How did Astronomers deduce that the Sun was not a ball of fire?

Question

Its common knowledge that people used to think that the sun is a ball of fire or molten metal, but when did science start to prove otherwise?

Answer 1

I think it's maybe not the case that there was a moment when the astronomy community conclusively rejected the ball-of-fire hypothesis; astronomers simply accumulated more and more evidence against it. If you want to put a rough date on it, you could put your finger somewhere in the middle of the 19th century, as by then, other ideas had taken hold.

Back in the classical period, Anaxagoras had proposed that the Sun was a heap of molten metal. I don't know whether this was widely accepted by his contemporaries. The idea of the Sun as a ball of metal or fire certainly persisted for some time, though perhaps largely for lack of any better ideas. We didn't even understand oxygen and combustion until the work of Lavoisier and others in the late 18th century, so detailed calculations were presumably out of the question for a millennium or two after Anaxagoras. I don't know when calculations of how long combustion could sustain the Sun were first done, but it appears to have been not more than several decades after the theory of combustion was developed.

Why? Well, we can say that by the middle of the 19th century, the predominant explanation for the Sun's luminosity was not the burning of coal but instead gravitational potential energy. By the 1860s, it was widely known that chemical reactions could only power the Sun for a few thousand years. We also now had a potentially viable alternative: a decade earlier, Hermann von Helmholtz had begun exploring the idea that gravitational contraction of some sort, by what we now call the Kelvin-Helmholtz mechanism, was the source of energy, with gravitational potential energy being transformed into heat$^{\dagger}$. Around the same time, Lord Kelvin suggested that meteors falling into the Sun provided the necessary energy, a similar mechanism to Helmholtz's. I believe astronomers continued with the contraction hypothesis through the turn of the century - I've seen an article written around 1900 to that effect.

However, during the early and mid- 1900s, quantum theory and nuclear physics were being developed, and the work of Eddington, Bethe and others would lay the groundwork for our current understanding of solar energy production. Previous models (including, finally, Kelvin-Helmholtz contraction) were now known to be insufficient because they allowed the Sun to shine for only thousands or millions of years, and geologists had established that Earth itself was much older than this. Fusion, on the other hand, allows the Sun to survive for billions of years - a timescale that matches up well with the age of the Earth. We also knew that hydrogen and helium were the dominant constituents of the Sun and other stars; while Wollaston and Fraunhofer had performed the first solar spectroscopy observations in the early 1800s, the true composition of the Sun was not accepted for more than a century, when Cecilia Payne made a detailed study of spectral lines.

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$^{\dagger}$ While this does produce heat in various bodies, including T Tauri stars, it is not significant in most stars beyond that stage.

Answer 2

Scientists figured that the sun couldn't be a ball of coal during the industrial age, because given the mass of the Sun, all the coal would have burned out before humans appeared on Earth. But we didn't know what else could be going on.

In 1904, Rutherford suggested radioactive decay as a possible process that could account for the Sun's energy. But it is only after Einstein and the discovery of $E=mc^2$ that they knew that fusion could be going on inside. And indeed, that's what Eddington suggested in 1920.

Finally, in 1925, Cecilia Payne-Gaposchkin suggested that the Sun might be mostly hydrogen.

Answer 3

Scientists used telescopes aligned with a prism in the 1860's and found that sunlight is the same continuous bands of color as plasma light from an electric tube lamp, plus some non-ionized elements visible as narrow bands.

Bunsen and Kirchhoff introduced spectroscopy as a lab method in 1860.

A video with images of Bunsen's apparatus and experiments is here:

Here is a summary:

Because the electrons of the sun are not bound to the nucleus on specific orbits, it's light spectrum is a continuous rainbow of all the colors.

It had already been found that the sun had black lines in it's spectrum. i.e. Wollaston from 1802 and Fraunhofer in 1812.

Bunsen wanted to measure the color of elemental flames using "color filters", and Kirchoff suggested the he use a prism instead. They were surprised to find color bands that gave every element a different optical fingerprint. It caused great wonder in the scientific world of the time.

They found that the spectrum of the sun was the same as an ionised plasma, and they found the optical fingerprints of various gas phase elements also in the sun's using telescopes aligned with prism and measuring devices.

Answer 4

It was not proven at the time, but in 1863 a famous article was published on this question.

https://www.scientificamerican.com/article/experts-doubt-the-sun-is-actually-burning-coal/

I don't know if fully quoting is allowed, so I'll quote the first paragraph.

“If the sun were composed of coal, it would last at the present rate only 5,000 years. The sun, in all probability, is not a burning, but an incandescent, body.