When starlight passes through a cold cloud of hydrogen gas, some hydrogen atoms absorb energy, then reradiate it in all directions. As a result, spectrum of the star shows dark absorption lines at the energies for which less energy from the star reaches us. How does the spectrum of dark absorption lines for very cold hydrogen differs from the spectrum of bright emission lines from very hot hydrogen?

Whenever the bright lines fall on the metal surface, the emission of a photon takes place, and when the dark lines fall on the metal surface, the absorption of a photon takes place.

In the case of cold hydrogen gas, at the initial stage, most of the electrons are in the ground state, due to which resultant photons have higher energy because of the difference of energy from the ground state to a first excited state.

Whereas, in the case of hot hydrogen gas, most of the electrons are in the excited state at the initial stage, resulting in lower energy because of the difference of energy from the ground state to the first excited state.

Therefore hydrogen gas with a lower temperature will absorb more photons than hydrogen gas with a higher temperature.