Cataclysmic Variable Stars
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	Most stars appear unchanging and ‘boring’ unless you watch them over billions of years. Observe a Cataclysmic Variable Star (CV) for half an hour and it can change in brightness by a factor of 10 to 100. The next night it may not be visible at all. Cataclysmic Variables (CVs) are binary star systems, which consist of a late type main sequence star (K or M type) and a white dwarf star. These are referred to as the secondary star and the primary star. The orbital period usually ranges from 1 to 10 hours. Both stars are close to each other, so close that they are semidetached and therefore material from the secondary star overfills its Roche lobe and matter falls toward the white dwarf forming an accretion disk. Where the matter meets the disk a bright spot or shock front occurs. Periodically the disk flares up brightening significantly How they evolve CVs evolve from 2 main sequence stars to semi-detached binaries. One star is a little more massive than the other star. They may orbit each other every few years and have a distance of 100 million kilometres or so [2]. The more massive star evolves more rapidly than the lesser massive star and forms into a red giant. It overflows its Roche lobe and matter is transferred to the secondary star. The orbital separation begins to decrease as the system loses angular momentum. As the stars move closer together the giant loses all of its hydrogen envelope onto the secondary and an envelope between the two. The giant turns into a white dwarf. The distance between the 2 stars is now around a million kms. This process may take a thousand years [3]. The matter now travels in the opposite direction from the secondary to the white dwarf. This is because the secondary has overfilled its Roche lobe. The secondary star now has a Roche lobe shape and becomes tidally locked i.e. the period of rotation is the same as the orbital period. The matter streams out from the inner Langrangian point and forms a disk around the white dwarf due to the coriolis force. The disk becomes very bright due to the friction of the gasses closer into the white dwarf against the gas slightly further out. Both travel at different speeds due to Keplers Law. The brightest part of the system is the disk about the white dwarf. The matter eventually accretes onto the white dwarf. Once the disk is formed, matter continues to fall from the secondary and onto the disk forming a bright spot. X-ray radiation is emitted. These features can be seen in the light curves of CVs. The spheroidal shape of the secondary can cause dips in the curve called spheroidal variations as it rotates. The bright spot on the disk can be seen as humps when it moves in front of the secondary. A large dip occurs when the white dwarf is eclipsed. As the white dwarf is small, the dip is sharp. The bright spot becomes eclipsed slightly later giving the dip a non-symmetrical shape. This shows that the disk is much larger than the white dwarf or the bright spot. The size of the disk can be interpreted from the light curve. The period between the dips determines the orbital period. Measuring radial velocities from absorption lines can also verify the orbital period. The light curve will also be different depending on if the system is edge onto us or at a higher angle. The lightcurve will also be different depending on what wavelength is measured. The secondary will give off more light at the red end of the spectrum since its around 3000 degrees K. The white dwarf at 12000K will be much brighter in the blue or ultraviolet. The disk can vary from 5000K at the outer edge to 30 000K at the inner edge [4]. The disk is highly turbulent and unstable. As matter falls on the disk it piles up as it is accreting faster than the material can move through the disk. This increases the angular momentum transport and in turn increases the accretion of matter onto the white dwarf. The disk will increase in brightness around 100 times at this stage. This is called an outburst. This changes the lightcurve in that the disk is the major player instead of the white dwarf. Soon after the disk is cleared and the system is back in quiescence. The outbursts happen periodically perhaps every few months but vary by several days. Lightcurves usually show non-periodic flickering. This demonstrates that the disk is turbulent and is thought to be generated from the inner disk. Some CVs show it maybe from the bright spot. Dwarf novae are a subclass and show outbursts 2 to 5 magnitudes above their quiescent brightness. The outbursts are thought to be accretion powered. It maybe due to thermal instabilities in the accretion disk which feeds the white dwarf or maybe due to modulated mass transfer from the secondary [6]. Subclasses are named after there prototype for example U Gem (UG), Z Cam, SU Uma, SS Cyg [2]. They have characteristic and different lightcurves. Another class is VY Scl stars which are unusually bright and fade unpredictably. These are sometimes referred to as antidwarf novae. UG CVs brighten10 to 250 times periodically. They also have smaller jumps in brightening in their quiet state in the order of 0.05 to 0.5 magnitudes in time scales of minutes or hours. This is called flickering. U Gem was the first observed dwarf nova by Russell Hind in 1855. UGSU CVs have the characteristic of having superoutbursts which are a magnitude brighter than their normal outbursts. They have a periodic small amplitude jump called superhumps which are superimposed on the outburst light curve. These superhumps have a slightly longer period than their orbital period. The CVs discussed so far involved are non magnetic. That is the magnetic field of the white dwarf interacts weakly on the accretion flow. Where the white dwarf is strongly magnetic the accretion process changes as the ionized gas will follow magnetic field lines and also the moving charges will create their own magnetic fields. These CVs are much brighter in x-rays. These Vs are called Polars [4] and have different sub groups depending on the spin of the white dwarf, field strength, and accretion rate.