If we restrict ourselves to serious study of the sun, the mid-19 The issue of where the sun's energy came from turned out to be a thorny one.
It was rapidly realized that none of our terrestrial energy sources would be adequate.
But when detailed calculations of the expected neutrino flux were confronted with measurements, about 30 years ago, a significant discrepancy was found.
Only about half of the expected neutrinos could be found.
This final section, with answers to creationist claims, is rather short; the bulk of the is in the lengthy introductory sections, containing all the background knowledge needed to understand the answers.
Alternatively, one might assume that star birth is an ongoing process, so that the sky is filled with stars of all different ages.
The potential gravitational energy of the cloud would be released as heat, as it contracted, and Hermann Helmholtz realized that this was a possible energy source for the sun, provided that it was still in the contracting phase.
William Thomson (better known as Lord Kelvin) elaborated and promulgated this theory during the last decades of the 19 century.
Larger, more massive stars should generally be more luminous than smaller ones, but there is no reason to expect a relation with temperature.
This stands in stark contradiction with the discovery by Hertzsprung and Russell (Hertzsprung 1905; Russell 1914) that the vast majority of stars do give such a pattern, when plotted in what is now known as a Hertzsprung-Russell diagram (e.g. Eddington (1924) developed these ideas further, and showed that the only reasonable conclusion is that stars start out contracting (and shining from gravitational energy), but that they then reach equilibrium along what is now known as the "main sequence".