Editing Binary star (section)

==Etymology==
The term ''binary'' was first used in this context by Sir [[William Herschel]] in 1802,<ref name=aitkenix /> when he wrote:<ref>{{cite journal | date = 1802 | title = Catalogue of 500 New Nebulae, Nebulous Stars, Planetary Nebulae, and Clusters of Stars; With Remarks on the Construction of the Heavens| first= William |last=Herschel | jstor=107131| url =https://zenodo.org/record/1432308 | journal = Philosophical Transactions of the Royal Society of London | volume = 92 | pages = 477–528 [481] |bibcode = 1802RSPT...92..477H | doi = 10.1098/rstl.1802.0021 | doi-access = free }}</ref>

{{blockquote|If, on the contrary, two stars should really be situated very near each other, and at the same time so far insulated as not to be materially affected by the attractions of neighbouring stars, they will then compose a separate system, and remain united by the bond of their own mutual gravitation towards each other. This should be called a real double star; and any two stars that are thus mutually connected, form the binary sidereal system which we are now to consider.}}

By the modern definition, the term ''binary star'' is generally restricted to pairs of stars which revolve around a common center of mass. Binary stars which can be [[optical resolution|resolved]] with a telescope or [[interferometry|interferometric]] methods are known as ''visual binaries''.<ref name=Heintz12>{{cite book | last=Heintz | first=W. D. | date=1978 | pages=[https://archive.org/details/DoubleStars/page/1 1–2] | title=Double Stars | publisher=D. Reidel Publishing Company | location=Dordrecht | isbn=978-90-277-0885-4 | url=https://archive.org/details/DoubleStars/page/1 }}</ref><ref name = "csep10">{{cite web | url = http://csep10.phys.utk.edu/astr162/lect/binaries/visual.html | title = Visual Binaries | publisher = University of Tennessee}}</ref> For most of the known visual binary stars one whole revolution has not been observed yet; rather, they are observed to have travelled along a curved path or a partial arc.<ref name=Heintz5>{{cite book | last=Heintz | first=W. D. | date=1978 | page=[https://archive.org/details/DoubleStars/page/5 5] | title=Double Stars | publisher=[[D. Reidel]] Publishing Company | location=Dordrecht | isbn=978-90-277-0885-4 | url=https://archive.org/details/DoubleStars/page/5 }}</ref>

[[File:Gwiazda podwójna zaćmieniowa schemat.svg|thumb|Eclipsing binary showing different phases of the smaller secondary relative to the primary star (center)]]

The more general term ''[[double star]]'' is used for pairs of stars which are seen to be close together in the sky.<ref name=aitkenix>''The Binary Stars'', [[Robert Grant Aitken]], New York: Dover, 1964, p. ix.</ref> This distinction is rarely made in languages other than English.<ref name=Heintz12 /> Double stars may be [[Binary system (astronomy)|binary system]]s or may be merely two stars that appear to be close together in the sky but have vastly different true distances from the Sun. The latter are termed ''optical doubles'' or ''optical pairs''.<ref>{{cite book | last=Heintz | first=W. D. | date=1978 | page=[https://archive.org/details/DoubleStars/page/17 17] | title=Double Stars | publisher=D. Reidel Publishing Company, Dordrecht | isbn=978-90-277-0885-4 | url=https://archive.org/details/DoubleStars/page/17 }}</ref>