What causes the extreme thermal signature of an SNR to subside over time during its evolution?

Answer

Radiative cooling mechanisms cause the material to radiate away its heat energy into space

As the remnant expands, radiative cooling allows the gas to radiate its heat away, causing the extreme thermal signature to subside and merge with the cooler ISM.

What causes the extreme thermal signature of an SNR to subside over time during its evolution?
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