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Glossary term: Binary Star

Description: A binary star is a system of two stars that orbit around their common center of mass as a result of being bound together by the force of gravity. Their orbits follow Kepler's laws of motion and are elliptical (shaped like a squashed circle) or circular in shape.

More than half of all stars in the Milky Way are in binary systems or part of systems with more than one companion star (known as higher order multiple-star systems). Due to their enormous distances from Earth, most binary stars and higher order multiple-star systems appear to the observer as a single star.

Binaries can be classified into a number of categories according to the observational method by which they were found to be binary stars. They may simultaneously belong to more than one category:

Visual binaries can be observed as two separate stars close together on the sky. Not all stars that appear close on the sky (double stars) are binaries bound by gravity, some may just be close on the sky by coincidence but not bound by gravity. Double stars that are not binary stars bound by gravity can be separated by hundreds of light years in distance.

Spectroscopic binaries are found due to the Doppler shift of the lines in the stars' spectrum as the stars orbit their common center of mass.

Eclipsing binaries can be detected when one of the component stars passes between its companion star and an observer, blocking some of the light from the companion star and causing the combined light of the system to look briefly dimmer.

Astrometric binaries are systems where only one stellar image is observed – either due to one of the stars being too faint to be observed or the two stars' images being blended together – but where the orbital motion of the stars in the binary system causes the brightest point of the stellar image to show a periodic change in position on the sky.

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Term and definition status: This term and its definition have been approved by a research astronomer and a teacher

The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".

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Related Diagrams


Carina appears as the hull of a line drawing of a ship which faces left (east). Canopus is on its right (west) edge

Carina Constellation Map

Caption: The constellation Carina along with its bright stars and surrounding constellations. Carina is surrounded by (going clockwise from the top): Vela, Puppis, Pictor, Volans, Chamaeleon, Musca and Centaurus. Carina is notable for Canopus, the second brightest star in the night sky and for the famous variable star Eta Carinae. This latter object is a binary star system of two young very massive stars. Its brightness has varied greatly over the past few centuries. At one point it outshone Canopus before dimming by a factor of four thousand to become invisible to the naked eye. In recent years it has brightened enough to again be visible without the aid of a telescope. Carina is a southern constellation and thus the whole constellation is visible at some point in the year to the entire southern hemisphere. The whole constellation is also visible from equatorial regions of the northern hemisphere with parts of the constellation visible from northern temperate regions. The open clusters NGC 2516, NGC 3532 and IC 2602 lie in Carina. These are marked here by yellow circles. The Carina Nebula, also known as NGC 3372, lies in the constellation. This large nebula contains many massive young stars, including Eta Carinae. This diagram maps an area around the north celestial pole. Here lines of constant right ascension converge. The right ascension values of these lines are marked on the x-axis above and below the diagram. Some of the lines of constant declination are marked on the y-axis. The sizes of the stars marked here relate to the star's apparent magnitude, a measure of its apparent brightness. The larger dots represent brighter stars. The Greek letters mark the brightest stars in the constellation. These are ranked by brightness with the brightest star being labeled alpha, the second brightest beta, etc., although this ordering is not always followed exactly. Carina was previously part of the larger Argo Navis constellation along with Vela and Puppis. As the letter designations for stars were created before this division took place, Greek letter designations are now divided between the three constellations with Carina having stars designated alpha and beta but no gamma or delta. The dotted boundary lines mark the IAU's boundaries of the constellations and the solid green lines mark one of the common forms used to represent the figures of the constellations. Neither the constellation boundaries, nor the lines joining the stars appear on the sky.
Credit: Adapted by the IAU Office of Astronomy for Education from the original by the IAU and Sky & Telescope

License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons