Issue 3.16

The Astrophysics Spectator

September 20, 2006

This issue's topic is cataclysmic variables, the compact binary systems that contain a degenerate dwarf. This class of binary include the classical nova, which is the original nova that prompted the prefix of super to distinguish it from the consequences of stellar collapse. Much of the radiation we see from these systems is released as visible and ultraviolet light by the accretion disk surrounding the degenerate dwarf. X-rays are leased behind a standing shock wave at the surface of the neutron star, as the gas in the accretion disk strikes the atmosphere of the degenerate dwarf.

The cataclysmic variable shares much of its physics with the low-mass x-ray binary, for both systems transfer mass from a low-mass star to a compact star through Roche lobe overflow. The evolution of such systems is stable. Unlike with the low-mass x-ray binaries, however, the path from a binary pair of main-sequence stars to a binary systems containing a degenerate dwarf star and a main-sequence star does not require an exotic phase where the two stars share a common envelope. The transition from main-sequence star to degenerate dwarf does not expel significant amounts of mass, which means that the binary system is not disrupted by the transition. As a consequence, while cataclysmic variables are less luminous than low-mass x-ray binaries, they are much more common, and so easily found.

I added a news item about the naming of the dwarf planet Eris, which had been nicknamed Xena, for the television warrior princess. The choice of names appears deliberately ironic, as Eris is the Greek goddess of strife, and strife describes the process of defining what is a planet. But the question I have is did someone at the International Astronomical Union intend to name Eris's moon after Lucy Lawless, the actress who played Xena, because the name they chose, Dysnomic, means lawlessness.

Next Issue: The next issue of The Astrophysics Spectator is planned for release in two weeks on October 4.

News

Largest Dwarf Planet Given Name Eris (September 13, 2006). The largest dwarf planet in our Solar System is now formally named Eris, after the Greek god of strife. The name is fitting, given that the discovery of Eris led to the demotion of Pluto to dwarf planet status. The moon of the Eris was given the name Dysnomic. (continue)

Stars

Evolution of Cataclysmic Variables. Cataclysmic variables are compact binary systems containing a degenerate dwarf and a main-sequence star. Mass is transferred in the system from the main-sequence star to the degenerate dwarf. The period of these systems changes as they lose orbital energy and angular momentum through two mechanisms: a stellar wind and gravitational radiation. At the beginning, the period of the binary system decreases over time. When the period reaches 3 hours, the stellar wind is thought to shut off, and the system goes dark. At 2 hours, they become bright again. At 80 minutes, the source of pressure at the core of the main-sequence star changes, causing the period to increase with further mass transfer, and giving us a lower bound in the orbital period of cataclysmic variables. (continue)