• Question: If stars are so bright then why is the night sky so dark?

    Asked by nessy123 to Adam, Joanna, Louise S, Marcus on 21 Nov 2012.
    • Photo: Joanna Giles

      Joanna Giles answered on 21 Nov 2012:

      I used to think about this all the time!! A really really great question!
      Because if you think about it – the universe is supposed to be infinite? There is supposed to be an infinite number of stars? Why then is there not a star in every space that you see? Why is it so dark?

      Well, it’s because those stars are so far away and the speed of light is constant. Did you know that when you are looking at the stars in the sky, you are actually looking into the past? The star that you see isn’t even there anymore because it took so long for the light to travel from the star to our planet, that it has actually burned up and collapsed and “died”.

      Basically, the stars are so bright, but because they are light years away we cannot see them. The sun is a star, and that is why it is light here during the day. The sun is close enough to be able to light up the whole sky. Further away from that and they are mere specks, like if you tried to look at a super bright torch from across a playing field – it would just be a little speck 🙂

    • Photo: Marcus Wilson

      Marcus Wilson answered on 21 Nov 2012:

      I dont think i understood that until I read Joanna’s description, Thanks!

    • Photo: Adam Paige

      Adam Paige answered on 22 Nov 2012:

      Joanna makes a great point. If space was infinite you would expect every part of the sky to show light from one star or another. The fact that it is dark in the night sky was one of the first clues to suggest that space is not infinite.

      As our telescopes improved we could analyse the light from lots of stars, not just to see how bright it was, but also to analyse colour. That was important because the discovery of the theory of gravity meant that the universe could not just be sitting still. It would be pulled inwards (imploding) by gravity, unless it was expanding outwards fast enough that gravity couldn’t stop it. You can tell if stars are moving away from us or towards us by the colour of the light from them. It is called a doppler shift (it is similar to how a police siren sounds like a different note if the car is coming towards you or away from you). Edwin Hubble (as in the Hubble telescope in space) was the first to show that the stars are all moving away from us (their light is shifted towards the red end of the spectrum). Which means our universe is expanding. A really important discovery – and expected if the universe was formed in a big bang.

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