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Issue 5.06

The Astrophysics Spectator

April 4, 2008

Binary stars are surprisingly common.  They are so common that the star without a companion star is the exception rather than the rule.  Often the stars in a binary system are very close to one-another, with the stars in about half of the binary systems separated by a distance smaller than the distance of Jupiter from the Sun.  Some of the binary systems have stars separated by less than 1 AU, which is why one finds binary systems oriented so that the stars eclipse one-another, and why there are binary systems with stars touching each-other.  The greatest separations are around 4,000 AU, which is still very small compared to the distances between the stars.  Binary systems are comparable in size to our Solar System.

The small size of binary systems is fortuitous, because the stars in these systems complete an orbit in one or two decades, rather than in the several thousands years required by stars separated by 100 AU.  The decade-long periods make binary stars useable scales for weighting stars.  The most tightly-bound binaries give birth to the compact binaries that light the x-ray sky; compact binaries give astronomers and astrophysicists their best laboratories for studying neutron stars and black holes. Because of binary stars, we have a good understanding of stellar evolution, of the Galactic disk's composition, and the physics of compact objects.

This issue of the web site adds the second of a series of articles discussing binary stars and their impact on theories for star formation.  This is an introductory article that provides some background for a planned page on the theories of binary-star birth.

Next Issue:  The next issue of The Astrophysics Spectator is scheduled for April 16.

Jim Brainerd


Binary Stars.   Of the stars in the Galactic disk, only about one-third are without a companion star.  Most stars are in binary star systems, and many are in systems containing three or four stars.  Binary star systems are surprisingly small, with most smaller than the Solar System.  Usually the two stars in a system are not dramatically different in mass.  Binary stars are interesting for three reasons: they enable us to weight stars, they give us a clue about how stars form, and they give rise to the compact binary systems, which are the x-ray-bright systems containing a degenerate dwarf, a neutron star, or a black hole.  (continue)

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