Fusion is the joining of two light nuclei into a heavier one

it is the source of energy of a star

hydrogen is the lightest element

a star is born when 2 hydrogens fuse to form helium

Conditions of fusion

1. High temp - helps nuclei overcome the electrostatic repulsion
2. High number density - number of nuclei per unit volume
3. Confinement time - long enough

specific energy of fusion is e= mc^2

Nebula stars as just H2 gas

gravity brings the atoms together to form a protostar

fusion starts to be in equilibrium with gravity forming a main sequence star