What model was used to explain stellar energy before nuclear fusion was discovered?

For thousands of years it was generally assumed that the Sun was burning - chemical combustion.

When geologists began to accept about 1800 or so that the world seemed to be millions of years old instead of a few thousand, the chemical combustion idea began to seem inadequate to explain how the Sun could shine for millions of years.

Astronomers and physicists began to calculate that if a astronomical object formed by a nebula collapsing into a solid object that would heat up the collapsing material and the astronomical object would be red hot and liquid and gradually cool down and solidify.

They could also measure the diameter of the Sun accurately with 19th century instruments and detect that it was not rapidly shrinking. But it was possible that the Sun was shrinking and condensing at a rate too slow to be detected, but still fast enough to generate enough heat to make it shine as brightly as it did.

Meanwhile geologists were making the problem much worse as they kept increasing the estimated age of the Earth.

Sir Charles Lyell (1797-1875) author of Principles of Geology (1830-33) believed that the earth was much older than the oldest estimate yet made by geologists. I once read that Darwin acquired from Lyell the belief that Earth was almost infinitely old - I remember reading somewhere about an age of a million million years or 1,000,000,000,000 years.

Anyway, geologists steadily changed their estimates of the ages of the oldest know rock formations, usually upwards. The estimated age of the Earth increased from millions of years to tens of millions of years and then to hundreds of millions of years. By about 1900 the age of the Earth, and thus of course the age of the Sun, was estimated to be at least one billion years - 1,000,000,000 years - if I remember correctly.

But physicists and astronomers couldn't find any way to make the Sun shine from gravitational contraction for more than a few tens of millions of years. They were highly annoyed when geologists claimed that the Sun must have shone for tens of times, even a hundred times, as long as the calculations of physical science said was possible.

I once read that an astronomer and a geologist argued about that at a scientific meeting and the astronomer punched the geologist!

The mass energy equivalence (e=mc2) proposed by Einstein in 1905 offered a way out of his scientific paradox. If the Sun could convert a tiny fraction of its mass into energy each second it could shine for at an almost steady rate for billions of years. But how could matter be converted into energy?

In 1920 Arthur Eddington proposed that fusion of hydrogen into helium could produce stellar energy. In the 1920s, 1930s, and 1940s, George Gamov, Hans Bethe, Fred Hoyle, and others worked out how elements were formed by fusion in the Big Bang and in stars and how stars produced their energy from that fusion.

And as Steve Bird said, you could probably get a better answer in the history of science stack exchange.

Looking for sources for the statements I made I start in Wikipedia:

In the early years of the modern scientific era, the source of the Sun's energy was a significant puzzle. Lord Kelvin suggested that the Sun is a gradually cooling liquid body that is radiating an internal store of heat.[179] Kelvin and Hermann von Helmholtz then proposed a gravitational contraction mechanism to explain the energy output, but the resulting age estimate was only 20 million years, well short of the time span of at least 300 million years suggested by some geological discoveries of that time.[179][180] In 1890 Joseph Lockyer, who discovered helium in the solar spectrum, proposed a meteoritic hypothesis for the formation and evolution of the Sun.[181]

Not until 1904 was a documented solution offered. Ernest Rutherford suggested that the Sun's output could be maintained by an internal source of heat, and suggested radioactive decay as the source.[182] However, it would be Albert Einstein who would provide the essential clue to the source of the Sun's energy output with his mass-energy equivalence relation E = mc2.[183] In 1920, Sir Arthur Eddington proposed that the pressures and temperatures at the core of the Sun could produce a nuclear fusion reaction that merged hydrogen (protons) into helium nuclei, resulting in a production of energy from the net change in mass.[184] The preponderance of hydrogen in the Sun was confirmed in 1925 by Cecilia Payne using the ionization theory developed by Meghnad Saha, an Indian physicist. The theoretical concept of fusion was developed in the 1930s by the astrophysicists Subrahmanyan Chandrasekhar and Hans Bethe. Hans Bethe calculated the details of the two main energy-producing nuclear reactions that power the Sun.[185][186] In 1957, Margaret Burbidge, Geoffrey Burbidge, William Fowler and Fred Hoyle showed that most of the elements in the universe have been synthesized by nuclear reactions inside stars, some like the Sun.[187]

https://en.wikipedia.org/wiki/Sun#History_of_observation1

Lord Kelvin's original 1862 article is here:

https://zapatopi.net/kelvin/papers/on_the_age_of_the_suns_heat.html2

Ernest Rutherford in 1904 showed that the radioactive decay of radium would produce heat and thus the internal heat of the Earth would last for many times as long as Lord Kelvin calculated.

http://faculty.philosophy.umd.edu/LDarden/sciinq/3

But how much radium as in the Sun? Could the decay of radium prolong the Sun's light emitting stage enough for Earth to be as old as geology required?

In 1919 the French physicist Jean Perrin took on the problem. He had previously helped to establish that atoms were a reality through his experiments, and would later win the Nobel Prize for his work proving the existence of molecules. Perrin noted that the atomic weight of helium weighed slightly less (about 1% less) than would four hydrogen atoms which he envisioned could be combined to form Helium4 . While the difference in mass was very small, the energy released would be enormous. Perrin had found the right answer but did not have the knowledge necessary to describe the process in detail. The repulsive electrical charge would keep the four protons of hydrogen from merging to create helium and setting free the energy equivalent to the lost mass.

Famed Astrophysicist Arthur Eddington was convinced. He had taken Kelvin’s calculations of stellar density and temperature much further and found the temperatures and pressures at the center of the stars would be hellish, reaching as high as 10 million degrees K. Though, like Perrin, he did not have a physical model for how four hydrogen atoms could collide with such energy that they would fuse into Helium, he was convinced that this occurred due to the fact that Helium existed in stars. In fact his belief in this was so strong, he challenged his critics to (Continued from page 7) “go find a hotter place” than the interior of a star.

Discovery of the mechanism for the creation of most of a stars energy would have to await the new paradigm of quantum mechanics. In 1928, Russian American particle physicist George Gamow, would realize that it was statistically possible for one proton to “tunnel” through the repulsive electric charge of another proton to fuse together.

Finally in 1938, physicist Hans Bethe5 would work out in detail the series of nuclear reactions that could occur in the center of a star which leads to the formation of Helium and release of energy. He and others would also establish another series of reactions that power stars much larger than our Sun, and ultimately produce all of the elements, including those we are made of. Truly we are stardust.

https://www.astronomyhouston.org/sites/default/files/guidestar/2012October.pdf4

So I have added a few quotations and sources to back up my statements.