Revise GCSE Physics


Question:What is meant by Stellar red-shift?

Stellar red shift is the method that is used to calculate the speed at which distant stars and galaxies are travelling away from us.

By studying the spectrum of light from a star, we can work out exactly which elements are in the star.

It turns out that the spectra of distant stars are very slightly different to what we would expect.

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The spectra from distant stars appear to have the characteristic lines in their absorption spectra (for the elements they contain), shifted towards the red end of the visible spectrum.

This phenomenon is called the "Stellar Red Shift".

The number and pattern of the lines is unchanged, they are just all moved towards the Red end of the spectrum.

The amount of Red shift corresponds to how fast the star or galaxy is moving away from the Earth. More redshift means moving away from us at a faster speed.

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The fact that the spectra of all the stars in the Universe are red-shifted proves that the Universe is expanding, because all the stars are moving away from us and each other.This is the most important piece of evidence in support of the Big Bang Theory.


If a star was to be moving towards us, then the opposite effect would happen. The spectrum would be blue shifted.

The explanation for this is that the Doppler effect causes all waves (including light) to change wavelength if their emitter is moving away from the receiver - shifting to a longer wavelength means moving towards the red end of the spectrum.