Double star

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, 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.

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.

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

ζ Ursae Majoris, Mizar, the second star in from the end of the handle of the Big Dipper, and the constellation's fourth-brightest star. Mizar, which means "girdle," forms a famous double star, with its optical companion Alcor (80 Ursae Majoris), the two of which were termed the "horse and rider" by the Arabs. The ability to resolve the two stars with the naked eye is often quoted as a test of eyesight, although even people with quite poor eyesight can see the two stars.

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.

Alpha Capricorni (α Capricorni, abbreviated Alpha Cap, α Cap) is an optical double star in the constellation of Capricornus.

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.

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.

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.

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.

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.

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.

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.

Gamma Crucis, called Gacrux, is an optical double star.

Alpha 1 and Alpha 2 Capricorni

Alpha 1 Capricorni, Latinized from α 1 Capricorni, is a double star in the southern constellation of Capricornus.

Alpha 1 and Alpha 2 Capricorni

It is visible to the naked eye with an apparent visual magnitude of +3.57, forming part of the double star Alpha Capricorni along with α¹ Capricorni.

Winnecke 4 (Messier 40)

Winnecke 4 (also known as Messier 40 or WNC 4) is a double star in the constellation Ursa Major.

In 1873 – 1874, he produced a catalog of double stars.

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.

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

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.

p Eridani

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

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.

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. Capella A is itself a spectroscopic binary with components Aa and Ab, both giants.

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.

Kappa 1 and Kappa 2 Coronae Australis

Kappa2 Coronae Australis is a visual double star with Kappa1 Coronae Australis, also known as HR 6952.

Theta Muscae and Theta Muscae B

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.