The Cataclysmic Variable is a binary star system that consists of a white dwarf primary and a normal star companion. Typically, the two stars orbit very close to each other and often have a short orbital period ranging between 1 to 10 hours. Astronomers generally refer to the white dwarf star as the primary star of the binary, while they call the normal star the secondary or companion. In a cataclysmic variable, the companion star, usually a normal star like our Sun loses material onto the nearby white dwarf through the process of accretion
Astronomers classify Cataclysmic Variables (CVs) according to the properties of the observed outbursts. The two major types of CVs are classical novae and dwarf novae. Classical novae erupt once, and the amplitude of their outbursts is usually the largest observed among the cataclysmic variables. Classical nova outbursts occur when the companion’s hydrogen-rich material collects on the surface of the white dwarf and suddenly undergoes thermonuclear fusion. Because a white dwarf is an essentially dense, hot cinder (the glowing remnant of a sun-like star), this cataclysmic nuclear burning occurs only in the layer of hydrogen accreted onto its surface.
A dwarf nova outburst results from temporary increases in the rate of accretion onto the surface of the nearby white dwarf. Dwarf nova outbursts may be smaller in amplitude and higher in frequency than classical novae. The first dwarf nova discovered is called U Geminorum (U Gem), and its behavior is typical of many other dwarf novae. The optical brightness of U Gem increases 100-fold every 120 days or so and then returns to the original level after a week or two. Optical astronomers have also categorized several other types of Cataclysmic Variables. There are recurrent novae, systems with eruptive outbursts that fall in between the behaviors of classical and dwarf novae. Then there are nova-like systems, stars that have visual light spectra similar to other types of Cataclysmic Variables but have not been observed erupting.
X-ray astronomers have observed that many Cataclysmic Variables are weak X-ray sources. This is not surprising, since matter accreting from the companion can easily reach 100 million K or so near the surface of the white dwarf. Orbiting X-ray observatories have collected data that reveal the details of the accretion process near the primary. In the majority of Cataclysmic Variables the flow of material from the companion to the primary proceeds by means of an accretion disk. This happens because the material leaving the secondary has angular momentum due to orbital motion in the binary system, and so accretion cannot take place in a straight line. Rather, a disk-like structure forms in the plane of the binary orbit. Astrophysicists speculate that the X-rays observed from Cataclysmic Variables come from the boundary layer of the white dwarf—the region where the accretion disk hits the surface of the white dwarf. The Cataclysmic Variables that are the strongest X-ray emitters appear to have a magnetic white dwarf as the primary.
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