Christian Doppler’s 1842 paper focused especially on the colored light of binary stars. Its German title translates as “On the coloured light of the binary stars and some other stars of the heavens.”
Doppler proposed that the observed frequency of a wave changes when its source or observer moves. He reasoned that orbital motion in a binary-star system could shift the frequency of starlight enough to alter its apparent color. In his model, an approaching star would shift toward shorter wavelengths, while a receding star would shift toward longer wavelengths.
The central Doppler principle was correct, but his explanation of stellar colors was not. Doppler assumed stars emitted broadly white light and that their orbital speeds could produce dramatic visible color changes. Modern astronomy shows that a star’s color is primarily related to its surface temperature, while motion is detected more reliably through measurable spectral shifts.
The effect was later tested with sound. In 1845, Christoph Buys Ballot demonstrated that approaching and receding sound sources changed the observed pitch, providing an early experimental confirmation of Doppler’s idea.