Why do Stars Twinkle?

We've all heard the song "Twinkle, Twinkle, Little Star". Stars do appear to twinkle, but why? First, what do we mean by twinkle? If you look at a star on a clear night for a few moments, you'll notice the light seems to move or waver just a bit. This is what is meant by twinkling.

Credit:ESA/Hubble & NASA

Stars are at great distances, with the closest star located just over 4 light years from our Solar System. Thus to our eyes the light from a star appears to come from a single point. As that light travels through our atmosphere, turbulence (movement of air) causes the light to change in brightness and intensity just a bit. This results in twinkling. This is not a real effect of the star, but a result of the atmosphere we need in order to survive. On some nights, if there is greater turbulence, twinkling will be greater. On other nights, the twinkling will be less, but it is still there. There's no way to remove turbulence in the atmosphere. The only way to stop star twinkling is to remove the atmosphere and that, well, isn't the greatest of ideas for what I hope is an obvious reason!!!

But why do planets not twinkle? I'll save the answer for that in a future post.