Science Questions

Why is it that stars appear spiky and not spherical? And why do they twinkle?

Sun, 15th Nov 2009

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Chris, Lowestoft asked:

Why is it that stars appear spiky and not spherical? And why do they twinkle?


The main reason that stars look spiky is that the optical instrument you are looking at them with is not perfect. Because light is a wave, if you shine it through a small hole you get a pattern known as an interference pattern, this is because different light waves interfere with each other, making patterns of light and dark.

If the hole was circular you would get a relatively clean picture, but if it isn't then you will get various visual artifacts. These are much dimmer than the object itself so during the day you can't see them, but if a bright object is surrounded by darkness they are very obvious.

So if you look at a streetlight you often see streaks coming off it. If you squint, you make your pupil even less circular so the streaks get much stronger. The interference pattern will have a similar symmetry to the hole, so if you squint your eyes into a slit you get 2 strong streaks. Your camera doesn't have a perfectly circular aperture so you get streaks, the number will depend on the shape of the aperture.

Large telescopes need structures to hold their secondary mirrors, and it is the diffraction from these that you see in pictures from the Hubble Space Telescope etc.

Chris -  But the reason they’re actually twinkly when we look at them is nothing to do with that.  They’re twinkly when we see them through the Earth’s atmosphere because the Earth’s atmosphere is not uniform.  There’s air which is at hotter temperatures, and therefore, less dense, and there’s air which is at colder temperatures, and therefore, more dense.  And when light goes from a medium, which is more dense, into a medium which is less dense, it changes speed.  In fact, it speeds up a bit.  And it’s that change in speed that causes the light to bend a little bit.  And that means that when you see rays of light coming from a long way away, they appear to be coming from one place, and then another place, and another place, and then another because the light rays are being bent alternately as it goes in and out of warm and cold patches of the Earth’s atmosphere.  And that’s what makes the star twinkle.  And you see the same thing happening if you look at the lights from a harbour across a sea harbour or a port, for example.

Diana -   And you can also get polarised filters to put on your camera.  They will make the stars appear to point in numerous different directions, as well.


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