Regulus

regulus star,alpha leonis

Regulus, Alpha Leonis (α Leo), is a multiple star system located in the constellation Leo. It lies at a distance of 79.3 light years from Earth. With a combined apparent magnitude of 1.36, it is the brightest star in Leo and the 21st brightest star in the sky. It is only slightly fainter than Deneb in the constellation Cygnus and Mimosa in Crux, and it just outshines Adhara in Canis Major, Castor in Gemini, and Shaula in Scorpius.

regulus star,alpha leonis
Regulus (Alpha Leonis). image: Wikisky

Regulus is part of a relatively bright asterism known as the Sickle of Leo, which outlines the celestial lion’s head, mane and shoulders. Regulus sits at the base of the Sickle, marking the lion’s heart. The star is also one of the vertices of the Spring Triangle, a conspicuous asterism visible in the evening sky during the northern hemisphere spring.

Star system

Regulus appears as a single star to the naked eye, but it is in fact a multiple star system consisting of two pairs of stars and possibly more components. The components are designated as Alpha Leonis (Regulus) A, B, C, and D.

Regulus A is a spectroscopic binary system consisting of a blue-white main sequence star and a dim companion that has not yet been seen directly, but is believed to be a white dwarf. The primary component has more recently been given the stellar classification B8 IVn, indicating a subgiant star, one almost out of its supply of hydrogen.  The star has a mass 3.8 times that of the Sun and a radius about three times solar. With an effective temperature of 12,460 K, it shines with 288 solar luminosities. The star is a very fast spinner, with a projected rotational velocity of 318 km/s at the equator. Its estimated age is at least a billion years.

As a result of its high rotational velocity, the star’s shape has been distorted into a highly oblate spheroid, with its equatorial diameter 32 percent larger than its diameter at the poles. The star’s rotational period is only 15.9 hours and its equatorial rotational velocity is 96.5 percent of its breakup velocity. As a result, the star is emitting polarized light.

Along with Vega, Altair and Achernar, Regulus is one of the best known examples of gravity darkening. Due to the star’s oblate shape, its poles are closer to the centre of mass and have a higher temperature and brightness, while the equatorial region is cooler and less bright. The poles are said to be “gravity brightened,” while the equator is “gravity darkened.” The poles of Regulus are five times brighter than the equator.

The main component in the Regulus A system is orbited by a companion with at least 0.3 solar masses. The companion is believed to be a white dwarf. The two components complete an orbit around a common centre of mass every 40.11 days.

The primary component was once the smaller of the two stars. Before it became a white dwarf, the companion is believed to have transferred a lot of its mass onto the other star through gravitational interaction, speeding up the primary component’s rotation rate to what it is now.

The other components are also still on the main sequence, but considerably fainter than Regulus A, with apparent magnitudes of 8.1 (Regulus B) and 13.5 (Regulus C). Regulus B is visible in binoculars, while Regulus C can only be resolved in a larger telescope. The Regulus BC pair can be observed in smaller instruments.

Regulus B has the stellar classification K2 V, indicating an orange main sequence star, while Regulus C appears red, with the spectral class M4 V.  The stars are also much less massive than the primary, with masses of 0.8 and 0.3 solar masses respectively.

The Regulus BC pair is separated by 177 seconds of arc, or about 5,000 astronomical units, from Regulus A and can be seen in small telescopes. The stars share a common proper motion with Regulus A and are believed to be in orbit with a period of 125,000 years or more.

Regulus B and Regulus C have an orbital period of 600 years. They are separated by about 100 astronomical units.

Regulus D is a 12th magnitude star located at a separation of 212 arcseconds. The 2018 Gaia data release 2 revealed it to be a background object, not physically related to the Regulus system.

Facts

Regulus and its Leo neighbour Denebola are among the 58 bright stars that have been given a special status in the field of celestial navigation. They are the only navigational stars in Leo.

Regulus is the faintest of the 22 first magnitude stars. These are the 22 stars with apparent magnitudes of less than +1.50, or in other words, the brightest stars in the sky.

Regulus is part of the Sickle of Leo, a sickle-shaped asterism that represents the Lion’s head. The Sickle is outlined by the stars Epsilon Leonis, Rasalas (Mu Leonis), Adhafera (Zeta Leonis), Algieba (Gamma Leonis), and Eta Leonis.

Regulus is the nearest bright star to the ecliptic, located only 0.465 degrees from it. The next brightest star to the ecliptic is Wasat, Delta Geminorum, with a visual magnitude of 3.53. Its proximity to the ecliptic means that Regulus is frequently occulted by the Moon. It can also be occulted by Mercury and Venus, but this only happens rarely. It was last occulted by Venus on July 7, 1959, and the next occultation, also by Venus, will not occur until October 1, 2044.

Regulus is also occulted by asteroids. The asteroid 166 Rhodope, discovered in August 1876, was observed occulting Regulus by a team of researchers on October 19, 2005. On March 20, 2014, the star was occulted by 163 Erigone, a relatively large asteroid first discovered in April 1876. Erigone extinguished the star’s light for 14 seconds, but the occultation was not observed due to unfavourable weather conditions.

The heliacal rising of Regulus (rising just before the dawn after a period of invisibility) occurs in early September. Every eight years, Venus passes near the star around that time.

The autumnal equinox, the point where the Sun crosses the ecliptic in early autumn, is located halfway between Regulus and Spica.

Regulus was one of the four Royal Stars of Persia, along with Fomalhaut, Antares and Aldebaran. These stars were considered to be the guardians of the sky in ancient Persia, around the year 3000 BCE. In Persian astronomy, the sky was divided into four districts and each district was guarded by one of the four bright stars.

In medieval astrology, Regulus was one of the 15 Behenian fixed stars, believed to hold special astrological power. Each of the 15 stars was associated with a planet, a plant and a gemstone and the latter two were used in rituals to bring out the star’s influence. Regulus was associated with Jupiter and Mars, and linked with granite and mugwort.

Name

The name Regulus means “little king” or “prince” in Latin. In ancient Greece, the star was known as Basiliskos (“little king”), while the Romans knew it as Basilica Stella. It was the Polish astronomer and mathematician Nicolaus Copernicus who translated these names into Latin as Regulus.

The name Regulus was officially approved by the International Astronomical Union’s (IAU) Working Group on Star Names (WGSN) on June 30, 2016. It formally applies only to the component Alpha Leonis A, but is informally used for the whole system.

Regulus was also known by the Arabic name Qalb al-Asad, the Greek name Kardia Leontos and Latin Cor Leonis, all meaning “the heart of the Lion.” In Europe, the star was known as Rex and Basiliscus.

The Chinese know Regulus as the Fourteenth Star of Xuanyuan, the Yellow Emperor (轩辕十四). In Chinese astronomy, Regulus is part of the Xuanyuan asterism, which consisting of about 17 stars, among them 10 Ursae Majoris, Alpha Lyncis, 38 Lyncis, Kappa Leonis, Alterf (Lambda Leonis), Epsilon Leonis, Rasalas (Mu Leonis), Adhafera (Zeta Leonis), Algieba (Gamma Leonis), Subra (Omicron Leonis), and Rho Leonis.

In Babylonian astronomy, Regulus was known as Sharru, “the King.” It was listed as Lugal (“king”) in the MUL.APIN, the Babylonian compendium of astronomy and astrology compiled around 1000 BCE.

The Persians knew the star as Miyan, “the Centre” and as Venant, the guardian of the north. In Indian astronomy, the star’s name was Maghā, “the Mighty,” after a Hindu lunar mansion that consisted of all the stars of the Sickle of Leo.

Location

Regulus is easy to find and identify because it is part of a prominent asterism known as the Sickle of Leo. The Sickle looks like a backward question mark and consists of the stars that outline the mane of the celestial Lion, with Regulus at the base.

The Sickle of Leo, image: Wikisky

The Sickle can be found using the Pointer Stars, Merak and Dubhe, in the constellation Ursa Major. These are the same stars that are used to find Polaris, the North Star. A line drawn in the opposite direction leads toward the Sickle. A line extended from Megrez through Phecda, the inner stars of the Big Dipper’s bowl, also leads in the direction of the asterism.

Together with the bright Spica in Virgo and Arcturus in Boötes, Regulus forms the Spring Triangle, another prominent asterism appearing in the evening sky from March to May.

The Spring Triangle and the Great Diamond, image: Wikisky

Regulus is best seen in late winter and spring in the northern hemisphere, when it rises high above the horizon in the evening. The star is visible at some time of night year-round, with the exception of the month around August 22-24, when it is too close to the Sun. In late February, it is visible throughout the night.

Regulus can be used to find several bright galaxies that lie in the region of the sky between the star and the fainter Denebola, Beta Leonis, the star that marks the Lion’s tail. These include the Messier 96 Group (Leo I) with member galaxies Messier 95, Messier 96 and Messier 105, and the Leo Triplet, which consists of the galaxies Messier 65, Messier 66 and NGC 3628.

Regulus, Messier 95, Messier 96 and Messier 105, image: Wikisky

Constellation  

Regulus is the luminary of the constellation Leo. Easily recognizable for the Sickle asterism that outlines the Lion’s mane and shoulders, Leo is known for its bright stars – mainly Regulus and Denebola – and a number of bright deep sky objects. The constellation is home to the Leo Triplet of galaxies, which consists of the galaxies Messier 65, Messier 66 and NGC 3628, the spiral galaxies Messier 95, Messier 96, the elliptical galaxy Messier 105, and the Cosmic Horseshoe, a gravitationally lensed system of two galaxies located at a distance of 5.2 and 10.3 gigalight years, discovered in 2007.

Leo constellation, image: Roberto Mura

Leo is also home to the red dwarf Wolf 359, one of the nearest stars to the Sun at a distance of 7.86 light years, the bright variable red giant star R Leonis, and the carbon star CW Leonis, the brightest star in the infrared N-band (10 μm).

The best time of year to observe the stars and deep sky objects of Leo is during the month of April.

The 10 brightest stars in Leo are Regulus (Alpha Leo, mag. 1.40), Algieba (Gamma Leo, mag. 2.08), Denebola (Beta Leo, mag. 2.113), Zosma (Delta Leo, mag. 2.56), Epsilon Leonis (mag. 2.98), Chertan (Theta Leo, mag. 3.324), Adhafera (Zeta Leo, mag. 3.33), Eta Leonis (mag. 3.486), Subra (Omicron Leo, mag. 3.52), and Rho Leonis (mag. 3.9).

Regulus – Alpha Leonis

Spectral classB8 IVn + K2 V + M4 V
Apparent magnitude1.36
Distance79.3 ± 0.7 light years (24.3 ± 0.2 parsecs)
Parallax41.13 ± 0.35 mas
Proper motionRA: −248.73 ± 0.35 mas/yr
Dec.: 5.59 ± 0.21 mas/yr
ConstellationLeo
DesignationsRegulus, Alpha Leonis, α Leo, 32 Leonis, HR 3982, HIP 49669, FK5 380, GJ 9316, GCTP 2384.00,

Regulus A

Spectral classB8 IVn
Variable typesuspected
U-B colour index-0.36
B-V colour index-0.11
Apparent magnitude1.40
Absolute magnitude0.57
Radial velocity+5.9 km/s
Mass3.8 M
Luminosity288 L
Radius3.092 ± 0.147 R
Temperature12,460 ± 200 K
Age≳1 billion years
Rotational velocity318 ± 8 km/s
Surface gravity3.54 ± 0.09 cgs
Right ascension10h 08m 22.311s
Declination+11° 58′ 01.95″
Designationsα Leo A, HD 87901, SAO 98967, LTT 12716, BD+12°2149, TYC 833-1381-1, GC 13926, GCRV 6417

Regulus BC

Spectral classK2 V + M4 V
U-B colour index+0.51
B-V colour index+0.86
Apparent magnitude8.13/13.50
Absolute magnitude6.3/11.6
Radial velocity+6.3 km/s
Mass0.8 M, 0.3 M
Luminosity (Regulus B)0.50 L
Temperature (Regulus B)4,885 K
Surface gravity (Regulus B)4.4 cgs
Right ascension10h 08m 12.8/14s
Declination+11° 59′ 48″
Designationsα Leo B/C, HD 87884, SAO 98966, LTT 12714, BD+12°2147, TYC 833-134-1, GC 13922, GCRV 6416, PPM 127133, Gaia DR2 3880785530720066176, Gaia DR1 3880785526425032832