Double star
Pairs of stars that appear close together from Earth.
A double star, also called a visual double, is what you get when two stars look close together in the sky from Earth—especially through a telescope. This can happen for two reasons: either they’re a true binary system, meaning they’re gravitationally bound and orbit each other, or they’re an optical double, a trick of perspective where two unrelated stars at different distances just line up by chance. For astronomers, binary stars are key because tracking their motions lets them directly calculate stellar masses and other properties.
The only possible naked-eye example of a binary star where both components are separately visible is Mizar and Alcor (though it’s actually a multiple-star system), but it’s not certain they’re gravitationally bound. Since the early 1780s, both professional and amateur observers have used telescopes to measure the distances and angles between double stars, tracking their relative motion. If that motion traces a curved orbital arc, or if it’s tiny compared to the stars’ shared proper motion, the pair is likely a binary. Otherwise, it’s optical. Multiple-star systems are studied the same way, though their dynamics are more complex.
There are three types of paired stars: optical doubles (unrelated stars that just appear close), visual binaries (gravitationally bound stars separately visible through a telescope), and non-visual binaries (whose binary nature is deduced by other means—like eclipses, spectroscopy, or proper-motion anomalies). Better telescopes can turn non-visual binaries into visual ones, as happened with Polaris A in 2006. The only thing that separates non-visual from visual binaries is the inability to see two separate stars telescopically.
Historically, Mizar was spotted as double by Benedetto Castelli and Galileo. Other discoveries followed: Robert Hooke found Gamma Arietis in 1664, and Fontenay found Acrux in 1685. Since then, the whole sky has been searched for double stars down to about magnitude 9.0. In the northern sky, at least 1 in 18 stars brighter than that are known to be double when viewed with a 36-inch telescope. Optical doubles and true binaries are lumped together for practical reasons—when Mizar was first found, it was hard to tell which was which. Better telescopes, spectroscopy, and photography later helped sort them out. Mizar’s components themselves turned out to be spectroscopic binaries.
W
- field
- Observational astronomy
- known_for
- Pairs of stars that appear close together; used to distinguish binary systems from optical alignments
- types
- Optical doubles, visual binaries, non-visual binaries
- first_recorded
- Mizar observed as double by Benedetto Castelli and Galileo
- catalog_size
- Over 100,000 entries in the Washington Double Star Catalog
Lore & Background
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 describes a curved arc of an orbit, or if the relative motion is small compared to the common proper motion of both stars, the pair is likely a binary star; otherwise, it is optical. Multiple stars are also studied in this way, though their dynamics are more complex.
Mizar, in Ursa Major, was observed to be double by Benedetto Castelli and Galileo. Other early discoveries include Gamma Arietis by Robert Hooke in 1664 and Acrux by Fontenay in 1685. Since then, the entire sky has been examined for double stars down to a limiting apparent magnitude of about 9.0. At least 1 in 18 stars brighter than 9.0 magnitude in the northern half of the sky are known to be double stars visible with a 36-inch telescope.
Confirmation of a visual double star as a binary can be achieved by observing relative motion. If the motion is part of an orbit, or if the stars have similar radial velocities or the difference in their proper motions is small compared to their common proper motion, the pair is probably physical. When observed over a short period, components of both optical doubles and long-period visual binaries appear to move in straight lines, making distinction difficult.
Reader's Guide
Double stars are significant because they provide a direct method for measuring stellar masses and other parameters through orbital motion. The distinction between optical doubles and true binaries is fundamental: optical doubles are unrelated stars aligned by chance, while visual binaries are gravitationally bound and reveal stellar properties. Improvements in telescopes can shift previously non-visual binaries into visual binaries, as happened with Polaris A in 2006. The Washington Double Star Catalog contains over 100,000 entries, though orbits have been computed for only a few thousand. The legacy of double star observation includes the development of techniques such as spectroscopy and astrometry to identify non-visual binaries. The Mizar system, a visual binary whose components are themselves spectroscopic binaries, illustrates the complexity of multiple-star systems. Double stars remain a key tool for understanding stellar evolution and dynamics.
Did You Know?
- The only possible case of a binary star whose two components are separately visible to the naked eye is Mizar and Alcor, though it is not known for certain whether they are gravitationally bound.
- Improvements in telescopes can shift previously non-visual binaries into visual binaries, as happened with Polaris A in 2006.
- At least 1 in 18 stars brighter than 9.0 magnitude in the northern half of the sky are known to be double stars visible with a 36-inch telescope.
- The Washington Double Star Catalog contains over 100,000 entries, each giving measures for the separation of two components.
More in Stars And Stellar Phenomena 1-24
Spotted an error? Know more?
This is a living reference — every entry is fact-audited, and reader corrections feed straight into our audit queue. Suggest an edit · See this site's audit record