Regulus or Alpha Leonis (Alp Leo) is the brightest naked eye star in the constellation Leo. With an apparent magnitude of 1.36, Regulus is the 21st brightest star in the entire sky (see: 50 Brightest Stars ). Its absolute magnitude is -0.52 and its distance is 78 light years. The Equinox J2000 equatorial coordinates are RA = 10h 08m 22.3s, Dec = +11° 58' 02".

Regulus has a spectral type of B7V, a surface temperature of 10,300° Kelvin and a luminosity 150 times the Sun. It has a mass of 3.5 solar masses and a diameter 3.2 times the Sun.

The image above shows the uncropped view of Regulus (North is up) through the Takahashi E-180 Astrograph.

Regulus is a multiple star system consisting of four stars. Regulus A is a binary star consisting of a blue-white main sequence star (spectral type B7V) which is orbited by a star suspected of being a white dwarf of at least 0.3 solar masses. The two stars take approximately 40 days to complete an orbit around their common center of mass.

The primary star of Regulus A is about 3.5 solar masses 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, the star's gravity would provide insufficient centripetal force to hold it together, and it would tear itself apart.

Given the extremely distorted shape of primary, the relative orbital motion of the binary may be notably altered with respect to pure two-body Keplerian motion because of long-term perturbations affecting its orbital period. In other words, the third Kepler law, which holds exactly only for two pointlike masses, would be no longer valid because of the highly distorted shape of the primary.

At a distance of around 4,200 Astonomical Units (AU) from Regulus A is a binary star system that shares a common proper motion. Designated Regulus B (spectral type K2V) and Regulus C (spectral type M4V), the pair have an orbital period of 2,000 years and are separated by about 100 AU.

The light output of the Regulus system is dominated by Regulus A. Regulus B, if seen in isolation, would be a binocular object of magnitude +8.1, and its companion, Regulus C, the faintest of the three stars which have been directly observed, would require a substantial telecope to be seen, at magnitude +13.5. Regulus A is itself a spectroscopic binary: the secondary star has not yet been directly observed as it is much fainter than the primary. The BC pair lies at an angular distance of 177 arc-seconds from Regulus A, making them visible in amateur telescopes.

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, as are occultations by asteroids. The last planetary occultation of Regulus was by Venus on July 7, 1959. 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.

Regulus is expected to be occulted by the asteroid 163 Erigone on March 20, 2014. This event will be visible along a path about 40 miles wide from New York City to Oswego in the United States, and extending approximately northwest into Canada on a track that includes Belleville and North Bay, Ontario.

Although best seen in the evening in northern hemisphere in late winter and spring, Regulus can be found at some time of night throughout the year except for about a month on either side of August 22, when the Sun is too near. 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.

The description above is based on the Regulus entry in Wikipedia. For more information about Regulus, see Stars (Jim Kaler).

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