Stars - CCEAHigher tier: Red shift and the Big Bang

You may have noticed that when an ambulance or police car goes past, its siren is high-pitched as it comes towards you, and then becomes low-pitched as it goes away.

This effect, where there is a change in frequency and wavelength, is called the Doppler effect.

It happens with any wave source that moves towards or away from an observer.

If the object is moving towards an observer, the waves are shifted to a higher frequency, shorter wavelength.

If the object is moving away from an observer, the waves are shifted to a lower frequency, longer wavelength.

This happens with light as well as sound.

Our Sun contains helium.

We know this because there are black lines in the spectrum of the light from the Sun where helium has absorbed light.

These lines form the absorption spectrum for helium.

When we look at the spectrum of a star in another galaxy, the absorption spectrum is there, but the pattern of lines has moved, or ‘shifted’, towards the lower frequency, longer wavelength, red end of the spectrum, as you can see below:

This is called red-shift.

Light from a star in a distant galaxy is shifted towards the long wavelength, red end of the spectrum.

This tells us that:

• other galaxies are moving away from our galaxy;
• the Universe is expanding.

Astronomers have found that the further from us a star is, the more its light is red-shifted.

This tells us that:

• The further away a galaxy is, the faster it is moving away.

Since we cannot assume that we have a special place in the Universe, it suggests that everything is moving away from everything else as you would expect with an explosion.

Summary

• There is a red shift in light observed from most distant galaxies.
• Red shift shows that a galaxy is moving away from us.
• The further away the galaxies are, the bigger the red shift.
• The further away a galaxy is, the faster it is moving away.
• Red shift gives us evidence that the universe is expanding.