“They are like a hot lump of coal slowly cooling”, Ashley Yeager describes white dwarfs in her piece, “Dead – But Not Duds” in Science News, Oct 11, 2008.

White dwarfs, the dead corpse of a star with a mass less than about 1.4 solar masses, create in their final days, spectacular planetary nebula, such as the Helix Nebula, the Ring Nebula and the Cat’s Eye Nebula.

They have become especially famous since the early 1990s as standard candles in the measurement of the acceleration of the universe and the primary evidence of dark energy.

All Type Ia supernova are believed to be caused when a companion star overflows onto a white dwarf. The white dwarf’s mass grows until it reaches the critical value of about 1.4 solar masses, at which point the thermal and degeneracy pressures can’t hold back the pull of gravity and its core collapses into a neutron star in a supernova explosion. If all white dwarf cores collapse at the same mass, the supernova explosions should have about the same absolute luminosity.

Understanding the nature of white dwarfs is all the more important to calibrate how standard the candle is. So far, three types of white dwarfs have been identified based on their surface chemistry, those with hydrogen outer shells, those with helium and those with carbon.

But perhaps the most intriguing property of white dwarfs is that they pulsate. More than 200 pulsating white dwarfs have been discovered with periods ranging from about 40 seconds up to 20 minutes and magnitude changes from less than 1% to almost 30%. The frequencies at which they pulsate may be related to the resonant frequencies of the cores, and reveal information about the density, speed of sound, rotation and mass.

Yeager says a better description of them than a hot lump of coal is they are a hot, ringing bell, slowly cooling. The pulsations are believed to be from ringing, or vibrations of the dense white dwarf material. A white dwarf is about the mass of the sun, compressed to a size about the earth, with a density more than 200,000 times that of the earth and a surface force of gravity about 100 million times stronger than on Earth. Studying the pulsations has been termed astroseismology.

The slowing period is a measure of the cooling rate. It’s possible to test cooling processes from the pulsation rate. In addition to radiative cooling, there appears to be a contribution from neutrinos and possibly by emission of axion particles, that are believed to make up dark matter. At least, it is possible to put an upper limit on the mass of the as yet undiscovered axion particle, a possible dark matter candidate, as less than 5 milli-eV.

The most intriguing possibility being searched for is variations in the pulsation periods that might be due to an orbiting planet. A Jupiter size planet might be detectable given another year of observing time.

Understanding the nature of white dwarfs will pay off locally, in gaining a glimpse of what will happen to our sun some day, and better calibrating the expansion of the universe to gain of glimpse of what its fate might be.

More information about white dwarfs is available from the White Dwarf Research Corporation, www.white dwarf.org.

Published in Dec 2008 Cosmic Messenger