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 anasterism that marks the head of Leo.
α Leonis (Latinized toAlpha Leonis) is the star system'sBayer designation. The traditional nameRēgulus isLatin for 'prince' or 'little king'. In 2016, theInternational Astronomical Union organized 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 includedRegulus for this star. It is now so entered in the IAU Catalog of Star Names.[23]
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 star in the night sky with anapparent 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 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 oftenocculted by 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 byMercury andVenus are possible but rare, as are occultations byasteroids. Seven other stars which have aBayer designation are 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.
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 Rhodope was 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 Erigone in the early morning of March 20, 2014.[29] The center of the shadow path passed throughNew York andeastern 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'sLASCO C3 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,Venus passes 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 Regulusbefore the star's heliacal rising).[citation needed]
Regulus is the brightest star in the constellation of Leo (right tip, below is bright Jupiter in 2004).
Regulus is a multiplestar system consisting 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-whitesubgiant star 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 purelyKeplerian scenario 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's mass.[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 highlyoblate shape.[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 light because of this.[17]
Regulus BC is 5,000 AU[36] from Regulus A. A and BC share acommon proper motion and are thought to orbit each other[5] taking several million years. Designated Regulus B and Regulus C, the pair hasHenry Draper Catalogue number 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 dwarf named SDSS J1007+1930 (full name: SDSS J100711.74+193056.2) may be bound to the Regulus system, it share similarproper motion andradial velocity and has a similarmetal abundance to 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 period assuming 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 (0.06 M☉),[note 1] aneffective temperature of1,600 K and aspectral type L9 or T0, making it aL dwarf orT 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]
^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.
Rēgulus isLatin for 'prince' or 'little king';[38] itsGreek equivalent 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 theArabic قلب 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 軒轅十四, the Fourteenth Star of Xuanyuan, theYellow Emperor. InIndian astronomy, Regulus corresponds to theNakshatraMagha ("the bountiful").
^Assuming an age of one billion years. This mass is below thehydrogen burning limit and make SDSS J100711.74+193056.2substellar, unable to fuse hydrogen and become a dim star.
^abHøg, E.; Fabricius, C.; Makarov, V. V.; Urban, S.; Corbin, T.; Wycoff, G.; Bastian, U.; Schwekendiek, P.; Wicenec, A. (2000). "The Tycho-2 catalogue of the 2.5 million brightest stars".Astronomy and Astrophysics.355: L27.Bibcode:2000A&A...355L..27H.doi:10.1888/0333750888/2862.ISBN978-0333750889.
^abcdDucati, J. R. (2002). "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system".CDS/ADC Collection of Electronic Catalogues.2237: 0.Bibcode:2002yCat.2237....0D.
^Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007–2013)".VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S.1: 02025.Bibcode:2009yCat....102025S.
^abcdMcAlister, H. A.; ten Brummelaar, T. A.; Gies; Huang; Bagnuolo, Jr.; Shure; Sturmann; Sturmann; Turner; Taylor; Berger; Baines; Grundstrom; Ogden; Ridgway; Van Belle; et al. (2005). "First Results from the CHARA Array. I. An Interferometric and Spectroscopic Study of the Fast Rotator Alpha Leonis (Regulus)".The Astrophysical Journal.628 (1):439–452.arXiv:astro-ph/0501261.Bibcode:2005ApJ...628..439M.doi:10.1086/430730.S2CID6776360.
^abcdMartin, E. L.; Magazzu, A.; Rebolo, R. (1992). "On the post-T-Tauri nature of late-type visual companions to B-type stars".Astronomy and Astrophysics.257: 186.Bibcode:1992A&A...257..186M.
^Sigismondi, Costantino; Troise, Davide (2008). "Asteroidal Occultation of Regulus:. Differential Effect of Light Bending".THE ELEVENTH MARCEL GROSSMANN MEETING on Recent Developments in Theoretical and Experimental General Relativity:2594–2596.Bibcode:2008mgm..conf.2594S.doi:10.1142/9789812834300_0469.ISBN9789812834263.
^Sigismondi, C.; Flatres, T.; George, T.; Braga-Ribas, F. (2014). "Stellar limb darkening scan during 163 Erigone asteroidal occultation of Regulus on March 20, 2014 at 6:06 UT".The Astronomer's Telegram.5987: 1.Bibcode:2014ATel.5987....1S.
^"Regulus 2014". International Occultation Timing Association. Archived fromthe original on 2020-11-21. Retrieved2019-06-23.
^Lindroos, K. P. (1985). "A study of visual double stars with early type primaries. IV Astrophysical data".Astronomy and Astrophysics Supplement Series.60: 183.Bibcode:1985A&AS...60..183L.
^Rogers, J. H. (February 1998). "Origins of the ancient constellations: I. The Mesopotamian traditions".Journal of the British Astronomical Association.108 (1):9–28.Bibcode:1998JBAA..108....9R.
