Regulus, also known as Alpha Leonis, is a quadruple star system located 77.5 light years away. It is the brightest star of the constellation Leo. In the night sky, with the unaided eye, it is seen as a single star with an apparent magnitude of 1.35. From our vantage point on the Earth, Regulus sits just off of the ecliptic, causing it to be regularly occulted by the Moon. The other planets also cause occultations of the star from time to time, with the next occurrence to happen on October 1, 2044, by the planet Venus.
Regulus in Culture and History
The name Regulus is a derivative form for the Latin word Rex, meaning "king". It is often translated as "prince" or "little king". The Greek name for the star is Βασιλίσκος, (Basiliscus), which is still commonly used, is also translated as "little king". In much earlier times the star was known as the "King of the Celestial Sphere" by the Akkadians. Arabic astronomers referred to the star as قلب لأسد or Qalb Al Asad, meaning "the heart of the lion", which the derivative names Kalbeleced or Kalbelasit for the star are from. The Chinese viewed the star as 轩辕十四, the Fourteenth Star of the Yellow Emperor, Huang-di (黄帝). Astrologers in medieval times saw Regulus as one of the fifteen Behenian fixed stars.
Regulus is really a multiple star system made up of four known stars grouped in two binary pairs. Of the four, only three can be visibly observable. Only the blue-white Regulus A is visible with the unaided eye, and is closely orbited by what is most likely a white dwarf. The other two companion stars of the Regulus system are cooler, main sequence stars some 4200 AU away from Regulus A.
Alpha Leonis A
Regulus A, the primary star, is a blue-white main sequence dwarf of spectral type B7 Va,ne. The star is very oblate, due to its extremely fast rotation, and thus is 3.14 times the Sun's diameter at the poles and 4.16 times at its equator.  The star has a mass 3.4 times that of our Sun. As a bright, blue-white star, Regulus A is 140 times as visually luminous as the Sun, but its total luminosity is 240 times as great when accounting for the ultraviolet.
Like many young, energetic stars, Regular A rotates extremely fast. At the equator, the star is rotating at 317 kilometers per second, distorting the star significantly. This tremendous rotary speed gives the star one complete rotation every 15.9 hours. (compared to the Sun's rotation period of 25.38 days). The equator of the star is much cooler due to the rapid rotation, having a surface temperature of 10,200 K, as opposed to the 15,400 K surface temperature at the poles.. The rotation period is so quick that if the star rotated only 16 percent faster, centripetal force would overcome its gravity and cause the star to break apart.
Regulus A has a binary companion, determined only Spectroscopically. The companion is only 0.35 AU away and orbits the primary in 40.11 days. The exact nature of the companion star is unknown, but analysis suggests the star is a low mass white dwarf that is only 30% as massive as the sun (0.3 solar). This is unusual because according to current theories, white dwarfs should have no less then a minimum mass of 0.55 solar (55 percent the Sun's mass). One theory is that when the companion evolved off the main sequence into a luminous giant, it loss much of its mass to Regulus A through tidal interaction.
For an Earth-like world to exist with liquid water on its surface, the hypothetical planet would need to be between 11.9 and 15.5 AU away from Regulus A. This is equivalent to between the orbits of Saturn and Uranus in our Solar System. Such a planet would take some 22 to 33 years to complete an orbit.
Alpha Leonis B
Regulus B is an orange-red main sequence star that is spectral type K1-2 V. With an apparent magnitude of only 8.14, it is invisible to the unaided eye. The star is around 80 percent our Sun's diameter while only 31 percent as luminous. The star and its binary companion (Regulus C) orbit each other with an average separation of 98.9 AU, taking some 2,000 years to complete. Together the two stars orbit Regulus A at a distance of 4200 AU, or some 100 times the distance of Pluto from our Sun, taking 125,000 years to complete one orbit.
An Earth-like planet with liquid water would require an orbit centered around 0.56 AU from Regulus B. Such an orbit would be between those of Venus and Mercury in our Solar System and would take 171 days to complete. From such a hypothetical world, Regulus A would appear as a star six times as luminous as the Full Moon here on Earth.
Alpha Leonis C
Regulus C is a red dwarf star with a spectral type M5 V. The star is around 20 percent of the Sun's mass but is only 0.31 percent its visual luminosity. An Earth-like world around the star would be centered around 0.056 AU in order to have liquid water on its surface. At such a close orbit, the planet's year would only be 10.7 days, causing the planet to be tidally locked.