# How did astronomers first come to understand that the Sun's core was a gas?

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?

• First of all, in the late 1960's the 5-minute solar oscillations on the surface of the sun have beend discovered. Those have been recognized as surface features of internal pressure waves with penetration depth $d$, wavenumber $k$ and their relation roughly $d \sim \frac{1}{\sqrt{k}}$.