At magnitude 1.35, Regulus (or α Leo) is the brightest star in the constellation Leo, the Lion. It is also the 21st-brightest star in the night sky. Regulus appears at the handle of a group of stars forming the Lion's head; this group is the asterism known as the "Sickle" or "Backwards Question Mark".
Regulus is located almost precisely on the ecliptic, the Sun's annual path around the celestial sphere. In fact, Regulus is closest to the ecliptic of the 100 brightest stars in the sky. Because of its position, Regulus is regularly occulted by the Moon, and sometimes by the planets Mercury and Venus. The last occultation of Regulus by a planet was on July 9, 1959 (by Venus); the next will occur on October 1, 2044 (also by Venus).
The Sun makes its closest approach to Regulus around August 23rd of each year. The heliacal rising of Regulus - that it, when it first emerges from the Sun's glare in the morning twilight sky - occurs in the first week of September. Every 8 years, Venus passes Regulus around the time of the star's heliacal rising, most recently in 2006.
Name and Mythology
The proper name Regulus was given to α Leo by Copernicus. Regulus is Latin for "prince" or "little king"; the Greek form of this name (Basilicus) is sometimes also used. The star was known both as the "Lion's Heart" and as the "Kingly Star" to the ancient Babylonians; it was known to the much earlier Akkadians of Mesopotamia as the legendary "King of the Celestial Sphere," who ruled before the Great Flood.
In Arabic, Regulus is known as "Qalb Al Asad" or "Qalbu Al-'asad", meaning "the heart of the lion". This phrase is sometimes approximated as Kabelaced, and translated into Latin as "Cor Leonis".
Regulus is known in Chinese as the Fourteenth Star of Xuanyuan, the Yellow Emperor. In Hindu astronomy, Regulus corresponds to the Nakshatra Magha. Persian astrologers around 3000 BC knew Regulus as Venant, one of the four royal stars.
Properties
Regulus is 77.5 light years from Earth's Solar System. It is a main-sequence star, like the Sun, that is fueled by the internal fusion of hydrogen into helium. Regulus is a blue-white star of spectral class B7 V, with a surface temperature of 12,000 K - twice as hot as the Sun. Visually, Regulus is 140 times more luminous than the Sun, climbing to 240 times more luminous when the star's ultraviolet radiation is taken into account. Regulus is 3.5 times larger than the Sun, a figure derived both its temperature and luminosity, and from a direct measure of angular diameter. Regulus contains about 3.5 times the Sun's mass, and is a young star with an age of only 50 million years.
Regulus spins extremely rapidly, with a rotation period of only 15.9 hours. This causes it to have a highly oblate shape, about 32% greater at its equator than between its poles. This results in so-called gravity darkening: the photosphere at Regulus' poles is considerably hotter, and five times brighter per unit surface area, than its equatorial region. If Regulus were rotating only 16% faster, centripetal force would overcome gravity, and the star would tear itself apart.
Companions
Regulus is a multiple star system, with a pair of small, faint companions. These companions (α Leo B and C) appear at 8th magnitude, 177" to the northwest of Regulus itself, and are easily seen in binoculars. The companion pair has an apparent separation of 4.163", which corresponds to about 100 AU at the system's distance. They have an orbital period of 2,000 years. The pair orbits Regulus A with a period of over 130,000 years, at a distance of some 4200 AU.
α Leo B is a orange-red main sequence star of spectral class K1-2 V. It contains around 80% of the Sun's mass, and has 31% of its luminosity. α Leo C is a main sequence red dwarf star of spectral class M5 V. It contains a 20% of the Sun's mass, and has 0.31% of its luminosity. From the little double, Regulus would look like a brilliant star six times brighter than our full Moon.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]