Double star

Astronomers have mistakenly reported observations of a double star in place of J 900 and a faint star in the constellation of Gemini.
Artist's impression of the discs around the young stars HK Tauri A and B.

Pair of stars that appear close to each other as viewed from Earth, especially with the aid of optical telescopes.

- Double star

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Binary star

System of two stars that are gravitationally bound to and in orbit around each other.

Binary system of two stars
Edge-on disc of gas and dust present around the binary star system HD 106906
Algol B orbits Algol A. This animation was assembled from 55 images of the CHARA interferometer in the near-infrared H-band, sorted according to orbital phase.
Artist's conception of a cataclysmic variable system
Artist's impression of the binary star system AR Scorpii
Artist rendering of plasma ejections from V Hydrae
Artist's impression of the sight from a (hypothetical) moon of planet HD 188753 Ab (upper left), which orbits a triple star system. The brightest companion is just below the horizon.
Schematic of a binary star system with one planet on an S-type orbit and one on a P-type orbit
The two visibly distinguishable components of Albireo
Luhman 16, the third closest star system, contains two brown dwarfs.
Planet Lost in the Glare of Binary Stars (illustration)

The more general term double star is used for pairs of stars which are seen to be close together in the sky.

Proper motion

Astrometric measure of the observed changes in the apparent places of stars or other celestial objects in the sky, as seen from the center of mass of the Solar System, compared to the abstract background of the more distant stars.

Relation between proper motion and velocity components of an object. A year ago the object was d units of distance from the Sun, and its light moved in a year by angle μ radian/s. If there has been no distortion by gravitational lensing or otherwise then μ = is the distance (usually expressed as annual velocity) transverse (tangential or perpendicular) to line of sight from the Sun. The angle is shaded light blue from the sun to the object's start point and its year later position as if it had no radial velocity. In this diagram the radial velocity happens to be one of the sun and object parting, so is positive.
The celestial north and south poles are above/below CNP, CSP; the origin of all 24 hours of Right Ascension (the measure of absolute celestial east–west position), the March equinox (center of the sun's position then) at the J2000 epoch, is vector V. In red the diagram adds the components of proper motion across the celestial sphere. An ideal time to measure exactly such a small annual shift is at culmination. The culmination of the star is daily reached when the observer (and earth) passes as shown by the blue arrows "beneath" the star. The positive axes of the two components of its usually annually measured or published shift in proper motion are the exaggerated red arrows, note: the right arrows point to the east horizon. One red annotation is subtly shorter as the cosine of a star resting at 0° declination is 1, so such a star's east or west shift would not need to be multiplied by the cosine of its declination. The proper motion vector is μ, α = right ascension, δ = declination, θ = position angle (simply the 90° compliment of declination).
Barnard's Star, showing position every 5 years 1985–2005.
Proper motion of 61 Cygni in one year intervals.

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.

Ursa Major

Constellation in the northern sky, whose associated mythology likely dates back into prehistory.

The constellation Ursa Major as it can be seen by the unaided eye.
Ursa Major and Ursa Minor in relation to Polaris
Ursa Major shown on a carved stone, c.1700, Crail, Fife
H. A. Rey's alternative asterism for Ursa Major can be said to give it the longer head and neck of a polar bear, as seen in this photo, from the left side.
Ursa Major as depicted in Urania's Mirror, a set of constellation cards published in London c.1825.
Johannes Hevelius drew Ursa Major as if being viewed from outside the celestial sphere.
Starry Night Over the Rhone by Vincent van Gogh (1888)
Polaris and the Big Dipper on the flag of Alaska.

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.

Mizar and Alcor

Mizar and Alcor in constellation Ursa Major

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

Small number of stars that orbit each other, bound by gravitational attraction.

Star system named DI Cha. While only two stars are apparent, it is actually a quadruple system containing two sets of binary stars.
Orbits of the HR 6819 hierarchical triple star system: an inner binary with one star (orbit in blue) and a black hole (orbit in red), encircled by another star in a wider orbit (also in blue).
Subsystem notation in Tokovinin's Multiple Star Catalogue
Sirius A (center), with its white dwarf companion, Sirius B (lower left) taken by the Hubble Space Telescope.
HD 98800 is a quadruple star system located in the TW Hydrae association.

A star system of two stars is known as a binary star, binary star system or physical double star.

Washington Double Star Catalog

The distribution of the objects of the catalog over the firmament is fairly even.

The Washington Double Star Catalog, or WDS, is a catalog of double stars, maintained at the United States Naval Observatory.

Apparent magnitude

Measure of the brightness of a star or other astronomical object observed from Earth.

Asteroid 65 Cybele and two stars, with their magnitudes labeled
Image of 30 Doradus taken by ESO's VISTA. This nebula has a visual magnitude of 8.
Graph of relative brightness versus magnitude

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

Friedrich Georg Wilhelm von Struve

Friedrich Georg Wilhelm von Struve (Василий Яковлевич Струве, trans.

Von Struve
Friedrich Georg Wilhelm von Struve

He is best known for studying double stars and for initiating a triangulation survey later named Struve Geodetic Arc in his honor.

Bayer designation

Stellar designation in which a specific star is identified by a Greek or Latin letter followed by the genitive form of its parent constellation's Latin name.

Detail of Bayer's chart for Orion showing the belt stars and Orion Nebula region, with both Greek and Latin letter labels visible
Orion constellation map

Usually these are double stars (mostly optical doubles rather than true binary stars), but there are some exceptions such as the chain of stars π1, π2, π3, π4, π5 and π6 Orionis.


Constellation of the southern sky that is centred on four bright stars in a cross-shaped asterism commonly known as the Southern Cross.

Depiction of the Crux by João Faras in May 1500
Southern Cross from New Zealand
Deep exposure of Crux, Coalsack Nebula, and IC 2944
Locating the south celestial pole
The constellation Crux as it can be seen by the naked eye
Crux with clouds, from Cape Town
Crux, appearing on a number of flags and insignia

γ Crucis or Gacrux is an optical double star. The primary is a red-hued giant star of magnitude 1.6, 88 light-years from Earth, and is one of the closest red giants to Earth. Its secondary component is magnitude 6.5, 264 light-years from Earth.