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.
Epoch J2000 Equinox J2000
|Right ascension||07h 34m 35.863s|
|Declination||+31° 53′ 17.79″|
|Apparent magnitude (V)||1.93|
|Right ascension||07h 34m 36.100s|
|Declination||+31° 53′ 18.57″|
|Apparent magnitude (V)||2.97|
|Right ascension||07h 34m 37.584s|
|Declination||+31° 53′ 17.8160″|
|Apparent magnitude (V)||9.83|
|Spectral type||A1V + dM1e|
|B−V color index||+0.03|
|Spectral type||Am + dM1e|
|B−V color index||+0.04|
|Spectral type||dM1e + dM1e|
|U−B color index||+1.04|
|B−V color index||+1.49|
|Variable type||BY Dra|
|Proper motion (μ)|| RA: –191.45 mas/yr |
Dec.: –145.19 mas/yr
|Parallax (π)||64.12 ± 3.75 mas|
|Distance||51 ± 3 ly |
(15.6 ± 0.9 pc)
|Radial velocity (Rv)||+6.0 km/s|
|Absolute magnitude (MV)||+0.986|
|Radial velocity (Rv)||–1.2 km/s|
|Absolute magnitude (MV)||+1.886|
|Radial velocity (Rv)||+2.5 km/s|
|Absolute magnitude (MV)||+8.950|
|α Geminorum Aa|
|Surface gravity (log g)||4.2 cgs|
|Metallicity [Fe/H]||0.98 dex|
|Rotational velocity (v sin i)||18 km/s|
|α Geminorum Ba|
|Surface gravity (log g)||4.0 cgs|
|Rotational velocity (v sin i)||33 km/s|
|α Geminorum Ca/Cb (components are identical)|
|Surface gravity (log g)||4.6317 cgs|
|Metallicity [Fe/H]||~0.0 dex|
|Rotational velocity (v sin i)||37 km/s|
|Primary||α Geminorum A|
|Companion||α Geminorum B|
|Period (P)||445 yr|
|Semi-major axis (a)||7.369″|
|Longitude of the node (Ω)||41.7°|
|Periastron epoch (T)||2401950.650|
|Argument of periastron (ω)|
|Primary||α Geminorum Aa|
|Companion||α Geminorum Ab|
|Period (P)||9.2128 days|
|Periastron epoch (T)||2427543.938|
|Argument of periastron (ω)|
|Primary||α Geminorum Ba|
|Companion||α Geminorum Bb|
|Period (P)||2.9283 days|
|Periastron epoch (T)||2427501.703|
|Argument of periastron (ω)|
|Primary||α Geminorum AB|
|Companion||α Geminorum C|
|Period (P)||14,000 yr|
|Primary||α Geminorum Ca|
|Companion||α Geminorum Cb|
|Period (P)||0.814 days|
|Inclination (i)||86.29 ± 0.10°|
|Longitude of the node (Ω)||7.315°|
|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|
|Castor C (YY Gem)|
Castor is a multiple star system made up of six individual stars; there are three visual components, all 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. 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. 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.
The combined apparent magnitude of all six stars is +1.58.
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.
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 star spots or other variations in their photospheres. The two red dwarfs of Castor C are almost identical, with masses around a half M☉and luminosities less than 10% of the Sun.
Etymology and cultureEdit
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.
In Chinese, 北河 (Běi Hé), meaning North River, refers to an asterism consisting of Castor, ρ Geminorum, and Pollux. Consequently, Castor itself is known as 北河二 (Běi Hé èr, English: the Second Star of North River.)
In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) 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.
Castor C also has the variable star designation YY Geminorum.
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