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Issue 3.13

The Astrophysics Spectator

August 2, 2006

This week, The Astrophysics Spectator continues its examination of close binary systems with an overview of the cataclysmic variables and the x-ray binaries added to the “Stars” topical path.

Many stars begin life as members of binary star systems. For most systems, this binary relationship has no effect on the evolution of either star in the system, but for the small fraction of systems that have a small separation between the stars—the compact binaries— the binary relationship dramatically alters the end evolution of both stars. In particular, one star can cannibalize the other. We see this happening today in the cataclysmic variables and the x-ray binaries.

Cataclysmic variables are compact binary star systems that contains a degenerate dwarf and a main-sequence star (a star powered by hydrogen fusion in its core). The degenerate dwarf in this system is larger than the main-sequence star. What makes these stars systems stand out is that the atmosphere of the main-sequence star is flowing onto the degenerate dwarf, which generates visible light, ultraviolet light, and x-rays.

The x-ray binaries are compact binary stars that contain a neutron star or a black hole candidate. These systems fall into two categories: low mass x-ray binaries, where the cannibalized star is about 1 solar mass or less, and the high mass x-ray binaries, where the cannibalized star is larger than about 10 solar masses.

The low mass x-ray binaries illuminate themselves in much the same way as the cataclysmic variable. The one difference is that while the fusion-powered star in the cataclysmic variable is always a main-sequence star, the fusion-powered star in the x-ray binary can be a main-sequence star, a star burning helium or carbon, or a degenerate dwarf. This wider variety of mass donors gives the x-ray binary systems a wider range of orbital periods. While the orbital period of a cataclysmic variable is no shorter than 80 minutes, the low mass x-ray binary can have an orbital period as short as about 10 minutes.

The mechanism that transfers mass from the fusion-powered star to the compact star in a high-mass x-ray binaries is generally much different than in the low mass x-ray binary. In most high-mass binaries, the compact star captures some of the wind blowing away from the cannibalized star. This makes the high-mass binary much more volatile than the low-mass binary.

This fortnight's edition also adds a commentary on freedom of speech, motivated by the controversy at the University of Wisconsin over the teaching of conspiracy theories.

Next Issue: The next issue of The Astrophysics Spectator is planned for release on August 16.

Jim Brainerd


Content-Free Academic Freedom. Professor Stanley Fish's idea of academic freedom is that the scholar can study anything that has an “intellectually payoff,” but he cannot proselytize his students to his beliefs. This view of academic freedom cannot work in the sciences, where an undergraduate education consists of nothing but proselytizing students to our basic understanding of science. (continue)


Compact Binary Stars. When the distance between two stars in a binary system is short, each star can affect the other's structure and evolution. In time, one of these stars exhausts its nuclear fuel and becomes a degenerate dwarf, neutron star, or black hole candidate. With this transition, the binary system can be a brilliant source of light as the compact star cannibalizes its companion. These bright compact binaries fall into two classes: the cataclysmic variables, which contain a degenerate dwarf star, and the x-ray binaries, which contain either a neutron star or a black hole candidate. (continue)

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