White Dwarfs and Novae

Stars low and intermediate mass, up to about 8 solar masses, end their life as a white dwarf - the collapsed dead core of a red giant that has shed its out layers in a planetary nebula. The core is no longer able to sustain nuclear fusion reactions and resists complete gravitational collapse only through electron degeneracy pressure when it reaches a size similar to that of the Earth. The energy released by contraction, coupled with the initially very hot core temperature, means that white dwarfs are born hot - up to 200,000 K. At these temperature, the white dwarf is a source of soft X-rays that has been detected by X-ray and EUV sky surveys carried out by satellites such as Einstein, ROSAT, and the Extreme Ultraviolet Explorer. High resolution X-ray spectrographs on board the Chandra X-ray Observatory enable the study of the surface properties of the white dwarf atmosphere, providing clues to internal structure and evolutionary origin. White dwarfs are generally thought to be born with a mass of about 0.6 solar masses. However, in very close interacting binary systems, the white dwarf can accrete material from a stellar companion. This material is either channeled directly onto the white dwarf by magnetic fields or else forms a disk as it gradually spirals in to the white dwarf surface. The gravitational energy released during the accretion process heats up a boundary layer to high temperatures, making these types of object strong UV and X-ray sources. Accretion is often quite dramatically variable and these objects are referred to as cataclysmic variables. Sometimes the accreted material becomes sufficiently hot and dense to initiate a thermonuclear runaway, during which nuclear reactions release sufficient energy to brighten the object dramatically. These are referred to as novae. If the white dwarf can grow to more than about 1.4 solar masses - known as the Chandrasekhar limit - it can further collapse against electron degeneracy triggering a conflagration known as a Type Ia supernova.