What is Red Shift?

Imagine a racing car coming towards you, driving past you then driving away. Think about what it would sound like.To the person driving the car the pitch of the noise made by the engine doesn't change but to you it does.

This happens because the sound waves coming from the car are squashed together if it is approaching you and stretched apart if it is receding. We call this the Doppler Effect. But what has this got to do with stars and galaxies?

All stars emit light. If we look at the spectrum of the light (e.g. if we make it go through a prism) we see black lines. These are there because some of the light is absorbed by elements in the outer layers of the stars. The are called absorption lines.

If we compare light from nearby galaxies with light from galaxies much further away we can see a difference. The black lines are shifted towards the red end of the spectrum. Their wavelength is bigger than it should be. What does this mean?

Could it be something like the Doppler effect? Are these galaxies moving away from us? YES.

By measuring the amount of red shift we can work out how quickly they are receding (moving away from us).

Imagine light coming from two galaxies, one not moving and the other moving away from us. See how the light from the moving galaxy is stretched as it moves away making its wavelength bigger.