- Dark or bright lines seen in the spectra of astronomical objects. The lines – emission lines if bright and absorption lines if dark – are observed at frequencies which represent the difference in energy levels between possible electron configurations of different atoms. It follows that the lines can be used to determine the presence or absence of different elements, and by observing their thickness the abundance of different elements can be determined. In addition, the velocity with which the emitting body is approaching or receding from the Earth can be found by measuring Doppler effects on the wavelengths of spectral lines. Spectral lines were first observed in optical astronomy but are now observed across almost the whole electromagnetic spectrum and are the very core of the methodology of modern astronomy and astrophysics.