What condition signifies a protostar's official birth and entry onto the Main Sequence?

Answer

Core temperature reaching 15 million degrees Celsius to ignite sustained nuclear fusion

The true birth of a star occurs when the core temperature reaches approximately 15 million degrees Celsius, enabling sustained nuclear fusion where hydrogen converts to helium, creating an outward pressure that balances gravity.

What condition signifies a protostar's official birth and entry onto the Main Sequence?
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