The double-degenerate, super-Chandrasekhar nucleus of the planetary nebula Henize 2-428

The probable evolution of the planetary nebula Henize 2-428 is examined, from a close binary system in which both stars ejected their envelopes to the white dwarf stage with a short orbital period and combined mass above the Chandrasekhar limit, suggesting that the system should merge in about 700 million years and trigger a type Ia supernova.