Aprotostar forms once the nebular cloud condenses and the core begins
to orbit
to cool
to heat
nuclear fusion

The answer is; to heat

A protostar is the earliest stage in star formation in the universe.  When the gas cloud begins to collapse on its own weight, the core begins to heat up. The heat is due to the force of compression of the dust and gas under its gravity. When the pressure and temperatures become high enough, the hydrogen in the core begins to fuse.