When does the Cooling Shell stage typically commence, resulting in prominent optical emission?

Answer

Once the gas temperature drops below approximately $20,000 ext{ Kelvin}$

The subsequent stage begins when the gas temperature falls below roughly $20,000 ext{ Kelvin}$, enabling electrons to recombine and allowing the remnant to radiate energy much more efficiently, primarily through optical emission.

When does the Cooling Shell stage typically commence, resulting in prominent optical emission?
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