Why does Hydrogen fusion begin at the cores outer edge?

Question

What is present there that allows H- fusion to take place?(And) Why it doesnt happen closer to the core?

Answer 1

Begin?  I understand that fusion does not take place in the very center, in mature stars.  This is because of the "ash" (helium, carbon, etc.)  But for for stars that are just beginning to form, there is no  ash, and fusion should begin in the center where the hydrogen concentration and temperatures are highest.

Please give a source for  "hydrogen fusion  begin at the cores  outer edge."