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
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
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
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
Not all accreting binaries are this
exotic, but mass accretion from
one star onto another is a
common and very important
phenomenon in astronomy.
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
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
The Gravitational Potential Energy Surface
The gravitational potential energy for a binary system is plotted in the
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.
X-ray Binary Animation
( 2.1 MB QuickTime
- Source ).
Here are some
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.
adjacent computer simulation by Professor
John Blondin and collaborators at North Carolina
State University illustrates very complex accretion in a binary system