Final answer:
Accretion of matter in an interstellar cloud leads to the formation of a protostar, marking the initial stage in the birth of a new star before the onset of nuclear fusion.
Step-by-step explanation:
Accretion of matter in an interstellar cloud leads to the formation of a protostar. This process starts with the gathering of gas and dust in the interstellar medium. As matter accretes, the density and temperature of the cloud's core increase, eventually leading to the collapse of the gas under gravity. This collapse forms a protostar, which is the embryonic stage of a star before nuclear fusion begins in its core. Over time, the protostar evolves and may enter the main sequence as a new star.