A dwarf nova is a symbiotic binary.
An accretion disk awakens a `retired' central star
(a white dwarf) to new life. The disk itself is fed by the companion.
Sometimes, i.e. more or less regularly, the disk exhibits strong outbursts.
It is this behavior which makes a dwarf nova disk an ideal
object to verify accretion disk theory. Of course, the theory for
non stationary disks is pretty tough, especially if the assumption of thermal
equilibrium does not hold anymore. But, if such a non-equilibrium disk
expands or contracts vertically in an homologue manner, even this
case proved to be tractable.
Stars (and planets?) originate from cold interstellar matter
after a gravitational collapse, provided angular momentum can
be carried away. Only turbulence (together with magnetic fields)
is effective enough for this job - and makes
the resulting accretion disk and its boundary layer radiate.
Additionally, the fast rotation of young T Tauri stars is
spun down to the observed relatively low rate by
complicated interaction with its disk.
