Final answer:
A star before it begins nuclear fusion in its core is known as a protostar. This early phase takes place as the gravitational collapse of a dense core within a molecular cloud transitions to a stage capable of sustaining nuclear fusion, marking the star's progression to the main sequence.
Step-by-step explanation:
The correct answer to the student's question is protostar. A protostar is a forming star that has not yet reached the point where sustained fusion can occur in its core. This phase occurs after a dense core of material in a molecular cloud begins to collapse under its own gravity and continues until the star reaches sufficient temperature and pressure in its core to start nuclear fusion, thereby entering the main sequence phase of its evolution.
Evolution of a Star
The evolution of a star like the Sun, from the main-sequence phase of its evolution until it becomes a white dwarf, involves several stages. Initially, the star fuses hydrogen to helium in its core. As the hydrogen supply diminishes, the core contracts and heats up, leading to the expansion of the outer layers and the formation of a red giant. After expelling its outer layers and leaving behind a core, the star becomes a white dwarf and ultimately cools over time.