Cataclysmic variable star
Binary stars that brighten dramatically via accretion-driven outbursts.
Cataclysmic variable stars are binary systems where the two stars are so close that the white dwarf's gravity distorts its companion, pulling matter from it. This companion is called the donor star and is usually less massive than the white dwarf. The infalling material, often rich in hydrogen, typically forms an accretion disk around the white dwarf. The loss of gravitational potential energy from this infalling matter powers strong ultraviolet and X-ray emissions from the disk. The shortest known orbital period for a hydrogen-rich system is 51 minutes.
These stars brighten irregularly by a large amount before fading back to a quiet state. They were originally called novae (Latin for "new") because outbursts visible to the naked eye, from an otherwise invisible star, made them look like new stars in the sky. When material from the inner edge of the disk falls onto the white dwarf's surface, it can accumulate. If the density and temperature at the bottom of this hydrogen layer become high enough, runaway hydrogen fusion ignites, rapidly converting the hydrogen to helium—this is a classical nova outburst. If accretion continues long enough to push the white dwarf near the Chandrasekhar limit, the interior density may ignite runaway carbon fusion, triggering a Type Ia supernova that destroys the white dwarf entirely.
The accretion disk itself can become unstable, leading to dwarf-nova outbursts. In these events, the outer part of the disk switches from a cool, dim state to a hotter, brighter one for a time, then returns to the cool state. Dwarf novae can recur on timescales from days to decades.
Cataclysmic variables are divided into subgroups, often named after a bright prototype star. In some systems, the white dwarf's magnetic field is strong enough to disrupt the inner disk or prevent disk formation entirely. These magnetic systems often show strong, variable polarization in optical light and are sometimes called polars; they frequently display small brightness fluctuations at what is thought to be the white dwarf's rotation period.
Over 1,600 cataclysmic variables are known, with the catalog frozen as of February 1, 2006, though more are discovered each year. They are among the most common astronomical objects found by amateurs because, during outburst, they become bright enough to detect with modest instruments.
- Known systems
- over 1600
- Catalog frozen date
- 1 February 2006
- Shortest observed orbit hydrogen rich
- 51 minutes in ZTF J1813+4251
- Galactic novae discovered per year
- around six
- Modeled galactic nova rate per year
- between 20 and 50
Lore & Background
Cataclysmic variables are binary stars with a white-dwarf primary and a donor secondary star. The white dwarf's gravity distorts the secondary, and matter rich in hydrogen accretes onto the white dwarf, usually forming an accretion disk. The infalling material loses gravitational potential energy, powering strong UV and X-ray emission. The shortest observed orbit in a hydrogen-rich system is 51 minutes in ZTF J1813+4251.
Outbursts occur via two main mechanisms. A classical nova outburst happens when the density and temperature at the bottom of the accumulated hydrogen layer ignite runaway hydrogen fusion, converting hydrogen to helium. If accretion continues until the white dwarf nears the Chandrasekhar limit, runaway carbon fusion may trigger a Type Ia supernova, destroying the white dwarf. Dwarf-nova outbursts arise from an instability in the accretion disk, where the outer portion temporarily shifts from a cool, dull mode to a hotter, brighter mode, recurring on timescales of days to decades.
Cataclysmic variables are subdivided into groups, often named after a bright prototype star. In magnetic systems, the white dwarf's magnetic field can disrupt or prevent disk formation; these systems often show strong, variable polarization and small-amplitude brightness fluctuations at the white dwarf's rotation period. Some CVs also experience superhumps—periodic brightenings caused by deformations of the accretion disk when its rotation resonates with the orbital period.
Reader's Guide
Cataclysmic variables are among the classes of astronomical objects most commonly found by amateurs, as their outbursts make them bright enough to detect with modest instruments; the only celestial objects easily confused with them are bright asteroids, whose movement from night to night is clear. Verifying a CV is straightforward: they are usually quite blue, exhibit rapid and strong variability, and have peculiar emission lines. They emit in ultraviolet and X-ray ranges, and are expected to emit gamma rays from positron annihilation, though this has not yet been detected. Around six galactic novae are discovered each year, while models based on observations in other galaxies suggest a rate of 20 to 50 per year; this discrepancy is attributed to obscuration by interstellar dust, a lack of observers in the southern hemisphere, and difficulties observing during daylight and full moon. The catalog of known CV systems was frozen as of 1 February 2006, though more are discovered each year.
Did You Know?
- The shortest observed orbit in a hydrogen-rich cataclysmic variable system is 51 minutes in ZTF J1813+4251.
- Dwarf-nova outbursts can recur on timescales of days to decades.
- Cataclysmic variables are expected to emit gamma rays from annihilation of positrons, but this has not yet been detected.
- Around six galactic novae are discovered each year, but models suggest 20 to 50 should occur.
More in Variable Stars 1-24
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