What slows down the contraction of a star-forming cloud when it makes a protostar?

The thermal energy trapped inside the protostar

Explanation:

A protostar is an early stage of a star being formed. A star forms in nebula which is a molecular cloud of dust and gases. As the star forming cloud contracts, it raises the pressure, density and temperature. The thermal energy stored inside slows down the contraction. Actually, there is rapid collision between molecules which later releases as photons.

pe = weightxheight = 2000x1000=2,000,000joules