Binaries systems can have very large separations, in which case the period, by Kepler's laws, is long. Some binaries have separations that are comparable in size to the stars themselves, however. Such systems are called close binaries. In close binaries the orbital period is small, and because the stars are so close together, matter may stream from one star onto the other star. These are called accreting binaries, and they lead to a broad range of very interesting phenomena.

An Exotic Example

The following artist's conception shows an accreting binary at the center of the globular cluster NGC 6624, which is about 28,000 light-years away in Sagittarius. The star is a source of powerful X-ray bursts.

One member of the binary is a neutron star and it has a less-massive white-dwarf star companion, seen at lower left. Matter appears to be accreting from the white dwarf onto the neutron star. Not all accreting binaries are this exotic, but mass accretion from one star onto another is a common and very important phenomenon in astronomy.

Gravitational Potentials

By the law of gravitation, every object attracts every other object in the Universe. In a binary star system, if the masses are large enough and the separations small enough, a gas particle at a large radius in one star may feel a gravitational force from the other star that is comparable to or even larger than that from its "own" star. In that case, the gas particle is unstable against being transferred from one star to the other (image adjacent right).

This can be illustrated by plotting contours of equal gravitational potential, as illustrated in the adjacent image. The center of mass is marked with an "x". The point labeled L1 is called the inner Lagrange point; it is a point where one unique potential energy contour intersects itself. If a star expands sufficiently to place matter near the inner Lagrange point, accretion can occur onto the other star.

Roche Lobes and Mass Accretion

As illustrated in the preceding image, there is a unique gravitational potential energy contour in a binary system that intersects itself in one point called the inner Lagrange point. This contour defines two regions, one around each star, called Roche lobes (see image adjacent right). Mass accretion can occur if one of the stars fills its Roche lobe, allowing matter to spill over the inner Lagrange point onto the other star. Here is an example of the Roche lobes for Algol.

The Gravitational Potential Energy Surface

The gravitational potential energy for a binary system is plotted in the adjacent diagram (Source). The stars are located at the two minima but we see that as the stars approach each other a saddle-shaped valley develops between them. The inner Lagrange point sits in this saddle, and mass can flow through this region between the two stars if they are close enough together.

Accretion Disks in Binary Systems

Because angular momentum must be conserved, accretion from one star in a binary to the other often involves the accreting matter going into orbit around the other. This matter in orbit forms an accretion disk surrounding the second star.

As the matter in the accretion disk undergoes collisions and interactions it is heated and loses energy by radiating light, with the wavelength depending on the temperature of the disk. This causes the matter to spiral from the accretion disk onto the second star.

Accreting X-ray Binary Animation ( 2.1 MB QuickTime - Source ).

Here are some supercomputer simulations of accretion in binary stars (click on the first image displayed to start).

Wind Driven Accretion

Accretion in binary systems can also take the form of a wind from the surface of one star, as opposed to a thin accretion stream flowing through the inner Lagrange point. Then the second star accumulates matter from the first star as it moves on its orbit through this wind.

In complex situations, both winds and tidal accretion streams may play a role. The adjacent computer simulation by Professor John Blondin and collaborators at North Carolina State University illustrates very complex accretion in a binary system (Source).

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