1. Introduction
Cataclysmic variable stars (CVs) are binary star systems containing a low-mass main sequence secondary and a white dwarf primary star. Due to the proximity of the stars to each other, the secondary star is distorted into a tear-drop shape. This also results in mass-transfer between the secondary star and the primary. As the mass flows toward the primary star, it forms an accretion disk around the white dwarf which results in a very dynamic system. To add to the dynamics of these systems, some CV systems undergo quasi-periodic outbursts. In all CV systems, the distance is so great that the stars themselves are nearly invisible in telescopes so the accretion disk is the source of most of the light visible from earth. CVs include systems such as novae, dwarf novae, polars, and intermediate polars.
Artist rendition of a cataclysmic variable star system, in this case, a dwarf-nova system.
Image permission courtesy of Mark A. Garlick.
Artist rendition of a polar. The magnetic field of the white-dwarf totally prevents the formation of an accretion disk. Instead, the magnetic field redirects all of the incoming gas toward the magnetic poles of the white dwarf.
Image permission courtesy of Mark A. Garlick.
Artist rendition of an intermediate polar. The magnetic field of the white-dwarf prevents the formation of a complete accretion disk. Instead, the magnetic field redirects some of the incoming gas toward the magnetic poles of the white dwarf.
Image permission courtesy of Mark A. Garlick.
Artist rendition of the view from a hypothetical planet in orbit around a CV system. Not only are CVs a worthy object of scientific study, but the visual imagery they can bring to mind provide a wonderful subject for planetarium use.
Image permission courtesy of Mark A. Garlick.
CVs are an excellent example of how modern technology has been instrumental in solving some of the
puzzles provided by these star systems. Astronomers are able to use tools such as photometry and
spectroscopy to extract an amazing amount of information from an object that can only be seen as a
point of light even in the largest telescopes. The use of these tools combined with the dynamics of
these systems make CVs an attractive example of what modern astronomers do.
Most of what we know about CV systems is due to the fact that there is always some kind of motion in
progress. In cataclysmic variables, a red dwarf is transferring matter onto a white dwarf. The matter
transfer results in the formation of an accretion disk around the primary star. Due to conservation of
angular momentum, the matter is unable to fall directly onto the surface of the white dwarf. Angular
momentum carries the gas stream in the direction of the orbit of the secondary star while at the same
time, falling toward the primary. Viscosity causes the gas to spread out forming an accretion disk. As
gas continues to fall towards the primary, it impacts the edge of the accretion disk where it is slowed
down. The loss in energy resulting from the decrease in speed manifests itself in a "hot-spot".
Figure 5.
Diagram of a typical dwarf-nova system. The diagram shows gas leaving the surface of the secondary star at the first Lagrange point (L1). As the gas impacts the accretion disk, it heats up creating a hot-spot.
Graphic created using Mathematica 4.2.
Figure 6.
Dwarf novae are a subclass of CVs which show quasi-periodic outbursts of 1 to 5 magnitudes. Outbursts are the result of two possible occurrences: the sudden transfer of matter from the secondary star or an instability in the accretion disk. These images show a CV (U Gem) before and during an outburst. U Gem is the lowest bright star in both images. The first image is from the online Digital Sky Survey and the second image was a short exposure taken using a CCD camera and the 31" NURO telescope, part of Lowell Observatory.