Editing Double star

{{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.

==History==
[[Mizar (star)|Mizar]], in [[Ursa Major]], was observed to be double by [[Benedetto Castelli]] and [[Galileo]].<ref name="newviewofmizar">[http://leo.astronomy.cz/mizar/article.htm A New View of Mizar] {{Webarchive|url=https://web.archive.org/web/20080307111656/http://leo.astronomy.cz/mizar/article.htm |date=2008-03-07 }}, Leos Ondra, accessed on line May 26, 2007.</ref> The identification of other doubles soon followed: [[Robert Hooke]] discovered one of the first double-star systems, [[Gamma Arietis]], in 1664,<ref>{{cite book | author = Aitken, Robert G. | title = The Binary Stars | year = 1935 | place = New York | publisher = McGraw-Hill | pages = 1 }}</ref> while the bright southern star [[Alpha Crucis|Acrux]], in the [[Crux|Southern Cross]], was discovered to be double by Fontenay in 1685.<ref name="aitken1"/> Since that time, the search has been carried out thoroughly and the entire sky has been examined for double stars down to a limiting [[apparent magnitude]] of about 9.0.<ref>See ''The Binary Stars'', [[Robert Grant Aitken]], New York: Dover, 1964, pp. 24–25, 38, and p. 61, The present status of double star astronomy, K. Aa. Strand, ''Astronomical Journal'' '''59''' (March 1954), pp. 61–66, {{bibcode|1954AJ.....59...61S}}.</ref> At least 1 in 18 stars brighter than 9.0 [[magnitude (astronomy)|magnitude]] in the northern half of the sky are known to be double stars visible with a {{convert|36|in|mm|adj=on}} [[telescope]].<ref name=aitken260>''The Binary Stars'', [[Robert Grant Aitken]], New York: Dover, 1964, p. 260.</ref>

The unrelated categories of optical doubles and true binaries are lumped together for historical and practical reasons. When Mizar was found to be a binary, it was quite difficult to determine whether a double star was a binary system or only an optical double. Improved telescopes, spectroscopy,<ref>[[Joseph von Fraunhofer|Fraunhofer]], 1814</ref> and photography are the basic tools used to make the distinction. After it was determined to be a visual binary, Mizar's components were found to be spectroscopic binaries themselves.<ref>[[Edward Charles Pickering|Pickering]], 1889</ref>

==Observation of double stars==
[[File:Masquerading as a double star.jpg|thumb|Astronomers have mistakenly reported observations of a double star in place of [[J 900]] and a faint star in the constellation of Gemini.<ref>{{cite news|title=Masquerading as a double star|url=http://www.spacetelescope.org/images/potw1312a/|access-date=25 March 2013|newspaper=ESA/Hubble Picture of the Week}}</ref> ]]

Observation of visual double stars by visual measurement will yield the ''separation'', or angular distance, between the two component stars in the sky and the ''[[position angle]]''. The position angle specifies the direction in which the stars are separated and is defined as the bearing from the brighter component to the fainter, where north is 0°.<ref name=argyle2>p. 2, ''Observing and Measuring Double Stars'', Bob Argyle, ed., London: Springer-Verlag, 2004, {{ISBN|1-85233-558-0}}.</ref> These measurements are called ''measures''. In the measures of a visual binary, the position angle will change progressively and the separation between the two stars will oscillate between maximum and minimum values. Plotting the measures in the plane will produce an ellipse. This is the ''apparent orbit'', the projection of the orbit of the two stars onto the celestial sphere; the true [[orbit]] can be computed from it.<ref name=argyle5355>p. 53–67, ''Observing and Measuring Double Stars'', Bob Argyle, ed., London: Springer-Verlag, 2004, {{ISBN|1-85233-558-0}}.</ref> Although it is expected that the majority of catalogued visual doubles are visual binaries,<ref name=Heintz12>{{cite book | last=Heintz | first=W. D. | year=1978 | page=[https://archive.org/details/DoubleStars/page/12 12] | title=Double Stars | publisher=D. Reidel Publishing Company | location=Dordrecht | isbn=90-277-0885-1 | url=https://archive.org/details/DoubleStars/page/12 }}</ref> orbits have been computed for only a few thousand of the over 100,000 known visual double stars.<ref name=wdsintro>"Introduction and Growth of the WDS", [http://ad.usno.navy.mil/wds/wdstext.html#intro The Washington Double Star Catalog] {{Webarchive|url=https://web.archive.org/web/20080917194922/http://ad.usno.navy.mil/wds/wdstext.html#intro |date=2008-09-17 }}, Brian D. Mason, Gary L. Wycoff, and William I. Hartkopf, Astrometry Department, [[United States Naval Observatory]], accessed on line August 20, 2008.</ref><ref>[http://ad.usno.navy.mil/wds/orb6.html Sixth Catalog of Orbits of Visual Binary Stars] {{Webarchive|url=https://web.archive.org/web/20090412084731/http://ad.usno.navy.mil/wds/orb6.html |date=2009-04-12 }}, William I. Hartkopf and Brian D. Mason, [[United States Naval Observatory]], accessed on line August 20, 2008.</ref>

==Distinction between binary stars and other double stars==
Confirmation of a visual double star as a binary star can be achieved by observing the relative motion of the components. If the motion is part of an [[orbit]], or if the stars have similar [[radial velocities]] or the difference in their [[proper motion]]s is small compared to their common proper motion, the pair is probably physical. When observed over a short period of time, the components of both optical doubles and long-period visual binaries will appear to be moving in straight lines; for this reason, it can be difficult to distinguish between these two possibilities.<ref name=Heintz1718>{{cite book | last=Heintz | first=W. D. | year=1978 | pages=[https://archive.org/details/DoubleStars/page/17 17–18] | title=Double Stars | publisher=[[D. Reidel]] Publishing Company | location=Dordrecht | isbn=90-277-0885-1 | url=https://archive.org/details/DoubleStars/page/17 }}</ref>

==Designations==
Some bright visual double stars have a [[Bayer designation]]. In this case, the components may be denoted by superscripts. An example of this is [[α Crucis]] (Acrux), whose components are α<sup>1</sup> Crucis and α<sup>2</sup> Crucis. Since α<sup>1</sup> Crucis is a [[spectroscopic binary]], this is actually a multiple star. Superscripts are also used to distinguish more distant, physically unrelated, pairs of stars with the same Bayer designation, such as [[Alpha Capricorni|α<sup>1,2</sup> Capricorni]], [[Xi Centauri|ξ<sup>1,2</sup> Centauri]], and [[Xi Sagittarii|ξ<sup>1,2</sup> Sagittarii]]. These optical pairs are resolvable by the naked eye.

Apart from these pairs, the components of a double star are generally denoted by the letters A (for the brighter, ''primary'', star) and B (for the fainter, ''secondary'', star) appended to the designation, of whatever sort, of the double star. For example, the components of [[α Canis Majoris]] (Sirius) are α Canis Majoris A and α Canis Majoris B (Sirius A and Sirius B); the components of [[44 Boötis]] are 44 Boötis A and 44 Boötis B; the components of [[ADS 16402]] are ADS 16402A and ADS 16402B; and so on. The letters AB may be used together to designate the pair. In the case of multiple stars, the letters C, D, and so on may be used to denote additional components, often in order of increasing separation from the brightest star, A.<ref name=Heintz19>{{cite book | last=Heintz | first=W. D. | year=1978 | page=[https://archive.org/details/DoubleStars/page/19 19] | title=Double Stars | publisher=D. Reidel Publishing Company | location=Dordrecht | isbn=90-277-0885-1 | url=https://archive.org/details/DoubleStars/page/19 }}</ref>

{| class="wikitable" : style="float:right; margin:1em 1em"
|+ Double star discoverer designations<ref name=argyle307308 />
|-
! Discoverer
! Traditional code
! WDS code
|-
| [[Parramatta Observatory|Brisbane Observatory]]
| Brs0
| BSO
|-
| [[S. W. Burnham]]
| β
| BU
|-
| [[James Dunlop]]
| Δ
| DUN
|-
| [[William Herschel]]
| H I, II, etc.
| H 1, 2, etc.
|-
| [[N. Lacaille]]
| Lac
| LCL
|-
| [[F. G. W. Struve]]
| Σ
| STF
|-
| Struve Appendix Catalogue I
| Σ I
| STFA
|-
| Struve Appendix Catalogue II
| Σ II
| STFB
|-
| [[Otto Struve]]
| OΣ
| STT
|-
| Pulkova Appendix Catalogue
| OΣΣ
| STTA
|}
Visual doubles are also designated by an abbreviation for the name of their discoverer followed by a catalogue number unique to that observer. For example, the pair α Centauri AB was discovered by Father Richaud in 1689, and so is designated ''RHD 1''.<ref name=aitken1 /><ref>Entry 14396-6050, discoverer code RHD 1AB,[http://ad.usno.navy.mil/wds/Webtextfiles/wdsnewframe3.html The Washington Double Star Catalog] {{Webarchive|url=https://archive.today/20120708194616/http://ad.usno.navy.mil/wds/Webtextfiles/wdsnewframe3.html |date=2012-07-08 }}, [[United States Naval Observatory]]. Accessed on line August 20, 2008.</ref> Other examples include Δ65, the 65th double discovered by [[James Dunlop]], and Σ2451, discovered by [[F. G. W. Struve]].

The [[Washington Double Star Catalog]], a large database of double and multiple stars, contains over 100,000 entries,<ref name=wdsintro /> each of which gives measures for the separation of two components. Each double star forms one entry in the catalog; multiple stars with ''n'' components will be represented by entries in the catalog for ''n''−1 pairs, each giving the separation of one component of the multiple star from another. Codes such as AC are used to denote which components are being measured—in this case, component C relative to component A. This may be altered to a form such as AB-D to indicate the separation of a component from a close pair of components (in this case, component D relative to the pair AB.) Codes such as ''Aa'' may also be used to denote a component which is being measured relative to another component, A in this case.<ref>[http://ad.usno.navy.mil/wds/wdsweb_format.txt Format of the current WDS] {{webarchive |url=https://web.archive.org/web/20080412092018/http://ad.usno.navy.mil/wds/wdsweb_format.txt |date=April 12, 2008 }}, Washington Double Star Catalog, [[United States Naval Observatory]]. Accessed on line August 26, 2008.</ref> Discoverer designations are also listed; however, traditional discoverer abbreviations such as Δ and Σ have been encoded into a string of uppercase Roman letters, so that, for example, Δ65 has become DUN&nbsp;&nbsp;65 and Σ2451 has become STF 2451. Further examples of this are shown in the adjacent table.<ref name=argyle307308>p. 307–308, ''Observing and Measuring Double Stars'', Bob Argyle, ed., London: Springer-Verlag, 2004, {{ISBN|1-85233-558-0}}.</ref><ref>[http://ad.usno.navy.mil/wds/wdsnewref.txt References and discoverer codes, The Washington Double Star Catalog] {{Webarchive|url=https://web.archive.org/web/20080413021722/http://ad.usno.navy.mil/wds/wdsnewref.txt |date=2008-04-13 }}, [[United States Naval Observatory]]. Accessed on line August 20, 2008.</ref>

==Examples==
[[File:Almach double star.jpg|thumb|The color-contrastant binary Gamma Andromedae seen by a telescope]]

===Visual binaries===
* [[Acrux]]
* [[Capella]]
* [[p Eridani]]
* [[Gamma Leonis]]
* [[Gamma Andromedae]]
* [[Polaris]]
* [[Procyon]]
* [[Sirius]]
* [[Alpha Centauri]] system (AB) and [[Proxima Centauri]] (thus α Cen C): Actually a three-star system

===Optical doubles===
* [[Alpha1 Capricorni|Alpha<sup>1</sup>]] and [[Alpha2 Capricorni|Alpha<sup>2</sup> Capricorni]]
* [[Theta Muscae]] and [[Theta Muscae B]]
* [[Zeta1 Scorpii|Zeta<sup>1</sup>]] and [[Zeta2 Scorpii|Zeta<sup>2</sup> Scorpii]]
* [[Eta1 Coronae Australis|Eta<sup>1</sup>]] and [[Eta2 Coronae Australis|Eta<sup>2</sup> Coronae Australis]]
* [[Winnecke 4]] ([[Winnecke 4|Messier 40]])

===Uncertain===
* [[Albireo|Albireo A]] and [[Albireo|Albireo B]]
* [[Kappa1 Coronae Australis|Kappa<sup>1</sup>]] and [[Kappa2 Coronae Australis|Kappa<sup>2</sup> Coronae Australis]]
* [[Omicron1 Centauri|Omicron<sup>1</sup>]] and [[Omicron2 Centauri|Omicron<sup>2</sup> Centauri]]
* [[Mizar (star)|Mizar]] system (Aa/Ab/Ba/Bb) and [[Alcor (star)| Alcor]] system (thus Mizar Ca/Cb), generally considered a physical system.

==References==
{{Reflist}}

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