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'''Regulus''', also known as '''Alpha Leonis''', is a quadruple star system located 77.5 light years away.  It is located in, and 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.  Because, from our vantage point on the [[Earth]], Regulus sits just off of the [[ecliptic]], it is regularly occulted by the [[Moon]].  The other planets also caused  occultations of the star from time to time, with the next occurrence to happen on October 1, 2044, by the planet [[Venus]].

'''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==

== Regulus in Culture and History==

The name Regulus is a diminutive 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).  [[Astrology|Astrologers]] in medieval times saw Regulus as one of the fifteen Behenian fixed stars.<ref>http://penelope.uchicago.edu/Thayer/E/Gazetteer/Topics/astronomy/_Texts/secondary/ALLSTA/Leo*.html</ref>

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).  [[Astrology|Astrologers]] in medieval times saw Regulus as one of the fifteen Behenian fixed stars.<ref>http://penelope.uchicago.edu/Thayer/E/Gazetteer/Topics/astronomy/_Texts/secondary/ALLSTA/Leo*.html</ref>

==Star System==

==Star System==

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 directly observed.  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.  The system was originally thought to be around 300 million years old, however recent data suggests the age of the system may be only around 50 million years.<ref name="McAlister">http://xxx.lanl.gov/abs/astro-ph/0501261</ref>

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.  The system was originally thought to be around 300 million years old, however recent data suggests the age of the system may be only around 50 million years.<ref name="McAlister">http://xxx.lanl.gov/abs/astro-ph/0501261</ref>

===Alpha Leonis 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]]. <ref name="McAlister">http://xxx.lanl.gov/abs/astro-ph/0501261</ref> 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]].   

'''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]]. <ref name="McAlister">http://xxx.lanl.gov/abs/astro-ph/0501261</ref> 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 rotation speed gives the star one completely rotation in 15.9 hours (compared to the Sun's rotation period of 25.38 days).  As a result, the equator region is much cooler at the surface, glowing only at 10,200K, compared to the poles, which have a surface temperature of 15,400 K.<ref name="illinois">http://stars.astro.illinois.edu/sow/Regulus.html</ref>.  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.

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.<ref name="illinois">http://stars.astro.illinois.edu/sow/Regulus.html</ref>.  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.<ref>http://adsabs.harvard.edu/abs/2008ApJ...682L.117G</ref> 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 it's mass to Regulus A through tidal interaction.<ref name="illinois">http://stars.astro.illinois.edu/sow/Regulus.html</ref>

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.<ref>http://adsabs.harvard.edu/abs/2008ApJ...682L.117G</ref> 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.<ref name="illinois">http://stars.astro.illinois.edu/sow/Regulus.html</ref>

For an [[terrestrial planet|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 and 33 years to complete an orbit.

For an [[terrestrial planet|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===

===Alpha Leonis B===

===Alpha Leonis C===

===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 [[terrestrial planet|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, as well as [[Tidal lock|tidally locked]].

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 [[terrestrial planet|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, nd the planet would be [[Tidal lock|tidally locked]].