• Question: If stars emit light energy and there is an infinite amount of stars than why is the sky dark at nighttime?

    Asked by to Mark, Matthew, Mike, Paul, Sabina on 13 Mar 2014. This question was also asked by .
    • Photo: Sabina Hatch

      Sabina Hatch answered on 13 Mar 2014:

      There are definitely a lot of stars out there, but they are extremely far away.
      Looking at a torch near your face would appear very bright, but then if you placed the torch facing you and walked backwards, the further you get from it – the dimmer the light appears. This is because not all the light is now reaching your eye, so the intensity is lower.

      The same goes for the stars, only a small amount of their light is actually reaching us from so far away. The light from stars can appear brighter if you focus the light into your eyes using a telescope.

    • Photo: Mike Lee

      Mike Lee answered on 13 Mar 2014:

      Nice question, there are two options.

      1) There is a limited number of stars.
      2) light from distant stars has not reached us.

      Both could be true. I don’t know if there is an infinite number of stars, there could be. But I do know that light from the most distance stars has not reached us yet. This is because the universe had a beginning, so there has only been 13 billion years for light to travel across the universe, which is not enough time for the furthest away stars’ light to get to us.

    • Photo: Paul Coxon

      Paul Coxon answered on 13 Mar 2014:

      This is a very famous puzzle called “Olbers’ Paradox”. It’s named after a German astronomer called Heinrich Olbers who wondered exactly the same thing: if there are lots are stars, then why is the sky at night dark?

      We can’t say if there is an infinite number are starts, but there certainly are lots and LOTS of them in space, shining out light towards Earth. But because light travels at a fixed speed it takes time for that light to reach us. In fact, light from our nearest star, the Sun, takes 9 minutes to reach Earth.

      The universe is very, very big, and is getting bigger. Also it’s only existed for 13.8 billion years, so the light from any distant stars more than 13.8 lightyears away is still travelling but hasn’t reached us yet.

    • Photo: Mark Jackson

      Mark Jackson answered on 15 Mar 2014:

      Fantastic question! This was actually a very difficult problem which stumped a lot of smart people. It was most famously asked by German astronomer Heinrich Olbers, and so it is called Olbers’ Paradox. The mainstream suggestion was actually given by poet Edgar Allen Poe: the Universe must have a beginning moment and so there are not an infinite number of stars which could be observed today, and in fact not even enough that the sky appears bright. Quoth the raven, Nevermore!

      The problem is actually even worse with the Big Bang Theory, since the Universe began extremely hot and dense. Where is all this radiation, if it has had the whole time to reach us? The answer is that the radiation actually *is* all around us, but the expansion of the Universe has also stretched the wavelength of the light so that we don’t see it. It is called the Cosmic Microwave Background and is one of the most interesting things in modern physics, as I explain more at http://goo.gl/v4jXBx.

Comments