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. 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 is regularly occulted by the Moon. Occultations by the planets Mercury and Venus are also possible but rare. 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. Other planets will not occult Regulus over the next few millennia because of their node positions.
The Sun makes its closest approach to Regulus around August 23 of each year. For most Earth observers, the heliacal rising of Regulus 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.
Regulus has about 3.5 times the Sun’s mass and is a young star of only a few hundred million years. 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 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 it were rotating only 16% faster, centripetal force would overcome gravity and the star would tear itself apart.
Regulus is a multiple star system composed of a hot, bright, bluish-white star with a pair of small, faint companions. The pair orbits the much-larger Regulus A with a period of over 130,000 years at a distance of some 4,200 AU. The companion pair have an orbital period of 2,000 years and are separated by about 100 AU.
Etymology and Cultural Associations
Regulus is Latin for 'prince' or 'little king'. The Greek variant Basiliscus is also used. It is known as Qalb Al Asad, from the Arabic قلب لأسد or Qalb[u] Al-´asad, meaning 'the heart of the lion'. This phrase is sometimes approximated as Kabelaced and translates into Latin as Cor Leonis. It is known in Chinese as 轩辕十四, 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'. It was one of the fifteen Behenian stars known to medieval astrologers
* Regulus in fiction
1. ^ McAlister, H. A., ten Brummelaar, T. A., et al. (2005). "First Results from the CHARA Array. I. An Interferometric and Spectroscopic Study of the Fast Rotator Alpha Leonis (Regulus).". The Astrophysical Journal 628: 439-452. doi:10.1086/430730.
* Fred Schaaf, “Horrorfications of the Lion’s heart,” Sky & Telescope, April 2006.
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