During which stage does a star become a main sequence star?

A star becomes a main sequence star during the stage when it heats up and begins to fuse hydrogen into helium in its core. This process of nuclear fusion is a key characteristic of main sequence stars, defining their stability and ability to produce energy.

During this stage, the gravitational forces pulling inward due to the star's mass are balanced by the outward pressure created from the nuclear fusion reactions occurring in its core. This balance is critical because it allows the star to maintain a stable size for an extended period, which is typically where stars spend the majority of their lifetimes.

The other stages outlined do not lead to the formation of a main sequence star. For example, if a star begins to cool and expand, it could be entering a different phase, such as becoming a red giant rather than a main sequence star. If nuclear fusion stops entirely, the star would no longer be able to maintain the necessary energy output, leading to contraction or transformation into another stellar state. Similarly, the formation from a red super giant would not directly address the primary phase of becoming a main sequence star, as it indicates a different evolutionary path.