The core of a star must react a temperature of 10,000,000 degree Celsius for hydrogen fusion to begin
Step-by-step explanation:
A star formed from a rotating cloud which is made up with completely with helium and hydrogen. The gravitational force pulls the cloud behind to collapse inward and due to this it shrinks and later it spins more and more quickly with the outer parts.
When the core and inner part become a rough spherical clump then it becomes disk. The star life cycles follow the patterns which is based on mostly on their initial mass.