Regulus

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For other uses, see Regulus (disambiguation).

**Regulus A/B/C**
Observation data Epoch J2000
Constellation	Leo
Right ascension	A: 10h 08 min 22.3s BC: 10h 08 min 12.8/14s
Declination	A: +11° 58' 02" BC: +11° 59' 48"
Apparent magnitude (V)	1.35/8.14/13.5
Spectral Characteristics
Spectral type	B7 V/K1-2 V/M5 V
U-B color index	−0.36/0.54
B-V color index	−0.11/0.87
Variable type	Slightly
Astrometry
Radial velocity (R_v)	+5.9 km/s
Proper motion (μ)	RA: 249 mas/yr Dec.: 2 mas/yr
Parallax (π)	42.09 ± 0.79 mas
Distance	77 ± 1 ly (23.8 ± 0.4 pc)
Absolute magnitude (M_V)	−0.52/4.2/9.5
Physical Characteristics
Mass	3.5/0.8/0.2 M_☉
Radius	3.15–4.15/0.5/? R_☉
Luminosity	350/0.31 L_☉
Temperature	10,300–15,400/? K
Metallicity
Rotation	315 km/s. (15.9 hours)/?
Age	5 × 10⁷ years
Other designations
Alpha Leonis, 32 Leo, Cor Leonis, Basilicus, Lion's Heart, Rex, Kalb al Asad, Kabeleced, GJ 9316, HR 3982, BD +12° 2149/2147, HD 87901/87884, GCTP 2384.00, LTT 12716/12714, SAO 98967/98966, FK5 380, HIP 49669.
Database references
SIMBAD	data

Regulus (α Leo / α Leonis / Alpha Leonis) is the brightest star in the constellation Leo and one of the brightest stars in the nighttime sky.

Regulus is approximately 77.5 light years from Earth's Solar System. Its name is Latin for "prince." It is also known as Qalb Al Asad, from Arabic قلب الأسد qalb[u] al-´asad, meaning "the heart of the lion." This phrase is sometimes approximated as Kabelaced, or translated into Latin as Cor Leonis. In Hindu astronomy, Regulus corresponds to the Nakshatra Magha. Regulus is considered the last first magnitude star in the sky because the next brightest star, Adhara, has a magnitude of 1.50, officially making it a second magnitude star. Of the brightest stars in the sky, Regulus is closest to the ecliptic, and the sun makes its closest approach around August 23 of each year. This fact also means that Regulus is regularly occulted by the Moon. It is known as 轩辕十四 (the Fourteenth Star of Xuanyuan) in Chinese. Xuanyuan is the name of Yellow Emperor.

Regulus has about 3.5 times the Sun's mass, and is a young star only a few hundred million years old. It is spinning extremely rapidly, with a rotation period of only 15.9 hours, which causes it to have a highly oblate shape. This results in gravity darkening: its poles are considerably hotter and five times as bright (per unit surface area) than its equator. If it were rotating only 16% faster the centripetal force of gravity would not be enough to keep it from tearing itself apart.

This star has a pair of small, faint companions in the form of a binary star system. These stars are separated by about 100 AUs and orbit each other every 2,000 years. This pair is separated from the much larger Regulus A by about 4,200 AU and orbit the main star with a period of over 130,000 years.