Why do stars sparkle?

Gazing up at the heavens at night, it’s easy to get lost in the beauty and immensity of the universe: the moon’s silvery glow, planets moving through their orbits, distant stars twinkling with light emitted millions of years ago.

This dancing of light we see when we look at stars has fascinated humans for centuries – even the smallest humans. “Twinkle twinkle little star, how I wonder what you are” is one of the first songs we learn as children. As we grow older, we learn the names of the stars and constellations.

But what about that twinkle? Why do stars “dance” in the night sky?

We must first turn our gaze to ground level. Picture the wavy motion of air just above burning hot sand. We see this effect because the hot air has a different density to the cooler air above it, and it moves up. As it does, the heat dissipates and alters the density of the air, making it refract light, bending and redirecting it in a slightly different way than the cooler air it meets. To our eyes, it almost appears as if the air is liquid.

The way light travels can be manipulated and redirected by forces altering something called the refractive index. Essentially, this index refers to how much a beam of light can bend in a new direction: a high refractive index means the light will be bent a lot; a low index, a little.

To get back to our star subject (pun intended): between us and the billions of suns above and around us is a thick film of atmosphere. While this is responsible for keeping us alive, it also distorts light.

The atmosphere is like a multilayered “cake”, with stacked levels of decreasing density the higher up you go. In the same way the heat of the sand causes interference in the air above, light doesn’t have a smooth journey as it makes its way from space to our eyes. Where the different layers meet, light is slightly refracted in a new direction, a process that repeats as it penetrates each successive layer.

This results in a zig-zag effect, creating the illusion that the star is slightly shifting. The scientific name is “stellar scintillation” and it’s the reason space-based telescopes produce far superior images to ground-based observatories.

But why, you may wonder, don’t all stars twinkle? There are two main reasons. One is that the light from stars closer to the horizon has to travel through more atmosphere to reach you and so is even more zig-zagged than light from stars higher up. The other is that what appears to be a star is sometimes, in fact, a planet.

A star twinkles not only because its light has to pass through the atmosphere, but also because its light is tiny. The distance of these stars from Earth is so huge that they appear as just a dot. Planets, on the other hand, are closer, so their light appears more as a disc than a dot. The atmospheric distortion is partly cancelled out by light from the planet zigging and zagging in opposite directions.

So the next time you gaze skyward at night, wondering about dancing stars, or when your child or grandchild asks you why that little star in the nursery rhyme twinkles, you’ll have an answer.