Wikipedia

Regulus

This article is about the star. For other uses, seeRegulus (disambiguation).

Regulusis the brightest object in theconstellationLeoand one of thebrightest starsin thenight sky.It has theBayer designationdesignatedα Leonis,which isLatinizedtoAlpha Leonis,and abbreviatedAlpha Leoorα Leo.Regulus appears singular, but is actually a quadruplestar systemcomposed of four stars that are organized into two pairs. Thespectroscopic binaryRegulus A consists of a blue-whitemain-sequencestar and its companion, which has not yet been directly observed, but is probably awhite dwarf.The system lies approximately 79light yearsfrom theSolar System.

Regulus
Location of Regulus (circled)
Observation data
EpochJ2000EquinoxJ2000
Constellation Leo
Pronunciation UK:/ˈrɛɡˌjulʊs/US:/ˈrɛɡˌjʊlʊsˌ/[1]
Regulus A
Right ascension 10h08m22.311s[2]
Declination +11° 58′ 01.95″[2]
Apparent magnitude(V) 1.40[3]
Regulus BC
Right ascension 10h08m12.8s[4]
Declination +11° 59′ 49″[4]
Apparent magnitude(V) 8.13[5]/13.50[5]
Characteristics
Regulus A
Evolutionary stage Subgiant
Spectral type B8 IVn[3]
U−Bcolor index –0.36[6]
B−Vcolor index –0.11[6]
Variable type Suspected[7]
Regulus BC
Evolutionary stage Main sequence
Spectral type K2 V[8]+ M4 V[8]
U−Bcolor index +0.51[6]
B−Vcolor index +0.86[6]
Astrometry
A
Radial velocity(Rv)4.39±0.09[9]km/s
Proper motion(μ)RA:−248.73±0.35[2]mas/yr
Dec.:5.59±0.21[2]mas/yr
Parallax(π)41.13 ± 0.35mas[2]
Distance79.3 ± 0.7ly
(24.3 ± 0.2pc)
Absolute magnitude(MV)–0.57[10]
BC
Radial velocity(Rv)+6.72[11]km/s
Proper motion(μ)RA:−254.399±0.028[11]mas/yr
Dec.:8.127±0.027[11]mas/yr
Parallax(π)41.2745 ± 0.0270mas[11]
Distance79.02 ± 0.05ly
(24.23 ± 0.02pc)
Absolute magnitude(MV)6.20/11.56[12]
Orbit[9]
Primaryα Leo Aa (HD 87901 A)
Companionα Leo Ab (HD 87901 B)
Period(P)40.102±0.002 d
Semi-major axis(a)6.00±0.17R☉projected
Eccentricity(e)0 (assumed)
Semi-amplitude(K1)
(primary)		7.58±0.12km/s
Details
A
Mass4.15±0.06[13]M☉
Radius4.21+0.07
−0.06(equatorial),3.22+0.05
−0.06(polar)[13]R☉
Luminosity341+27
−28[13]L☉
Surface gravity(logg)3.54±0.09[14]cgs
Temperature11,010 (equatorial), 14,520 (polar)[13]K
Metallicity[Fe/H]+0.21[15]dex
Rotation15.9 hours[16]
Rotational velocity(vsini)318±8[17]km/s
Age≳1[18]Gyr
Ab
Mass0.31±0.10[9]M☉
Radius0.061±0.011[9]R☉
Temperature20,000±4,000[9]K
B
Mass0.8[19]M☉
Radius0.83[12]R☉
Luminosity0.50[19]L☉
Surface gravity(logg)4.4[19]cgs
Temperature4,885[19]K
Metallicity[Fe/H]−0.21[20]dex
C
Mass0.3[5]M☉
Radius0.37[12]R☉
Temperature3,242[12]K
Other designations
α Leonis,32 Leonis,GJ9316,HR3982,ADS7654,WDSJ10084+1158
α Leo A:BD+12°2149,FK5380,HD87901,HIP49669,SAO98967,LTT12716
α Leo B/C:BD+12°2147,HD87884,SAO98966,LTT12714
Database references
SIMBADRegulus
BC

HD 87884 is separated from Regulus by176″and is itself a close pair. Regulus, along with five slightly dimmer stars (Zeta Leonis,Mu Leonis,Gamma Leonis,Epsilon Leonis,andEta Leonis) have collectively been called 'the Sickle', which is anasterismthat marks the head of Leo.

Nomenclature

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α Leonis(Latinized toAlpha Leonis) is the star system'sBayer designation.The traditional nameRēgulusisLatinfor 'prince' or 'little king'. In 2016, theInternational Astronomical Unionorganized aWorking Group on Star Names(WGSN)[21]to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[22]included a table of the first two batches of names approved by the WGSN; which includedRegulusfor this star. It is now so entered in the IAU Catalog of Star Names.[23]

Observation

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Regulus through Celestron CGEM DX 1100 @ F6.3, Canon T3i, Televue 4X Powermate, ISO 800, 30 sec exposure

The Regulus system as a whole is the twenty-firstbrightest starin the night sky with anapparent magnitudeof +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 telescope 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.[24]

Regulus as viewed through a 110mm refractor in full daylight.

Regulus is 0.465 degrees from theecliptic,[25]the closest of the bright stars, and is oftenoccultedby theMoon.This occurs in spates every 9.3 years, due tolunar precession.The last spate was around 2017, with occultations every month from December 2016 till July 2017, each one limited to certain areas on Earth.[26]Occultations byMercuryandVenusare possible but rare, as are occultations byasteroids.Seven other stars which have aBayer designationare less than 0.9° from the ecliptic (perfected, mean plane of Earth's orbit and mean apparent path of the Sun) the next brightest of which isδ (Delta) Geminorum,of magnitude +3.53. As Regulus closely aligns to the mean orbits of large bodies of theSolar Systemand involves more light reaching the Earth than such other stars, the system has advanced telescopic use (to study and identify objects occulting and casting their shadow on a telescope, including known or unknown asteroids of the Solar System such asTrojans,being in line by definition with their associated planetary plane).

The last occultation of Regulus by a planet was on July 7, 1959, by Venus.[27]The next will occur on October 1, 2044, also by Venus. Other planets will not occult Regulus over the next few millennia because of theirnode positions.An occultation of Regulus by the asteroid166 Rhodopewas filmed in Italy on October 19, 2005. Differential bending of light was measured to be consistent withgeneral relativity.[28]Regulus was occulted by the asteroid163 Erigonein the early morning of March 20, 2014.[29]The center of the shadow path passed throughNew Yorkandeastern Ontario,but no one is known to have seen it, due to cloud cover. The International Occultation Timing Association recorded no observations at all.[30]

Although best seen in the evening in the northern hemisphere's late winter and spring, Regulus appears at some time of night throughout the year except for about a month (depending on ability to compensate for the sun's glare, ideally done so in twilight) on either side of August 22–24, when the Sun is too close.[31]The star can be viewed the whole night, crossing the sky, in late February. Regulus passes throughSOHO'sLASCOC3 every August.[32]

For Earth observers, theheliacal rising(pre-sunrise appearance) of Regulus occurs late in the first week of September, or in the second week. Every 8 years,Venuspasses very near the star system around or a few days before the heliacal rising, as on 5 September 2022 (the superior conjunction of Venus happens about two days earlier with each turn of its 8-year cycle, so as this cycle continues Venus will more definitely pass Regulusbeforethe star's heliacal rising).[citation needed]

Stellar system

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Regulus is the brightest star in the constellation of Leo (right tip, below is bright Jupiter in 2004).

Regulus is a multiplestar systemconsisting of at least four stars and asubstellar object.Regulus A is the dominant star, with a binary companion 177 "distant that is thought to be physically related. Regulus D is a 12th magnitude companion at 212",[33]but is an unrelated background object.[34]

Regulus A is a binary star consisting of a blue-whitesubgiantstar of spectral type B8, which is orbited by a star of at least 0.3 solar masses, which is probably awhite 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 purelyKeplerianscenario because of non-negligible long-term orbital perturbations affecting, for example, itsorbital period.In other words,Kepler's third law,which holds exactly only for two point-like masses, would no longer be valid for the Regulus system. Regulus A was long thought to be fairly young, only 50–100 million years old, calculated by comparing its temperature, luminosity, and mass. The existence of a white dwarf companion would mean that the system is at least 1 billion years old, just to account for the formation of the white dwarf. The discrepancy can be accounted for by a history of mass transfer onto a once-smaller Regulus A.[18]

The primary of Regulus A has about 4.15 times theSun'smass.[13]It is spinning extremely rapidly, with a rotation period of only 15.9 hours (for comparison, the rotation period of the Sun is 25 days[35]), which causes it to have a highlyoblateshape.[16]This results in so-calledgravity darkening:the photosphere at Regulus' poles is considerably hotter, and five times brighter per unit surface area, than its equatorial region.[18]The star's surface at the equator rotates at about 320 kilometres per second (199 miles per second), or 96.5% of its critical angular velocity for break-up. It is emittingpolarized lightbecause of this.[17]

Regulus BC is 5,000AU[36]from Regulus A. A and BC share acommon proper motionand are thought to orbit each other[5]taking several million years. Designated Regulus B and Regulus C, the pair hasHenry Draper Cataloguenumber HD 87884. The first is a K2V star, while the second is about M4V.[16]The companion pair has an orbital period of about 600 years[5]with a separation of 2.5 "in 1942.[16]

A far more distantbrown dwarfnamed SDSS J1007+1930 (full name: SDSS J100711.74+193056.2) may be bound to the Regulus system, it share similarproper motionandradial velocityand has a similarmetal abundanceto Regulus B, which hints for a physical connection between both systems. The estimated distance from Regulus is3.9+0.6
−0.5parsecs(12.6+2.0
−1.5ly), and theorbital periodassuming an circular orbit would be around 200 million years, comparable to the Sun's orbital period around the Milky Way (galactic year). It is estimated to have a mass of roughly60MJ(0.06M☉),[note 1]aneffective temperatureof1,600Kand aspectral typeL9 or T0, making it aL dwarforT dwarf.In the future it will either be stripped away by stellar encounters because it is so weakly bound to the system, or it was once closer and got ejected by dynamical interactions. The extreme distance makes it uncertain to conclude whether it is gravitationally bound to Regulus.[37]

Approximate true-color reconstruction of Regulus based on interferometric imaging.[13]
Regulus system
Separation
(arcsec) 		Projected
separation
(AU) 		Orbital
period 		Spectral
type 		Mass
(M☉) 		App. mag.
(V)
Regulus ABC Regulus A[orbit note 1] Regulus Aa 0.015 0.356 40.1 days B8 IVn 3.44 1.4
(combined)
Regulus Ab WD? 0.31
Regulus BC[orbit note 1] Regulus B 2.1 60 600 years K2V 0.78 8.1
Regulus C M4V 0.32 13.5
SDSS J1007+1930 27,200 800,000 200 million
years		 L9 0.06 26
  1. ^abRegulus A and B are separated by 180 arcseconds, resulting in a projected separation of 4400 AU/0.07 light years. The combined binary system may have an approximate orbital period of 130,000 years.

Etymology and cultural associations

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RēgulusisLatinfor 'prince' or 'little king';[38]itsGreekequivalent is Basiliskos or, in Latinised form, Basiliscus.[39][40][41]The name Regulus first appeared in the early 16th century.[41]It is also known as Qalb al-Asad, from theArabicقلب الأسد, meaning 'the heart of the lion', a name already attested in the Greek Kardia Leontos[39][42]whose Latin equivalent is Cor Leōnis. The Arabic phrase is sometimes approximated as Kabelaced.[citation needed]In Chinese it is known as hiên viên thập tứ, the Fourteenth Star of Xuanyuan, theYellow Emperor.InIndian astronomy,Regulus corresponds to theNakshatraMagha( "the bountiful" ).

Babylonianscalled itSharru( "the King" ), and it marked the 15th ecliptic constellation. InIndiait was known asMaghā( "the Mighty" ), inSogdianaMagh( "the Great" ), inPersiaMiyan( "the Centre" ) and also as one of the four 'royal stars' of the Persian monarchy.[43]It was one of the fifteenBehenian starsknown tomedievalastrologers,associated withgranite,mugwort,and thekabbalisticsymbol.

In the BabylonianMUL.APIN,Regulus is listed asLugal,meaning king, with co-descriptor, "star of the Lion's breast".[44]

See also

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Notes

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  1. ^Assuming an age of one billion years. This mass is below thehydrogen burning limitand make SDSS J100711.74+193056.2substellar,unable to fuse hydrogen and become a dim star.

References

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