Beta Capricorni

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
Right ascension	20^h 21^m 0.7^s ^[citation needed]
Declination	14° 46′ 53″ ^[citation needed]
Apparent magnitude (V)	+3.05/+6.09 ^[citation needed]
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)	8.3966 ± 0.6348 mas[1]
Distance	390 ± 30 ly (119 ± 9 pc)
Absolute magnitude (MV)	−2.03/+1.03[2]
Other designations
Dabih, Dabikh, Dikhabda, 9 Capricorni, FK5 762, WDS J20210-1447AB
β^1 Cap: BD−15° 5629, HD 193495, HIP 100345, HR 7776, WDS J20210-1447A
β^2 Cap: BD−15° 5626, HD 193452, HIP 100325, HR 7775, WDS J20210-1447B
Database references
SIMBAD	β Cap
	β^1 Cap
	β^2 Cap

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,^[3] and also (rarely) by planets.

The system is believed to consist of five stars.^[4] 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)^[5][6] 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).^[7]

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.^[8] In 2016, the IAU organized a Working Group on Star Names (WGSN)^[9] to catalogue and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems.^[10] 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.^[6]

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.^[11] Consequently, the Chinese name for Beta Capricorni itself is 牛宿一 (Niú Su yī, English: the First Star of Ox).^[12]

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 blue-white B-Type main sequence dwarf Beta Capricorni Ab1 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 radius 35 times that of the Sun, and a luminosity 600 times that of the Sun. The Ab1 component's unseen companion, Beta Capricorni Ab2, orbits Ab1 with an orbital period of 8.7 days.

Beta¹ Capricorni is 4.6 degrees north of the ecliptic, so it can be occulted by the Moon.

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.

Beta Capricorni D and E

These lie 112 arcseconds away from Beta¹ Capricorni.

References

1. Brown, A. G. A.; et al. (Gaia collaboration) (2021). "Gaia Early Data Release 3: Summary of the contents and survey properties". Astronomy & Astrophysics. 649: A1. arXiv:2012.01533. Bibcode:2021A&A...649A...1G. doi:10.1051/0004-6361/202039657. S2CID 227254300. (Erratum: doi:10.1051/0004-6361/202039657e). Gaia EDR3 record for this source at VizieR.
2. Anderson, E.; Francis, Ch. (2012), "XHIP: An extended hipparcos compilation", Astronomy Letters, 38 (5): 331, arXiv:1108.4971, Bibcode:2012AstL...38..331A, doi:10.1134/S1063773712050015, S2CID 119257644.
3. White, Nathaniel M.; Feierman, Barry H. (September 1987), "A Catalog of Stellar Angular Diameters Measured by Lunar Occultation", Astronomical Journal, 94: 751, Bibcode:1987AJ.....94..751W, doi:10.1086/114513.
4. "Displaying next number in catalog HIP => 100345". Multiple Star Catalog. Archived from the original on 2018-07-26. Retrieved 2018-07-26.
5. Kunitzsch, Paul; Smart, Tim (2006). A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations (2nd rev. ed.). Cambridge, Massachusetts: Sky Pub. ISBN 978-1-931559-44-7.
6. "Naming Stars". IAU.org. Retrieved 18 June 2018.
7. Hessman, F. V.; Dhillon, V. S.; Winget, D. E.; Schreiber, M. R.; Horne, K.; Marsh, T. R.; Guenther, E.; Schwope, A.; Heber, U. (2010). "On the naming convention used for multiple star systems and extrasolar planets". arXiv:1012.0707 [astro-ph.SR].
8. Allen, Richard Hinckley (1963) [1899]. Star Names: Their Lore and Meaning (Reprint ed.). New York, NY: Dover Publications Inc. p. 140. ISBN 0-486-21079-0.
9. IAU Working Group on Star Names (WGSN), International Astronomical Union, retrieved 22 May 2016.
10. "WG Triennial Report (2015-2018) - Star Names" (PDF). p. 5. Retrieved 2018-07-14.
11. (in 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.
12. (in Chinese) 香港太空館 - 研究資源 - 亮星中英對照表 Archived 2009-09-29 at the Wayback Machine, Hong Kong Space Museum. Accessed on line November 23, 2010.