In the Center of the GALAXIES ; How does the Nuclear Reaction Get Initiated ?
Formation of galaxies is analogous to the formation of Cyclones. ( Ref. Whirlpool Model of Galaxy Formation Thread )
Differential Velocity, temperature, and pressure of the different layers of Air Currents cause Cyclones, that spirals dust, vapors and objects from ground to the sky.
Similarly, the Black Holes in the center of the galaxies suck interstellar matter from space and pumps them to its bottom. From there it rises upward in a great cyclone like swirl and reach to the top of the galaxy. During the process there is a great friction between the upstream and the down streams layers. It gives rise to violent increase in temperature making the center a nuclear furnace. This furnace fuses the subatomic particles into hydrogen. ( Source 'The Universe' ISBN 9788190950206)
Astronomers and scientists consider compression caused due to gravity be the reason for the start of nuclear reaction that lack details and is far from reality.