After a thermonuclear supernova explodes, the debris expands at roughly 1/10 the speed of light and the supernova brightens to maximum luminosity about 20 days later. At maximum, the supernova is about 10 billion times as luminous as the Sun. Then it fades, rapidly for the first month or so, then more slowly. Since the observed fading rate at late times is very close to the rate at which radioactive cobalt-56 decays to iron-56 in the laboratory, we believe that the optical light of the supernova at late time results from this radioactivity.

Many mysteries remain in understanding thermonuclear supernovae. The most serious puzzle is this: under what circumstances does the mass transfer and surface explosion trigger the explosion of the entire white dwarf star? Why don't these binary star systems undergo repeating nova explosions indefinitely? One idea is that a white dwarf binary system may become a supernova only after the donor star has transferred all the hydrogen in its outer envelope and is now transferring pure helium onto the white dwarf. Since much higher temperatures and pressures are required to ignite helium than to ignite hydrogen, a much thicker surface layer of helium must accumulate on the white dwarf before it can explode. The explosion of this thick layer of helium is more violent than the explosion of a thin hydrogen layer, violent enough to trigger the explosion of the carbon/oxygen core. But theorists are not confident that this mechanism can account for the number of thermonuclear supernovae that are observed. Another idea is that a thermonuclear supernova is the result of a merger of two white dwarf stars in a binary system.