Capella

Capella
	Location of Capella Aa, as the brightest star (circled)
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
	Alhajoth, Hokulei, α Aurigae, 13 Aurigae, BD+45 1077, FK5 193, GC 6427, HD 34029, HIP 24608, HR 1708, SAO 40186, PPM 47925, ADS 3841, CCDM J05168+4559, WDS 05167+4600, LTT 11619, NLTT 14766
	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

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.

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)^{[note 2]} 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.

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[note 1]

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[note 2]

Observation data Epoch J2000.0 Equinox J2000.0 (ICRS)
Location of Capella Aa, as the brightest star (circled)
Constellation	Auriga
Pronunciation	/kəˈpɛlə/^[1]
A
Right ascension	05^h 16^m 41.35871^s^[2]^{[note 1]}
Declination	+45° 59′ 52.7693″^[2]^{[note 1]}
Apparent magnitude (V)	+0.08^[3] (+0.03 – +0.16^[4])
H
Right ascension	05^h 17^m 23.728^s^[5]
Declination	+45° 50′ 22.97″^[5]
Apparent magnitude (V)	10.16^[6]
L
Right ascension	05^h 17^m 23.943^s^[7]
Declination	+45° 50′ 19.84″^[7]
Apparent magnitude (V)	13.7^[8]
Characteristics
A
Spectral type	G3III:^[9]
U−B color index	+0.44^[3]
B−V color index	+0.80^[3]
V−R color index	−0.3^[3]
R−I color index	+0.44^[3]
Variable type	RS CVn^[10] (suspected^[11])

Aa
Evolutionary stage	Red clump^[12]
Spectral type	K0III^[13]

Ab
Evolutionary stage	Subgiant^[12]
Spectral type	G1III^[13]

H
Evolutionary stage	Main sequence (red dwarf)^[14]
Spectral type	M2.5 V^[15]
U−B color index	1.24^[16]
B−V color index	1.50^[14]
R−I color index	0.91^[14]

L
Evolutionary stage	Main sequence (red dwarf)
Spectral type	M4:^[17]
Astrometry
A
Radial velocity (R_v)	+29.9387±0.0032^[12] km/s
Proper motion (μ)	RA: 75.52^[2] mas/yr Dec.: −427.11^[2] mas/yr
Parallax (π)	76.20 ± 0.46 mas^[2]
Distance	42.919±0.049 ly (13.159±0.015^[12] pc)


Aa
Absolute magnitude (M_V)	+0.296^[12]
Ab
Absolute magnitude (M_V)	+0.167^[12]

HL

Radial velocity (R_v)	31.63±0.14^[12] km/s

H
Proper motion (μ)	RA: 88.57^[18] mas/yr Dec.: −428.91^[18] mas/yr
Parallax (π)	74.9521 ± 0.0188 mas^[19]
Distance	43.52 ± 0.01 ly (13.342 ± 0.003 pc)
Absolute magnitude (M_V)	9.53^[20]
L
Proper motion (μ)	RA: 54.1^[21] mas/yr Dec.: −417.5^[21] mas/yr
Parallax (π)	75.1838 ± 0.0534 mas^[22]
Distance	43.38 ± 0.03 ly (13.301 ± 0.009 pc)
Absolute magnitude (M_V)	13.1^[23]
Orbit^[12]
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^[12]
Primary	H
Companion	L
Period (P)	300 yr
Semi-major axis (a)	3.5" (40 AU^[24])
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^[12]
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^[12]^[24] `M`_☉
Radius	0.54±0.03^[20] `R`_☉
Luminosity (bolometric)	0.05^[20] `L`_☉
Surface gravity (log g)	4.75±0.05 cgs
Temperature	3,700±150^[20] K
Metallicity [Fe/H]	+0.1^[20] dex
L
Mass	0.53^[12] `M`_☉
Other designations
Alhajoth, Hokulei, α Aurigae, 13 Aurigae, BD+45 1077, FK5 193, GC 6427, HD 34029, HIP 24608, HR 1708, SAO 40186, PPM 47925, ADS 3841, CCDM J05168+4559, WDS 05167+4600, LTT 11619, NLTT 14766^[25]^[3]^[26]^[27]^[16]
A: GJ 194
HL: GJ 195^[16]
H: PPM 47938, G 96-29, LTT 11622, NLTT 14788, 2MASS J05172386+4550229^[28]
L: 2MASS J05172394+4550198^[29]
Database references
SIMBAD	Capella
	Capella H
	Capella L