Question

How does the reaction within the center of every star start?

Answer 1

A stellar core is in the extremely hot, dense region at the middle of a star. For a standard main sequence star, the core region is the volume where the temperature and pressure conditions leave energy production through thermonuclear fusion of hydrogen into helium.

Complete answer:
Stars are powered by fusion in their cores, mostly converting hydrogen into helium. The assembly of the latest elements via nuclear reactions is named nucleosynthesis. A star's mass determines what other sort of nucleosynthesis occurs in its core (or during explosive changes in its life cycle).
Fusion is the joining of nuclei to form larger nuclei, with the discharge of energy. A ball of contracting interstellar gas becomes a star just like the Sun when fusion reactions start in its center.
Gravity is that the force that makes the pressure to fuse atoms, which makes the celebs shine. Eventually the temperature is high enough that the star starts fusing hydrogen into helium. When the outward pressure produced by the heating of the gas by fusion energy balances gravity a stable star is made.

Note:
All stars begin their lives from the collapse of fabric during a giant molecular cloud. These clouds are clouds that form between the celebs and consist primarily of molecular gas and mud. Turbulence within the cloud causes knots to make which may then collapse thereunder is its own gravity.