Castor (star)
Castor is the second-brightest object in the zodiac constellation of Gemini. It has the Bayer designation α Geminorum, which is Latinised to Alpha Geminorum and abbreviated Alpha Gem or α Gem. With an apparent visual magnitude of 1.58, it is one of the brightest stars in the night sky. Castor appears singular to the naked eye, but it is actually a sextuple star system organized into three binary pairs. Although it is the 'α' (alpha) member of the constellation, it is fainter than 'β' (beta) Geminorum, Pollux.
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Gemini |
| Pronunciation | /ˈkæstər/ |
| 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 | |
| AB | |
| Radial velocity (Rv) | 5.40 km/s |
| 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) |
| Absolute magnitude (MV) | +0.986 / +1.886 |
| C | |
| Radial velocity (Rv) | +2.5 km/s |
| Proper motion (μ) | RA: –201.490 mas/yr Dec.: –97.104 mas/yr |
| Parallax (π) | 66.2323 ± 0.0511 mas |
| Distance | 49.24 ± 0.04 ly (15.10 ± 0.01 pc) |
| Absolute magnitude (MV) | +8.950 |
| Details | |
| α Geminorum Aa | |
| Mass | 2.37 M☉ |
| Radius | 2.089 R☉ |
| Surface gravity (log g) | 4.2 cgs |
| Temperature | 10,286 K |
| Rotational velocity (v sin i) | 18 km/s |
| Age | 290 Myr |
| α Geminorum Ba | |
| Mass | 1.79 M☉ |
| Radius | 1.648 R☉ |
| Surface gravity (log g) | 4.0 cgs |
| Temperature | 8,842 K |
| Rotational velocity (v sin i) | 33 km/s |
| α Geminorum Ca/Cb (components are identical) | |
| Mass | 0.5992 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 Geminorum A |
| Companion | α Geminorum B |
| Period (P) | 459 yr |
| Semi-major axis (a) | 6.722″ |
| Eccentricity (e) | 0.338 |
| Inclination (i) | 115.1° |
| Longitude of the node (Ω) | 41.3° |
| Periastron epoch (T) | 2436785 |
| Argument of periastron (ω) (secondary) | 251.84° |
| Orbit | |
| Primary | α Geminorum Aa |
| Companion | α Geminorum 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 Geminorum Ba |
| Companion | α Geminorum 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 Geminorum AB |
| Companion | α Geminorum C |
| Period (P) | 14,000 yr |
| Orbit | |
| Primary | α Geminorum Ca |
| Companion | α Geminorum Cb |
| Period (P) | 0.814 days |
| Eccentricity (e) | 0 |
| Inclination (i) | 86.29 ± 0.10° |
| Longitude of the node (Ω) | 7.315° |
| Semi-amplitude (K1) (primary) | 121.0 km/s |
| Semi-amplitude (K2) (secondary) | 119.0 km/s |
| Other designations | |
| A: BD+32°1581A, HD 60179, HR 2891, PLX 1785.00 | |
| B: BD+32°1581B, HD 60178, HR 2890 | |
| C: YY Geminorum, BD+32°1582 | |
| Database references | |
| SIMBAD | data |
| Castor A | |
| Castor B | |
| Castor C (YY Gem) | |
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