What is the main sequence stage in stellar evolution characterized by?

The main sequence stage in stellar evolution is primarily characterized by energy production through hydrogen fusion. During this phase, a star fuses hydrogen nuclei into helium in its core, releasing a significant amount of energy in the process. This energy production is crucial because it generates the outward pressure that balances the gravitational pull trying to collapse the star. Stars, including our Sun, spend the majority of their lifecycle in this stable state, where the rate of fusion and the energy produced are in equilibrium with the forces acting within the star.

While other processes like helium fusion may occur in later stages of a star's life cycle (after the main sequence), they are not characteristic of this particular phase. The expansion and cooling of a star typically occur when it has exhausted its hydrogen fuel and transitioned into later evolutionary stages. Likewise, the formation of planetary systems is a different aspect of stellar evolution that relates more to the early stages of star formation rather than the main sequence stage itself.