The life-cycle of stars
A nebula consists of dust and gas, and there are places where gravity causes these to clump together. This means their gravitational attraction to other atoms increases, pulling more atoms into the clump. The process where atoms fall into the clump and become part of the protostar is called accretion. To become a star, the protostar will need to achieve hydrostatic equilibrium by balancing the gravity, pulling atoms in, and the radiation pressure pushing heat and light out. When equilibrium is achieved, if a specific mass is not reached (around 0.08 times the mass of the Sun), the protostar will become a brown dwarf, but if this critical mass is reached, then nuclear fusion is able to begin, and the star is born, entering the main sequence.