Regulus 3

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� AAO/ROE, photo by David Malin
(Used with permission)

Larger labelled image of Pleiades,
or a color photo of Regulus A.

Regulus A is a blue-white star
that is larger, brighter, and
hotter than Sirius A, like six
of the brightest stars of the
Pleiades Cluster at left.
(See a Digitized Sky Survey
image of Regulus from
NASA's NStars Database.)

System Summary

The Regulus, or Alpha Leonis, triple star system is located around 77.5 light-years (ly) from Sol (HIPPARCOS Plx of 42.09, +/- e_Plx of 0.79 mas). The star lies in the eastern part (10:8:22.3+11:58:1.9 for Star A, 10:8:12.8+11:59:48.4 for Star B, and 10:8:14+11:59.8 for Star C, ICRS 2000.0) of Constellation Leo (see chart and photo), the Lion -- north of Constellation Sextans. In the venerable Star Names: Their Lore and Meaning, Richard Hinckley Allen noted that its proper name, Regulus, is the diminutive form of the Latin "Rex" (King), in turn derived from the Greek for "Star of the King" and the much earlier Akkadians of Mesopotamia as the legendary "King of the Celestial Sphere" who ruled before the Great Flood. According to Professor Jim Kaler's Stars page on Regulus, Star A is one of the dimmest of the first magnitude stars (see a color photo of Regulus A by Akira Fujii). It lies only half a degree from the ecliptic on the path of the Sun and so is regularly occulted by the Moon. In addition to wide binary stellar companions (stars B and C), Regulus also has a well known optical companion (Wo or GJ 9316 D).

Alpha Leonis A

Regulus is a blue-white main sequence star of spectral and luminosity type B7 Va,ne (Mahipal Singh, 1982). The star may have a mass 3.5 times greater than Sol's, 3.6 times its diameter (Henson, et al, 1989; and angular diameter in two colors from a Lunar occultation from Richard R. Radick, 1982), and 140 times its visual luminosity (240 times with ultraviolet). It is probably is less than 300 million years old.

NASA Observatorium

Regulus A is hotter,
brighter, and bluer
than Vega or Sirius A,
but is cooler, dimmer,
and smaller than Spica.

See a discussion of
the "main sequence"
as part of stellar
evolution and death.

As a very young star, Regulus A appears to be a fast rotator with a rotational period of 11 to 22 hours and a substantial magnetic field around one kG (Henson, et al, 1989; and Wolstencroft et al, 1981). According to the Yale Bright Star Catalogue, 1991 5th Revised Edition notes entry for HR 3982, excess infrared emission indicative of dust has been detected. Regulus A is a variable star designated CSV 101099 and NSV 4750. Useful catalogue numbers and designations for the star include: Alp or Alf Leo A, 32 Leo A, HR 3982, GJ 9316 A, Hip 49669, HD 87901, BD+12 2149, SAO 98967, FK5 380, Wo 9316 A, LTT 12716, and ADS 7654 A.

According to Robert Burnham, Jr. (1931-93), Regulus A has relatively distant, orange-red and red dwarf binary companions BC that were last observed to be separated by around 4,200 AUs. Their wide separation implies an orbital period of at least 130,000 years, according to Professor Jim Kaler's Stars page on Regulus. The Astronomiches Rechen-Institut at Heidelberg's ARICNS entry for Star B notes that the observed separation between Star A and its binary companions has been 177" at 307� from 1836 to 1960. (A review of young B-type stars with wide binaries is available from: Robert E. Murphy, 1969, pp. 1086-1087).

The distance from Regulus A where an Earth-type planet would be "comfortable" with liquid water is between 11.9 and 15.5 AUs -- between the orbits of Saturn and Uranus in the Solar System. At such distances from the star, such a planet would have an orbital period between 22 and 33 Earth years. If there is life on any Earth-type planet that has had time to cool to hold water orbiting very youthful Regulus A, it is likely to be primitive single-cell, anaerobic (non-oxygen producing) bacteria under constant bombardment by meteorites and comets as Earth was for the first billion years. Since there is unlikely to be free oxygen in the atmosphere of such a planet, it probably would not have an ozone layer (O₃) although Regulus A puts out a lot more hard radiation (especially ultraviolet) than Sol. Astronomers would find it very difficult to detect an Earth-sized planet around this star using present methods.

Alpha Leonis B

Star "B" is a orange-red sequence dwarf star of spectral and luminosity type K1-2 V. It may have around 80 percent of Sol's mass, somewaht less than its diameter, and 31 percent of its luminosity.

� Torben Krogh & Mogens Winther,

(Amtsgymnasiet and EUC Syd Gallery,
student photo used with permission)

Regulus B is an orange-red dwarf
star, like Epsilon Eridani at left
center of the meteor.

According to Robert Burnham, Jr. (1931-93), stars B and C have a combined visual luminosity of around 31 percent of Sol's. The most recently observation reported implied a separation around 98.9 AUs (4.163" at 77.5 ly) with a period around 2,000 years, based on the Yale Bright Star Catalogue, 1991 5th Revised Edition notes entry for HR 3982*. According to the Astronomiches Rechen-Institut at Heidelberg's ARICNS entry Star C, the separation of stars B and C has varied between 3.9" at 93� in 1867 to 2.5" at 86� in 1943.

The orbit of an Earth-like planet (with liquid water) around Star B would be centered around 0.56 AU -- between the orbits of Mercury and Venus in the Solar System -- with an orbital period around 171 days. On a planet orbiting stars B or C, Regulus A would be so look like an extremely bright star that's six times as luminous as a Full Moon on Earth. Useful catalogue numbers and designations for the star include: Alp or Alf Leo B, 32 Leo B, GJ 9316 B, HD 87884, BD+12 2147, SAO 98966, Wo 9316 B, LTT 12714, and ADS 7654 B.

Alpha Leonis C

NASA -- larger image

Regulus C is a dim red dwarf star, like
Gliese 623 A (M2.5 V) and B (M5.8Ve) at lower right.

Star "C" is a red sequence dwarf star of spectral and luminosity type M5 V. It may have a fifth of Sol's mass, substantially less than its diameter, and 0.31 percent of its luminosity. The orbit of an Earth-like planet (with liquid water) around Star B would be centered around 0.056 AU -- well inside the orbit of Mercury in the Solar System -- with an orbital period of 10.7 days. Useful catalogue numbers and designations for the star include: Alp or Alf Leo C, 32 Leo C, GJ 9316 C, Wo 9316 C, and ADS 7654 C.

Closest Neighbors

The following table includes all star systems known to be located within 10 light-years (ly), plus more bright stars within 10 to 20 ly, of Regulus.

Star System	Spectra & Luminosity	Distance (light-years)
Steph 852	M0 V	6.6
LTT 12663	M V	7.9
LTT 12680	DC7 /VII	8.2
Steph 823	M0 V	8.6
LTT 12694	M0 V	9.8
* plus bright stars *		. . .
BD+12 2700	G5 V	11
40 Leonis	F6 IV	13
39 Leonis AB	F8 Vbw M1 V	15
BD+16 2116 AB	G2-5 V ?	17

Other Information

Try Professor Jim Kaler's Stars site for other information about Regulus at the University of Illinois' Department of Astronomy.

Up-to-date technical summaries on this star can be found at: the Astronomiches Rechen-Institut at Heidelberg's ARICNS for Star A Star B, and Star C; and NASA's NStar Database. Additional information may be available at Roger Wilcox's Internet Stellar Database.

The Constellation Leo represents the Lion that Hercules (also a constellation) had to kill as one of his 12 tasks. For more information and an illustration of the constellation, go to Christine Kronberg's Leo. For another illustration, see David Haworth's Leo.

For more information about stars including spectral and luminosity class codes, go to ChView's webpage on The Stars of the Milky Way.