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Binary and Multiple Star Systems
X-ray image of
Zeta Orionis A (upper
right)
and B (lower left)
(NASA/CXC/
W.Waldron, J.Cassinelli)
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In many binary systems the stars orbit their common center of mass under the
influence of their mutual gravitational force, but they evolve independently.
These are called wide binaries, and are analogous to friends that are far apart
and stay in touch with an occasional telephone call or e-mail on holidays. The
hot upper atmospheres, or coronas, of these stars
can produce X-rays, but not nearly so spectacularly as the X-ray binaries
discussed below and elsewhere. Wide binaries are nevertheless important because
they provide the best means for measuring the masses of stars by observing the
size and period of the orbit and then applying the theory of gravity.
In some binary systems, called close binaries, the stars are so close together
that they can transfer matter to each other and change the way the stars look and
evolve. They are like very close friends or family members who strongly affect
each other's lives.
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