Regulus A/B/C

Regulus is located at the lower right on this map of the constellation.
Observation data Epoch J2000      Equinox J2000
Constellation	Leo
Right ascension	A: 10h 08m 22.3s BC: 10h 08m 12.8/14s
Declination	A: +11° 58′ 02″ BC: +11° 59′ 48″
Apparent magnitude (V)	1.35/8.14/13.5
Characteristics
Spectral type	B7 V + K1-2 V + M5 V
U−B color index	–0.36/+0.54
B−V color index	–0.11/+0.87
Variable type	Slightly
Astrometry
Radial velocity (Rv)	+5.9 km/s
Proper motion (μ)	RA: 249 mas/yr Dec.: 2 mas/yr
Parallax (π)	42.09 ± 0.79 mas
Distance	77 ± 1 ly (23.8 ± 0.4 pc)
Absolute magnitude (MV)	–0.52/6.3/11.6
Details
α Leo A
Mass	3.8[1] M☉
Radius	3.15–4.15[1] R☉
Luminosity	288[1] L☉
Surface gravity (log g)	3.54 ± 0.09[2]
Temperature	12,460 ± 200[1] K
Rotational velocity (v sin i)	315 km/s
Age	≳1[3] Gyr
α Leo B/C
Mass	0.8/0.2 M☉
Radius	0.5/? R☉
Luminosity	0.31 L☉
Other designations
Alpha Leonis, 32 Leo, Cor Leonis, Basilicus, Lion’s Heart, Rex, Kalb al Asad, Kabeleced, BD+12 2149/2147, GCTP 2384.00, GJ 9316, HD 87901/87884, HIP 49669, HR 3982, LTT 12716/12714, SAO 98967/98966, FK5 380.
Database references
SIMBAD	data

Regulus (α Leo, α Leonis, Alpha Leonis) is the brightest star in the constellation Leo and one of the brightest stars in the night sky, lying approximately 77.5 light years from Earth. Regulus is a multiple star system composed of four stars that are organized into two pairs. The spectroscopic binary Regulus A consists of a blue-white main-sequence star and its companion, which has not yet been directly observed, but is probably a white dwarf star.^[4] Located farther away is the pair Regulus B and Regulus C, which are dim main-sequence stars.

Observations[link]

Ground track of the March 20, 2014 occultation of Regulus by 163 Erigone.

Of the brightest stars in the sky, Regulus is closest to the ecliptic, and is regularly occulted by the Moon. Occultations by the planets Mercury and Venus are also possible but rare, as are occultations by asteroids. The last occultation of Regulus by a planet was on July 7, 1959, by Venus. The next will occur on October 1, 2044, also by Venus. Other planets will not occult Regulus over the next few millennia because of their node positions.

Regulus is expected to be occulted by the asteroid 163 Erigone on March 20, 2014.^[5] This event will be visible along a path about 40 miles wide from New York City to Oswego in the United States, and extending approximately northwest into Canada on a track that includes Belleville and North Bay, Ontario.^[6]

An occultation of Regulus by the asteroid (166) Rhodope was observed by 12 observers from Portugal, Spain, Italy, and Greece on 2005-10-19. Details are at http://www.euraster.net/results/2005/index.html#1019-166

Although best seen in the evening in northern hemisphere in late winter and spring, Regulus can be found at some time of night throughout the year except for about a month on either side of August 22, when the sun is too near.^[7] For most Earth observers, the heliacal rising of Regulus occurs in the first week of September. Every 8 years, Venus passes Regulus around the time of the star's heliacal rising, most recently in 2006.

The primary of Regulus A has about 3.5 times the Sun’s mass and is a young star of only a few hundred million years. It is spinning extremely rapidly, with a rotation period of only 15.9 hours, which causes it to have a highly oblate shape.^[8] This results in so-called gravity darkening: the photosphere at Regulus' poles is considerably hotter, and five times brighter per unit surface area, than its equatorial region. If it were rotating only 16% faster, the star's gravity would provide insufficient centripetal force to hold it together, and it would tear itself apart.

System[link]

Regulus is a multiple star system consisting of four stars. Regulus A is a binary star consisting of a blue-white main sequence star of spectral type B7V, which is orbited by a star of at least 0.3 solar masses, which is probably a white dwarf. The two stars take approximately 40 days to complete an orbit around their common centre of mass. Given the extremely distorted shape of the primary, the relative orbital motion may be notably altered with respect to the two-body purely Keplerian scenario because of non-negligible long-term orbital perturbations affecting, for example, its orbital period. In other words, the third Kepler law, which holds exactly only for two pointlike masses, would be no longer valid because of the highly distorted shape of the primary.

At a distance of around 4,200 AU^{[citation needed]} from Regulus A is a binary star system that shares a common proper motion. Designated Regulus B and Regulus C, the Henry Draper Catalogue number for this pair is HD 87884. The first is a K2V star, while the companion is approximately M4V.^[8] The companion pair has an orbital period of 2,000 years and is separated by about 100 AU.^{[citation needed]}

Visibility[link]

The Regulus system as a whole is the twenty-first brightest star in the night sky with an apparent magnitude of +1.35. The light output is dominated by Regulus A. Regulus B, if seen in isolation, would be a binocular object of magnitude +8.1, and its companion, Regulus C, the faintest of the three stars that has been directly observed, would require a substantial telecope to be seen, at magnitude +13.5. Regulus A is itself a spectroscopic binary: the secondary star has not yet been directly observed as it is much fainter than the primary. The BC pair lies at an angular distance of 177 arc-seconds from Regulus A, making them visible in amateur telescopes^{[citation needed]}.

Etymology and cultural associations[link]

Rēgulus is Latin for 'prince' or 'little king'. The Greek variant Basiliscus is also used. It is known as Qalb al-Asad, from the Arabic قلب الأسد, meaning 'the heart of the lion'. This phrase is sometimes approximated as Kabelaced and translates into Latin as Cor Leōnis. It is known in Chinese as 轩辕十四, the Fourteenth Star of Xuanyuan, the Yellow Emperor. In Hindu astronomy, Regulus corresponds to the Nakshatra Magha.

Persian astrologers around 3000 BC knew Regulus as Venant, one of the four 'royal stars'.^{[citation needed]} It was one of the fifteen Behenian stars known to medieval astrologers, associated with granite, mugwort, and the kabbalistic symbol .

In MUL.APIN, Regulus listed as LUGAL, meaning "the star that stands in the breast of the Lion:the King.".^[9]

Regulus

See also[link]

• Leo constellation
• Table of stars with Bayer designations
• Regulus in fiction

References[link]

• Fred Schaaf, “Horrorfications of the Lion’s heart,” Sky & Telescope, April 2006.

External links[link]

Wikimedia Commons has media related to: Regulus (star)

• Portrait of a Star on the Edge
• Egg-Shaped Regulus is Spinning Fast
• APOD Pictures: 1) Regulus Occulted 2) Regulus & Leo 1 Dwarf Galaxy photo 1 [photo 2 3) Regulus, Mars & Coma Star Cluster
• "Regulus 3". SolStation. http://www.solstation.com/stars2/regulus3.htm. Retrieved December 1, 2005.