I’m pretty sure the general theory was that the sun was a ball of incandescent gas and that it was by the burning of this gas that the sun could give off energy. How something the size of the sun could give off the amount of energy it did, though, wasn’t known.
I think that gravitational collapse was the big theory. According to a Historical Note from An Introduction to the Theory of Stellar Structure and Evolution by Prialnik, in 1862, Lord Kelvin calculated how long an object like the Sun could maintain its luminosity with only gravitational collapse. This “Kelvin-Helmholtz timescale” is only a few million years, and caused no small bit of contention between astronomers and geologists, who believed from the fossil record that the Earth was more like billions of years old. There’s a quote from Eddington (a big-time astrophysicist) in 1920 which more or less says that gravitational collapse is out, and nuclear burning is in, but it wasn’t completely settled until about 10 years later.
Gravitational collapse is, of course, the conversion of gravitational potential energy into light and heat as the star shrinks. I’ve never heard of this collisional heating, and honestly, it sounds too far-fetched even for the real scientists of 1875. But I’m not an expert in this.
Until the 1900’s astronomers weren’t even certain what the sun and stars were made of. That made it difficult to define a mechanism for the production of heat and light. The spectroscopes of the day led them to propose the existence of several unearthly elements in cosmic bodies: Nebulium Coronium
The confusion didn’t end with the sun – one cientist (I forget who) calculated the age of the earth based upon how long it would take to cool down to its present-day temperature. He got an age that was far too short (and I think he knew it). There’s radioactve decay contributing to the heat of the core hat he knew nothing about.
And it’s not the contribution of radioactive decay to the core, but to the crust, that is the problem. He determined how much heat was escaping from the surface, and calculated that at that rate, the Earth would have taken a short time to cool.
A lot of that heat is due to radioactive decay, since radioactive material has concentrated in the Earth crust.
I thought that Verne’s explanation would sound silly even to a 1875 scientist. It is good to know that they had a few better sounding hypothesi, and that they knew they were missing something.
(After all, nothing that we know of could keep the sun’s flame burning for billions of years except fission/fusion.)
As for Kelvin’s calculations, if I can hijack my own thread, I don’t see any adjustment for the fact that the earth is receiving input heat from the sun. Surely, the earth would stabilize at a temperature where it emits as much radiation as it absorbs. Or am I missing something about his set-up?
I don’t know when “they” first realized the Sun was a star, but over 300 years ago Huygens tried to calculate the distance to Sirius by assuming the Sun was a star.
His method was to view the Sun through smaller and smaller pinholes until its apparent brightness matched that of Sirius (or at least his recollection of how bright Sirius was the night before!). Then, knowing that an object’s brightness decreases as the square of its distance, he could determine how far Sirius was in terms of the distance to the Sun.
It might coincide with when the “sphere of immovable stars” was debunked as a concept in astronomy. I think I recall something about postulating that the stars were other suns. Even somebody proposing maybe other peopl lived around them…
Kelvin used his estimate of the age of the earth (something like 100,000 years–or was 100 million–to attempt to refute evolution. As noted above, he didn’t know about radioactive decay. I do not know if his computation was otherwise sound. Is radioactive decay the only thing that has kept the earth warm for 4.6 billion years?