1) the pressure of the hydrogen gas increases and the gas reaches extreme temperatures
2) bare nuclei collide with each other due to massive amounts of heat
3) the colliding neuclei fuse to form larger helium atoms having two protons and two neutrons
4) hydrogen atoms shed their electrons
This could be wrong so don't fully trust this answer
Green house effect is the trapping of heat by some gases ! mainly in our earth this is increasing due to CO2 !