REGULUS (Alpha Leonis). Regulus, glowing at the heart of Leo the Lion, one of the great constellations of the
zodiac, is near the end of the list of first magnitude stars. At
a distance of only 77 light years, it shines in our sky at
magnitude 1.35, just marginally brighter than the next one down, Adhara, the second brightest star of Canis Major. The Latin name means "the
little king," the reference to a kingly star going back to ancient
times. Regulus marks the bottom of an asterism called the "sickle of
Leo," a sickle-shaped figure that outlines the head of the
celestial lion. The star is almost exactly on the ecliptic, the
path of the Sun, and is regularly occulted, or covered over, by the
Moon. Down and to the left of Regulus, find the brighter star Spica. The autumnal equinox, where the Sun
crosses the ecliptic in September, lies right between the two.
Regulus is a "main sequence" star, a so-called dwarf that like the
Sun is fueled by the internal fusion of
hydrogen into helium. Though technically a dwarf, Regulus is still
visually 140 times brighter than the Sun, the luminosity climbing
to 240 time brighter when the star's ultraviolet radiation is taken
into account. Its luminosity and a temperature of 12,000 Kelvin
show from theory that it has a mass some 3.5 times solar.
Consistently, it is 3.5 times larger than the Sun, the figure
derived both from temperature and luminosity and from a direct
measure of angular diameter. Regulus has a distant lower mass
companion located at least 4200 astronomical units away from it
(100 times Pluto's distance from the Sun), which orbits Regulus
with a period of at least 130,000 years. The companion is ITSELF
a double separated by at least 95 astronomical units) in a thousand
year orbit. Both stars are less massive and dimmer than the Sun.
The brighter is an orange dwarf similar to the lesser component of
Alpha Centauri, while the fainter is
a red (class M) dwarf. From the little double, Regulus would look
like a brilliant star six times brighter than our full Moon.