Alternative Models
The accretion mechanism described in the left panel is the one that is most consistent with
data for Type Ia supernovae. However, there are competing models that cannot be ruled
out entirely
by present data. In the most popular alternative
model, both stars of the binary are white
dwarfs. As they orbit around each other, they emit energy in the form of
gravitational waves and spiral closer together. Eventually they merge and the two white dwarfs
together exceed the limiting mass and the star explodes in a manner similar to that described
in the left frame.