Editing Double star (section)

{{short description|Pair of stars that appear close to each other}}
{{About||the Heinlein novel|Double Star|other uses}}
{{Distinguish|Binary star}}
[[File:Optical Double.jpg|thumb|The Hubble Telescope's 100,000th capture showing two stars that appear close together. The star on the right is a nearby Milky Way star while the star in the middle is a quasar 9 billion light years away, its extreme luminosity making just as bright despite vast distances. This is an example of an optical double.]]
In [[observational astronomy]], a '''double star''' or '''visual double''' is a pair of [[star]]s that [[apparent distance|appear close]] to each other as viewed from [[Earth]], especially with the aid of [[optical telescope]]s.

This occurs because the pair either forms a [[binary star]] (i.e. a binary system of stars in mutual [[orbit]], [[gravitationally]] bound to each other) or is an ''optical double'', a chance line-of-sight alignment of two stars at different distances from the observer.<ref name=aitken1>{{cite book |title=The Binary Stars |last=Aitken |first=R. G. |author-link=Robert Grant Aitken |location=New York |publisher=Dover |year=1964 |page=1}}</ref><ref name=heintz17>{{cite book |last=Heintz |first=W. D. |year=1978 |page=[https://archive.org/details/DoubleStars/page/17 17] |title=Double Stars |url=https://archive.org/details/DoubleStars/page/17 |publisher=[[D. Reidel]] |location=[[Dordrecht]] |isbn=90-277-0885-1 }}</ref> Binary stars are important to stellar astronomers as knowledge of their motions allows direct calculation of stellar mass and other stellar parameters. The only (possible) case of "binary star" whose two components are separately visible to the naked eye is the case of [[Mizar and Alcor]] (though actually a multiple-star system), but it is not known for certain whether Mizar and Alcor are gravitationally bound.<ref name=zimmerman>{{cite journal|bibcode=2010ApJ...709..733Z|arxiv=0912.1597|title=Parallactic Motion for Companion Discovery: An M-Dwarf Orbiting Alcor|journal=The Astrophysical Journal|volume=709|issue=2|pages=733–740|last1=Zimmerman|first1=Neil|last2=Oppenheimer|first2=Ben R|last3=Hinkley|first3=Sasha|last4=Brenner|first4=Douglas|last5=Parry|first5=Ian R|last6=Sivaramakrishnan|first6=Anand|last7=Hillenbrand|first7=Lynne|last8=Beichman|first8=Charles|last9=Crepp|first9=Justin R|last10=Vasisht|first10=Gautam|last11=Roberts|first11=Lewis C|last12=Burruss|first12=Rick|last13=King|first13=David L|last14=Soummer|first14=Rémi|last15=Dekany|first15=Richard|last16=Shao|first16=Michael|last17=Bouchez|first17=Antonin|last18=Roberts|first18=Jennifer E|last19=Hunt|first19=Stephanie|year=2010|doi=10.1088/0004-637X/709/2/733|s2cid=6052794}}</ref>

Since the beginning of the 1780s, both professional and amateur double star observers have telescopically measured the distances and angles between double stars to determine the relative motions of the pairs.<ref name=heintz410>{{cite book |last=Heintz |first=W. D. |year=1978 |pages=[https://archive.org/details/DoubleStars/page/4 4–10] |title=Double Stars |url=https://archive.org/details/DoubleStars/page/4 |publisher=[[D. Reidel]] |location=[[Dordrecht]] |isbn=90-277-0885-1 }}</ref> If the relative motion of a pair determines a curved arc of an [[orbit]], or if the relative motion is small compared to the common [[proper motion]] of both stars, it may be concluded that the pair is in mutual orbit as a binary star. Otherwise, the pair is optical.<ref name=heintz17 /> [[Multiple star]]s are also studied in this way, although the dynamics of multiple [[stellar system]]s are more complex than those of binary stars.

The following are three types of paired stars:
* '''Optical doubles''' are unrelated stars that appear close together through chance alignment with Earth.
* '''[[Binary star#Visual binaries|Visual binaries]]''' are gravitationally bound stars that are separately visible with a telescope.
* '''Non-visual binaries''' are stars whose binary status was deduced through more esoteric means, such as [[occultation]] ([[Binary star#Eclipsing binaries|eclipsing binaries]]), [[astronomical spectroscopy|spectroscopy]] ([[Binary star#Spectroscopic binaries|spectroscopic binaries]]), or anomalies in [[proper motion]] ([[Binary star#Astrometric binaries|astrometric binaries]]).

Improvements in telescopes can shift previously non-visual binaries into visual binaries, as happened with [[Polaris]] A in 2006.<ref>{{cite web |url=http://hubblesite.org/newscenter/archive/releases/2006/02/image/f/ |title=Artist's Concept of Polaris System - Annotated |work=HubbleSite |publisher=[[Space Telescope Science Institute]] }}</ref> It is only the inability to telescopically observe two separate stars that distinguishes non-visual and visual binaries.