Variable star
There are several types and subclasses of variable stars.
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Type I Variables
Type One Variables are stars that regularly pulsate in size and change in brightness. As the star increases in size, its brightness decreases; then, the reverse occurs. Type One Variables may not be permanently variable; the fluctuations may just be an unstable phase the star is going through.
Type II Variables
A Type Two Variable star is a variable star whose brightness and size cycle over a very long time period, in the order of many standard months. Type Twos are pulsating red giants that vary in magnitude as much as a factor of many hundred.
Cataclysmic Variables
Cataclysmic Variables (CVs) are interacting binary stars in which a main sequence star rotates around a white dwarf, i.e. a compact star of stellar mass but planetary size. The two stars are so close that the main sequence star, also called a red dwarf, reaches to the point in which the gravitational forces and the centrifugal force are exactly in balance. At this point it is very easy for the stellar material to transfer from the big star into the realm of the smaller one.
The fate of the material depends on the rate with which the main sequence star loses matter and on the presence of a magnetic field of the white dwarf. If there is no significant magnetic field the matter will spiral around the white dwarf, because it has too much angular momentum to fall directly onto the compact little star. Only slowly can it transfer the angular momentum to some material moving outwards, the main part of the matter slowly spirals down onto the white dwarf and forms thereby a disc, called an accretion disc.
Nova-likes
If the mass transfer rate is low these discs undergo sudden, nearly regular brightenings, called outbursts. For an explanation of these phenomena see the article on accretion discs in dwarf novae. If the mass transfer is high, such outbursts are prevented and the disc is believed to be in a steady state. These systems are called Nova-likes.
Polars
In the case that the white dwarf has a strong magnetic field, the formation of an accretion disc is prevented. Instead, the matter in the stream from the main sequence star is connecting to the field lines of the white dwarf. These may also be known as polars.
Intermediate Polars
If the magnetic field lies between the extremes, the formation of an accretion disc is not completely prevented. The material still starts to spiral around the white dwarf, but when it arrives at a certain radius, it is drawn out of the orbital plane and falls along picturesquely called accretion curtains, onto both poles of the white dwarf. These systems are called intermediate polars.
