Issue 3.19

The Astrophysics Spectator

November 8, 2006

This issue is light, adding a single page that describes degenerate dwarf stars, which are more commonly called white dwarf stars.

Degenerate dwarf stars are the evolutionary end of all main-sequence less massive than about 10 solar masses. They are the second most common type of star in our universe, outnumbered only by the low-mass main-sequence stars. A degenerate dwarf star cannot generate energy, because it can neither gravitationally contract nor burn thermonuclear fuel. A degenerate star simply cools, losing whatever heat is left from its earlier life of thermonuclear fusion and gravitational contraction. Even when young, a degenerate dwarf is much dimmer than a main-sequence star with a comparable surface temperature, because the degenerate dwarf, despite having a mass comparable to the Sun's, has a radius comparable to Earth's. Most of the numerous old degenerate dwarf stars in our Galaxy are invisible to our telescopes. These stars comprise part of the “dark matter” of the galaxy, matter whose existence is betrayed by the Galaxy's gravitational field.

Degenerate dwarfs are the engines of cataclysmic variables, pulling gas from their companion stars and pulling that gas onto themselves. Distant degenerate dwarf stars are only visible through this mechanism.

Isolated degenerate dwarfs provide a tool for measuring the age of the galaxy. The coolest degenerate dwarf stars in our Galaxy were created in the Galaxy's initial burst of star formation; by modeling how these degenerate dwarf stars cool, and comparing the model to the observed photosphere temperature, one can estimate the age of our Galactic disk.

Next Issue: The next issue of The Astrophysics Spectator is planned for release in two weeks on November 22.

Stars

Degenerate Dwarf Stars. Degenerate dwarf stars, more commonly called white dwarf stars, are the evolutionary endpoint for main-sequence stars of less than 10 solar masses. These stars generate no energy; instead, they are stable stars that are gradually cooling. They are supported from gravitational collapse by electron degeneracy pressure, the pressure exerted by a cold gas of electrons because of the Pauli exclusion principle. Degenerate dwarfs are dim and difficult to find, but they are one of the largest components of mass within the Galaxy. (continue)