6666s[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 | |
| β1 Cap | |
| Spectral type | K0II+B8V[5] |
| U−B color index | +0.27[6] |
| B−V color index | +0.79[6] |
| β2 Cap | |
| Evolutionary stage | main sequence[7][3] |
| Spectral type | A0III[5] |
| U−B color index | −0.11[8] |
| B−V color index | −0.02[8] |
| Astrometry | |
| β1 Cap | |
| Radial velocity (Rv) | −19.0±0.6[9] 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] |
| β2 Cap | |
| Proper motion (μ) | RA: +44.411 mas/yr[3] Dec.: −0.637 mas/yr[3] |
| Parallax (π) | 9.8983±0.3071 mas[3] |
| Distance | 330 ± 10 ly (101 ± 3 pc) |
| Absolute magnitude (MV) | +1.03[4] |
| Orbit[5] | |
| Primary | Aa |
| Name | Ab |
| Period (P) | 3.762 yr |
| Semi-major axis (a) | 0.049″ |
| Eccentricity (e) | 0.452 |
| Inclination (i) | 75.1° |
| Semi-amplitude (K1) (primary) | 22.4 km/s |
| Semi-amplitude (K2) (secondary) | 19.6 km/s |
| Orbit[5] | |
| Primary | Ab1 |
| Name | Ab2 |
| Period (P) | 8.677 days |
| Eccentricity (e) | 0.340 |
| Semi-amplitude (K1) (primary) | 35.1 km/s |
| Orbit[5] | |
| Primary | Ba |
| Name | Bb |
| Period (P) | 400 yr |
| Semi-major axis (a) | 0.689″ |
| Eccentricity (e) | 0.458 |
| Inclination (i) | 125.2° |
| Details | |
| β1 Cap Aa | |
| Mass | 3.69±0.20[10] M☉ |
| Radius | 31.4[10] R☉ |
| Luminosity | 501[10] L☉ |
| Temperature | 4,870[10] K |
| Age | 230[10] Myr |
| β Cap Ab1 | |
| Mass | 4.22[5] M☉ |
| Luminosity | 112[10] L☉ |
| Age | 230[10] Myr |
| β Cap Ab2 | |
| Mass | 0.94[5] M☉ |
| Age | 230[10] Myr |
| β Cap Ba | |
| Mass | 2.53[5] M☉ |
| Radius | 1.98[3] R☉ |
| Luminosity | 55[3] L☉ |
| Surface gravity (log g) | 4.18[3] cgs |
| Temperature | 11,188[3] K |
| Age | 230[10] Myr |
| β Cap Bb | |
| Mass | 1.23[5] M☉ |
| Age | 230[10] Myr |
| Other designations | |
| Dabih, β Cap, 9 Capricorni, FK5 762, WDS J20210-1447AB | |
| β1 Cap: Dabih Major, BD−15°5629, HD 193495, HIP 100345, HR 7776, WDS J20210-1447A | |
| β2 Cap: Dabih Minor, BD−15°5626, HD 193452, HIP 100325, HR 7775, WDS J20210-1447B | |
| Database references | |
| SIMBAD | β1 Cap |
| β2 Cap | |
The system is believed to consist of five stars.[5] 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)[12][13] 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,[14] it is unclear whether they are simply optical doubles or part of the Beta Capricorni system.[15]
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).[16]
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.[17] In 2016, the IAU organized a Working Group on Star Names (WGSN)[18] to catalogue and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems.[19] 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.[13]
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.[20] Consequently, the Chinese name for Beta Capricorni itself is 牛宿一 (Niú Su yī, English: the First Star of Ox).[21]
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. If gravitationally bound, they would take around a million years to complete one orbit.[5]
| 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, and the close binary system Beta Capricorni Ab. They are separated by 0.04 arcseconds (5 AU[22]) and have an orbital period of 3.77 years.[5] Beta1 Capricorni is sufficiently close to the ecliptic to be occulted by the Moon.[22]
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.[22] The Ab1 and Ab2 components are separated by about 0.1 au[22] 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 have a published spectral class, but is estimated to have 0.94 times the mass of the Sun.[5]
Beta2 Capricorni
Beta2 Capricorni is a simpler single-lined spectroscopic binary. The visible component, Beta Capricorni Ba, is an A-type star with 40 times the luminosity of the Sun. The companion, Beta Capricorni Bb, is approximately 3 arcseconds from Ba and has a mass slightly higher than the Sun. Ba is classified as a mercury-manganese star, a chemically peculiar star with unusually strong mercury and manganese lines in its spectrum.[23]
Despite the luminosity class of III, the visible component of Beta2 Capricorni is apparently a main sequence star.[3][7]