Only objects of known luminosity called standard candles can be used to measure distances. There are lots of things we use as standard candles for instance, Cepheid Variable stars get brighter and dimmer in a regular way that tells us how luminous they are, so we can work out there distance to the galaxy they live in. But there are limits to living standard candles, like if something is in the way blocking the lighter.
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Imagine a fire engine. Its siren emits sound waves. As it rushes along a road, the crests of the sound waves in front of it are closer together than those behind it. This is due to something called the Doppler Effect it effects light waves emitted from moving sources too. Shorter waves appear bluer where as longer waves appear redder. We can use this red-shift and blue-shift of light to study the motions of things in space.
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Looking at galaxies containing Standard Candles astronomers found that the light from all distant galaxies was red-shift. The more distant the galaxy, the greater the red-shift.
Light waves travelling to us from distant galaxies are red-shifted, not because they are moving away from us but because the have stretched out as they travelled to us through our expanding universe. We know how far away things are in the distant universe by measuring how red-shifted their light is. |