Star Facts: Regulus

Regulus Star
Image Credit: New Forest Observatory

Regulus (Alpha Leonis), meaning “Prince” or “Little King” in Latin, is the brightest star in the constellation Leo, and the 21st brightest star in the entire sky. It is actually a four star system made up of two binary pairs, with the principal star in the system, Regulus A, a blue-white main sequence star; and its companion, which has not been observed directly, thought to be a white dwarf. They are so close together, however, such that their colors can only be separated spectroscopically.

Quick Facts

• Constellation: Leo
• Coordinates: RA 10h 8m 22s | Dec -11° 58′ 2?
• Distance to Earth: 79.3 light years
• Star Type: B8 IVn (Regulus A)
• Mass: 3.8 sol
• Radius: 3.092 sol
• Apparent Magnitude: +1.40
• Luminosity: 288 sol
• Surface Temperature: 12,460 K
• Rotational Velocity: 347 km/s
• Age: 1 billion years
• Other Designations: Alpha Leonis, 32 Leo, Cor Leonis, Aminous Basilicus, Lion’s Heart, Rex, Kalb al Asad, Kabeleced, FK5 380, GCTP 2384.00, GJ 9316, HIP 49669, HR 3982.

Location

Finding RegulusRegulus in Leo can be seen from latitudes between +90 and -65°, with the constellation found by following the pointer stars of the Big Dipper in the opposite direction of Polaris to the striking lionesque figure of Leo. The star Regulus is found at the bottom of the constellation’s distinctive sickle-shaped asterism, and marks the lion’s heart. Regulus is best seen during the late winter and spring from northern hemisphere, but except for a month on either side of August 22nd, the star is visible in the sky at different elevations and positions throughout the year. However, Regulus’ heliacal rising, which is when the star becomes visible in the east for a short while just before sunrise, occurs in the first week of September each year, and is visible to most observers on Earth.

Visibility

The combined Regulus system has an apparent magnitude of +1.35, which makes it the twenty-first most luminous “star” in the entire night sky. By way of contrast, Regulus B would be a binocular object with a magnitude of +8.1 if it could be seen in isolation. Even dimmer is Regulus C, the faintest star of the Regulus system that has been observed directly, which at magnitude +13.5 requires a substantial telescope to be seen. However, the combined Regulus BC pair is visible in medium-sized amateur telescopes about 177 seconds of arc from Regulus A.

Age

Based on Regulus’ color, temperature, luminosity, and temperature, it was first thought by investigators that Regulus was no more than between 50 and 100 million years old, especially given its mass. However, the fact that its companion turned out to be a white dwarf meant that this calculation had to be revised upward, just to account for the formation and evolution of the white dwarf. Therefore, revised estimates put the stars’ age at around 1 billion years.

Physical Properties

Primary Pair

The principal star in the primary system (Regulus A) and its companion (Regulus AB), which has a mass of about 0.3 sol, are separated by 0.35 AU, and have an orbital period of about 40 days. However, since Regulus A has a rotational velocity of 347 km/s, its highly distorted shape is thought to have serious implications for the stability of the pair’s orbital period, and the orbit itself. At issue is the fact that Kepler’s Third Law holds true only for two point-like masses, which means that Regulus’ highly oblate shape may noticeably alter the mechanics of the two stars’ mutual orbit around their common centre of mass. As a point of interest, Regulus’ spin rate is just 15% or so below the value at which the star would fly apart under the enormous centrifugal forces generated by its high rotational velocity.

One other major effect of Regulus’ high spin rate is “gravity darkening”, or a phenomenon which has resulted in the photosphere of Regulus being lot hotter at the polar regions than elsewhere, and these regions being at least five times as bright as the stars’ equatorial regions per any given unit of area.

Secondary Pair

The second pair of stars in the system, designated Regulus CD (HD 87884) in the Henry Draper Catalogue, is about 5,000 AU distant from the principal pair. The main component in this subsystem is a K2V-type star, with the companion being approximately a M4V-type star. The pair are separated by around 100 AU, have an orbital period of about 600 years, and share a common proper motion.

Occultation

Located only 0.46 degrees from the ecliptic, Regulus is regularly occulted by the Moon, but less so by the planets Venus and Mercury, and even less frequently by asteroids. Nonetheless, the last occultation of Regulus by Venus occurred on July 7th, 1959, and the next occultation by Venus will only occur in October of 2044. Due to the nodal positions of the other planets, including that of Mercury, Regulus will not be occulted by any planet for at least the next several millennia. Note, though, that the star was seen to be occulted by the asteroid 166 Rhodope on October 19th, 2005 by twelve observers from Portugal, Spain, Italy, and Greece. More recently, Regulus was also occulted by the asteroid 163 Erigone in the wee hours of March 20th, 2014, although the event is not known to have been observed by anyone.

History

Throughout history, Regulus has been associated with royal power, and to the ancient Babylonians was known as Sharru (the King), to the Akkadians as Amil-gal-ur (King of the Celestial Sphere), and to the Persians as Miyan (the Centre), or as Venant, one of the four “royal stars” that watched over all the other stars in the form of a Fixed Cross, the others being Aldebaran, Fomalhaut, and Antares. In old India, the star was known as Magha (the Mighty), while the Greek astronomer Ptolemy used the word Basiliskos (“little king”) to describe it, which was later translated into Latin by Nicolaus Copernicus as Regulus.

Medieval European astrologers also knew Regulus as one of the fifteen Behenian stars, which were stars that were endowed with magical properties through being associated with plants and gemstones. In the case of Regulus, its magical properties derived from its association with granite and the mugwort plant.

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