Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: stana on 04/01/2008 19:28:32

Title: Why and How do Stars Twinkle?
Post by: stana on 04/01/2008 19:28:32
What is the science behind stars appearing to twinkle in the sky?

Or was the nursery rhyme just a lie [:P]
Title: Re: Why and How do Stars Twinkle?
Post by: lightarrow on 04/01/2008 19:32:08

Or was the nursery rhyme just a lie [:P]
Athmosphere movement or changes in temperature and so refraction index, along the light's path.
Title: Re: Why and How do Stars Twinkle?
Post by: Soul Surfer on 05/01/2008 00:06:00
A star is essentially a point source of light so even the tiniest movements in the earth's atmosphere will move and distort the image and this causes the light of the star to be magnified and reduced like lights through bumpy glass. Very occsaionally you get very still nights when the stars hardly twinkle at all.  I have only known one of these in  my life so far and that was in Autumn 2007.
Title: Re: Why and How do Stars Twinkle?
Post by: syhprum on 05/01/2008 05:22:54
Modern telescopes use a laser beam to measure the effect and via a computer system adjust the curvature of an auxiliary mirror to compensate.
Title: Re: Why and How do Stars Twinkle?
Post by: chris on 06/01/2008 12:06:15
The twinkling effect is caused by the star's light changing speed as it passes through hotter and colder regions of the Earth's atmosphere. As the light changes speed its path also bends slighty and this is called refraction. You can see the same effect happening on a smaller scale if you look above a burning candle or a bonfire, or even your toaster when it's switched on - the objects in the background appear to be shimmering and twisting.

The effect occurs because light slightly travels faster in a less dense medium, and hot air is less dense than cold air. Consequently, as light from a distant star passes through the atmosphere it alternately hits patches of  cold air and less dense warmer air, bending back and forth as it does so.

As a result the light doesn't reach your eye in a straight line or with a consistent intensity, so it appears to shimmer, or twinkle. A more Earthly example of this phenomenon can be seen in the form of an apparent ocean hovering above a hot road in summer, or as a mirage in a desert.

We talked about mirages and refraction a few years ago on the Naked Scientists radio show. Here's the link:

We also recently discussed why stars appear to twinkle but nearby planets don't:


Title: Why and How do Stars Twinkle?
Post by: lyner on 06/01/2008 16:12:36
You can always distinguish a planet from a star because the planets don't twinkle. This is because their light arrives from a (albeit small) range of directions and the diffraction (yes - diffraction; it's phase addition of the light through a range of path lengths as it reaches you eye.) effect due to the atmospheric irregularities are much less. The diffraction effect is diluted when the light source has finite width.