Novae, X-Ray Bursts,
and Supernovae
Accretion in binary star systems, illustrated schematically in the adjacent
diagram, can have spectacular consequences for
the evolution of the two stars. In particular, there are three classes of
violent explosions that are thought to result from accretion in binary star systems:
novae, X-ray bursts, and type Ia supernovae. All
involve the accretion of matter in a binary system from a more normal star onto a
highly compact object (either a white dwarf or a neutron star).
Nova Outbursts
In a nova outburst, the matter accretes from a more normal star onto a white dwarf. This
eventually triggers a thermonuclear runwaway on the surface of the white dwarf which blows
off a hot, bright surface layer. We shall discuss novae again when we discuss the death of
stars.
X-Ray Bursters
In an X-ray burst, the mechanism is thought to be similar to that for a nova, except the
accretion is onto a neutron star. We shall discuss neutron stars and X-ray bursts further
in the chapter on neutron stars.`
Type Ia Supernova Explosions
In a type Ia supernova, matter accretes onto a white dwarf in a binary, similar to the case
for a nova. But instead of
triggering a thermonuclear runaway that blows off a surface layer, the explosion consumes the
entire white dwarf, converting its mass to energy in a gigantic explosion that is much more
energetic than a nova or X-ray burst. We shall discuss
supernovae further in the chapter on the death of stars.