Capella

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
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	42.919 ± 0.049 ly (13.159 ± 0.015pc)
Aa
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
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
Other designations
Alhajoth, Hokulei, α Aurigae, α Aur, Alpha Aurigae, Alpha Aur, 13 Aurigae, 13 Aur, ADS 3841, BD+45°1077, CCDM J05168+4559, FK5 193, GC 6427, GJ 194, HD 34029, HIP 24608, HR 1708, IDS 05093+4554, LTT 11619, NLTT 14766, PPM 47925, SAO 40186, WDS 05167+4600A.
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. A prominent star in the winter sky of the northern hemisphere, 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. The Capella system is relatively close, at only 42.8 light-years (13.1 pc) from the Sun.

Although it appears to be a single star to the naked eye, Capella is actually a system of four stars in two binary pairs. The first pair consists of two bright yellow giant stars, both of which are around 2.5 times as massive as the Sun. They have exhausted their core hydrogen, and cooled and swollen, moving off the main sequence. Designated Capella Aa and Capella Ab, they are in a very tight circular orbit some 0.76 astronomical units (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 ageing red clump star, it is fusing helium to carbon and oxygen in its core. 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 a brief evolutionary phase known as the Hertzsprung gap as it expands and cools further to become a red giant. The Capella system is one of the brightest sources of X-rays in the sky, thought to come primarily from the corona of the more massive giant. The second pair, around 10,000 au from the first, consists of two faint, small and relatively cool red dwarfs. They are designated Capella H and Capella L. Several other stars in the same visual field have been catalogued as companions but are physically unrelated.