Regulus, whose name means “little king,” is the brightest star in the constellation Leo and the 21st-brightest star in the night sky. Although it appears as a single blue-white point of light, it is actually a four-star system dominated by one rapidly rotating star.
Easy to Find at the Base of Leo’s Sickle
Regulus marks the base of the Sickle, a distinctive backward-question-mark pattern forming the head and mane of Leo the Lion. From mid-northern latitudes, the star is especially prominent on late-winter and spring evenings. Its position at the bottom of the Sickle traditionally represents the Lion’s heart, which explains another of its historic names: Cor Leonis, meaning “Heart of the Lion.”
The 21st-Brightest Star in the Night Sky
Regulus has an apparent magnitude of approximately 1.35, making it the 21st-brightest star in the night sky. Located about 79 light-years from Earth, its dominant star is a hot blue-white B-type star with roughly four times the Sun’s mass and several hundred times its luminosity. Its prominence reflects both this enormous energy output and its relative proximity to Earth.
Spins Once in Just 16 Hours
Regulus completes one rotation in approximately 15.9 hours, compared with about 25 days for the Sun at its equator. Material around the star’s equator travels at roughly 320 kilometers (200 miles) per second, placing Regulus among the most rapidly rotating bright stars in the night sky. Regulus spins close to its critical, or “breakup,” speed—the point at which material at its equator would begin escaping because the outward effect of rotation would overcome the star’s gravity.
Roughly One-Third Wider at Its Equator
Regulus’s extreme rotation prevents it from maintaining a spherical shape. Centrifugal effects push its equatorial regions outward, making its equatorial diameter approximately 32% greater than its polar diameter. Regulus is therefore roughly one-third wider across its equator than it is from pole to pole, while more slowly rotating stars such as the Sun remain almost perfectly spherical.
Its Poles Are Hotter and Brighter
Regulus’s temperature varies considerably across its surface. Its poles reach approximately 14,500 kelvins, while its equator is closer to 11,000 kelvins—a difference of about 3,500 degrees. This variation is caused by gravity darkening. Regulus’s rapid rotation reduces the effective gravity at its bulging equator, allowing the surface there to expand and cool. Its more compact polar regions experience stronger effective gravity, producing greater pressure and energy flow that make them hotter and brighter.
A Hidden Companion Made Regulus Spin Faster
The secret behind Regulus’s extraordinary rotation lies in a faint companion orbiting it every 40 days. This companion was originally the more massive star, but as it aged and swelled, it transferred most of its outer material to Regulus. The stream of gas carried angular momentum with it, increasing Regulus’s mass and spinning the star up to nearly its breakup speed. All that fresh material also rejuvenated Regulus, making it appear far younger than the system’s true age of at least one billion years. The stripped companion is now evolving into a white dwarf.
Regulus Is Actually a Four-Star System
Regulus is not one star but a system of four arranged in two very unequal pairs. The brilliant star visible to the naked eye and its pre-white-dwarf companion form the close central pair, separated by approximately 52 million kilometers (32 million miles), or about one-third of the distance between Earth and the Sun. Around 4,500 astronomical units away lies another pair consisting of a smaller orange K-type star and a faint red dwarf. The brighter of these distant companions can be seen through binoculars or a small telescope, but almost all the light we associate with Regulus comes from the dominant blue-white star.
The Moon Can Make Regulus Disappear
Regulus lies only about half a degree from the ecliptic—the apparent path followed by the Sun, Moon and planets across the sky. As a result, the Moon periodically passes directly in front of it, temporarily hiding the star in an event called an occultation. Planetary occultations are much rarer, although Venus concealed Regulus in 1959 and is expected to do so again on October 1, 2044.