Red Shift

Red shift is a shift in the wavelength of light emitted by a cosmic object toward the longer (red) wavelengths of the object's spectrum. Light acts like a wave, and its wavelength is the distance between crests of successive waves. The term red shift comes from the shifts first detected in wavelengths of light, but such shifts also occur at radio and other electromagnetic wavelengths. When a red shift occurs, all wavelengths are lengthened by the same fraction. A red shift is expressed as a percentage increase over the normal wavelength.

An example of a red shift can be seen in the spectra of quasars, extremely powerful sources of radio and light waves. A series of bright spectral lines caused by hydrogen appears in the spectrum of Quasar 3C 273 (Object 273 in the 3rd Cambridge catalog of radio sources). The wavelength of each line of 3C 273 is 15.8 percent longer than the normal wavelength. Thus, the red shift of the quasar is 15.8 percent.

Most astronomers believe red shifts occur in cosmic objects because the earth and the objects are speeding away from each other. The amount of red shift of an object indicates its velocity. Objects with small red shifts move at a percentage of the speed of light equal to the red shift percentage. To calculate the velocity of objects that have larger red shifts, astronomers must take into account the effects of the special theory of relativity. The red shift of 3C 273 indicates a speed of 27,000 miles (43,500 kilometers) per second, which is 14.5 percent of the speed of light. Some quasars have red shifts of more than 300 percent, indicating that they have a speed of over 88 percent of the speed of light.

In 1929, Edwin Hubble, an American astronomer, discovered that the red shift of galaxies increases uniformly with the increasing distance of the galaxies from the earth. Thus, the red shift could be used to estimate the distance of the galaxies from the earth. Hubble's discovery has led astronomers to believe that galaxies are rapidly moving away from one another as in an explosion. His findings form the basis for the theory that the universe is expanding.

Contributor: Thomas E. Lutz, Ph.D., Prof. of Astronomy, Washington State Univ.

Red Shift

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