I was reading a very old book (around 1850) on astronomy and the author explains Sun spots as a phenomena where you can see the black planetary like hard core of the Sun through two different atmospheres. It got me interested that somewhere between 1850 and now it became understood that the Sun's core is a gas. How was this determined conclusively and approximately when in history?
Well simply said: All data fits the premise that it should be a gas.
But what data?