Question: If the closest star to earth is Proxima Centauri and the Canis Major Dwarf Galaxy is the closest galaxy if there are billions of stars in just our universe how can astronomers tell if they're definatley the closest?

James Holloway answered on 14 Mar 2013:

Redshift of the light we see.

If something is moving away from us the light it gives off appears more red to us. If something is moving towards us the light appears more blue. This is because of the Doppler Effect.

Hubble discovered that the universe is expanding. That means if something is close to us it is drifting away slowly, if something is very far away it is moving away from us very fast (imagine drawing black dots on a balloon and then blowing the balloon up, think about the speed at which the dots move away from each other).

We can look at a star and identify what are called ‘absorption lines’ in the colour of the light given off. (this is when a certain colour is missing because a chemical in the sun has absorbed it all). These absorption lines will always be at the same colour if you look at them in the laboratory.

When we look at a star and see that a certain ‘absorption line’ is a little bit more red than what we see in the laboratory, we know that is because the star is moving slowly away from us. Hubble then tells us that it must be close if it is moving slowly away from us.

When we look at a star and see that a certain ‘absorption line’ is A LOT more red than what we see in the laboratory, we know that is because the star is moving QUICKLY away from us. Hubble then tells us that it must be very far away if it is moving fast away from us.

You may ask:
‘well how did Hubble know how far things were away in the first place? how did he realise that the things moving fast away from us *were* far away?’

Well he noticed something very clever. He already knew that when stars die they sometimes explode in whats called a super nova (a truly huge explosion). What he realised was is that a certain type of super novae exploded with the same brightness each time – the explosions were always the same in size.

He then got his telescope and pointed them at distant galaxies, waiting for a star to explode in them. He recorded how bright it appeared to him. Dim explosions meant the galaxy was far away, bright explosions meant the explosion was close. Once he had enough data he could then look at the redshift of these galaxies and tell other scientists how rapidly the universe was expanding, and how much redshift to expect on distant objects.

Really clever guy!

I hope that was not too much info.