1. The study of spectral lines from different atoms and molecules which is an important part of studying the chemistry that goes on in stars and in interstellar clouds.
2. The splitting up of electromagnetic radiations into the constituent wavelengths.

Atoms can exist in a number of discreet energy levels and they emit or absorb photons when they make transitions from one level to another.

The energies of the photons emitted or absorbed by one atom are different from those of all of the other atoms.

The photon energies are directly related to their frequencies, which set their colors in the spectrum, so by observing the colors of the photons, it is possible to determine which atoms are being observed.

This can be done in a laboratory, and it can also be done with the light reaching us from stars, near or distant, which enables us to identify the atoms that stars are made of.