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Spectral Lines

Doppler Shift

Another way that spectral lines are used in astronomy is to determine the velocity of an object. An object which is moving away from Earth will have its spectral lines shifted to longer wavelengths due to the Doppler shift acting on the emitted photons. Similarly, objects moving towards Earth will be shifted to shorter wavelengths. By measuring the shift of a spectrum, the velocity with which the object is moving with respect to the earth can be determined. A shift to longer wavelengths is called a red shift, since red light appears on the long wavelength side of the visible spectrum, while a shift to shorter wavelengths is called a blue shift.

Doppler shift measurements of spectral lines have been used to measure the velocities of winds in stars, the speeds of outflowing gases from stars and other objects; the rotational motion of material in the center of galaxies, and the recession of galaxies due to the expansion of the universe. The latter measurements are particularly important, since they allow astronomers to probe the structure of the Universe.



Aller, Lawrence H. Atoms, Stars, and Nebulae. New York: Cambridge University Press, 1991.

Kaufmann, William J. III. Universe. New York: W. H. Freeman and Company, 1991.

David Sahnow


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Absorption spectrum

—The record of wavelengths (or frequencies) of electromagnetic radiation absorbed by a substance; the absorption spectrum of each pure substance is unique.

Bohr atom

—A model of the atom, proposed by Niels Bohr, that describes the electrons in well-defined energy levels.

Doppler shift

—The shift in wavelength of a spectroscopic line from its zero velocity wavelength to longer (redder) wavelengths if the source of the line is moving away from the observer or to shorter (bluer) wavelengths if the source is approaching the observer.

Emission spectrum

—A spectrum containing narrow spectral lines at frequencies corresponding to the photon energies of the atoms making up the object being observed.

Energy level

—An allowed energy state of an electron in the Bohr model of the atom.


—A single quantum of light.

Planck's law

—A relationship describing the proportionality between the frequency of light and the energy of a photon.


—The ability of a spectrograph to separate two adjacent spectral lines.


—Instrument for dispersing light into its spectrum of wavelengths then photographing that spectrum.


—A display of the intensity of radiation versus wavelength.

Additional topics

Science EncyclopediaScience & Philosophy: Adam Smith Biography to Spectroscopic binarySpectral Lines - History, Spectrographs, Doppler Shift