Helium fusion does not begin until the star has entered the giant region of the H-R diagram.

Question

Quizplus · Accepted Answer

This is because helium fusion requires a higher temperature and pressure than hydrogen fusion, and these conditions are only reached in the core of a star once the hydrogen fuel has been depleted and the core has contracted and heated up. This happens when the star evolves off the main sequence and enters the giant region of the H-R diagram.

Helium Fusion Does Not Begin Until the Star Has Entered