A protostar is an early stage in the formation of a star, occurring when a cloud of gas and dust collapses under its own gravity. During this phase, the material gathers in a central core, heating up as it accumulates more mass. This process can take millions of years and is crucial for the eventual birth of a star. As the protostar continues to evolve, it may develop a surrounding disk of material, which can lead to the formation of planets. Eventually, when the core temperature becomes high enough for nuclear fusion to begin, the protostar transitions into a main-sequence star, marking the next stage in its life cycle.