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
- 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
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
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).