Binaries

A binary star is a pair of stars that appear close together in the sky. If there are 3 it is called a triple.

There are two kinds of binary:

Optical Doubles
They aren't actually close together but appear so from our line of sight. The majority fall into this category.

True Binaries
By measuring the amount of Doppler shift from the light coming from binary pairs we can measure their speed of rotation about a point somewhere between them. Where this point is, the barycentre, will depend on the relative masses of the stars involved. It will be closer to the more massive star. This is not necessarily the brighter of the pair. The first pair was discovered by Herschel in 1804, a pair known as Castor.

True binaries are difficult to find. Ones which are relatively far apart, so appear as separate stars, generally have a slow rotation velocity. One way of distinguishing a binary is by examining its light curve. If the two stars eclipse each other in our line of sight then the amount of light we detect will fall. The magnitude of Algol is usually 2.3 but every 10 hours it falls to 3.4, becoming 68% dimmer. (It is the winking eye of the Gorgon Medusa)

Binaries are actually quite common. Many may include three or more stars rotating around each other. Occasionally one sees two pairs of stars acting as a multiple binary.

An amazing type of binary is a recurrent nova. It involves a large star, such as a red giant, and a white dwarf. Matter from the large star spirals towards the white dwarf forming an accretion disc, a disc of material orbiting the smaller star. Occasionally a large amount of hydrogen from the disc falls into the white dwarf and a sudden runaway fusion reaction occurs. A violent eruption occurs producing a burst of energy and blowing away the accretion disc. This is a dwarf nova. The process starts again, with the formation of a new accretion disc, and repeats itself until the larger star runs out of sufficient matter.

A binary pair	Castor in Gemini	Algol in Perseus	A red giant and white dwarf nova pair