Why helium fusion require much higher temperatures than hydrogen fusion

1.) Why does helium fusion require much higher temperatures than hydrogen fusion?

2.) Explain how and why the turnoff point on the H-R diagram of a cluster is related to the cluster’s age.

3.) What is the difference between Population I and Population II stars? In what sense can the stars of one population be considered the “children” of the other?

4.) Why are the pulsating stars known as Cepheids important to astronomers who study the galaxies beyond the Milky Way?

5.) Sometimes when two stars are near each other, the more massive one can end up being the less massive on after a period of time. How does this happen??