Star of the Month
Learn more about Regulus, Leo’s brightest star
Regulus (Alpha (_) Leonis) is one of the principal stars of the spring sky. It’s easy to spot at the base of the asterism known as the Sickle, a pattern representing the head of Leo. If you need further affirmation, follow the lines of both sides of the Plough’s blade down and away from Polaris. With a bit of artistic license, they converge at Regulus.
Regulus is 79 lightyears from the Sun and appears to us at mag. +1.4, located 0.5° from the ecliptic. This location means that Solar System objects appear to interact with it and, as is the case currently, the Moon can occult Regulus. Planetary conjunctions with Regulus aren’t uncommon but planetary occultations are rare. The next will occur on 1 October 2044, when Venus passes in front of the star.
Regulus is a multiple star system. An obvious mag. +8.2 companion sits 177 arcseconds away in a northwest direction (position angle 307°). This orange companion, Regulus B, is a double with a separation of 2.5 arcseconds between it and mag. +13.5 Regulus C. The B-C pair’s orbital period is an estimated 600 years. This pales into insignificance compared to the 125,000 years it takes B-C to orbit the main Regulus A star.
Spectroscopic analysis of Regulus A reveals that it too is a double. Its so-far-unseen companion is believed to be a white dwarf star, orbiting Regulus A once every 40.11 days at a distance of just 52 million kilometres.
Another interesting characteristic of Regulus is its fast rotation rate of 317km/s. At this speed, Regulus will be flattened into an oblate spheroid, approximately one-third wider at the equator than its polar diameter. Its equatorial diameter is estimated to be 4.3 times that of the Sun.