• Question: Why do stars twinkle?

    Asked by to Laurence, Dave, Chris on 18 Jun 2014. This question was also asked by .
    • Photo: Laurence Perreault Levasseur

      Laurence Perreault Levasseur answered on 18 Jun 2014:

      This is a very good question!!
      The reason why stars twinkle is that as their light passes through the Earth’s atmosphere to reach us, it gets slightly deflected in a lot of random directions because or turbulence in the air. These are things like hot air pockets, etc. that make the atmosphere constantly churn. When light passes through, it gets refracted a bit. From our point of view, this make it look like the star image constantly changes in position and brightness, or ‘twinkle’!

      That’s why it’s a lot easier (but a lot more expensive, so we can’t make too many of those) to have a telescope in space, like the Hubble space telescope. Up there there is no atmosphere, and so stars do not twinkle. This mean that the Hubble telescope can get much clearer images of distant galaxies!

      Also, things like planets do not twinkle. That’s because stars are so far away that from our point of view they appear just like a point in the sky. But planets are much closer, so they appear to be bigger and to have a finite size. Because of that, the random motion kind of averages out over the whole surface of the planet, and they don’t twinkle! This difference is one of the easiest way to figure what is a planet and what is a star 🙂

    • Photo: Dave Jones

      Dave Jones answered on 18 Jun 2014:

      Really good question! Have you ever looked out on a hot day, towards the horizon, maybe over a road or something, and seen that things in the distance seem to be shimmering? Or even over a kettle when the steam is beginning to escape out of the spout? It is the same shimmering that we see there that makes the stars twinkle.

      When the light from the stars gets to Earth, before it can enter your eye it has to pass through the atmosphere. The air in the atmosphere is moving around a lot and as the light passes through it gets knocked off course just a little bit, first in one direction, then back, just sort of jiggling it about like the air is jiggling about. The amount that it gets jiggled varies just like the movement of the air varies, so from one moment to the next the stars looks like it’s moving around just a little and that’s what we see as the twinkling. Astronomers actually measure this twinkling and use it to define how good a particular place (or even night, because it can vary between nights too) is for doing astronomy – we call it “the seeing”. Good seeing means the stars don’t twinkle, bad seeing means they twinkle a lot.

      That’s why the best telescopes are built up mountains, where the light from stars doesn’t have to pass through much atmosphere and so the atmosphere has less chance to make them twinkle, and in places where the atmosphere is very stable, because in those places the stars just don’t twinkle as much meaning astronomers like me can take better images of them! That’s one of the reasons I work in the desert in Chile and not back in the UK, the stars in the UK twinkle too much (when they aren’t hidden by clouds!).

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