Cataclysmic variable star

Cataclysmic variable stars are unique binary star systems. A binary system consists of two stars orbiting one another. These stars undergo irregular increases in brightness. They swell to a very high brightness and then drop back to a quiet state. This quiet state is called quiescence. These systems are important because they show us how stars interact through gravity. They are often classified by how much their brightness changes during an outburst.

The mechanism of a cataclysmic variable relies on the relationship between two specific stars. The system includes a white-dwarf primary and a mass-transferring secondary star. The secondary is often called the donor star. It is usually less massive than the white dwarf. Because the stars are so close, the gravity of the white dwarf distorts the donor star. This gravity pulls material away from the donor. Most of this material is rich in hydrogen. As the hydrogen falls toward the white dwarf, it forms an accretion disk. This is a rotating disk of matter. The material loses gravitational potential energy as it falls. This process powers strong ultraviolet and X-ray emissions from the disk. Eventually, the material at the inner edge of the disk hits the surface of the white dwarf.

There are several distinct types of outbursts in these systems. A classical nova occurs when hydrogen accumulates on the white dwarf. The density and temperature at the bottom of this hydrogen layer rise. This triggers runaway hydrogen fusion reactions. These reactions rapidly convert the hydrogen into helium. Some systems experience dwarf novae, also known as U Geminorum stars. These are smaller outbursts caused by instabilities in the accretion disk. The outer part of the disk switches from a cool, dull mode to a hotter, brighter mode. These dwarf novae can recur on timescales ranging from days to decades.

Astronomers classify these stars into many subgroups based on their unique behaviors. Some are called polars, such as AM Herculis stars. In these systems, the white dwarf has a very strong magnetic field. This field can disrupt the inner accretion disk or prevent it from forming. Instead, the magnetic field channels the matter directly onto the white dwarf. Another group is called intermediate polars, or DQ Herculis stars. These have slightly weaker magnetic fields that create substructure within the disk. There are also recurrent novae like T Pyxidis and RS Ophiuchi. These stars have outbursts that repeat every 10 to 80 years. Other types include SU Ursae Majoris stars, which have bright superoutbursts, and Z Camelopardalis stars, which temporarily halt at certain brightness levels.

Discovery of these stars often involves amateur astronomers. When a cataclysmic variable is in its outburst phase, it is quite bright. It can be detected with modest instruments. Scientists can verify these stars because they are usually blue. They also show rapid variability and peculiar emission lines. They emit light in the ultraviolet and X-ray ranges. While models suggest they should emit gamma rays from positron annihilation, this has not been detected yet. In our own Milky Way galaxy, about six galactic novae are discovered each year. However, models suggest the true rate might be between 20 and 50 per year. This gap exists because of interstellar dust and difficulties observing during certain times of the year.

The scale of these events can be massive. A classical nova outburst can change a star's brightness by 6 to 19 magnitudes. Recurrent novae have smaller outbursts of 4 to 9 magnitudes. In extreme cases, a white dwarf can reach the Chandrasekhar limit. This is a specific mass limit for white dwarfs. If the accretion process continues, the increasing density may ignite runaway carbon fusion. This triggers a Type Ia supernova. This is a massive explosion that completely destroys the white dwarf.

Cataclysmic variables connect to many different areas of science. Some systems, like AM Canum Venaticorum stars, consist of two white dwarfs. These are important because they are sources of gravitational waves. Other systems, like Z Andromedae, are known as symbiotic variables. These involve a large, cool component losing mass to a hot, compact component. There are over 1,600 known cataclysmic variable systems in catalogs. These stars help scientists understand the life cycles of stars and the physics of extreme gravity.