Aprotostar forms once the nebular cloud condenses and the core begins

Explanation:

A protostar forms once the nebular cloud condenses and the core begins HEAT.

A protostar is a very young star that is still gathering mass from its parent molecular cloud. The protostellar phase is the earliest one in the process of stellar evolution. 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.

found in the nucleus

has mass of one amu

and 1and 1