Final answer:
Gas clouds contract to form stars primarily due to gravitational attraction within dense cores, overcoming internal pressure, leading to a rapid collapse and the formation of a protostar. Triggered star formation can also occur when massive stars heat and compress surrounding gas. Ultimately, gravitational force and subsequent heating lead to nuclear reactions, birthing new stars.
Step-by-step explanation:
What makes a gas cloud contract to form stars? The key factor is the gravitational attraction that causes all parts of the cloud to be attracted to each other, ultimately collapsing the cloud. This process initiates when the gravitational force of the infalling gas within a molecular cloud becomes strong enough to overwhelm the pressure exerted by the cold material in dense cores, leading to a rapid collapse. As a result, the core increases in density significantly, and a protostar is formed in this stage.
Additionally, events such as the formation of a massive star can trigger further star formation. When a massive star emits significant ultraviolet radiation or explodes at the end of its life, the energy can heat and compress the surrounding gas, triggering the formation of new stars in a process called triggered star formation. The accumulation of material continues until the protostar develops a strong enough stellar wind, which shape the remaining material and can contribute to the formation of Herbig-Haro objects. Ultimately, the protostar will continue to heat up until nuclear reactions start, marking the birth of a new star.