Castor (star)
 Castor within the constellation Gemini |
|
| Observation data Epoch J2000 Equinox J2000 |
|
|---|---|
| Constellation | Gemini |
| A | |
| Right ascension | 07h 34m 35.863s |
| Declination | +31° 53′ 17.79″ |
| Apparent magnitude (V) | 1.93 |
| B | |
| Right ascension | 07h 34m 36.100s |
| Declination | +31° 53′ 18.57″ |
| Apparent magnitude (V) | 2.97 |
| C | |
| Right ascension | 07h 34m 37.584s |
| Declination | +31° 53′ 17.8160″ |
| Apparent magnitude (V) | 9.83 |
| Characteristics | |
| A | |
| Spectral type | A1V + dM1e |
| B−V color index | +0.03 |
| B | |
| Spectral type | Am + dM1e |
| B−V color index | +0.04 |
| C | |
| Spectral type | dM1e + dM1e |
| U−B color index | +1.04 |
| B−V color index | +1.49 |
| Variable type | BY Dra |
| Astrometry | |
| Proper motion (μ) |
RA: –191.45 mas/yr Dec.: –145.19 mas/yr |
| Parallax (π) | 64.12 ± 3.75mas |
| Distance | 51 ± 3 ly (15.6 ± 0.9 pc) |
| A | |
| Radial velocity (Rv) | +6.0 km/s |
| Absolute magnitude (MV) | +0.986 |
| B | |
| Radial velocity (Rv) | –1.2 km/s |
| Absolute magnitude (MV) | +1.886 |
| C | |
| Radial velocity (Rv) | +2.5 km/s |
| Absolute magnitude (MV) | +8.950 |
| Details | |
| α Gem Aa | |
| Mass | 2.76 M☉ |
| Radius | 2.4 R☉ |
| Surface gravity (log g) | 4.2 cgs |
| Temperature | 10,286 K |
| Metallicity [Fe/H] | 0.98 dex |
| Rotational velocity (v sin i) | 18 km/s |
| α Gem Ba | |
| Mass | 2.98 M☉ |
| Radius | 3.3 R☉ |
| Surface gravity (log g) | 4.0 cgs |
| Temperature | 8,842 K |
| Rotational velocity (v sin i) | 33 km/s |
| Ca/Cb | |
| Mass | 0.5992 / 0.5971 M☉ |
| Radius | 0.6191 R☉ |
| Luminosity | 0.0733 L☉ |
| Surface gravity (log g) | 4.6317 cgs |
| Temperature | 3,820 K |
| Metallicity [Fe/H] | ~0.0 dex |
| Rotational velocity (v sin i) | 37 km/s |
| Age | 370 Myr |
| Orbit | |
| Primary | α Gem A |
| Companion | α Gem B |
| Period (P) | 445yr |
| 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) (secondary) |
239.8° |
| Orbit | |
| Primary | α Gem Aa |
| Companion | α Gem Ab |
| Period (P) | 9.2128 days |
| Eccentricity (e) | 0.5 |
| Periastron epoch (T) | 2427543.938 |
|
Argument of periastron (ω) (primary) |
266.4° |
|
Semi-amplitude (K1) (primary) |
12.9 km/s |
| Orbit | |
| Primary | α Gem Ba |
| Companion | α Gem Bb |
| Period (P) | 2.9283 days |
| Periastron epoch (T) | 2427501.703 |
|
Argument of periastron (ω) (primary) |
94.7° |
|
Semi-amplitude (K1) (primary) |
31.9 km/s |
| Orbit | |
| Primary | α Gem AB |
| Companion | α Gem C |
| Period (P) | 14,000 yr |
|
Semi-amplitude (K1) (primary) |
121.0 km/s |
|
Semi-amplitude (K2) (secondary) |
119.0 km/s |
| Orbit | |
| Primary | α Gem Ca |
| Companion | α Gem Cb |
| Period (P) | 0.814 days |
| Eccentricity (e) | 0 |
| Inclination (i) | 86.29 ± 0.10° |
| Longitude of the node (Ω) | 7.315° |
| Other designations | |
| A: BD+32°1581A, PLX 1785.00, HR 2891, HD 60179 | |
| B: BD+32°1581B, HR 2890, HD 60178 | |
| C: YY Gem, BD +32° 1582 | |
| Database references | |
| SIMBAD | Castor A |
| Castor B | |
| Castor C (YY Gem) | |
Castor, also designated Alpha Geminorum (α Geminorum, abbreviated Alpha Gem, α Gem) is the second-brightest star in the constellation of Gemini and one of the brightest stars in the night sky. Although it has the identifier 'alpha', it is fainter than Beta Geminorum (Pollux).
Castor was recorded as a double star in 1718 by James Pound. It may have been resolved in 1678 by Cassini. The separation of the two stars has increased from 2" in 1907 to 7" in 1997. The two stars have magnitudes of 1.9 and 3.0.
A third star is 73" distant from the main components. It was discovered to vary in brightness with a regular period and was thought to be an eclipsing binary, but the variations are now considered to be due to areas of different brightness on the surface of one or both stars. It was given the variable star designation YY Geminorum.
All three of the visual components are spectroscopic binaries and Castor is a complex multiple star system made up of six individual stars. Castor A and B both have orbits of a few days with a much fainter companion. The Castor C components orbit in less than a day. Castor C is believed to be in orbit around the bright pair, but with an extremely long period of several thousand years.
The combined apparent magnitude of all six stars is +1.58.
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. The two red dwarfs of Castor C are almost identical, with masses around a half M☉ and luminosities less than 10% of the Sun.
Castor B is an Am star, with particularly strong spectral lines of certain metals.
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