What Happens when light from a star passes through a cool cloud of sodium?

When light from a star passes through a cool cloud of sodium, specific wavelengths of the light are absorbed by the sodium atoms in the cloud. This absorption results in dark lines, known as absorption lines, appearing in the spectrum of the star's light. These lines correspond to the energy levels of the sodium atoms and provide valuable information about the composition and properties of the cloud. This phenomenon is a key principle in spectroscopy, allowing astronomers to study the chemical makeup of interstellar matter.