What is Binary star?
[[File:Sirius A and B Hubble photo.jpg|thumb|[[Hubble Space Telescope|Hubble]] image of the [[Sirius]] binary system, in which [[Sirius#Sirius B|Sirius B]]<nowiki/> can be seen at lower left]] [[File:Albireo.jpg|thumb|The two visibly distinguishable components of [[Albireo]].]] [[File:Animation binaire eclipses 2.gif|thumb|right|Animation of binary eclipsing stars]] [[File:Algol AB movie imaged with the CHARA interferometer - labeled.gif|thumb|175px|right|Algol B orbits Algol A. This animation was assembled from 55 images of the CHARA [[interferometer]] in the near-[[infrared]] H-band]] A '''binary star''' is two [[star]]s which orbit around each other. For each star, the other is its ''companion star''. Many stars are part of a system with two or more stars. The brighter star is called the ''primary star'', and the other is the ''secondary star''. Binary stars are important in [[astrophysics]] because looking at their orbits allows [[scientist]]s to find out their [[mass]]es. From this, scientists found the mass&ndash;[[luminosity]] relationship, and used it to find the masses of single stars. Binary stars are not the same as line-of-sight ''optical double stars'', which look close together but are not connected by [[gravity]]. Optical double stars may actually be far apart in space, but binary stars are quite close together. The first person to discover and prove true binary stars was the Anglo-German astronomer [[William Herschel]]. He published the first catalogue of binary stars, and his son [[John Herschel]] found several thousand more and updated the catalogue.<ref>Buttmann, Gunther 1974. ''In the shadow of the telescope: a biography of John Herschel''. Lutterworth, Guilford. p50 & 197</ref> [[John Michell]] first suggested that double stars might be physically attached to each other when he argued in 1767 that the probability that a double star was due to a chance alignment was small. <ref>pp. 10–11, ''Observing and Measuring Double Stars'', Bob Argyle, ed., London: Springer, 2004, {{ISBN|1-85233-558-0