In 1842 the Austrian physicist Christian Doppler described what came to be known as the Doppler shift - an example of which is the change in frequency of a police car siren as it moves towards and away from an observer standing on the pavement.
Doppler realised that light, like sound, will change its frequency depending on the relative movement of the object emitting it and the person observing it. Light frequency changes are observed as changes in colour, so a star that moves away from Earth appears redder; if it moves towards us, it appears bluer. Using a technique called spectroscopy, astronomers measure the red shift or blue shift of stars to understand their relative motion.
Image: Christian Doppler (credit: SPL)
An Austrian physicist shows how the light of moving stars changes.
The Doppler shift shows how stars are moving.
Dr Francisco Diego explains how the Doppler shift can be used to work out whether stars are moving towards us or away from us.
Prof Brian Cox studies the colour of stars to understand how the Universe began.
Prof Brian Cox explains how we can understand the origins of the Universe through differing wavelengths of light emitted by stars.
Christian Andreas Doppler (//; 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.