What has to happen for a star to join the main sequence ? 1) Nuclear Fusion

2) Shell Heating

3) Use up most of its available fuel

4) Hydrostatic Equilibrium

Nuclear fusion

Explanation:

This is because main sequence of star is powered by fusion of hydrogen to helium atoms together and this process releases energy. The energy released when the gas collapse into a protostar make the center of the protostar to be extremely hot. When the core becomes very hot, nuclear fusion can start.

the answer is d. 4

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