This tells us that distant galaxies are moving away from us, and that the further away a galaxy is, the faster it is moving away. Astronomers have found that the further from us a star is, the more its light is red-shifted.
It is a change in frequency of the position of the lines. We know this because there are black lines in the spectrum of the light from the Sun where helium has absorbed light. This happens with light, too, and is called ‘red-shift’. It happens with any wave source that moves relative to an observer. This effect, where there is a change in frequency and wavelength, is called the Doppler effect. You may have noticed that when an ambulance or police car goes past, its siren is high-pitched as it comes towards you, then becomes low-pitched as it goes away.
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