What process causes the orbit of two white dwarfs to decay, potentially leading to a Type Ia supernova?

Answer

Gravitational radiation causing orbital decay

In a binary system involving two white dwarfs, the emission of gravitational radiation causes their orbit to shrink over time until they eventually spiral inward and merge, potentially triggering the thermonuclear runaway.

What process causes the orbit of two white dwarfs to decay, potentially leading to a Type Ia supernova?
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