A protostar is a very young star that is still gathering mass from its parent molecular cloud. It is the earliest phase in the process of stellar evolution. During this phase, the protostar is not yet hot enough for fusion to take place, so its luminosity comes exclusively from the heating of the protostar as it contracts. The protostar is usually surrounded by dust, which blocks the light that it emits, making it difficult to observe in the visible spectrum.

The formation of a protostar begins when gravity begins to pull the gases together into a ball, a process known as accretion. As the cloud collapses, it begins to spin, and by the time a protostar is formed, the cloud flattens, and there is a protostellar disk spinning around the protostar. These disks probably slow the rotation of the protostar and sometimes coalesce into planetary systems.

A protostar becomes a main sequence star when its core temperature exceeds 10 million K, which is the temperature needed for hydrogen fusion to operate. If the protostar can reach this temperature, the hydrogen fusion process will start, and it will become an actual star. However, some protostars never get hot enough to start the hydrogen fusion process and are known as brown dwarfs. Brown dwarfs are generally smaller than our sun, but larger than the planet Jupiter, and they continue to shine dimly for millions of years as they cool down.

In summary, a protostar is a young star that is still gathering mass from its parent molecular cloud and has not yet depleted its surrounding molecular gas through gravitational contraction.