When was gravitational contraction an important energy-generation mechanism for the Sun?

Gravitational contraction played a crucial role during the early formation of the Sun, specifically when it was forming from a collapsing interstellar cloud of gas. In this phase, the cloud of dust and gas exerted gravitational attraction on itself, leading to a decrease in size and an increase in density and temperature. As the gravitational forces pulled the particles closer together, potential energy converted into thermal energy, raising the temperature of the core.

This process continued until the temperatures and pressures at the center became sufficient to initiate nuclear fusion, which marks the transition to the next phase of the Sun's life cycle—its main sequence phase. The energy generated through gravitational contraction was essential at this stage because the Sun had not yet established stable nuclear fusion reactions.

In contrast, during the main sequence phase, the Sun generates energy primarily through nuclear fusion, and when it becomes a red giant or is formed from a supernova remnant, other phenomena dominate its energy generation and evolution. Thus, the significance of gravitational contraction is chiefly recognized during the early formation of the Sun rather than in its later life stages.