Require discrete amounts of energy to switch levels. Absorb or emit at specific wavelengths only. Fingerprints for atoms, can recognize composition from spectra. Emission vs. absorption spectra. Emission lines are in same position as absorption lines for a give species of atom.
Emission lines are always at same wavelength for a given atom anywhere in the universe.
So, can use as reference for Doppler measurements. Assumption that Physics is same everywhere in Universe. Longer focal length, more magnification, but dimmer image. Must compromise some for others. Earth as a Planet.