Born on 29 November 1803 in Salzburg, Austria, physicist Christian Doppler proposed the phenomenon that enables astronomers to determine the velocity of distant objects. He studied mathematics at the Vienna Polytechnic Institute and math, mechanics, and astronomy at the University of Vienna. Doppler had shaky health throughout his life, and he had trouble securing a stable professorship. Nonetheless he produced several important papers, none more influential than “On the colored light of the double stars and certain other stars of the heavens” in 1842. He theorized that the colors of two stars orbiting each other would appear to change slightly, due to one star moving away from the observer and the other moving closer. More generally, Doppler proposed that the apparent frequency of light or sound waves emanating from an object changes depending on the object’s motion toward or away from the observer. Scientists in many fields, particularly astronomy, depend on the Doppler effect. Stars and galaxies moving away from Earth shift toward the red part of the color spectrum (redshifted), while astronomical objects moving toward Earth are blueshifted. Scientists analyzing the distance to and motion of stars and galaxies have determined that the universe is expanding at an ever-faster rate.
