Cataclysmic variable star
Encyclopedia
Nova
A nova is a cataclysmic nuclear explosion in a star caused by the accretion of hydrogen on to the surface of a white dwarf star, which ignites and starts nuclear fusion in a runaway manner...
e, from the Latin 'new', since ones with an outburst brightness visible to the naked eye and a quiescent brightness invisible appeared as new stars in the sky.
They consist of two component stars; a white dwarf
White dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...
primary, and a mass transfer
Mass transfer
Mass transfer is the net movement of mass from one location, usually meaning a stream, phase, fraction or component, to another. Mass transfer occurs in many processes, such as absorption, evaporation, adsorption, drying, precipitation, membrane filtration, and distillation. Mass transfer is used...
ring secondary. The stars are so close to each other that the gravity of the white dwarf distorts the secondary, and the white dwarf accretes matter from the companion. Therefore, the secondary is often referred to as the donor star. The infalling matter, which is usually rich in hydrogen, forms in most cases an accretion disc
Accretion disc
An accretion disc is a structure formed by diffuse material in orbital motion around a central body. The central body is typically a star. Gravity causes material in the disc to spiral inward towards the central body. Gravitational forces compress the material causing the emission of...
around the white dwarf. Strong UV and X-ray emission is often seen from the accretion disc. The accretion disk may be prone to an instability
Instability
In numerous fields of study, the component of instability within a system is generally characterized by some of the outputs or internal states growing without bounds...
leading to dwarf nova
Dwarf nova
A U Geminorum-type variable star, or dwarf nova is a type of cataclysmic variable starhttp://www.sai.msu.su/groups/cluster/gcvs/gcvs/iii/vartype.txt consisting of a close binary star system in which one of the components is a white dwarf, which accretes matter from its companion...
outbursts, when a portion of the disk material falls onto the white dwarf; the cataclysmic outbursts occur when the density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
and temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
at the bottom of the accumulated hydrogen layer rise high enough to ignite nuclear fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
reactions, which rapidly convert the hydrogen layer to helium.
If the accretion process continues long enough to bring the white dwarf close to the Chandrasekhar limit
Chandrasekhar limit
When a star starts running out of fuel, it usually cools off and collapses into one of three compact forms, depending on its total mass:* a White Dwarf, a big lump of Carbon and Oxygen atoms, almost like one huge molecule...
, the increasing interior density can ignite runaway carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
fusion and trigger a Type Ia supernova
Type Ia supernova
A Type Ia supernova is a sub-category of supernovae, which in turn are a sub-category of cataclysmic variable stars, that results from the violent explosion of a white dwarf star. A white dwarf is the remnant of a star that has completed its normal life cycle and has ceased nuclear fusion...
explosion, which completely destroys the white dwarf.
Classification
Cataclysmic variables are subdivided into several smaller groups, often named after a bright prototype star characteristic of the class. In some cases the magnetic fieldMagnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
of the white dwarf is strong enough to disrupt the inner accretion disk or even prevent disk formation altogether. Magnetic systems often show strong and variable polarization in their optical light, and are therefore sometimes called polars
Polar (cataclysmic variable)
A Polar is a type of cataclysmic variable binary star system with a very strong magnetic field.In most cataclysmic variables, matter from a main-sequence companion star is gravitationally stripped by a white dwarf star in the form of an accretion disk. In polar systems, the magnetic field of the...
; these often exhibit small-amplitude brightness fluctuations at what's presumed to be the period of rotation of the white dwarf
| (Classical) nova Nova A nova is a cataclysmic nuclear explosion in a star caused by the accretion of hydrogen on to the surface of a white dwarf star, which ignites and starts nuclear fusion in a runaway manner... e |
These cataclysmic variables have very large outbursts, of 6 to 19 magnitudes, caused by thermonuclear fusion of material accreted onto the white dwarf. | ||||||
| Recurrent novae | These have outbursts of about 4 to 9 magnitudes, repeating every 10 to 80 years. Examples include T Pyxidis T Pyxidis T Pyxidis is a binary star system in the constellation Pyxis estimated at about from Earth. It contains a sun-like star and a white dwarf. Because of their close proximity and the larger mass of the white dwarf, it draws matter from the larger, less massive star which causes periodic... and RS Ophiuchi. |
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| Dwarf nova Dwarf nova A U Geminorum-type variable star, or dwarf nova is a type of cataclysmic variable starhttp://www.sai.msu.su/groups/cluster/gcvs/gcvs/iii/vartype.txt consisting of a close binary star system in which one of the components is a white dwarf, which accretes matter from its companion... e |
Dwarf novae, or U Geminorum stars, are cataclysmic variables which are observed to brighten repeatedly, though by a smaller amount than classical novae.
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| Polars Polar (cataclysmic variable) A Polar is a type of cataclysmic variable binary star system with a very strong magnetic field.In most cataclysmic variables, matter from a main-sequence companion star is gravitationally stripped by a white dwarf star in the form of an accretion disk. In polar systems, the magnetic field of the... |
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| VY Sculptoris | These are stars which occasionally drop in brightness by more than one magnitude, with very occasional dwarf-nova-type outbursts during the dim state. They may be a subclass of polars. | ||||||
| AM Canum Venaticorum | These are cataclysmic variables both of whose components are white dwarfs; the accretion disc is composed primarily of helium, and they are of interest as sources of gravitational waves. | ||||||
| SW Sextantis | These are like dwarf novae but have the accretion disc in a steady state, so don't show outbursts; the disc emits non-uniformly. They are usually also eclipsing variables, though this appears to be a selection artefact. |
There are over 1600 known CV systems. The catalog was frozen as of 1 February 2006 though more are discovered each year.
Discovery
Cataclysmic variables are among the classes of astronomical objects most commonly found by amateurs, since a cataclysmic variable in outburst is bright enough to be detectable with very modest instruments, and the only celestial objects easily confused with them are bright asteroids whose movement from night to night is clear.Around six novae are discovered each year, whilst models based on observations in other galaxies suggest that the rate of occurrence ought to be between 20 and 50; this discrepancy is due partly to obscuration by interstellar dust, and partly to a lack of observers in the southern hemisphere and to the difficulties of observing while the Sun is up and at Full Moon.
Verifying that an object is a cataclysmic variable is also fairly straightforward: they are usually quite blue objects, they exhibit rapid and strong variability, and they tend to have peculiar emission lines. They emit in the ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
and X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
ranges; they are expected also to emit gamma rays, from annihilation of positrons from proton-rich nuclei produced in the fusion explosion, but this has not yet been detected
External links
- The Catalog and Atlas of Cataclysmic Variables
- TPP/CVcat - a catalogue of Cataclysmic Variable Stars
- RKcat (Ritter and Kolb), 7th edition
- CVNet, a web site and community for CV enthusiasts and researchers - features announcements of new discoveries
- A Beginner's Guide to Cataclysmic Variables - features a very good categorisation of the different classes of stars
- Cataclysmic Variables, NASA's HEASARC page