General Astronomy/The Doppler Effect

The Doppler effect or Doppler shift describes a phenomenon in which the wavelength of radiated energy from a body approaching the observer is shifted toward shorter wavelengths, whereas the wavelengths are shifted to longer values when the emitting object is receding from the observer. This happens with any form of any energy emitted in waves, including sound and light. Sound propagates in a different manner from electromagnetic energy, but the effect is analogous.

With sound, the shortening of wavelength of a approaching train horn makes it higher in pitch, a effect that rapidly changes to a lowered pitch as the train passes. In terms of visible light, approaching objects exhibit a shift toward shorter (bluer) wavelengths, whereas the light of receding objects shofts toward the red wavelengths. With light, typically the effect is not noticeable to the human eye, but easily detectable with a spectroscope.

The relative speed of stars moving toward or away from the Sun, as detected through the Doppler effect, gives clues to the Sun's motion through the Milky Way Galaxy as well as other information about the motion of stars, star clusters and gas clouds in space. The relative motions of binary stars (two stars orbiting around a common center of gravity) can be detected in the variations of their light, and in fact even some binary stars have been detected that cannot be seen as separate stars, but whose binary nature is known from the variations in their combined spectra.

The Doppler effect as applied to distant galaxies led to Edwin Hubble's discovery that all galaxies other than a few of our nearest neighbors are receding from the Milky Way. This, in turn, led to the realization that the Universe is expanding. Since all the distant galaxies showed shifts to longer wavelengths, this became known as the Red Shift, or more specifically the Cosmological Red Shift because of its implications for cosmology, the study of the origin and evolution of the Universe.