Spectral Classification
"The sun is a mass of incandescent gas
A gigantic nuclear furnace
Where hydrogen is built into helium
At a temperature of millions of degrees" -They Might Be Giants
It's so ingrained in us that the Sun is a nuclear furnace powered by hydrogen atoms fusing into heavier elements that it's difficult to remember that, just 100 years ago, we didn't even know what the Sun was made out of!
Image credit: Landscape Photography by Barney Delaney.
The conventional wisdom at the time, believe it or not, was that the Sun was made out of pretty much the same elements that the Earth is! Although that…