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.