Castor (star)

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Gemini constellation map.svg
Castor within the constellation Gemini
Observation data
Epoch J2000      Equinox J2000
Constellation Gemini
Pronunciation /ˈkæstər/[1]
Right ascension 07h 34m 35.863s[2]
Declination +31° 53′ 17.79″[2]
Apparent magnitude (V) 1.93[2]
Right ascension 07h 34m 36.100s[2]
Declination +31° 53′ 18.57″[2]
Apparent magnitude (V) 2.97[2]
Right ascension 07h 34m 37.584s[2]
Declination +31° 53′ 17.8160″[2]
Apparent magnitude (V) 9.83[2]
Spectral type A1V + dM1e[3]
B−V color index +0.03[4]
Spectral type Am + dM1e[3]
B−V color index +0.04[4]
Spectral type dM1e + dM1e[3]
U−B color index +1.04[5]
B−V color index +1.49[5]
Variable type BY Dra[6]
Radial velocity (Rv)5.40[7] km/s
Proper motion (μ) RA: –191.45[8] mas/yr
Dec.: –145.19[8] mas/yr
Parallax (π)64.12 ± 3.75[8] mas
Distance51 ± 3 ly
(15.6 ± 0.9 pc)
Absolute magnitude (MV)+0.986 / +1.886[4]
Radial velocity (Rv)+2.5[9] km/s
Proper motion (μ) RA: –201.490[10] mas/yr
Dec.: –97.104[10] mas/yr
Parallax (π)66.2323 ± 0.0511[10] mas
Distance49.24 ± 0.04 ly
(15.10 ± 0.01 pc)
Absolute magnitude (MV)+8.950[11]
α Geminorum Aa
Mass2.76[12] M
Radius2.4[13] R
Surface gravity (log g)4.2[13] cgs
Temperature10,286[14] K
Metallicity [Fe/H]0.98[14] dex
Rotational velocity (v sin i)18[15] km/s
α Geminorum Ba
Mass2.98[12] M
Radius3.3[13] R
Surface gravity (log g)4.0[13] cgs
Temperature8,842[14] K
Rotational velocity (v sin i)33[15] km/s
α Geminorum Ca/Cb (components are identical)
Mass0.5992[11] M
Radius0.6191[11] R
Luminosity0.0733[11] L
Surface gravity (log g)4.6317[11] cgs
Temperature3,820[11] K
Metallicity [Fe/H]~0.0[11] dex
Rotational velocity (v sin i)37[11] km/s
Age370[11] Myr
Primaryα Geminorum A
Companionα Geminorum B
Period (P)445[12] yr
Semi-major axis (a)7.369″
Eccentricity (e)0.360
Inclination (i)112.9°
Longitude of the node (Ω)41.7°
Periastron epoch (T)2401950.650
Argument of periastron (ω)
Primaryα Geminorum Aa
Companionα Geminorum Ab
Period (P)9.2128 days
Eccentricity (e)0.5
Periastron epoch (T)2427543.938
Argument of periastron (ω)
Semi-amplitude (K1)
12.9 km/s
Primaryα Geminorum Ba
Companionα Geminorum Bb
Period (P)2.9283 days
Periastron epoch (T)2427501.703
Argument of periastron (ω)
Semi-amplitude (K1)
31.9 km/s
Primaryα Geminorum AB
Companionα Geminorum C
Period (P)14,000 yr
Semi-amplitude (K1)
121.0 km/s
Semi-amplitude (K2)
119.0 km/s
Primaryα Geminorum Ca
Companionα Geminorum Cb
Period (P)0.814 days
Eccentricity (e)0
Inclination (i)86.29 ± 0.10°
Longitude of the node (Ω)7.315°
Other designations
Castor, α Gem, 66 Geminorum, FK5 287, Gliese 278, HIP 36850, SAO 60198
A: BD+32°1581A, PLX 1785.00, HR 2891, HD 60179
B: BD+32°1581B, HR 2890, HD 60178
C: YY Geminorum, BD+32°1582
Database references
Castor A
Castor B
Castor C (YY Gem)

Castor is the second-brightest object in the zodiac constellation of Gemini and one of the brightest stars in the night sky. It has the Bayer designation α Geminorum, which is Latinised to Alpha Geminorum and abbreviated Alpha Gem or α Gem. It appears singular to the naked eye, but it is actually a sextuple star system organized into three binary pairs, made up of the stars Castor Aa, Castor Ab, Castor Ba, Castor Bb, Castor Ca, and Castor Cb. Although it is the 'α' (alpha) member of the constellation, it is fainter than 'β' (beta) Geminorum, Pollux.

Stellar system[edit]

Period = 9.2128 d
Separation = 3.9″
Period = 2.9283 d
Separation = 71″
Period = 0.814 d

Hierarchy of orbits in the Castor system[17]

Castor is a multiple star system made up of six individual stars; there are three visual components, all of which are spectroscopic binaries. Appearing to the naked eye as a single star, Castor was first recorded as a double star in 1718 by James Pound, but it may have been resolved into at least two sources of light by Cassini as early as 1678. The separation between Castor A and Castor B has increased from about 2″ (2 arcseconds of angular measurement) in 1970 to about 6″ in 2017.[18][16] These two binary pairs have magnitudes of 1.9 and 3.0.

Castor Aa and Ba both have orbits of a few days with a much fainter companion.

Castor C, or YY Geminorum, was discovered to vary in brightness with a regular period. It is an eclipsing binary with additional variations due to areas of different brightness on the surface of one or both stars, as well as irregular flares.[11] The Castor C components orbit in less than a day. Castor C is believed to be in orbit around Castor AB, but with an extremely long period of several thousand years. It is 73″ distant from the bright components.[16]

The combined apparent magnitude of all six stars is +1.58.

Physical properties[edit]

Castor is 51 light-years away from Earth, determined from its large annual parallax.

The two brightest stars are both A-class main-sequence stars, more massive and brighter than the Sun. The properties of their red dwarf companions are difficult to determine, but are both thought to have less than half the mass of the Sun.[16]

Castor B is an Am star, with particularly strong spectral lines of certain metals.

Castor C is a variable star, classified as a BY Draconis type. BY Draconis variables are cool dwarf stars which vary as they rotate due to starspots or other variations in their photospheres. The two red dwarfs of Castor C are almost identical, with masses around a half Mand luminosities less than 10% of the Sun.[11]

All the red dwarfs in the Castor system have emissions lines in their spectra, and all are flare stars.[13]

Etymology and culture[edit]

α Geminorum (Latinised to Alpha Geminorum) is the star system's Bayer designation.

Castor and Pollux are the two "heavenly twin" stars that give the constellation Gemini (meaning twins in Latin) its name. The name Castor refers specifically to Castor, one of the twin sons of Zeus and Leda in Greek and Roman mythology.

The star was annotated by the Arabic description Al-Ras al-Taum al-Muqadim, which translates as the head of the foremost twin. In the catalogue of stars in the Calendarium of Al Achsasi Al Mouakket, this star was designated Aoul al Dzira, which was translated into Latin as Prima Brachii, meaning the first in the paw.[19]

In Chinese, 北河 (Běi Hé), meaning North River, refers to an asterism consisting of Castor, ρ Geminorum, and Pollux.[20] Consequently, Castor itself is known as 北河二 (Běi Hé èr, English: the Second Star of North River.)[21]

In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[22] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN; which included Castor for the star α Geminorum Aa.[23]

Castor C also has the variable star designation YY Geminorum.

Long-exposure observation[edit]

Stars that can set (not in a circumpolar constellation for the viewer) culminate at midnight – noticeable where viewed away from any polar region experiencing midnight sun – when at opposition, meaning they can be viewed from dusk until dawn. This applies to α Geminorum on 14 January, in the current astronomical epoch.[24]

Half of the year from this date, 14 July, the star will be at conjunction above or below, the sun – apart by the star's declination (angle set out in table, right). The nearby days and months have most of the star's risen time being during daylight.


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External links[edit]

Coordinates: Sky map 07h 34m 36s, +31° 53′ 18″