Capella
Capella is the brightest star in the northern constellation of Auriga. It has the Bayer designation α Aurigae, which is Latinised to Alpha Aurigae and abbreviated Alpha Aur or α Aur. Capella is the sixth-brightest star in the night sky, and the third-brightest in the northern celestial hemisphere after Arcturus and Vega. A prominent object in the northern winter sky, it is circumpolar to observers north of 44°N. Its name meaning "little goat" in Latin, Capella depicted the goat Amalthea that suckled Zeus in classical mythology. Capella is relatively close, at 42.9 light-years (13.2 pc) from the Sun. It is one of the brightest X-ray sources in the sky, thought to come primarily from the corona of Capella Aa.
| Observation data Epoch J2000.0 Equinox J2000.0 (ICRS) | |
|---|---|
| Constellation | Auriga |
| Pronunciation | /kəˈpɛlə/ |
| A | |
| Right ascension | 05h 16m 41.35871s |
| Declination | +45° 59′ 52.7693″ |
| Apparent magnitude (V) | +0.08 (+0.03 – +0.16) |
| H | |
| Right ascension | 05h 17m 23.728s |
| Declination | +45° 50′ 22.97″ |
| Apparent magnitude (V) | 10.16 |
| L | |
| Right ascension | 05h 17m 23.943s |
| Declination | +45° 50′ 19.84″ |
| Apparent magnitude (V) | 13.7 |
| Characteristics | |
| A | |
| Spectral type | G3III: |
| U−B color index | +0.44 |
| B−V color index | +0.80 |
| V−R color index | −0.3 |
| R−I color index | +0.44 |
| Variable type | RS CVn (suspected) |
| Aa | |
| Evolutionary stage | Red clump |
| Spectral type | K0III |
| Ab | |
| Evolutionary stage | Subgiant |
| Spectral type | G1III |
| H | |
| Evolutionary stage | Main sequence (red dwarf) |
| Spectral type | M2.5 V |
| U−B color index | 1.24 |
| B−V color index | 1.50 |
| R−I color index | 0.91 |
| L | |
| Evolutionary stage | Main sequence (red dwarf) |
| Spectral type | M4: |
| Astrometry | |
| A | |
| Radial velocity (Rv) | +29.9387±0.0032 km/s |
| Proper motion (μ) | RA: 75.52 mas/yr Dec.: −427.11 mas/yr |
| Parallax (π) | 76.20 ± 0.46 mas |
| Distance | 42.919±0.049 ly (13.159±0.015 pc) |
| Aa | |
| Absolute magnitude (MV) | +0.296 |
| Ab | |
| Absolute magnitude (MV) | +0.167 |
| HL | |
| Radial velocity (Rv) | 31.63±0.14 km/s |
| H | |
| Proper motion (μ) | RA: 88.57 mas/yr Dec.: −428.91 mas/yr |
| Parallax (π) | 74.9521 ± 0.0188 mas |
| Distance | 43.52 ± 0.01 ly (13.342 ± 0.003 pc) |
| Absolute magnitude (MV) | 9.53 |
| L | |
| Proper motion (μ) | RA: 54.1 mas/yr Dec.: −417.5 mas/yr |
| Parallax (π) | 75.1838 ± 0.0534 mas |
| Distance | 43.38 ± 0.03 ly (13.301 ± 0.009 pc) |
| Absolute magnitude (MV) | 13.1 |
| Orbit | |
| Primary | Aa |
| Companion | Ab |
| Period (P) | 104.02128±0.00016 d |
| Semi-major axis (a) | 0.056442±0.000023" (0.74272±0.00069 AU) |
| Eccentricity (e) | 0.00089±0.00011 |
| Inclination (i) | 137.156±0.046° |
| Longitude of the node (Ω) | 40.522±0.039° |
| Periastron epoch (T) | 2,448,147.6±2.6 JD |
| Argument of periastron (ω) (primary) | 342.6±9.0 JD° |
| Semi-amplitude (K1) (primary) | 25.9611±0.0044 km/s |
| Semi-amplitude (K2) (secondary) | 26.860±0.0017 km/s |
| Orbit | |
| Primary | H |
| Companion | L |
| Period (P) | 300 yr |
| Semi-major axis (a) | 3.5" (40 AU) |
| Eccentricity (e) | 0.75 |
| Inclination (i) | 52° |
| Longitude of the node (Ω) | 288° |
| Periastron epoch (T) | 2,220 |
| Argument of periastron (ω) (secondary) | 88° |
| Details | |
| A | |
| Metallicity [Fe/H] | −0.04±0.06 dex |
| Age | 590–650 Myr |
| Aa | |
| Mass | 2.5687±0.0074 M☉ |
| Radius | 11.98±0.57 R☉ |
| Luminosity (bolometric) | 78.7±4.2 L☉ |
| Surface gravity (log g) | 2.691±0.041 cgs |
| Temperature | 4,970±50 K |
| Rotation | 104±3 days |
| Rotational velocity (v sin i) | 4.1±0.4 km/s |
| Ab | |
| Mass | 2.4828±0.0067 M☉ |
| Radius | 8.83±0.33 R☉ |
| Luminosity (bolometric) | 72.7±3.6 L☉ |
| Surface gravity (log g) | 2.941±0.032 cgs |
| Temperature | 5,730±60 K |
| Rotation | 8.5±0.2 days |
| Rotational velocity (v sin i) | 35.0±0.5 km/s |
| H | |
| Mass | 0.57 M☉ |
| Radius | 0.54±0.03 R☉ |
| Luminosity (bolometric) | 0.05 L☉ |
| Surface gravity (log g) | 4.75±0.05 cgs |
| Temperature | 3,700±150 K |
| Metallicity [Fe/H] | +0.1 dex |
| L | |
| Mass | 0.53 M☉ |
| Other designations | |
| A: GJ 194 | |
| HL: GJ 195 | |
| H: PPM 47938, G 96-29, LTT 11622, NLTT 14788, 2MASS J05172386+4550229 | |
| L: 2MASS J05172394+4550198 | |
| Database references | |
| SIMBAD | Capella |
| Capella H | |
| Capella L | |
Although it appears to be a single star to the naked eye, Capella is actually a quadruple star system organized in two binary pairs, made up of the stars Capella Aa, Capella Ab, Capella H and Capella L. The primary pair, Capella Aa and Capella Ab, are two bright-yellow giant stars, both of which are around 2.5 times as massive as the Sun. The secondary pair, Capella H and Capella L, are around 10,000 astronomical units (AU) from the first and are two faint, small and relatively cool red dwarfs.
Capella Aa and Capella Ab have exhausted their core hydrogen, and cooled and expanded, moving off the main sequence. They are in a very tight circular orbit about 0.74 AU apart, and orbit each other every 104 days. Capella Aa is the cooler and more luminous of the two with spectral class K0III; it is 78.7 ± 4.2 times the Sun's luminosity and 11.98 ± 0.57 times its radius. An aging red clump star, it is fusing helium to carbon and oxygen in its core. Capella Ab is slightly smaller and hotter and of spectral class G1III; it is 72.7 ± 3.6 times as luminous as the Sun and 8.83 ± 0.33 times its radius. It is in the Hertzsprung gap, corresponding to a brief subgiant evolutionary phase as it expands and cools to become a red giant. Several other stars in the same visual field have been catalogued as companions but are physically unrelated.