Capella (star)

Capella Aa/Ab

Capella is the brightest star in Auriga
Observation data Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation	Auriga
Right ascension	05h 16m 41.3591s
Declination	+45° 59′ 52.768″
Apparent magnitude (V)	0.08(0.03 - 0.16)
Characteristics
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
Aa
Spectral type	K0III
Ab
Spectral type	G1III
Astrometry
Aa
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 ± 0.46mas
Distance	13.159 ± 0.015pc
Absolute magnitude (MV)	0.296
Ab
Absolute magnitude (MV)	0.167
Orbit
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)	2448147.6 ± 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
Details
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
Metallicity	−0.04 ± 0.06
Rotation	104 ± 3
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
Rotational velocity (v sin i)	35.0 ± 0.5 km/s
Age	590 - 650 Myr
Other designations
Alhajoth, Capella, Hokulei, α Aurigae, α Aur, Alpha Aurigae, Alpha Aur, 13 Aurigae, 13 Aur, ADS 3841 AP, BD+45°1077, CCDM J05168+4559AP, FK5 193, GC 6427, GJ 194, HD 34029, HIP 24608, HR 1708, IDS 05093+4554 AP, LTT 11619, NLTT 14766, PPM 47925, SAO 40186, WDS 05167+4600Aa/Ab.
Database references
SIMBAD	data

Capella, also designated Alpha Aurigae (α Aurigae, abbreviated Alpha Aur, α Aur), is the brightest star in the constellation of Auriga; the sixth-brightest in the night sky and the third-brightest in the northern celestial hemisphere, after Arcturus and Vega.

Although it appears to be a single star to the naked eye, it is actually a star system of four stars in two binary pairs. The first pair consists of two bright, type-G giant stars, designated Capella Aa and Capella Ab, in a very tight circular orbit some 0.76 AU apart and a derived orbital period of 104 days. Capella Aa is the brighter of the two at spectral class G8III (G8 Giant) whereas Ab is slightly smaller and of spectral class G0III (G0 Giant). Aa has a calculated mass of 3.05 times that of the Sun and Ab some 2.57 times that of the Sun. These two stars have both exhausted their core hydrogen fuel and become giant stars, though it is unclear exactly what stage they are on the stellar evolutionary pathway.

The second pair, around 10,000 astronomical units from the first, consists of two faint, small and relatively cool red dwarfs. They are designated Capella H and Capella L. The stars labelled Capella C through to G and I through to K are actually unrelated stars in the same visual field. The Capella system is relatively close, at only 42.8 light-years (13.1 pc) from the Sun.

α Aurigae (Latinised to Alpha Aurigae) is the star's Bayer designation. The traditional name Capella (Latin for small female goat) is a diminutive of "Capra" (Latin for female goat). 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 Capella for this star. It is now so entered in the IAU Catalog of Star Names.