Repulsive force and collapse force, these two forces are balanced for most of a star's lifetime.
If fusion increases, the outward repulsive force becomes greater.
The star will tend to expand, the central pressure decreases, and fusion returns to a balanced rate.
If fusion decreases, the outward repulsive force diminishes.
The star will then collapse inward, increasing the central pressure, accelerating fusion, and again returning to a balanced rate.
It's somewhat like a seesaw.
This stable phase of stars is called the main sequence star—our current Sun is in this phase, as do most stars.
From birth to becoming a main sequence star, the evolutionary process is the same for most stars, differing only in the length of time.
If the initial mass is large.
The collapsing force will be large, and the corresponding fusion rate will be high.
If the initial mass is small.
The gravitational collapse will be small, and the corresponding fusion rate will be low.
Simply put.
