When does a white dwarf trigger a Type Ia supernova instead of a nova?

Answer

When it accretes so much material that it crosses the Chandrasekhar Limit.

The critical tipping point for total destruction is reaching the Chandrasekhar Limit, quantified as about 1.4 times the mass of the Sun, which destabilizes the entire white dwarf structure.

When does a white dwarf trigger a Type Ia supernova instead of a nova?

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