Regulus

Regulus is the brightest object in the constellation Leo.

It is also one of the brightest stars visible in the night sky. Astronomers use the Bayer designation Alpha Leonis to identify it. This name is often abbreviated as Alpha Leo. The star is the twenty-first brightest star in the sky. It has an apparent magnitude of +1.35. This number helps scientists measure how bright an object appears from Earth.

While Regulus looks like a single point of light, it is actually a quadruple star system. This means it is composed of four stars organized into two distinct pairs. The first pair is known as Regulus A. This is a spectroscopic binary system. A spectroscopic binary is a pair of stars that are so close together they appear as one. In this pair, a blue-white main-sequence star orbits a companion that is a pre-white dwarf. These two stars complete one orbit around their common center of mass every 40 days.

Regulus A is a very massive and unusual star. It has about 4.15 times the mass of our Sun. The star rotates extremely rapidly, completing a full turn in just 15.9 hours. For comparison, our Sun takes about 25 days to rotate once. This fast rotation causes the star to take on an oblate shape. An oblate shape is one that is flattened at the poles, like an egg. This shape leads to a phenomenon called gravity darkening. Because of the shape, the poles are much hotter and five times brighter than the equatorial region.

The second pair of stars is called Regulus BC. This pair is also known as HD 87884. Regulus BC is separated from Regulus A by about 5,000 AU. An AU, or astronomical unit, is a unit used to measure vast distances in space. Regulus B and Regulus C orbit each other every 600 years. Regulus B is a K2V type star. Regulus C is an M4V type star and is the faintest of the stars directly observed. The separation between Regulus B and C was measured at 2.5 arc-seconds in 1942.

Regulus has been studied by many cultures throughout history. Its traditional name, Rēgulus, is Latin for "little king."

In ancient Babylon, the star was called Lugal, which also means king. In Arabic, it is known as Qalb al-Asad, meaning "the heart of the lion." In China, it is identified as the Fourteenth Star of Xuanyuan. It is also called Nüzhu, representing the queen. In India, the star is part of the nakshatra Maghā, which means "the bountiful." These various names show how important the star has been to people across the world.

Observers on Earth can see Regulus at many different times. It is best seen in the evening during the late winter and spring in the northern hemisphere. It is visible most of the year, except for a short time around August 22–24. During this time, the Sun is too close to the star to see it clearly. Regulus is very close to the ecliptic, which is the path the Sun appears to take. Because of this, the Moon often performs an occultation of the star. An occultation happens when a celestial body like the Moon passes in front of a star, hiding it from view.

Occultations of Regulus are not just caused by the Moon. Planets like Venus can also occult the star. The last time Venus hid Regulus was on July 7, 1959. The next time this will happen is October 1, 2044. Asteroids can also cause occultations. In 2005, the asteroid 166 Rhodope passed in front of Regulus. Scientists used this event to measure the bending of light. This measurement was consistent with the theory of general relativity. Such events help astronomers understand the physics of our universe.