Editing Sirius (section)

== Stellar system ==
[[File:Orbit Sirius B arcsec.png|thumb|upright=1.2|The orbit of Sirius&nbsp;B around A, as seen from Earth (slanted ellipse). The wide horizontal ellipse shows the true shape of the orbit (with an arbitrary orientation) as it would appear if viewed straight on.]]
[[File:Sirius A & B X-ray.jpg|thumb|right|A [[Chandra X-ray Observatory]] image of the Sirius star system, where the spike-like pattern is due to the support structure for the transmission grating. The bright source is Sirius&nbsp;B. Credit: NASA/SAO/CXC]]

Sirius is a [[binary star]] system consisting of two white stars orbiting each other with a separation of about 20&nbsp;AU{{efn|
Semi-major axis in AU {{=}} {{sfrac|semimajor axis in seconds| parallax}} {{=}} {{sfrac|7.56″|0.37921}} {{=}} 19.8&nbsp;AU; as the eccentricity is 0.6, the distance fluctuates between 40% and 160% of that, roughly from 8 AU to 32 AU.
}}
(roughly the distance between the Sun and [[Uranus]]) and a period of 50.1&nbsp;years. The brighter component, termed Sirius&nbsp;A, is a [[main sequence|main-sequence]] star of [[A-type main-sequence star|spectral type early&nbsp;A]], with an estimated surface temperature of 9,940&nbsp;[[Kelvin|K]].<ref name="Adelman2004"/> Its companion, Sirius&nbsp;B, is a star that has already evolved off the main sequence and become a white dwarf. Currently 10,000&nbsp;times less luminous in the visual spectrum, Sirius&nbsp;B was once the more massive of the two.<ref name="Holberg2007-214"/> The age of the system has been estimated at 230&nbsp;million years. Early in its life, it is thought to have been two bluish-white stars orbiting each other in an elliptical orbit every 9.1&nbsp;years.<ref name="Holberg2007-214"/> The system emits a [[infrared excess|higher than expected level of infrared radiation]], as measured by [[IRAS]] space-based observatory. This might be an indication of dust in the system, which is considered somewhat unusual for a binary star.<ref name="SolStation"/><ref name="Backman1986"/> The [[Chandra X-ray Observatory]] image shows Sirius&nbsp;B outshining its partner as an X-ray source.<ref name="Brosch2008-126"/>

In 2015, Vigan and colleagues used the [[VLT Survey Telescope]] to search for evidence of substellar companions, and were able to rule out the presence of giant planets 11&nbsp;times more massive than Jupiter at 0.5&nbsp;AU distance from Sirius&nbsp;A, 6–7&nbsp;times the mass of Jupiter at 1–2&nbsp;AU distance, and down to around 4&nbsp;times the mass of Jupiter at 10&nbsp;AU distance.<ref>{{cite journal | last1= Vigan | first1= A. | last2= Gry | first2= C. | last3= Salter | first3= G. | last4= Mesa | first4= D. | last5= Homeier | first5= D. | last6= Moutou | first6= C. | last7= Allard | first7= F. |year= 2015 |title= High-contrast imaging of Sirius&nbsp;A with VLT/SPHERE: looking for giant planets down to one astronomical unit | journal= Monthly Notices of the Royal Astronomical Society |volume=454 |issue=1 |pages=129–143 |doi=10.1093/mnras/stv1928 | doi-access= free |bibcode= 2015MNRAS.454..129V |arxiv = 1509.00015 | s2cid= 119260068 }}</ref> Similarly, Lucas and colleagues did not detect any companions around Sirius&nbsp;B.<ref>{{cite journal | last1=Lucas | first1=Miles | last2=Bottom | first2=Michael | last3=Ruane | first3=Garreth | last4=Ragland | first4=Sam | year=2022 | title=An imaging search for post-main-sequence planets of Sirius&nbsp;B | journal=The Astronomical Journal | volume=163 | issue=2 | page=81 | arxiv=2112.05234 | bibcode=2022AJ....163...81L | doi=10.3847/1538-3881/ac4032 | s2cid=245117921 | doi-access=free }}</ref>

=== Sirius A ===
[[File:Relative sizes of the Alpha Centauri components and other objects (artist’s impression).tif|thumb|Relative sizes of [[List of nearest stars and brown dwarfs|local stars]], incl. Sirius, the Sun and Jupiter (artist’s impression)]]
[[File:Sirius A-Sun comparison2.png|thumb|Comparison of Sirius&nbsp;A and the Sun, to scale and relative surface brightness]]

Sirius&nbsp;A, also known as the Dog Star, has a mass of {{Solar mass|2.063}}.<ref name=bond/><ref name="Liebert2005"/><ref name="Braganca2003"/> The radius of this star has been measured by an [[astronomical interferometer]], giving an estimated angular diameter of 5.936±0.016&nbsp;[[milliarcsecond|mas]]. The [[stellar rotation|projected rotational velocity]] is a relatively low 16&nbsp;km/s,<ref name="Royer2002"/> which does not produce any significant flattening of its disk.<ref name="Kervella2003"/> This is at marked variance with the similar-sized [[Vega]], which rotates at a much faster 274&nbsp;km/s and bulges prominently around its equator.<ref name="Aufdenberg2006"/> A weak [[magnetic field]] has been detected on the surface of Sirius&nbsp;A.<ref name=petit2011/>

Stellar models suggest that the star formed during the collapsing of a [[molecular cloud]] and that, after 10&nbsp;million years, its internal energy generation was derived entirely from nuclear reactions. The core became [[convection zone|convective]] and used the [[CNO cycle]] for energy generation.<ref name="Kervella2003"/> It is calculated that Sirius&nbsp;A will have completely exhausted the store of hydrogen at its core within a billion ({{10^|9}})&nbsp;years of its formation, and will then evolve away from the main sequence.<ref>{{cite AV media |title=Stellar mass and lifetime on the main sequence |medium=diagram |website=NASA's cosmos |url=https://ase.tufts.edu/cosmos/view_picture.asp?id=1406 |access-date=8 February 2021 }}</ref> It will pass through a [[red giant]] stage and eventually become a white dwarf.{{sfn|Brosch|2008|p=198}}

Sirius&nbsp;A is classed as a type {{nobr|[[Am star]],}} because the spectrum shows deep metallic [[absorption line]]s,<ref name=auriere2010/> indicating an enhancement of its surface layers in elements heavier than helium, such as iron.<ref name="SolStation"/><ref name="Kervella2003"/> The spectral type has been reported as {{nobr|A0mA1 Va,}} which indicates that it would be classified as A1 from hydrogen and helium lines, but A0 from the metallic lines that cause it to be grouped with the Am&nbsp;stars.<ref name=nstars/> When compared to the Sun, the proportion of iron in the atmosphere of Sirius&nbsp;A relative to hydrogen is given by <math chem>\textstyle\ \left[\frac{\ce{Fe}}{\ce{H}}\right] = 0.5\ ,</math><ref name="Qiu2001"/> meaning iron is 316% as abundant as in the Sun's atmosphere. The high surface content of metallic elements is unlikely to be true of the entire star; rather the iron-peak and heavy metals are radiatively levitated towards the surface.<ref name="Kervella2003"/>

=== Sirius B ===
{{Other uses|Sirius B (disambiguation)}}
[[File:Sirius B-Earth comparison2.png|thumb|Size comparison of Sirius B and Earth]]

Sirius&nbsp;B (sometimes called "the Pup"<ref>{{Cite web|url=https://astrobites.org/2018/10/02/measuring_redshift_sirius_b/|title=Gravitational Redshift and the Pup: Measuring the Mass of Sirius B|first=Daniel|last=Berke|date=October 2, 2018|website=astrobites.org}}</ref>) is one of the most massive [[white dwarf]]s known. With a mass of {{Solar mass|1.02}}, it is almost double the {{Solar mass|0.5–0.6}} average. This mass is packed into a volume roughly equal to the Earth's.<ref name="Barstow2005"/> The current surface temperature is 25,200&nbsp;K.<ref name="Liebert2005"/> Because there is no internal heat source, Sirius&nbsp;B will steadily cool as the remaining heat is radiated into space over the next two billion years or so.<ref name="Imamura1995"/>

A white dwarf forms after a star has evolved from the main sequence and then passed through a [[red giant]] stage. This occurred when Sirius B was less than half its current age, around 120&nbsp;million years ago. The original star had an estimated {{Solar mass|5}}<ref name="Liebert2005"/> and was a [[B-type star]] (most likely B5V for {{Solar mass|5}})<ref name="Siess2000"/><ref name="Palla2005"/> when it was still on the main sequence, potentially burning around 600–1200 times more luminous than the sun. While it passed through the red giant stage, Sirius&nbsp;B may have enriched the [[metallicity]] of its companion, explaining the very high metallicity of Sirius&nbsp;A.

This star is primarily composed of a carbon–oxygen mixture that was generated by helium fusion in the progenitor star.<ref name="Liebert2005"/> This is overlaid by an envelope of lighter elements, with the materials segregated by mass because of the high surface gravity.<ref name="Koester1990"/> The outer atmosphere of Sirius&nbsp;B is now almost pure hydrogen—the element with the lowest mass—and no other elements are seen in its spectrum.<ref name="Holberg2004"/>

Although Sirius A and B compose a binary system that is reminiscent of those that can undergo [[Type Ia supernova]], the two stars are believed to be too far apart for it to occur, even if Sirius A swells into a [[red giant]]. [[Nova|Novas]], however, may be possible.<ref>{{Cite web |last1=Ahmad |first1=Pervaiz |last2=Weis |first2=Kerstin |date=January 2021 |title=Could the star Sirius B undergo a nova? |url=https://www.researchgate.net/post/Could_The_Star_Sirius_B_Undergo_A_Nova |access-date=23 July 2024 |website=[[ResearchGate]]}}</ref>{{Better source needed|reason=Citation is not based on actual research, just researcher's response to questions|date=July 2024}}

=== Apparent third star ===
{{redirect|Sirius C|the mythological star|Emme Ya}}
Since 1894, irregularities have been tentatively observed in the orbits of Sirius&nbsp;A and B with an apparent periodicity of 6–6.4&nbsp;years. A 1995 study concluded that such a companion likely exists, with a mass of roughly 0.05&nbsp;solar mass—a small [[red dwarf]] or large [[brown dwarf]], with an apparent magnitude of more than 15, and less than 3 arcseconds from Sirius A.<ref name="Benest1995"/>

In 2017, more accurate astrometric observations by the Hubble Space Telescope ruled out the existence of a stellar mass sized Sirius&nbsp;C, while still allowing a substellar mass candidate such as a lower mass [[Brown dwarf]]. The 1995 study predicted an astrometric movement of roughly 90&nbsp;[[Milliarcsecond|mas]] (0.09&nbsp;arcsecond), but Hubble was unable to detect any location anomaly to an accuracy of 5&nbsp;mas (0.005&nbsp;arcsec). This ruled out any objects orbiting Sirius&nbsp;A with more than 0.033&nbsp;solar mass (35&nbsp;Jupiter masses) in 0.5&nbsp;years, and 0.014 (15 Jupiter masses) in 2&nbsp;years. The study was also able to rule out any companions to Sirius&nbsp;B with more than 0.024&nbsp;solar mass (25&nbsp;Jupiter masses) orbiting in 0.5&nbsp;year, and 0.0095 (10&nbsp;Jupiter masses) orbiting in 1.8&nbsp;years. Effectively, there are almost certainly no additional bodies in the Sirius system larger than a small brown dwarf or large exoplanet.<ref name="siriusCruledout">{{cite news |last1=Andrew |first1=le&nbsp;Page |date=6 April 2017 |title=New Hubble observations of the Sirius system |website=drewexmachina.com |url=https://www.drewexmachina.com/2017/04/06/new-hubble-observations-of-the-sirius-system/ |access-date=21 March 2018}}</ref><ref name=bond/>

=== {{Anchor|Sirius supercluster}}Star cluster membership ===
In 1909, [[Ejnar Hertzsprung]] was the first to suggest that Sirius was a member of the [[Ursa Major Moving Group]], based on his observations of the system's movements across the sky. The Ursa Major Group is a set of 220&nbsp;stars that share a common motion through space. It was once a member of an [[open cluster]], but has since become gravitationally unbound from the cluster.<ref name="Frommert2003"/> Analyses in 2003 and 2005 found Sirius's membership in the group to be questionable: the Ursa Major Group has an estimated age of 500 ± 100&nbsp;million years, whereas Sirius, with metallicity similar to the Sun's, has an age that is only half this, making it too young to belong to the group.<ref name="Liebert2005"/><ref name="King2003"/><ref name="Croswell2005"/> Sirius may instead be a member of the proposed Sirius Supercluster, along with other scattered stars such as [[Beta Aurigae]], [[Alpha Coronae Borealis]], [[Beta Crateris]], [[Beta Eridani]] and [[Beta Serpentis]].<ref name="Eggen1992"/> This would be one of three large clusters located within {{convert|500|ly|pc}} of the Sun. The other two are the [[Hyades (star cluster)|Hyades]] and the [[Pleiades]], and each of these clusters consists of hundreds of stars.<ref name="Olano2001"/>

=== Distant star cluster ===
{{Main|Gaia 1}}

In 2017, a massive star cluster was discovered only 10&nbsp;[[Minute and second of arc|arcminutes]] from Sirius, making the two appear to be [[Angular distance|visually close]] to one other when viewed from the point of view of the [[Earth]]. It was discovered during a statistical analysis of [[Gaia (spacecraft)|''Gaia'']] data. The cluster is over a thousand times further away from us than the star system, but given its size it still appears at magnitude 8.3.<ref>{{cite journal |last1=Koposov |first1=Sergey E. |last2=Belokurov |first2=V. |last3=Torrealba |first3=G. |year=2017 |title=Gaia&nbsp;1 and 2. A pair of new galactic star clusters |journal=Monthly Notices of the Royal Astronomical Society |volume=470 |issue=3 |pages=2702–2709 |doi=10.1093/mnras/stx1182|doi-access=free |s2cid=119095351 |bibcode=2017MNRAS.470.2702K |arxiv=1702.01122}}</ref>