Twinkling of Stars

Starlight travels an enormous distance before it even reaches Earth's atmosphere — and once it does, it doesn't travel in a straight line all the way down. The atmosphere's density keeps changing (layers of air at different temperatures and densities), so the light keeps refracting continuously, bending slightly more each layer it passes through.

Because the atmosphere is never perfectly still — pockets of air are constantly shifting — the exact path the light takes keeps changing too. Since stars are so far away they act like tiny point sources, so this constant, tiny shifting makes the amount of light reaching your eye flicker: sometimes brighter, sometimes fainter. That flickering is what we see as twinkling.

Planets, on the other hand, are much closer to Earth and appear as extended sources of light rather than a single point — essentially like a tiny disc made of countless point sources. The twinkling from all those individual points averages out, cancelling itself, so planets appear to shine with a steady light.