What event forces the Sun's helium-rich core to reach the 100 million Kelvin threshold later in its life?
Answer
Gravitational contraction during the red giant phase
When core hydrogen is exhausted, the star contracts gravitationally, which drastically increases core density and temperature, eventually driving it past the critical threshold needed to ignite the accumulated helium.
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Nuclear fusion in the sun. The 4 steps from hydrogen to helium.
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