Sigma Aurigae

σ Aurigae

Location of σ Aurigae (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Auriga[1]
Right ascension	05h 24m 39.141s[2]
Declination	+37° 23′ 07.26″[2]
Apparent magnitude (V)	4.99[3]
Characteristics
Spectral type	K3III CN+2[4]
U−B color index	+1.75[3]
B−V color index	+1.42[3]
Astrometry
Radial velocity (Rv)	−19.7[5] km/s
Proper motion (μ)	RA: −0.564 mas/yr[2] Dec.: −10.953 mas/yr[2]
Parallax (π)	6.3543±0.0892 mas[2]
Distance	513 ± 7 ly (157 ± 2 pc)
Absolute magnitude (MV)	−0.75[1]
Details
Radius	43.72+1.08 −3.24[6] R☉
Luminosity	496±20[6] L☉
Surface gravity (log g)	1.75[7] cgs
Temperature	4,120+162 −50[6] K
Metallicity [Fe/H]	−0.03[7] dex
Rotational velocity (v sin i)	10[8] km/s
Other designations
σ Aur, 21 Aurigae, BD+37°1175, GC 6636, HD 35186, HIP 25292, HR 1773, SAO 57981, PPM 70308, WDS J05247+3723A[9]
Database references
SIMBAD	data

Sigma Aurigae is a star in the northern constellation of Auriga. Its name is a Bayer designation that is Latinized from σ Aurigae, and abbreviated Sigma Aur or σ Aur. This star is faintly visible to the naked eye with an apparent visual magnitude of 4.99.^[3] With an annual parallax shift of 6.35 mas,^[2] it is approximately 530 light-years (160 parsecs) distant from the Earth. It is drifting closer to the Sun with a radial velocity of −20 km/s.^[5]

This is an evolved giant star with a stellar classification of K3III CN+2,^[4] indicating that it has exhausted the supply of hydrogen at its core. The 'CN+2' notation indicates anomalously strong lines of the CN molecule in the spectrum. This star has expanded to 44 times the radius of the Sun and is radiating nearly 500^[6] times the Sun's luminosity from its enlarged photosphere at an effective temperature of 4,120 K.^[6] There is a 12th magnitude companion at an angular separation of 8 arcseconds, as well as two fainter companions at 28 and 35" respectively.^[10] All are background objects, stars much further away than Sigma itself.^{[citation needed]}

Sigma Aurigae, along with λ Aur and μ Aur, were Kazwini's Al Ḣibāʽ (ألحباع), the Tent.^[11] According to the catalogue of stars in the Technical Memorandum 33-507 – A Reduced Star Catalog Containing 537 Named Stars, Al Ḣibāʽ were the title for three stars: λ Aur as Al Ḣibāʽ I, μ Aur as Al Ḣibāʽ II and σ Aur as Al Ḣibāʽ III.^[12]

• HR 1773
• CCDM J05247+3723
• Image Sigma Aurigae