Behind the wave source, the waves spread out and the pitch is lower. When energized atoms and molecules vibrate, they give off massless light particles called photons. These photons travel as a wave, but because of quantum energy effects, a particular type of atom or molecule gives off only certain wavelengths of photon light. For example, when hydrogen atoms are giving off energy in the form of light, they emit light specifically at wavelengths of 410.2 nm, 434.0 nm, 486.1 nm, and 656.3 nm. This is called the emission spectra of hydrogen.
Scientists can use the emission spectra of atoms and molecules to study the composition of stars.
Scientists need simply to look very carefully at the intensity and wavelengths of the light given off by the star. A star containing hydrogen, for example, would have intense peaks of energy at 410.17 nm, 434.05 nm, 486.13 nm, and 656.28 nm. These hydrogen emission peaks would be in addition to the ones associated with the other elements contained in the star. In the 1920's, Edwin Hubble, while studying the stars of distant galaxies, found that for some, their emission spectra had peaks at 411.54 nm, 435.50 nm, 487.75 nm, and 658.47 nm. Hubble knew that these wavelengths did not correspond to any known element and that it was not likely that a combination of other elements or molecules was responsible.