What Are Binary Stars and Double Stars

Binary Stars and Double Stars are star systems consisting of two stars orbiting around their common centre of mass.

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 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).

As the number of binary stars being found grew, it became necessary to divide the category up further to clarify exactly what sort of double star it was. To understand the first category, optical doubles.

Classification of Double and Binary Stars

Space by its very nature functions in three dimensions. Stars are sprinkled all over the place. From our viewpoint, one star may appear very close to another star. This can be because the stars just happen to lie in the same direction as us in space. The stars may not be linked in any way. They could in fact be many light years apart. By looking at them when stargazing, we have no way of knowing because everything in the night sky looks the same distance away from us. We call these stars which appear to be double but are separated by great distances an optical double, or just a double.

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? You will need to use a star chart or star catalogue. By simply observing the stars there is no way we can tell whether the stars are gravitationally bound to each other or not.

Interacting Binary Stars

If you are looking at a binary star system, it's fascinating to wonder what could be happening with the stars themselves. This is because sometimes the stars in a binary system can interact especially when one of the stars is 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 binary stars are still amazing to aim at. Some binaries show startling colour differences between the two stars, for example, a shimmering yellow next to a vivid blue, while other binary 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 several 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.

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.

Two binary stars can 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 then their outer atmosphere will "merge" and both stars will fill their Roche lobes. The uppermost part of the stellar atmosphere 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 intentionally 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.

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 colour difference, 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 Binary Stars