About half of the
stars visible in the
night sky are actually multiple star systems or double stars. The
gravitational force between the two stars in a binary system keeps them in
orbit about each
Our star "The Sun" is not in a binary system, although it is
considered to be a typical single star. The next nearest star is Alpha
Centauri which is 3 x 1013
When astronomers can actually see the two stars orbiting each other, the
binary is called a visual binary (see below). This binary system is Kruger 60
in the constellation Cepheus and has an orbital period of 44.5 years.
In most binary systems, both stars follow an elliptical orbit about their
common center of
Below is an illustration of a model of the elliptical pattern of a binary
system. Remember, this is just a sample model, it is not the only model.
Nevertheless, it sure seems to illustrate our point!
After many years of patient observation, astronomers can plot the orbits of
the stars in a visual binary (see below). As you can see, an astronomer has
drawn the orbit of one star with respect to the other. This illustration
shows the orbit of a faint visual double star in the constellation Ophiucus.
X-ray binaries are a special class of binary star systems which emit X-rays.
X-ray binaries are made up of a normal star and a collapsed star (a
or black hole).
These pairs of stars produce X-rays if the stars are close enough together
that material is pulled off the normal star by the gravity of the dense,
collapsed star. The X-rays come from the area around the collapsed star
where the material that is falling toward it is heated to very high
temperatures (over a million degrees!).