Editing Binary star (section)

===Cataclysmic variables and X-ray binaries===
[[File:Accretion Disk Binary System.jpg|thumb|left|Artist's conception of a [[Cataclysmic variable star|cataclysmic variable system]]]]

When a binary system contains a [[compact star|compact object]] such as a [[white dwarf]], [[neutron star]] or [[stellar-mass black hole|black hole]], gas from the other (donor) star can [[accretion (astronomy)|accrete]] onto the compact object. This releases [[gravitational potential energy]], causing the gas to become hotter and emit radiation. [[Cataclysmic variable star]]s, where the compact object is a white dwarf, are examples of such systems.<ref>{{cite journal |author=Smith, Robert Connon |title=Cataclysmic Variables |journal=Contemporary Physics |date=November 2006 |volume=47 |issue=6 |pages=363–386 |bibcode=2007astro.ph..1654S |doi=10.1080/00107510601181175 |arxiv=astro-ph/0701654 |s2cid=2590482 |url=http://sro.sussex.ac.uk/2256/1/cp06_12-15_cvreview.pdf |type=Submitted manuscript |df=dmy-all}}</ref> In [[X-ray binaries]], the compact object can be either a [[neutron star]] or a [[stellar-mass black hole|black hole]]. These binaries are classified as [[low-mass X-ray binary|low-mass]] or [[high-mass X-ray binary|high-mass]] according to the mass of the donor star. High-mass X-ray binaries contain a young, [[Stellar classification|early-type]], high-mass donor star which transfers mass by its [[stellar wind]], while low-mass X-ray binaries are semidetached binaries in which gas from a [[Stellar classification|late-type]] donor star or a white dwarf overflows the Roche lobe and falls towards the neutron star or black hole.<ref>{{cite thesis |chapter-url=http://www.mporzio.astro.it/~gianluca/phdthesis/node11.html |chapter=Neutron Star X-ray binaries |archive-url=https://web.archive.org/web/20081210180216/http://www.mporzio.astro.it/~gianluca/phdthesis/node11.html |archive-date=2008-12-10 |df=dmy-all |title=A Systematic Search of New X-ray Pulsators in ROSAT Fields |first=Gian Luca |last=Israel |type=Ph.D. thesis |location=Trieste |date=October 1996}}</ref> Probably the best known example of an X-ray binary is the [[high-mass X-ray binary]] [[Cygnus X-1]]. In Cygnus&nbsp;X-1, the mass of the unseen companion is estimated to be about nine times that of the Sun,<ref>{{cite journal |last=Iorio |first=Lorenzo |year=2008 |journal=Astrophysics and Space Science |title=On the orbital and physical parameters of the HDE&nbsp;226868 / Cygnus&nbsp;X-1 binary system |bibcode=2008Ap&SS.315..335I |doi=10.1007/s10509-008-9839-y |volume=315 |issue=1–4 |pages=335–340 |arxiv=0707.3525|s2cid=7759638 }}</ref> far exceeding the [[Tolman–Oppenheimer–Volkoff limit]] for the maximum theoretical mass of a neutron star. It is therefore believed to be a black hole; it was the first object for which this was widely believed.<ref>{{cite web |url=http://imagine.gsfc.nasa.gov/docs/science/know_l2/black_holes.html |title=Black Holes |series=Imagine the Universe! |publisher=[[NASA]] |access-date=August 22, 2008 |df=dmy-all}}</ref>