[pfdietz]

We can compare the fraction of the Sun's energy that has come from fusion vs. coming from gravitational collapse (that is, the release of energy as the material becomes more tightly gravitationally bound, starting from the diffuse gas cloud that formed the Sun). It's about two orders of magnitude in favor of fusion.

This is related to the historical question of the age of the Earth. Before the discovery of fusion, it was thought the Sun was powered by gravity, which put an upper limit on the age of the Sun of some tens of millions of years. This was close to Lord Kelvin's limit on the age of the Earth as modeled as a solid sphere cooling by conduction, which led him to believe both estimates were correct. As it turns out, both estimates were flawed, but for different reasons, and it was only coincidence they were similar.