Final answer:
The initial gravitational contraction of a new star similar to our Sun is halted by the buildup of heat, which becomes trapped as the protostar becomes dense and opaque. This raises the gas pressure, slows contraction, and eventually leads to a more stable phase where the star has a roughly constant luminosity.
Step-by-step explanation:
When a new star like our Sun is forming, the initial gravitational contraction is halted by the accumulation of heat within the protostar, which prevents the gas pressure from falling and slows the contraction. This happens as the protostar becomes dense and opaque enough to trap the heat released by gravitational contraction. At this point, the rapid contraction stops, and the star reaches a state where its surface temperature increases, and its luminosity becomes roughly constant. Additionally, the action of stellar winds helps to clear away the surrounding dust and gas, making the star visible at various stages of its contraction.