When stars with eight times or less the mass of the Sun exhaust the nuclear fuel (e.g., helium, hydrogen) in their centers, they contract under their own gravity and transform into white dwarfs, which have almost the same mass as the Sun but almost the same size as the earth. White dwarfs, unlike other ordinary stars, support their own gravity by the degenerate pressure of electrons. The “cataclysmic variable” describes the binary system where a white dwarf and an ordinary star (hereinafter referred to as “companion star”) rotate in extremely close orbits and matter from the companion star escapes its gravitational sphere and flows to the white dwarf.
Cataclysmic variables are variable stars whose brightness changes drastically as their name suggests. They come in many types and the first group recognized by astronomers is called “novae.” Novae increase their brightness more than 10,000-fold over approximately one day and then decrease it gradually over several months. During this change, the strong gravity of the white dwarf surface forces the accreted matter from the companion star to contract, and then hydrogen, the major constituent of the matter, begins an unbounded nuclear fusion reaction.
Currently, the majority of cataclysmic variables are group called “dwarf novae,” not novae. Like novae, dwarf novae increase their brightness within a day or so, but the brightness increase level is less than 100-fold, or 1/100 that of the novae, hence the name “dwarf.” Another difference between the dwarf novae and novae is the frequency of explosions. Every dwarf nova repeats explosions intermittently at a rate of one per several weeks to several months. Meanwhile, the past records of nova explosions show that the shortest interval is 12 years and most of the explosions take place once in their histories. It is thought that a typical explosion cycle is 10,000 years.
Fig.1 shows an image of a typical cataclysmic variable. Since the matter flowing out from the companion star’s gravity sphere has angular momentum, it accretes, as forming a thin disk that moves in Kepler rotation locally, to the surface of the white dwarf. The disk is called an "accretion disk.” The temperature of the disk increases as it comes close to the white dwarf and reaches up to 1 billion kelvin at its innermost area. Cataclysmic variables are generally a strong blue color. This is because most of the light they emit originates from the high-temperature accretion disks. Accretion disks are also present around neutron stars and blackholes and they cause drastic changes in X-ray intensity. So, they are very closely related to X-ray astronomy, and their basic nature has been elucidated by observations of those in cataclysmic variables.