Novae
Some stars suddenly increase their brightness by very large amounts over a period of days, and
then slowly dim back to obscurity over a matter of months. The increase in brightness can be
as large as factors of a million. We call such a star a
nova. The name nova means "new" (the plural is novae): the
earliest observations could not see the original star because it was too faint, so it was thought to be
a new
star.
Some Clues
Some clues to the nature of novae are offered by three key observations.
First, novae seem to be
associated with binary systems in which one star is a white dwarf.
Second, Doppler shifts indicate an expanding
shell of gas emitting the light being observed from a nova.
Third, there are recurrent novae (novae that repeat
after some period of time). This
suggests the following nova mechanism, which has now been confirmed by
abundant observational data.
The Nova Mechanism
A nova can occur in a binary system in which one star is a more normal
star and it has a white dwarf companion.
Matter from the first star accretes in a thin layer on the surface of
the white dwarf, either because of the other star filling its Roche lobe and spilling
matter onto the white dwarf (typically through an accretion disk), or because of a strong wind
from the other star that the white dwarf captures onto its surface.
Eventually this layer ignites in a thermonuclear explosion under
degenerate conditions.
The resulting thermonuclear runaway (recall the earlier discussion of the helium flash in red
giant stars) blows a
thin surface layer off into space, causing a large rise in light output from the
system. The mechanism for a
nova outburst is illustrated schematically in the
figures shown above and below.
