Comment by Aransentin

The article about the Sun was quite fun; even though they didn't know about fusion, the article dismisses most theories about how it could generate such a large amount of energy (like chemical combustion or gravitational contraction).

IT says the most likely cause is some sort of "rearrangement of the structure of the elements' atoms" and "supposing a gaseous nebula is destined to condense into a sun, the elementary matter of which it is composed will develop in the process into our known terrestrial and solar elements, parting with energy as it does so". Pretty much as bang on as one could reasonably be given what they knew.