The Sun shines only because gravity is slowly making it contract.

When chemical burning proved too weak to explain the Sun, gravity offered a much better energy reservoir.

Hermann von Helmholtz and Lord Kelvin showed that a contracting Sun would convert gravitational potential energy into heat. The calculation was physically sound and could keep the Sun luminous far longer than ordinary combustion.

But not long enough.

The Kelvin–Helmholtz timescale was only on the order of tens of millions of years. Geologists and evolutionary biologists were accumulating evidence that Earth and life had existed for vastly longer intervals.

The age conflict became a major clue that another energy source was missing.

Nuclear physics supplied it. Hydrogen fusion in the core can power a main-sequence star like the Sun for billions of years.

Gravitational contraction was not discarded as nonsense. It really does heat forming stars and remains crucial before stable hydrogen fusion begins.

The superseded claim is the word only. Contraction helped build and heat the young Sun, but nuclear fusion is what sustains the mature Sun today.