The brightest X-ray sources in our galaxy are X-ray binaries. These X-ray
binaries are two stars which rotate around each other. One of the two is
a normal star; but the other is a collapsed star, such as a neutron star
or a black hole, which has about the same mass as our Sun but has shrunk
to ten kilometers or less in radius.
Material is drawn from the normal star and spirals in
via an accretion disk onto the compact star. Intense X-ray
emission is released from the inner region of the accretion
disk where it falls onto the collapsed star.