Beta Capricorni

β¹/β² Capricorni

Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Capricornus
Right ascension	20h21m00.7s
Declination	−14°46′53″
Apparent magnitude (V)	+3.05/+6.09
Characteristics
Spectral type	K0II+B8V/A0III
U−B color index	0.28/−0.11
B−V color index	0.79/−0.02
Variable type	none
Astrometry
Radial velocity (Rv)	−19/−18 km/s
Proper motion (μ)	RA: 48.42/42.64 mas/yr Dec.: 14.00/0.37 mas/yr
Parallax (π)	9.94 ± 1.00 mas
Distance	328 ly (101 pc)
Absolute magnitude (MV)	−1.96/+1.08
Other designations
Dabih, Dabikh, Dikhabda, 9 Capricorni, HR 7776/7775, HD 193495/193452, BD−15°5629/−15°5626, FK5 762, HIP 100345/100325.

Beta Capricorni (β Cap, β Capricorni) is a star system in the constellation Capricornus. It has the traditional name Dabih, which comes from the Arabic الذابح al-dhābiḥ, meaning "the butcher". The β Capricorni system is located 328 light years from Earth. Because it is near the ecliptic, β Capricorni can be occulted by the Moon, and also (rarely) by planets.

In Chinese, 牛宿 (Niú Su), meaning Ox (asterism), refers to an asterism consisting of β Capricorni, α² Capricorni, ξ² Capricorni, π Capricorni, ο Capricorni and ρ Capricorni.^[1] Consequently, β Capricorni itself is known as 牛宿一 (Niú Su yī, English: the First Star of Ox.)^[2]

Visual double

With binoculars or a small telescope, β Capricorni can be resolved into a double star. The brighter of these two components, β¹ Capricorni or Dabih Major, has an apparent magnitude of +3.05, while the dimmer one, β² Capricorni or Dabih Minor, has an apparent magnitude of +6.09. The two components are separated by 3.5 arcminutes on the sky, putting them at leat 21,000 AU (0.34 light years) apart. They take approximately 700,000 years to complete one orbit. Both of these components are themselves made up of multiple stars.

Due to the complexity of this system, several different schemes have arisen to denote the subcomponents. This article follows the naming used in the Multiple Star Catalogue.^[3]

β¹ Capricorni

The brighter component, β¹ Capricorni, is the more complex of the pair. It has at least three components, and its spectrum is difficult to interpret. It is dominated by a pair of stars, the orange K-type bright giant β Capricorni Aa, with an apparent magnitude of +3.08, and the blue-white B-Type main sequence dwarf, β Capricorni Ab with an apparent magnitude of +7.20. These two components 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 has another, unseen companion, β Capricorni Ac, which orbits Ab with an orbital period of 8.7 days. It is thought that the Aa component is itself multiple.

β² Capricorni

The dimmer of the visual double star, β¹ Capricorni, is simpler and more studied. It is a binary star, whose brighter component, β Capricorni B, has a magnitude of 6.1. It is a A0-giant with 40 times the luminosity of the Sun. The companion, β Capricorni C, is approximately 3 arcseconds from B. β Capricorni B is unusual for having large amounts of mercury and manganese in its atmosphere.

Other components

Two other nearby stars were discovered by John Herschel. These lie 112 arcseconds away from β¹ Capricorni, and it is unclear whether they are simply optical doubles or part of the β Capricorni system. They are sometimes referred to as β Capricorni D and E.

References

1. ^ (Chinese) 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN 978-986-7332-25-7.
2. ^ (Chinese) 香港太空館 - 研究資源 - 亮星中英對照表, Hong Kong Space Museum. Accessed on line November 23, 2010.
3. ^ Tokovinin, A. A. (1997). "MSC - a catalogue of physical multiple stars". A&AS 124 (1): 75–84. Bibcode 1997A&AS..124...75T. doi:10.1051/aas:1997181. 

External links

• Dabih article on Jim Kaler's Star of the Week