Christian Andreas Doppler /ˈdɒplər/ (German: [ˈdɔplɐ]; 29 November 1803 – 17 March 1853) was an Austrian mathematician and physicist. He is celebrated for his principle – known as the Doppler effect – that the observed frequency of a wave depends on the relative speed of the source and the observer. He used this concept to explain the color of binary stars.