The short answer
Stars twinkle because their light passes through Earth’s turbulent atmosphere on the way to your eyes. Pockets of air at different temperatures bend light by slightly different amounts, and as those pockets drift past, the star’s light is bent this way and that many times a second. The star itself isn’t flickering. Astronomers call the effect scintillation, and it is strongest for stars low in the sky and on nights when the air is unsteady.
What the air does to starlight
Air bends light, and how much it bends depends on the air’s temperature and density. The atmosphere is never perfectly uniform: wind, warm ground, cold air aloft and the jet stream mix it into countless small cells, each bending light a little differently.
Starlight crosses many of these cells on its way down. The result is that the light reaching your eye is constantly being focused and defocused, and nudged from side to side. The star seems to brighten and dim, and sometimes to jump slightly in position. Those quick changes are what you see as twinkling.
Stars are so far away that, to our eyes, each is effectively a single point of light. A point source has nothing to average out the disturbance, so every wobble in the air shows up directly.
Why planets usually don’t twinkle
Planets are much closer than stars and show a tiny disk rather than a single point. You can’t see the disk without magnification, but it is there. Light from different parts of that disk passes through slightly different paths in the air, and their wobbles partly cancel out. The planet’s overall brightness stays steadier.
That gives a handy rule of thumb: a bright “star” that shines with a steady light is often a planet, such as Jupiter or Venus. It isn’t a perfect test. On very turbulent nights, or when a planet is close to the horizon, planets can twinkle too, and bright stars high overhead on a calm night may barely twinkle at all. Use it together with the other clues in how to find planets, or check planets visible tonight.
Why stars near the horizon flash colors
Low in the sky, starlight travels through far more air than when it comes from overhead, so twinkling is much stronger there. Something else happens too: the atmosphere acts a little like a prism, bending blue light slightly more than red. Near the horizon, the colors of a star’s light take slightly different paths through the turbulence, so the star can appear to flash red, green and blue.
Sirius is famous for this when it is low on winter evenings from the Northern Hemisphere. It is the brightest star in the night sky, so its color changes are easy to notice, and it is regularly mistaken for something stranger. A brilliant, color-changing light that stays in the same place among the stars, and moves only with them over the night, is almost always a bright star low in turbulent air.
What twinkling means for observing
The same turbulence that makes stars twinkle blurs the view through a telescope. Astronomers call the steadiness of the air seeing. Strong twinkling high in the sky usually means poor seeing: planets look like they are underwater, and fine detail such as the cloud belts of Jupiter or the gap in Saturn’s rings comes and goes.
On such nights, switch to targets that don’t need sharp detail: wide star clusters, the Milky Way and, for the naked eye, the constellations themselves. Turbulence barely affects how many faint stars you can see; that depends on transparency, a separate quality of the sky. Seeing and transparency explains the difference and how to judge each.
Not all the turbulence is high up. Heat rising from rooftops, paved areas, and even the telescope itself after you carry it out of a warm house adds its own shimmer. Letting a telescope cool for a while and observing over grass rather than over buildings can help noticeably.
Do stars twinkle from space?
No. Above the atmosphere, starlight arrives undisturbed, and stars shine as steady points. That is one of the main reasons for putting telescopes in space. Large ground-based observatories also use a technique called adaptive optics, which measures the turbulence and reshapes a flexible mirror many times a second to cancel much of it out.
From the ground, twinkling is part of the experience of the night sky. On a calm night high in the sky it is gentle; low near the horizon on a windy winter night it can be dramatic. Either way, it’s a view of our own atmosphere as much as of the stars. For what’s up tonight, see tonight’s sky.