Editing Regulus (section)

==Stellar system==
[[File:LeoCC.jpg|thumb|left|upright|Regulus is the brightest star in the constellation of Leo (right tip, below is bright Jupiter in 2004).]]

Regulus is a multiple [[star system]] consisting of at least four stars and a [[substellar 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",<ref name="wds" /> but is an unrelated background object.<ref name="dr2" />

Regulus&nbsp;A is a binary star consisting of a blue-white [[subgiant]] star of spectral type B8, which is orbited by a star of at least 0.3&nbsp;solar masses, which is probably a [[white dwarf]]. The two stars take approximately 40&nbsp;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, [[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.<ref name="rappaport2009" />

The primary of Regulus A has about 4.15 times the [[Sun|Sun's]] mass.<ref name=che/> 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<ref>{{Cite web |title=Sun Fact Sheet |url=https://nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.html |access-date=2023-12-15 |website=nssdc.gsfc.nasa.gov}}</ref>), which causes it to have a highly [[oblate spheroid|oblate]] shape.<ref name="apj628"/> 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.<ref name="rappaport2009" /> 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 emitting [[Polarization (waves)|polarized light]] because of this.<ref name="Cotton2017"/>

Regulus BC is 5,000&nbsp;[[astronomical unit|AU]]<ref name=lindroos/> from Regulus&nbsp;A. A and BC share a [[common proper motion]] and are thought to orbit each other<ref name="tokovinin" /> taking several million years. Designated Regulus&nbsp;B and Regulus&nbsp;C, the pair has [[Henry Draper Catalogue]] number HD 87884. The first is a K2V star, while the second is about M4V.<ref name="apj628" /> The companion pair has an orbital period of about 600 years<ref name="tokovinin" /> with a separation of 2.5" in 1942.<ref name="apj628" />

A far more distant [[brown dwarf]] named SDSS J1007+1930 (full name: SDSS J100711.74+193056.2) may be bound to the Regulus system, since it shares a similar [[proper motion]] and [[radial velocity]] and has a similar [[metallicity|metal abundance]] to Regulus B, which hints for a physical connection between both systems. The estimated distance from Regulus is {{Val|3.9|0.6|0.5|u=[[parsec]]s}} ({{Val|{{convert|3.866|pc|ly|disp=number|sigfig=3}}|{{convert|0.607|pc|ly|disp=number|sigfig=2}}|{{convert|0.473|pc|ly|disp=number|sigfig=2}}|ul=ly}}), and the [[orbital 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 roughly {{jupiter mass|60|link=y}} ({{Solar mass|0.06}}),{{Efn|Assuming an age of one billion years. This mass is below the [[hydrogen burning limit]] and make SDSS J100711.74+193056.2 [[substellar object|substellar]], unable to fuse hydrogen and become a dim star.|group=note}} an [[effective temperature]] of {{val|1600|ul=K|fmt=commas}} and a [[spectral type]] L9 or T0, making it a [[L dwarf]] or [[T 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.<ref name="Mamajek+Burgasser2024"/>

[[File:Regulus in true color.jpg|left|thumb|169x169px|Approximate true-color reconstruction of Regulus based on interferometric imaging.<ref name="che" />]]

{|class="wikitable" align=center
|+Regulus system
!colspan=3|
!Separation<br />([[minute and second of arc|arcsec]])
!Projected<br />separation<br />([[astronomical unit|AU]])
!Orbital<br />period
![[stellar classification|Spectral<br />type]]
!Mass<br />([[solar mass|M<sub>☉</sub>]])
![[apparent magnitude|App. mag.]]<br />(V)
|-
|align=center rowspan=4| Regulus ABC ||align=center rowspan=2 style="background-color: #EEEEEE"|Regulus A<ref group="orbit note" name="orbit">Regulus 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.</ref> ||align=center style="background-color: #EEEEEE"| Regulus Aa ||align=center rowspan=2 style="background-color: #EEEEEE"| 0.015 ||align=center rowspan=2 style="background-color: #EEEEEE"| 0.356 ||align=center rowspan=2 style="background-color: #EEEEEE"| 40.1 days ||align=center style="background: {{star-color|B}}; | B8 IVn ||align=center| 3.44 ||align=center rowspan=2| 1.4<br />(combined)
|-
|align=center style="background-color: #EEEEEE"| Regulus Ab ||align=center style="background: {{star-color|D}}; | pre-WD ||align=center| 0.31
|-
|align=center rowspan=2 style="background-color: #DDDDDD"| Regulus BC<ref group="orbit note" name="orbit"/> ||align=center style="background-color: #DDDDDD"| Regulus B ||align=center rowspan=2 style="background-color: #DDDDDD"| 2.1 ||align=center rowspan=2 style="background-color: #DDDDDD"| 60 ||align=center rowspan=2 style="background-color: #DDDDDD"| 600 years ||align=center style="background: {{star-color|K}}; | K2V ||align=center| 0.78 ||align=center| 8.1
|-
|align=center style="background-color: #DDDDDD"| Regulus C ||align=center style="background: {{star-color|M}}; | M4V ||align=center| 0.32 ||align=center| 13.5
|-
|align=center colspan=3| SDSS J1007+1930 ||align=center| 27,200 ||align=center| 800,000 ||align=center| 200 million<br />years ||align=center style="background: {{star-color|L}}; | L9 ||align=center| 0.06 ||align=center| 26
|}
{{reflist|group="orbit note"}}