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The Andromeda galaxy (also known as M31) is one of the nearest galaxies
to our own at a distance of 2.2 million light years. It is just visible
with the naked eye on exceptionally dark nights. The X-ray image is quite
different from an optical image. The
X-ray sources that you see are not ordinary stars, like our sun, but
X-ray binaries: two stars in close orbit around each other. X-ray
binaries are special in that one of the stars is a neutron star (or black
hole in some cases). As a result of the interaction between the two stars
gas is pulled from the normal star's atmosphere onto the neutron star.
The gas swirls around the neutron star in an accretion disk. Towards the
centre of the disk the gas reaches temperatures of 10-100 million
degrees, powering a very bright X-ray source. The sources that you see
here are about a million times brighter in X-rays than stars like our sun
or those detected in the Hyades star cluster.
Around 100 X-ray binaries have been discovered in our own galaxy, and at
least as many in Andromeda. Because they are distant objects, we cannot
see the structure of an X-ray binary directly, but we can work out what
is going in these systems in a number of ways. For example the X-ray
output varies periodically in some X-ray binaries due to the fact that
the neutron star is eclipsed by the other star. The length of the orbital
period tells us the size of the orbit, and the masses of the stars whilst
the length of the eclipse tells us the relative size of the other star in
the binary that is eclipsing the neutron star.