Technique of splitting electromagnetic radiation (light) into its constituent wavelengths (a spectrum), in much the same way as a prism splits light into a rainbow of colors. Spectra are not smooth but punctuated by ‘lines’ of absorption or emission caused by interaction with matter. The energy levels of electrons in atoms and molecules are quantized, and the absorption and emission of electromagnetic radiation only occurs at specific wavelengths. Spectra contain an abundance of information.
The precise position (wavelength) at which known emission and absorption lines are detected can be used to measure the redshift of the observed object. For example, if the spectral line of Hβ (486.2 nm) is detected at 487.8 nm, one can calculate that the object has a recession velocity of 1,000 km/sec. This type of analysis can also be used to detect spectroscopic binaries and extra-solar planets. Spectral line broadening of absorption and emission lines can be used to measure the internal velocity dispersion of complex objects (e.g. the average velocity of stars within galaxies). Most absorption and emission lines are produced by metals. The depths or heights of these lines can be used to estimate the abundances of the metals responsible. In stars, these also permit the measurement of the temperature and pressure of the stellar atmosphere.