Double star
Encyclopedia
In observational astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

, a double star is a pair of star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s that appear close to each other in the sky as seen from Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

 when viewed through an optical telescope
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...

. This can happen either because the pair forms a binary star
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...

, i.e. a binary system of stars in mutual orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...

, gravitationally bound to each other, or because it is an optical double, a chance alignment of two stars in the sky that lie at different distances. Binary stars are important to stellar astronomers as knowledge of their motions allows direct calculation of stellar mass and other stellar parameters.

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. If the relative motion of a pair determines a curved arc of an orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...

, or if the relative motion is small compared to the common proper motion
Proper motion
The proper motion of a star is its angular change in position over time as seen from the center of mass of the solar system. It is measured in seconds of arc per year, arcsec/yr, where 3600 arcseconds equal one degree. This contrasts with radial velocity, which is the time rate of change in...

 of both stars, it may be concluded that the pair is in mutual orbit as a binary star. Otherwise, the pair is optical. Multiple star
Multiple star
A multiple star consists of three or more stars which appear from the Earth to be close to one another in the sky. This may result from the stars being physically close and gravitationally bound to each other, in which case it is physical, or this closeness may be merely apparent, in which case...

s are also studied in this way, although the dynamics of multiple stellar systems are more complex than those of binary stars.

There are three types of paired stars:
  • optical doubles — unrelated stars which appear close together through chance alignment with Earth
  • visual binaries — gravitationally-bound stars which are separately visible with a telescope
  • non-visual binaries — stars whose binary status was deduced through more esoteric means such as occultation
    Occultation
    An occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The word is used in astronomy . It can also refer to any situation wherein an object in the foreground blocks from view an object in the background...

    , spectroscopy
    Astronomical spectroscopy
    Astronomical spectroscopy is the technique of spectroscopy used in astronomy. The object of study is the spectrum of electromagnetic radiation, including visible light, which radiates from stars and other celestial objects...

    , or anomalies in proper motion
    Proper motion
    The proper motion of a star is its angular change in position over time as seen from the center of mass of the solar system. It is measured in seconds of arc per year, arcsec/yr, where 3600 arcseconds equal one degree. This contrasts with radial velocity, which is the time rate of change in...

    .

Conceptually, there is no difference between the latter two categories, and improvements in telescopes can shift previously non-visual binaries into the visual class, as happened with Polaris
Polaris
Polaris |Alpha]] Ursae Minoris, commonly North Star or Pole Star, also Lodestar) is the brightest star in the constellation Ursa Minor. It is very close to the north celestial pole, making it the current northern pole star....

 in 2006. Thus it is only our inability to observe the third group telescopically that makes the difference.

History

Mizar
Mizar (star)
The Mizar–Alcor stellar sextuple system consists of the quadruple system Mizar and the binary system Alcor.- Description :Mizar is a quadruple system of two binary stars in the constellation Ursa Major and is the second star from the end of the Big Dipper's handle. Its apparent magnitude is 2.23...

, in Ursa Major
Ursa Major
Ursa Major , also known as the Great Bear, is a constellation visible throughout the year in most of the northern hemisphere. It can best be seen in April...

, was observed to be double by Giovanni Battista Riccioli
Giovanni Battista Riccioli
Giovanni Battista Riccioli was an Italian astronomer and a Catholic priest in the Jesuit order...

 in 1650 (and probably earlier by Benedetto Castelli
Benedetto Castelli
Benedetto Castelli , born Antonio Castelli, was an Italian mathematician. He took the name "Benedetto" upon entering the Benedictine Order in 1595....

 and Galileo). The identification of other doubles soon followed: Robert Hooke
Robert Hooke
Robert Hooke FRS was an English natural philosopher, architect and polymath.His adult life comprised three distinct periods: as a scientific inquirer lacking money; achieving great wealth and standing through his reputation for hard work and scrupulous honesty following the great fire of 1666, but...

 discovered one of the first double-star systems, Gamma Arietis
Gamma Arietis
Gamma Arietis is a triple star system, 204 light years distant, in the constellation Aries. It has the traditional name Mesarthim, of obscure origin, and has been called "the First Star in Aries" as having been at one time the nearest visible star to the equinoctial point...

, in 1664, while the bright southern star Acrux
Alpha Crucis
Acrux is the brightest star in the constellation Crux, the Southern Cross, and, at a combined visual magnitude 0.77, is the twelfth brightest star in the night sky...

, in the Southern Cross
Crux
Crux is the smallest of the 88 modern constellations, but is one of the most distinctive. Its name is Latin for cross, and it is dominated by a cross-shaped asterism that is commonly known as the Southern Cross.-Visibility:...

, was discovered to be double by Fontenay in 1685. 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
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...

 of about 9.0. At least 1 in 18 stars brighter than 9.0 magnitude
Magnitude (astronomy)
Magnitude is the logarithmic measure of the brightness of an object, in astronomy, measured in a specific wavelength or passband, usually in optical or near-infrared wavelengths.-Background:...

 in the northern half of the sky are known to be double stars visible with a 36 inches (914.4 mm) telescope
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...

.

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, 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. Additionally, Mizar forms an optical double with the star Alcor
Horse and Rider
The Horse and Rider is an informal name given to the asterism composed of the two stars Mizar and Alcor because of their close proximity in the sky. Mizar is the second star in from the end of the handle of the Big Dipper. Mizar has apparent magnitude 2.27 and spectral class A1 V. With good...

. With a separation of 3 lightyears, it is doubted, but not absolutely proven, that there is no significant gravitational interaction between them. Thus "double star" remains a useful term for pairs whose status is not fully known.

Observation of double stars

Visual double stars are defined as double stars which are visible in an optical telescope. This is the majority of all known double stars. If visual doubles show similar properties, such as similar proper motion
Proper motion
The proper motion of a star is its angular change in position over time as seen from the center of mass of the solar system. It is measured in seconds of arc per year, arcsec/yr, where 3600 arcseconds equal one degree. This contrasts with radial velocity, which is the time rate of change in...

 through space, trigonometric parallaxes, or radial velocities
Radial velocity
Radial velocity is the velocity of an object in the direction of the line of sight . In astronomy, radial velocity most commonly refers to the spectroscopic radial velocity...

, this is evidence that they are gravitationally attached and form a binary system
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...

; in this case, the visual double star is called a visual binary.

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
Position angle
Position angle, usually abbreviated PA, is a measurement derived from observing visual binary stars. It is defined as the angular offset in degrees of the secondary star to the primary, relative to the north celestial pole....

. 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°. 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
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...

 can be computed from it. Although it is expected that the majority of catalogued visual doubles are visual binaries, orbits have been computed for only a few thousand of the over 100,000 known visual double stars.

Distinction between binary stars and other double stars

Visual double stars may be distinguished from binary stars by observing their relative motion. If the motion is part of an orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...

, or if the stars have similar radial velocities or the difference in their proper motion
Proper motion
The proper motion of a star is its angular change in position over time as seen from the center of mass of the solar system. It is measured in seconds of arc per year, arcsec/yr, where 3600 arcseconds equal one degree. This contrasts with radial velocity, which is the time rate of change in...

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.

Designations

Some bright visual double stars have a Bayer designation
Bayer designation
A Bayer designation is a stellar designation in which a specific star is identified by a Greek letter, followed by the genitive form of its parent constellation's Latin name...

. In this case, the components may be denoted by superscripts. An example of this is α Crucis (Acrux), whose components are α1 Crucis and α2 Crucis. Since α1 Crucis is a spectroscopic binary, this is actually a multiple star
Multiple star
A multiple star consists of three or more stars which appear from the Earth to be close to one another in the sky. This may result from the stars being physically close and gravitationally bound to each other, in which case it is physical, or this closeness may be merely apparent, in which case...

. Superscripts are also used to distinguish more distant, physically unrelated, pairs of stars with the same Bayer designation, such as α1,2 Capricorni
Alpha Capricorni
Alpha Capricorni is an optical double star in the constellation Capricornus. It has the traditional names Algiedi, Al Giedi, Algedi or Giedi; however, Giedi is sometimes also associated with β Capricorni....

 (separated by 0.11°), ξ1,2 Centauri
Xi Centauri
The Bayer designation Xi Centauri is shared by two star systems, in the constellation Centaurus:* ξ¹ Centauri* ξ² CentauriThey are separated by 0.66° on the sky....

 (separated by 0.66°), and ξ1,2 Sagittarii
Xi Sagittarii
The Bayer designation Xi Sagittarii is shared by two stars, ξ¹ Sagittarii and ξ² Sagittarii, in the constellation Sagittarius, separated by 0.46° in the sky. Because they are close to the ecliptic, they can be occulted by the Moon and, very rarely, by planets...

 (separated by 0.46°.) 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
ADS 16402
ADS 16402 is a binary star system, composed of two sun-like stars located 450 light-years away in the constellation Lacerta. The two stars are separated by 1500 AUs. The star system is estimated to be 3.6 billion years old...

 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.
Double star discoverer designations
Discoverer Traditional code WDS code
Brisbane Observatory Brs0 BSO
S. W. Burnham β BU
James Dunlop
James Dunlop
James Dunlop was an astronomer's assistant who was hired by Sir Thomas Brisbane to work at his private observatory, once located at Paramatta , New South Wales, about twenty-three kilometres west of Sydney, Australia during the 1820s and 1830s...

Δ DUN
William Herschel
William Herschel
Sir Frederick William Herschel, KH, FRS, German: Friedrich Wilhelm Herschel was a German-born British astronomer, technical expert, and composer. Born in Hanover, Wilhelm first followed his father into the Military Band of Hanover, but emigrated to Britain at age 19...

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 I Σ II STFB
Otto Struve
Otto Struve
Otto Struve was a Russian astronomer. In Russian, his name is sometimes given as Otto Lyudvigovich Struve ; however, he spent most of his life and his entire scientific career in the United States...

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. Other examples include Δ65, the 65th double discovered by James Dunlop
James Dunlop
James Dunlop was an astronomer's assistant who was hired by Sir Thomas Brisbane to work at his private observatory, once located at Paramatta , New South Wales, about twenty-three kilometres west of Sydney, Australia during the 1820s and 1830s...

, and Σ2451, discovered by F. G. W. Struve.

The Washington Double Star Catalog
Washington Double Star Catalog
The Washington Double Star Catalog, or WDS, is a catalog of double stars, maintained at the United States Naval Observatory. The catalog contains positions, magnitudes, proper motions and spectral types and has entries for 102,387 pairs of double stars. The catalog also includes multiple stars...

, a large database of double and multiple stars, contains over 100,000 entries, 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. 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  65 and Σ2451 has become STF2451. Further examples of this are shown in the table to the right.

Visual binaries

  • Alpha Centauri
    Alpha Centauri
    Alpha Centauri is the brightest star in the southern constellation of Centaurus...

  • Sirius
    Sirius
    Sirius is the brightest star in the night sky. With a visual apparent magnitude of −1.46, it is almost twice as bright as Canopus, the next brightest star. The name "Sirius" is derived from the Ancient Greek: Seirios . The star has the Bayer designation Alpha Canis Majoris...

  • Procyon
    Procyon
    Procyon is the brightest star in the constellation Canis Minor. To the naked eye, it appears to be a single star, the seventh brightest in the night sky with a visual apparent magnitude of 0.34...

  • Capella
    Capella (star)
    Capella is the brightest star in the constellation Auriga, the sixth brightest star in the night sky and the third brightest star in the northern celestial hemisphere, after Arcturus and Vega. Although it appears to be a single star to the naked eye, it is actually a star system of four stars in...

  • p Eridani
    P Eridani
    p Eridani is a binary star system in the constellation of Eridanus whose distance is approximately 26 light-years. It was found to be a double star in December 1825 by James Dunlop in Australia at his home at Paramatta, now spelt Parramatta.- Naming :The name "p Eridani", according to Nature,...

  • Polaris
    Polaris
    Polaris |Alpha]] Ursae Minoris, commonly North Star or Pole Star, also Lodestar) is the brightest star in the constellation Ursa Minor. It is very close to the north celestial pole, making it the current northern pole star....

  • Acrux

Optical doubles

  • Alpha¹ Capricorni
    Alpha¹ Capricorni
    Alpha¹ Capricorni is a binary star in the constellation Capricornus. It also has the traditional star name Prima Giedi or Algiedi Prima, and shares the name Algedi with α² Capricorni...

     and Alpha² Capricorni
    Alpha² Capricorni
    Alpha² Capricorni is a star in the constellation Capricornus. It also has the traditional star name Secunda Giedi or Algiedi Secunda, and shares the name Algedi with α¹ Capricorni.In Chinese, , meaning Ox , refers to an asterism consisting of α² Capricorni, β Capricorni, ξ2...

  • Xi1,2 Sagittarii
    Xi Sagittarii
    The Bayer designation Xi Sagittarii is shared by two stars, ξ¹ Sagittarii and ξ² Sagittarii, in the constellation Sagittarius, separated by 0.46° in the sky. Because they are close to the ecliptic, they can be occulted by the Moon and, very rarely, by planets...

  • Winnecke 4
    Winnecke 4
    Winnecke 4 is a double star in the constellation Ursa Major. It was discovered by Charles Messier in 1764 while he was searching for a nebula that had been reported in the area by Johann Hevelius. Not seeing any nebulae, Messier catalogued this double star instead...


Uncertain

  • Alpha Centauri
    Alpha Centauri
    Alpha Centauri is the brightest star in the southern constellation of Centaurus...

     system (AB) and Proxima Centauri
    Proxima Centauri
    Proxima Centauri is a red dwarf star about 4.2 light-years distant in the constellation of Centaurus. It was discovered in 1915 by Robert Innes, the Director of the Union Observatory in South Africa, and is the nearest known star to the Sun, although it is too faint to be seen with the naked eye...

     (thus α Cen C): association is generally considered a physically connected system
  • Castor
    Castor (star)
    Castor is the second brightest star in the constellation Gemini and one of the brightest stars in the night sky. Although it has the Bayer designation "alpha", it is actually fainter than Beta Geminorum...

     system (Aa/Ab/Ba/Bb) and YY Geminorum (thus Castor Ca/Cb) is generally considered a physical system
  • Mizar
    Mizar (star)
    The Mizar–Alcor stellar sextuple system consists of the quadruple system Mizar and the binary system Alcor.- Description :Mizar is a quadruple system of two binary stars in the constellation Ursa Major and is the second star from the end of the Big Dipper's handle. Its apparent magnitude is 2.23...

    system (Aa/Ab/Ba/Bb) and Alcor (itself a binary, thus Mizar Ca/Cb, though generally not considered physical until 2009)
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