Editing Binary star (section)

==Evolution==
[[File:Artist's impression of the evolution of a hot high-mass binary star.ogv|thumb|upright=1.2|Artist's impression of the evolution of a hot high-mass binary star]]

===Formation===
While it is not impossible that some binaries might be created through [[gravitational capture]] between two single stars, given the very low likelihood of such an event (three objects being actually required, as [[conservation of energy]] rules out a single gravitating body capturing another) and the high number of binaries currently in existence, this cannot be the primary formation process. The observation of binaries consisting of stars not yet on the [[main sequence]] supports the theory that binaries develop during [[star formation]]. Fragmentation of the [[molecular cloud]] during the formation of [[protostar]]s is an acceptable explanation for the formation of a binary or multiple star system.<ref>{{cite book | first = A. P. | last = Boss | chapter = Formation of Binary Stars | title = The Realm of Interacting Binary Stars | editor = J. Sahade | editor2 = G. E. McCluskey | editor3 = Yoji Kondo | date = 1992 | page = 355 | isbn = 978-0-7923-1675-6 | publisher = Kluwer Academic | location = Dordrecht}}</ref><ref>{{cite web | url = http://www.phys.lsu.edu/astro/nap98/bf.final.html | title = The Formation of Common-Envelope, Pre-Main-Sequence Binary Stars | first = J. E. | last = Tohline | author-link = Joel E. Tohline | author2 = J. E. Cazes | author3 = H. S. Cohl | publisher = Louisiana State University | access-date = 2006-06-25 | archive-date = 2016-06-04 | archive-url = https://web.archive.org/web/20160604021623/http://www.phys.lsu.edu/astro/nap98/bf.final.html | url-status = dead }}</ref>

The outcome of the [[three-body problem]], in which the three stars are of comparable mass, is that eventually one of the three stars will be ejected from the system and, assuming no significant further perturbations, the remaining two will form a stable binary system.

===Mass transfer and accretion===
As a [[Main sequence|main-sequence star]] increases in size during its [[stellar evolution|evolution]], it may at some point exceed its [[Roche lobe]], meaning that some of its matter ventures into a region where the [[Gravitation|gravitational pull]] of its companion star is larger than its own.<ref>{{cite book | first = Z. | last = Kopal | title = The Roche Problem | publisher = Kluwer Academic | date = 1989 | isbn = 978-0-7923-0129-5}}</ref> The result is that matter will transfer from one star to another through a process known as Roche lobe overflow (RLOF), either being absorbed by direct impact or through an [[accretion disc]]. The mathematical point through which this transfer happens is called the first [[Lagrangian point]].<ref>"[http://demonstrations.wolfram.com/ContactBinaryStarEnvelopes/ Contact Binary Star Envelopes]" by Jeff Bryant, [[Wolfram Demonstrations Project]].</ref> It is not uncommon that the accretion disc is the brightest (and thus sometimes the only visible) element of a binary star.

If a star grows outside of its Roche lobe too fast for all abundant matter to be transferred to the other component, it is also possible that matter will leave the system through other Lagrange points or as [[stellar wind]], thus being effectively lost to both components.<ref>"[http://demonstrations.wolfram.com/MassTransferInBinaryStarSystems/ Mass Transfer in Binary Star Systems]" by Jeff Bryant with Waylena McCully, [[Wolfram Demonstrations Project]].</ref>
Since the evolution of a star is determined by its mass, the process influences the evolution of both companions, and creates stages that cannot be attained by single stars.<ref>{{cite journal | first = C.B. | last = Boyle | title = Mass transfer and accretion in close binaries – A review | journal = Vistas in Astronomy | date = 1984 | volume = 27 | issue = 2 | pages = 149–169 | doi = 10.1016/0083-6656(84)90007-2|bibcode = 1984VA.....27..149B }}</ref><ref>{{cite book | first = D. | last = Vanbeveren |author2=W. van Rensbergen|author3=C. de Loore | title = The Brightest Binaries | publisher = Springer | date = 2001 | isbn = 978-0-7923-5155-9}}</ref><ref>{{cite journal | first = Z | last = Chen |author2=A. Frank|author3=E. G. Blackman|author4=J. Nordhaus|author5=J. Carroll-Nellenback| title = Mass Transfer and Disc Formation in AGB Binary Systems| journal = Monthly Notices of the Royal Astronomical Society | date = 2017 | doi = 10.1093/mnras/stx680 | volume=468 | issue = 4 | pages=4465–4477| doi-access = free |arxiv = 1702.06160 |bibcode = 2017MNRAS.468.4465C | s2cid = 119073723 }}</ref>

Studies of the eclipsing ternary [[Algol]] led to the ''[[Algol paradox]]'' in the theory of [[stellar evolution]]: although components of a binary star form at the same time, and massive stars evolve much faster than the less massive ones, it was observed that the more massive component Algol A is still in the [[main sequence]], while the less massive Algol B is a [[subgiant]] at a later evolutionary stage. The paradox can be solved by [[mass transfer]]: when the more massive star became a subgiant, it filled its [[Roche lobe]], and most of the mass was transferred to the other star, which is still in the main sequence. In some binaries similar to Algol, a gas flow can actually be seen.<ref>{{cite web | url = http://www.haydenplanetarium.org/hp/vo/ava/avapages/S1200algolbpi.html | title = Mass Transfer in the Binary Star Algol | first = J. M. | last = Blondin | author2 = M. T. Richards | author3 = M. L. Malinowski | publisher = American Museum of Natural History | url-status = dead | archive-url = https://web.archive.org/web/20060408135501/http://haydenplanetarium.org/hp/vo/ava/avapages/S1200algolbpi.html | archive-date = 2006-04-08 }}</ref>

===Runaways and novae===
[[File:Artist's illustration of scenario for plasma ejections from V Hydrae.jpg|thumb|Artist rendering of [[coronal mass ejection|plasma ejection]]s from [[V Hydrae]]]]
It is also possible for widely separated binaries to lose gravitational contact with each other during their lifetime, as a result of external perturbations. The components will then move on to evolve as single stars. A close encounter between two binary systems can also result in the gravitational disruption of both systems, with some of the stars being ejected at high velocities, leading to [[runaway star]]s.<ref>{{cite journal | first1 = R. | last1 = Hoogerwerf | first2 = J.H.J. | last2 = de Bruijne | first3 = P.T. | last3 = de Zeeuw | title = The Origin of Runaway Stars | journal = Astrophysical Journal | date = December 2000 | volume = 544 | issue = 2 | pages = L133 | doi = 10.1086/317315 | bibcode=2000ApJ...544L.133H |arxiv = astro-ph/0007436 | s2cid = 6725343 }}</ref>

If a [[white dwarf]] has a close companion star that overflows its [[Roche lobe]], the white dwarf will steadily [[accretion (astrophysics)|accrete]] gases from the star's outer atmosphere. These are compacted on the white dwarf's surface by its intense gravity, compressed and heated to very high temperatures as additional material is drawn in. The white dwarf consists of [[degenerate matter]] and so is largely unresponsive to heat, while the accreted hydrogen is not. [[Nuclear fusion|Hydrogen fusion]] can occur in a stable manner on the surface through the [[CNO cycle]], causing the enormous amount of energy liberated by this process to blow the remaining gases away from the white dwarf's surface. The result is an extremely bright outburst of light, known as a [[nova]].<ref>{{cite book | first = D. | last = Prialnik | chapter = Novae | title = Encyclopaedia of Astronomy and Astrophysics | date = 2001 | pages = 1846–1856}}</ref>

In extreme cases this event can cause the white dwarf to exceed the [[Chandrasekhar limit]] and trigger a [[supernova]] that destroys the entire star, another possible cause for runaways.<ref>{{cite book | first = I. | last = Icko | chapter = Binary Star Evolution and Type I Supernovae | title = Cosmogonical Processes | date = 1986 | page = 155}}</ref><ref>{{Cite book | title = Relativistic Flows in Astrophysics | first = R. | last = Fender| chapter = Relativistic Outflows from X-ray Binaries ('Microquasars') |bibcode = 2002LNP...589..101F | date = 2002 | volume = 589 | issue = 101 | arxiv = astro-ph/0109502 | pages = 101–122 | doi = 10.1007/3-540-46025-X_6 | series = Lecture Notes in Physics | isbn = 978-3-540-43518-1 }}</ref> An example of such an event is the supernova [[SN 1572]], which was observed by [[Tycho Brahe]]. The [[Hubble Space Telescope]] recently{{when|date=September 2023}} took a picture of the remnants of this event.