On 25th May 1842 Christian Doppler presented his book “On the coloured light of binary stars” to the Royal Bohemian Society and was catapulted to world renown.

The Doppler effect and coloured stars

Physicists had already learned how to measure the frequency of light (as an electromagnetic oscillation) and had realised that red light had a lower frequency (from 390 THz, i.e. 390 followed by 12 zeros oscillation per second) than blue light (up to 770 THz).

The faster a star is travelling away from us, the more its light is shifted to lower frequencies, and thus towards the red. Light from certain elements undergoing this shift oscillates at a lower frequency than light from a stationary source, or light from closer stars such as our small yellow sun.

It is extremely helpful that some elements can only emit (or absorb) specific colours (frequencies of light). Investigating light using a glass prism reveals these “spectral lines”.

Turning to binary star systems, these usually consist of a smaller and a larger star orbiting a common centre of gravity. When their orbits line up precisely with the Earth, the larger star regularly blocks our view of the smaller one. At that moment, the maximum speed of both stars at the point of their greatest distance from each other, can be calculated very precisely using the Doppler shift effect. Although in Doppler’s time there was a lack of knowledge about the colours of different types of stars, he proposed a theory which would be very useful to astronomy, and which would only later be proved correct.

Astronomy today: the applications of the Doppler effect

Today the Doppler effect allows us to not only calculate the speed of the stars and galaxies in relation to us, but also to determine their distance, their speed of rotation, the ejection velocities of supernovae and much more. It has also made research into X-ray stars possible.

Modern astronomy would be inconceivable without Christian Doppler’s contributions.

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