To change state from gas to a liquid

im doing a crossword puzzle and i cant figure out what it is plz

answer

fusion: the energy source of stars. the energy released from the collapse of the gas into a protostar causes the center of the protostar to become extremely hot. when the core is hot enough, nuclear fusion commences. fusion is the process where two hydrogen atoms combine to form a helium atom, releasing energy.

hope it

answer; i do believe the correct answer is (river); /// good luck