When a star runs out of thermonuclear fuel and exits the main sequence, it has a limited number of options for what to do. Depending on its mass, it could end lives as a white dwarf, a neutron star, a black hole, or it could go supernova on its way to one of those ending states. A large fraction of stars will end their life as a white dwarf, so when a new type of white dwarf is discovered it can reveal more about what happens at the end of a star's life on the main sequence.

Published in today's issue of Astrophysical Journal Letters, a trio of astronomers from the University of Texas Austin reported on the discovery of a new type of white dwarf. When a star ends as a white dwarf, it is classified by the type of material in its outer most shell. The majority of white dwarfs, about 80 percent according to the article, have an outermost gas shell that consists of hydrogen. The remaining 20 percent have had their shell of hydrogen stripped away for some reason, and have a outer layer of helium. Last year, a new discovery reported that astronomers found a third type of white dwarf, a "hot carbon white dwarf." It is theorized that these carbon white dwarfs are the heaviest white dwarfs, they are believed to be just slightly too small to go out with a bang as a supernova.





Credit: K. Williams/ T. Jones/McDonald Obs

Along with the discovery of carbon white dwarfs, it was calculated that pulsations could occur in these stars. With the belief that these newly discovered types of stars should pulsate, their light output should be variable—changing in relation to the pulsations. Using the Struve Telescope at McDonald Observatory, the astronomers started a systematic search for one of these pulsating stars. About 800 light-years away from Earth, the team found SDSS J142625.71+575218.3.

This pulsating white dwarf lives in the constellation Ursa Major—about ten degrees east northeast of Mizar, the middle star in the handle of the Big Dipper. Its intensity fluctuated by about two percent over a period of eight minutes. This find becomes important, according to the astronomers, because it can reveal some of the internal dynamics of the star and what happens at this stage of a star's life. The astronomers speculate that the pulsations are a result of ionized carbon cooling down as it becomes neutral carbon in the star's atmosphere. The authors state that other causes may be responsible for the observed behavior. Further study, and the discovery of more stars of this type will help confirm or deny any current hypotheses.

Astrophysical Journal Letters, 2008. DOI: 10.1086/588286