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 79 light
years (second Hipparcos reduction),
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 end 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. To the southeast of Regulus, find the
brighter star Spica. The autumnal equinox, where the Sun crosses the ecliptic in
late September, lies right between the two. Regulus is a class B
(B7) "main sequence" star, a so-called dwarf that like the Sun is fueled by the internal fusion of
hydrogen into helium, though recent classification has it as a
subgiant whose dwarf-life is coming to an end. Though technically
a dwarf, Regulus is still visually 150 times brighter than the Sun.
The measurement of total luminosity (in which invisible ultraviolet
radiation must be accounted for) is complicated by the star's
extremely fast equatorial rotation speed of 317 kilometers per
second, which distorts it into an oblate spheroid with an
equatorial diameter of 4.3 times that of the Sun (determined
through interferometry), 32 percent larger than the polar diameter.
As a result, the rotation poles, with a temperature of 15,400
Kelvin, are much hotter than the equator, which glows at 10,200
Kelvin. When the temperature variation is taken into account,
Regulus is seen to shine with a luminosity of 360 times that of the
Sun, which leads to a mass of 3.4 solar and verification that the
star is near the end of its hydrogen-fusing life, its age roughly
250 million years. Rotation speed combined with radius tells of a
star that spins with a period of just 16 hours. Regulus is
actually a quadruple star. Most obviously, it has a distant lower
mass companion 175 seconds of
arc (at least 4200 Astronomical Units from it, some 100 times
Pluto's distance from the Sun) that orbits Regulus with a period of
at least 125,000 years. The companion, however, is ITSELF a double
separated by at least 97 astronomical units apart in a minimum 880-
year orbit. Both stars are less massive and dimmer than the Sun.
The brighter is an orange K2 dwarf similar to the lesser component
of Alpha Centauri, while the fainter
is a red (class M4) dwarf. From the little double, Regulus would
look like a brilliant star four times brighter than our full Moon.
Much more intriguing is a tight fourth companion detected only spectroscopically that orbits
Regulus proper with a period of a mere 40.11 days. Analysis
suggests that it is white
dwarf with an anomolously low mass of just 0.3 solar, far below
the minimum of 0.55 allowed by current stellar evolution. Kepler's laws then give a separation of about 0.35
Astronomical Units. Astronomers speculate that when the white
dwarf was a luminous giant
(far larger and brighter than Regulus is now) that it transferred
much of its mass (through tidal interaction) to the star that is
now Regulus, and in doing so, sped it up to its current fast
rotation rate (which fits with the white dwarf scenario.
Much the same seems to be happening to Algol. Off in the distance, 217 seconds
away, is a fifth "companion" that seems to be only a line-of-sight
coincidence. (Description based in part on papers by D. R. Gies et
al. and H. A. McAlister et al. in the Astrophysical Journal)
Written by Jim Kaler 11/26/99; revised
9/19/08. Return to STARS.