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Published 8th May 2009 by

Binary stars are a star system consisting of two stars orbiting around their common centre of mass. The brighter star is called the primary and the other is its companion star or secondary.
Astronomical Objects Series
  1. Guide to the Constellations and Mythology
  2. What are Asteroids, Meteors and Comets?
  3. Binary Stars and Double Stars
  4. Variable Stars
  5. Supernova and Supernovae
  6. Types of Nebula and Nebulae
  7. What Is a Black Hole? Black Holes Explained - From Birth to Death
  8. Quasars
  9. Pulsars - The Universe's Gift to Physics
  10. What is inside a Neutron Star?
  11. Gamma Ray Bursts
  12. Kuiper Belt
  13. What is an Exoplanet?
  14. Galaxy Types and Galaxy Formation
  15. The Messier Catalogue
  16. The Caldwell Catalogue
  17. 25 Stunning Sights Every Astronomer Should See

The sight of binary stars partnered together is stunning, especially when they have vibrant colours.

After the invention of the telescope in the 17th century, the true nature of the night's sky became apparent. What had been mere fuzzy blobs to the unaided eye now had formed, and suddenly a whole new world of nebular, galaxies and star clusters could be observed.

When telescopes were trained on the star an interesting discovery was made - not all the stars we see as single points with our eyes are in fact alone. Some were revealed to be two stars or maybe even more. Double stars and multiple star systems were discovered.

Discovery of Binary Stars

Binary Stars were first discovered in 1767 by John Michell when he suggested that double stars might be physically attached to each other. In 1779 Sir William Herschel began observing double stars and soon thereafter published catalogues of double stars. He was the first to coin the phrase Binary Star in 1802. The modern definition restricts the term to pairs of stars which revolve around a common centre of mass (barycenter).

Hubble image of the Sirius binary system, in which Sirius B can be clearly distinguished (lower left).

Hubble image of the Sirius binary system, in which Sirius B can be clearly distinguished (lower left).


As the number of double stars being found grew, it became necessary to divide the category up further to clarify exactly what sorts of double star it was. To understand the first category, optical doubles. Imagine the 3D nature of space with stars sprinkled all over the place. From our viewpoint, one star may appear very close to another star, but this is only because the stars happen to lie in the same direction from us in space. In fact, these stars are not linked in any way. One of them could be much farther away from us than the other, but stargazing wise we have no way of knowing because everything in the night's sky looks the same distance away from us.

The second category is double stars linked by gravity. If you see one of these you are looking at a binary star. It's no coincidence that the stars of a double appear to be in the same place. They are both the same distance from us and orbit around each other. It is estimated that perhaps half of the stars in our galaxy are binary systems, although binaries account for only 5 per cent of stars observed so far.

How do you know which is which? Unless your star chart tells you so, simply observing the stars there is no way we can gauge whether the stars are gravitationally bound to each other or not.

Interacting Binary Stars

If you are looking up at a binary star system, it's fascinating to know what could be happening with the stars themselves. This is because sometimes the stars in a binary system can interact especially when on the stars are more massive than the other. In this case, the gasses can be pulled off the smaller companion, which can lead to tremendously destructive stellar explosions called novae.

You won't be able to see this kind of interaction when looking through a telescope, but double stars are still amazing to aim at. Some doubles show startling colour differences between the two stars, for example, a shimmering yellow next to a vivid blue, while others double stars will be more or less the same brightness, yet startlingly close to each other.

Importance of Binary Stars

Binary star systems are important to astrophysicists because their orbits allow the masses of two stars to be calculated. This allows other parameters such as radius and density to be estimated. These calculations allow an empirical mass-luminosity relationship to be created which allows the masses of single stars to be estimated.

Detection of Binary Stars

There are a number of ways that a binary star can be detected. Luckily most stars are large enough and bright enough that they can be directly observed. These are called visual binaries and usually have very long orbital periods (centuries or millennia) which make orbital calculations difficult.

An eclipsing binary, with an indication of the variation in intensity.

An eclipsing binary, with an indication of the variation in intensity.


If a binary star happens to orbit in a plane along our line of sight, its components will mutually eclipse and transit each other; these pairs are called eclipsing binaries or photometric binaries.

Mass Transfer and Accretion between Binary Stars

Most binary stars orbit at large distances and there is relatively little gravitational pull from one to the other. In this situation the stars are said to be detached.

It is possible for two binary stars to be close enough together that the gravitational pull of one is enough to "pull" matter from the other. The Roche Lobe is the region of space around a star in a binary system within which orbiting material is gravitationally bound to that star. If the star expands past its Roche lobe, then the material outside of the lobe will fall into the other star through a process known as Roche Lobe overflow (RLOF). These are called Semidetached binary stars.

If the stars are very close to each other than their outer atmosphere will "merge" and both stars will fill their Roche lobes. The uppermost part of the stellar atmospheres forms a common envelope that surrounds both stars. These are contact binaries.

Examples of Binary Stars

The image above right shows the most famous binary star Sirius A and B. This image was taken by the Hubble Space Telescope and was intentionality over exposed so that the much dimmer Sirius B can be seen in the lower left-hand corner. While Sirius A and B can be observed with amateur telescopes, the difference in magnitude between the two means that it is very difficult.

Albireo photographed by Jim Spinner on October 26, 2004

Albireo photographed by Jim Spinner on October 26, 2004

Jim Spinner

One of the easiest binary systems to image is Albireo in the constellation of Cygnus. Albireo is the brightest star in Cygnus and the binary pair shows a very striking difference in colour, with Albireo A glowing yellow and Albireo B a little fainter glowing in the blue light. The image right was photographed using an 8" Yeovil SCT and Philips Toucam WebCam by Jim Spinner on October 26th, 2004.

Some must see Double Stars

Mizar & Alcor13h 23.9m+54° 56'11.8'
Albireo19h 30.7m+27° 58'34"
Epsilon Lyrae18h 44.3m+39° 40'3.5'
Almach02h 03.9m+42° 20'9.8"
Ras Algethi17h 14.6m+14° 23'4.6"
Trapezium05h 35.3m-05° 23'25"
Castor07h 34.6m+31° 53'3"
Izar14h 45.0m+27° 04'2.8"
Tutorial Series

This post is part of the series Astronomical Objects. Use the links below to advance to the next tutorial in the couse, or go back and see the previous in the tutorial series.

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