Physics

The dark absorption line tells you that cool hydrogen gas in the star's outer atmosphere is selectively absorbing photons at that specific wavelength—meaning electrons are jumping from the $n=2$ to higher energy levels. This reveals both the presence of hydrogen and its temperature: only if the gas is cool enough will most electrons sit in the $n=2$ state ready to absorb H$\alpha$ photons, rather than being ionized or excited to higher states. The sharpness and depth of the line also hint at the gas's density and composition.

The dark absorption line tells you that cool hydrogen gas in the star's outer atmosphere is selectively absorbing photons at that specific wavelength—meaning electrons are jumping from the $n=2$ to higher energy levels. This reveals both the presence of hydrogen and its temperature: only if the gas is cool enough will most electrons sit in the $n=2$ state ready to absorb H$\alpha$ photons, rather than being ionized or excited to higher states. The sharpness and depth of the line also hint at the gas's density and composition.