Editing Binary star (section)

===={{Anchor|Eclipsing binary}}Eclipsing binaries====<!-- This section is linked from [[Eclipsing binary]] -->
{{redirect|Eclipsing binaries|the novel|E. E. Smith bibliography#Family D'Alembert}}<!-- Redirect necessary as link arrives here -->
An ''eclipsing binary star'' is a binary star system in which the orbital plane of the two stars lies so nearly in the line of sight of the observer that the components undergo mutual [[eclipse]]s.<ref name=B-EBS>{{cite web |url=http://www.physics.sfasu.edu/astro/ebstar/ebstar.html |first=D. |last=Bruton |title=Eclipsing Binary Stars |publisher=Stephen F. Austin State University |url-status=dead |archive-url=https://web.archive.org/web/20070414144827/http://www.physics.sfasu.edu/astro/ebstar/ebstar.html |archive-date=2007-04-14 |df=dmy-all}}</ref> In the case where the binary is also a spectroscopic binary and the [[parallax]] of the system is known, the binary is quite valuable for stellar analysis. [[Algol]], a triple star system in the [[Perseus (constellation)|constellation Perseus]], contains the best-known example of an eclipsing binary.

[[File:Artist’s impression of eclipsing binary.ogv|upright=1.2|thumb|This video shows an artist's impression of an eclipsing binary star system. As the two stars orbit each other they pass in front of one another and their combined brightness, seen from a distance, decreases.]]
Eclipsing binaries are variable stars, not because the light of the individual components vary but because of the eclipses. The [[light curve]] of an eclipsing binary is characterized by periods of practically constant light, with periodic drops in intensity when one star passes in front of the other. The brightness may drop twice during the orbit, once when the secondary passes in front of the primary and once when the primary passes in front of the secondary. The deeper of the two eclipses is called the primary regardless of which star is being occulted, and if a shallow second eclipse also occurs it is called the secondary eclipse. The size of the brightness drops depends on the relative brightness of the two stars, the proportion of the occulted star that is hidden, and the [[surface brightness]] (i.e. [[effective temperature]]) of the stars. Typically the occultation of the hotter star causes the primary eclipse.<ref name=B-EBS/>

An eclipsing binary's period of orbit may be determined from a study of its [[light curve]], and the relative sizes of the individual stars can be determined in terms of the radius of the orbit, by observing how quickly the brightness changes as the disc of the nearest star slides over the disc of the other star.<ref name=B-EBS/> If it is also a spectroscopic binary, the [[orbital elements]] can also be determined, and the mass of the stars can be determined relatively easily, which means that the relative densities of the stars can be determined in this case.<ref>{{cite web |url=http://www.physics.sfasu.edu/markworth/ast105/Binary-Stars.ppt |archive-url= https://web.archive.org/web/20030903072148/http://www.physics.sfasu.edu/markworth/ast105/Binary-Stars.ppt |url-status=dead |archive-date=2003-09-03 |format=[[Microsoft PowerPoint|PowerPoint]] |title=Binary Stars |first=M |last=Worth |publisher=Stephen F. Austin State University |df=dmy-all}}</ref>

Since about 1995, measurement of extragalactic eclipsing binaries' fundamental parameters has become possible with 8-meter class telescopes. This makes it feasible to use them to directly measure the distances to external galaxies, a process that is more accurate than using [[standard candle]]s.<ref name="wilson2008">{{cite journal |journal=The Astrophysical Journal |volume=672 |issue=1 |title=Eclipsing binary solutions in physical units and direct distance estimation |bibcode=2008ApJ...672..575W |doi=10.1086/523634 |author=Wilson, R.E. |date=1 January 2008 |pages=575–589 <!-- |access-date=4 July 2013 --> |df=dmy-all|doi-access=free }}</ref> By 2006, they had been used to give direct distance estimates to the [[Large Magellanic Cloud|LMC]], [[Small Magellanic Cloud|SMC]], [[Andromeda Galaxy]], and [[Triangulum Galaxy]]. Eclipsing binaries offer a direct method to gauge the distance to galaxies to an improved 5% level of accuracy.<ref name="Bonanos2006">{{cite journal |author=Bonanos, Alceste Z. |title=Eclipsing binaries: Tools for calibrating the extragalactic distance scale |year=2006 |journal=Proceedings of the International Astronomical Union |volume=2 |pages=79–87 |arxiv=astro-ph/0610923 |citeseerx=10.1.1.254.2692 |doi=10.1017/S1743921307003845 |bibcode=2007IAUS..240...79B|s2cid=18827791 }}</ref>