Dwarf Novae are cataclysmic variables -- variable stars -- whose brightness often changes "cataclysmically" over short periods of time. They were first discovered by the English Astronomer J.R. Hind on December 15, 1855, who observed an outburst by a star now known as U Geminorum

The dwarf novae can increase their luminosity by a factor of 100 (five magnitudes) for short periods of time, and they can exhibit these changes as often as once every few weeks or months. The dwarf novae are binary stars where one star is a white dwarf, and the other is a dwarf star overflowing its Roche lobe and spilling some of its mass onto the white dwarf. This overflowing gas forms an accretion disk around the white dwarf, and eventually spirals down onto its surface. The outbursts of dwarf novae are caused by drastic changes in the accretion disk, which result in additional matter spilling onto the white dwarf and heating of the accretion disk. The dwarf nova outburst occur periodically on timescales of days, weeks, or months, and the behavior of the outbursts can change from cycle to cycle. This outburst mechanism is very important, as it distinguishes the dwarf novae from the classical novae. The classical novae brighten by a factor of ten thousand or more because of a huge thermonuclear explosion on the white dwarf's surface, and do not repeat.

The dwarf novae exhibit a very broad range of behavior, despite their all being physically similar (a binary star system containing a white dwarf and a normal star). These differences in behavior can be attributed to differences in the mass of the normal star, the orientation of the accretion disk with respect to our line of sight, the separation and orbital period of the binary stars, the kinematic behavior of the accretion disk, and on and on. Nearly all of the white dwarfs in dwarf novae systems are weakly magnetic or not magnetic at all. This means that the flow of gas from the secondary star to the white dwarf is a purely mechanical, hydrodynamic (as opposed to a magnetohydrodynamic) process. A few dwarf novae are known to be intermediate polars, but there are no true polars among them.

There are three major subclasses of dwarf novae, each named after the class prototype:

  • U Geminorum or SS Cygni stars -- the prototypical dwarf novae stars. These binary star systems are typically oriented so that we can see the inner accretion disk, and even the white dwarf itself. This results in very complex behavior at all wavelengths from the infrared to the X-ray.

  • SU Ursa Majoris stars -- the secondary (normal) star is massive enough that its gravitational field makes the accretion disk around the white dwarf precess, which triggers a "superoutburst" -- an extended period of high brightness.

  • Z Camelopardalis stars -- these objects exhibit long "standstills", where the luminosity remains at a roughly constant, elevated level for years at a time. Apparently, they can occasionally go into outbursts which change the equilibrium structure of the accretion disk. This stops the cycle of outbursts, and leaves the disk at a state of high-but-steady mass accretion.

Like many of the other cataclysmic variables, the dwarf novae are interesting because they tell us an awful lot about the physics of accretion, and the hydrodynamics of accretion disks. They're also interesting because they are the dominant contributors to the ultraviolet and x-ray radiation background in the galaxy (and probably in most other galaxies, too). They may eventually wind up as type Ia supernovae, too, so they provide potentially useful laboratories for stars which eventually become supernovae. Of course, they're also fun to watch -- they're perennial targets for amateur astronomers who enjoy monitoring variable stars.

I used the great book, X-ray Binaries (Cambridge Astrophysics Series, Lewen, Van Paradijs, and Van Den Heuvel, editors, Cambridge U. Press; Cambridge, 1995) for some background information, as well as Coel Hellier's Cataclysmic Variable Stars (Springer-Praxis; Chicester, UK, 2001).

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