Hermann Carl Vogel: The Sixth Bruce Medalist

The determination of the speeds with which the stars are moving toward us or away from us (their radial velocities) began with an error. To explain why stars had different colors, Christian Doppler at the University of Prague proposed in 1842 that stars have different colors because shifts of wavelength due to radial motion. Doppler had correctly shown that the colors of light experience a shift due to the radial motion of the source, and that this shift is proportional to the speed with which the source is moving. The shift is toward longer wavelengths (a redshift) if the source is receding, toward shorter wavelengths (a blueshift) if it is approaching. Thus a star approaching at half the speed of light would have its red light shifted to violet and its blue light shifted beyond the visible range to ultraviolet, and that is why, Doppler said, some stars are blue.