Cataclysmic variable star

Two stars live very close together.

One star is small and heavy. It pulls gas from its partner star. This gas forms a bright ring around the small star. The ring glows with heat and light. Sometimes the stars get very bright all at once. This is called an outburst. Then the stars go back to being dim. This can happen many times. It is a big show in space!

Some stars act like a pair of partners. These are called cataclysmic variable stars. They are binary stars, which means they are two stars close together.

One star is a white dwarf. This is a small, heavy star. The other star is a donor star. The white dwarf uses gravity to pull gas from the donor. This gas forms a ring called an accretion disk. The gas in the disk gives off power. This power comes from the gas falling inward.

Sometimes, these stars have an outburst. An outburst is a sudden increase in brightness. A classical nova is a type of outburst. This happens when the hydrogen gas gets very hot. The heat causes a fusion reaction. This reaction turns hydrogen into helium. This makes the star look very bright for a time.

Other stars have dwarf-nova outbursts. These are smaller and happen more often. Some outbursts can happen every few days. Others happen every few decades. If the white dwarf gets too heavy, it might explode. This is called a Type Ia supernova. Such a big explosion would destroy the star completely.

Cataclysmic variable stars are special pairs of stars. These are called binary stars. They consist of two parts working together. One part is a white dwarf. This is a small, heavy star. The other part is a donor star. This star is usually less massive than the first one. These stars stay very close to each other. This closeness makes them very interesting to study. They change brightness in a strange way. They grow very bright and then fade back down.

Gravity is the engine that makes this work. The white dwarf uses its gravity to pull at the donor star. This pull distorts the shape of the donor. As a result, matter flows from the donor to the white dwarf. This matter is usually rich in hydrogen. It does not fall straight down. Instead, it forms a ring called an accretion disk. This disk gives off strong UV and X-ray light. This light comes from the energy of the falling material. The gas eventually hits the surface of the white dwarf.

Sometimes, these stars have big outbursts. A classical nova happens when hydrogen builds up. The bottom of the hydrogen layer gets very dense. It also gets very hot. This heat starts a runaway fusion reaction. This reaction turns the hydrogen into helium very fast. Some stars have smaller outbursts called dwarf novae. These happen when the accretion disk changes modes. It moves from a cool mode to a hot, bright mode. These can happen every few days or every few decades.

Scientists have found over 1,600 of these systems. They group them into different types based on how they act. Some are called polars because they have strong magnetic fields. These fields can stop a disk from forming. Other stars, like T Pyxidis, are recurrent novae. They repeat their outbursts every 10 to 80 years. One hydrogen-rich system, ZTF J1813+4251, has a very short orbit. Its orbit lasts only 51 minutes. There are many other types like SU Ursae Majoris stars.

These stars are often found by amateur astronomers. They are bright enough to see with modest tools. They are usually blue in color. They also change brightness very quickly. You can tell them apart from asteroids by how they move. If a white dwarf gets too heavy, something huge happens. It might reach the Chandrasekhar limit. This can trigger a Type Ia supernova. This explosion is so big that it destroys the white dwarf. It is a powerful way for stars to end.

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.