Double star

visual companiondouble starsoptical doubleoptical companiondoubleoptical binaryopticaloptical companionsoptical componentsoptical double star
Not to be confused with Binary star.wikipedia
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Mizar and Alcor

MizarAlcorAlcor–Mizar
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 sure whether Mizar and Alcor are gravitationally bound.
Mizar and Alcor are two stars forming a naked eye double in the handle of the Big Dipper (or Plough) asterism in the constellation of Ursa Major.

Star system

multiple star systemtriple starmultiple systems
Multiple stars are also studied in this way, although the dynamics of multiple stellar systems are more complex than those of binary stars.
A star system of two stars is known as a binary star, binary star system or physical double star.

Mizar

Zeta Ursae Majorisζ Ursae Majoris80 Ursae Majoris
Mizar, in Ursa Major, was observed to be double by Benedetto Castelli and Galileo.
It forms a well-known naked eye double star with the fainter star Alcor, and is itself a quadruple star system.

Optical telescope

telescopetelescopesoptical
In observational astronomy, a double star or visual double is a pair of stars that appear close to each other as viewed from Earth, especially with the aid of optical telescopes.
In the ideal case, the two components of a double star system can be discerned even if separated by slightly less than \alpha_R.

Ursa Major

Great BearOrsa MaggioreUrsa Major constellation
Mizar, in Ursa Major, was observed to be double by Benedetto Castelli and Galileo.

Proper motion

proper motionsproper-motionhigh proper motion star
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.
Two or more stars, double stars or open star clusters, which are moving in similar directions, exhibit so-called shared or common proper motion (or cpm.), suggesting they may be gravitationally attached or share similar motion in space.

Gamma Arietis

γγ AriMesarthim
The identification of other doubles soon followed: Robert Hooke discovered one of the first double-star systems, Gamma Arietis, in 1664, while the bright southern star Acrux, in the Southern Cross, was discovered to be double by Fontenay in 1685.
The double star nature of this system was discovered by Robert Hooke in 1664.

Binary star

spectroscopic binaryeclipsing binarybinary
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.
The term double star is often used synonymously with binary star; however, double star can also mean optical double star.

Alpha Capricorni

α CapricorniAl GiediAlgiedi
Superscripts are also used to distinguish more distant, physically unrelated, pairs of stars with the same Bayer designation, such as α 1,2 Capricorni, ξ 1,2 Centauri, and ξ 1,2 Sagittarii.

Observational astronomy

astronomical observationobservationsobservational
In observational astronomy, a double star or visual double is a pair of stars that appear close to each other as viewed from Earth, especially with the aid of optical telescopes.
The position or cross-wire micrometer is an implement that has been used to measure double stars.

Apparent magnitude

apparent visual magnitudemagnitudevisual magnitude
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.
For example, photometry on closely separated double stars may only be able to produce a measurement of their combined light output.

Crux

Southern CrossSouthern Cross constellationCrux Australis
The identification of other doubles soon followed: Robert Hooke discovered one of the first double-star systems, Gamma Arietis, in 1664, while the bright southern star Acrux, in the Southern Cross, was discovered to be double by Fontenay in 1685.
Other members include the blue-white stars Zeta, Lambda and both the components of the visual double star, Mu.

Friedrich Georg Wilhelm von Struve

StruveFriedrich Wilhelm StruveWilhelm Struve
Other examples include Δ65, the 65th double discovered by James Dunlop, and Σ2451, discovered by F. G. W. Struve.
He is best known for studying double stars and for initiating a triangulation survey later named Struve Geodetic Arc in his honor.

Washington Double Star Catalog

WDSWashington Double Star CatalogueWDS catalog
The Washington Double Star Catalog, a large database of double and multiple stars, contains over 100,000 entries, each of which gives measures for the separation of two components.
The Washington Double Star Catalog, or WDS, is a catalog of double stars, maintained at the United States Naval Observatory.

Robert Hooke

HookeDr Robert HookeHooke, Robert
The identification of other doubles soon followed: Robert Hooke discovered one of the first double-star systems, Gamma Arietis, in 1664, while the bright southern star Acrux, in the Southern Cross, was discovered to be double by Fontenay in 1685.
Hooke also was an early observer of the rings of Saturn, and discovered one of the first observed double-star systems, Gamma Arietis, in 1664.

Sherburne Wesley Burnham

S. W. BurnhamSherburne W. BurnhamBurnham
In 1873 – 1874, he produced a catalog of double stars.

Otto Struve

O. StruveOttoOtto Lyudvigovich Struve
As early as December 1923, Struve defended his PhD thesis on short-period spectroscopic double stars at the University of Chicago.

P Eridani

6 Erip EriHD 10360
It was found to be a double star in December 1825 by James Dunlop in Australia at his home at Paramatta, now spelt Parramatta.

James Dunlop

Dunlop
Other examples include Δ65, the 65th double discovered by James Dunlop, and Σ2451, discovered by F. G. W. Struve.
Dunlop's other major observational work was of 256 southern double stars or "pairs" below the declination of about 30° South.

Bayer designation

BayerdesignationBayer letter
Some bright visual double stars have a Bayer designation.
Usually these are double stars (mostly optical doubles rather than true binary stars), but there are some exceptions such as the chain of stars π 1, π 2, π 3, π 4, π 5 and π 6 Orionis.

Winnecke 4

M40Messier 40WNC4
Winnecke 4 (also known as Messier 40 or WNC 4) is a double star consisting of two unrelated stars in the constellation Ursa Major.

Capella

AlhaiotAlpha Aurigaealpha Aurigae (Capella)
Most are only line-of-sight companions, but the close pair of red dwarfs H and L are at the same distance as the bright component A and moving through space along with it.

Castor (star)

CastorCastor Aα Gem
Appearing to the naked eye as a single star, Castor was first recorded as a double star in 1718 by James Pound, but it may have been resolved into at least two sources of light by Cassini as early as 1678.

Alpha Centauri

Alpha Centauri BAlpha Centauri Aα Centauri
For example, in about 6,200 AD, α Centauri's true motion will cause an extremely rare first-magnitude stellar conjunction with Beta Centauri, forming a brilliant optical double star in the southern sky.

Theta Muscae

θ Musθ Muscae
To small telescopes, Theta Muscae appears as a double star, with a blue-cream brighter star and an O9III companion of magnitude 7.3 some 5.3 arcseconds away.