The short answer
Stars twinkle because of Earth’s atmosphere. Starlight travels through the vacuum of space for years without any flicker, then spends its last fraction of a second passing through several miles of moving air. That air bends the light slightly, and because the bending keeps changing, the star seems to waver in brightness and sometimes in position. The effect has a technical name, atmospheric scintillation.
The stars themselves are not flickering at the rate you see. Their light is essentially steady, and the twinkle is added at the very end of the journey.
Why moving air bends light
Light changes direction a little when it passes from one material into another or through air of different density. This bending is called refraction, a basic part of the science of light. It is the same effect that makes a straw look bent in a glass of water. Air is not the same everywhere: warm air is less dense than cool air, and the atmosphere is full of layers and pockets at different temperatures.
These pockets, often called turbulent cells, are carried along by winds at various heights. They can be a few centimeters to tens of centimeters across. As they drift across the path of a star’s light, they act like weak, constantly changing lenses. Some of the time they focus a bit more light toward your eye, so the star brightens. A moment later, they spread it out, and the star dims. Because the atmosphere is churning all the time, this happens many times per second.
It is also the same kind of turbulence that shapes weather systems on a much larger scale, although what matters for twinkling is the small-scale mixing.
Why a star is so easily disturbed
Even the nearest stars are so far away that, to the naked eye, each is just a point of light, much smaller than the eye can resolve. All the light from that point has to squeeze through a very narrow path in the air. If one turbulent cell lies in the way, it can affect all of that light at once. The star’s brightness, and sometimes its apparent position, jumps around.
You may also notice stars flashing different colors, especially when low in the sky. The atmosphere bends different colors of light by slightly different amounts, so for a moment one color may be directed toward you more strongly than the others. Sirius, the brightest star in the night sky, is a familiar example of a star that often seems to flicker in red, blue and white when it is near the horizon.
Why planets usually do not twinkle
Planets are much closer to us than stars. Although they look like points of light to the unaided eye, they are tiny disks rather than true points. You can see this with a small telescope. The light from a planet comes from many slightly different spots across that disk, and each travels along its own slightly different path through the air.
When one path is dimmed or brightened by turbulence, the others are likely to be affected differently. The ups and downs cancel out on average, so the total light that reaches your eye is much steadier. That is why a bright object in the night sky that glows with a steady light, rather than flickering, is often a planet such as Venus, Jupiter or Mars. It is a good rule of thumb for beginners, not a perfect test. Planets near the horizon can flicker, too, because the light then passes through much more air.
The Moon does not twinkle at all in normal viewing, since it is large enough that the effect averages out completely.
When twinkling is stronger
Several conditions make stars twinkle more:
- Near the horizon. Light from a low star passes through far more atmosphere than light from a star overhead, so there is more air to disturb it.
- Windy or unstable nights. Strong high-altitude winds and temperature differences stir up more turbulence.
- Over hot ground or rooftops. Rising warm air just above the surface adds its own flicker.
Calm, clear nights with a steady atmosphere give what astronomers call good “seeing”, and the stars twinkle less and look sharper in a telescope. Observatories are often built on high mountains partly because there is less air above them and the air tends to be more stable.
Twinkling and telescopes
For professional astronomers, twinkling is a nuisance, since the same turbulence blurs images of faint objects. Two main approaches deal with it:
- Go above the atmosphere. Space telescopes avoid the problem completely, which is one reason their images are so sharp. In orbit, stars do not twinkle, and astronauts on the space station describe the stars as steady points.
- Correct it in real time. Some ground-based telescopes use adaptive optics, in which a flexible mirror changes shape many times each second to cancel out the distortion. A bright guide star, real or created with a laser, is used to measure the turbulence.
An everyday comparison
Look at a coin lying on the bottom of a swimming pool. As ripples cross the water surface, the coin seems to shimmer and wobble. The coin is not moving; the water between you and it is. Starlight meets the same kind of distortion in the air. Another familiar example is the shimmer above a hot road in summer, which is the same effect on a smaller scale.
What you can learn from it
Twinkling reminds us that what we see in the night sky is filtered by the air around us. It also gives an easy field test: a point of light that stays calm night after night while flickering stars drift around it is probably a planet, and watching it over several weeks shows it wandering against the background stars. Twinkling is a property of the atmosphere, not a sign of anything unusual about the star itself.