What Makes Stars Twinkle?

Scientific American

16-05-2025

Ah, it's a lovely night for enjoying the outdoors. You go outside in the warm summer air to listen to the crickets and breathe in the scents of verdant life and then turn your head to the heavens. You see hundreds of stars in the sky, and the brightest are conspicuously twinkling and gleaming.
Some are even shifting their colors across the rainbow, delighting your eyes and mind—unless you're out there to do some observing with a telescope. That twinkling is lovely for any average stargazer to behold, but scientifically it's a pain in the astronomer.
Twinkling is the apparent rapid variation of brightness and color of the stars. It's technically called scintillation, from the Latin for 'sparkle,' which is apt. While it is admittedly lovely, it's still the bane of astronomers across the world.
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For millennia, twinkling was misunderstood. As with so many scientific principles, it was misdiagnosed by ancient Greeks such as Aristotle, who attributed it to human vision. At that time, he and his peers believed that the eye actively created vision by sending out beams that illuminated objects and allowed us to see them. But these beams were imperfect, so the belief went, and the farther away an object was, the more the beam would be distorted; stars, being very far away, suffered this flaw greatly, causing them to twinkle. It was Isaac Newton, through his studies of optics, who finally determined the true cause.
A fundamental property of light—true of all waves, in fact—is that it bends when it goes from one medium to another. You're familiar with this: a spoon sitting in a glass of water looks bent at the top of the liquid. This is called refraction, and in the case of the spoon, it happens when the light goes from the water in the glass to the air on its way to your eye, distorting the shape of the otherwise unbent spoon. The amount of refraction depends on the properties of the materials through which the light travels. Density, for instance, can dictate the degree of refraction for light moving through gas—so light traveling through air alone will still bend if the air has different densities from one spot to the next.
If Earth's atmosphere were perfectly static and homogeneous, then the refraction of starlight would be minimal. Our air is always in motion, however, and far from smooth. Winds far above the planet's surface stir the air, creating turbulence. This roils the gases, creating small air packets of different densities that move to and fro.
Starlight passing through one such parcel of air will bend slightly. From our point of view on Earth, the position of the star will shift slightly when that happens. The air is also in motion, so from moment to moment, the starlight will pass through different parcels on its way to your eye or your detector, shifting position each time, usually randomly because of the air's turbulent motion. What you see on the ground, then, is the star rapidly shifting left, right, up and down, and all directions in between, several times per second—in other words, twinkling.
The amount of the shift is confusingly called 'seeing' by astronomers, and it's actually quite small. It's usually only a few arcseconds, a very small angle on the sky—the full moon, for example, is about 1,800 arcseconds wide. Stars, though, are so far away from us that they appear to be a minuscule fraction of an arcsecond wide, a tiny point of light to the eye, so even this minuscule arcsecond-scale shifting makes them appear to dance around.
Note that this is why planets typically don't twinkle. Jupiter, for example, is usually several dozen arcseconds wide, so twinkling doesn't affect its position as much, and we perceive its light to be steady.
Twinkling is usually more obvious for stars near the horizon than overhead. The atmosphere is a shell of air surrounding Earth. When we look straight up, we're looking through roughly 100 kilometers of air, but toward the horizon, that length increases to more than 1,000 km! That gives the air many more chances to refract the starlight, increasing twinkling.
But it's not just position that twinkling affects. Different wavelengths—colors—of light refract by different amounts. This is why a prism or a raindrop breaks light up into separate colors to create a rainbow. For a star, which can emit light at essentially all colors, this means sometimes its red light is bent toward you and blue is bent away, so the star appears ruddy. A fraction of a second later, a different parcel of air refracts the blue light toward you, and the star sparks azure.
This effect is most apparent for white stars near the horizon. Sirius is white, and it's the brightest star in the night sky; when rising or setting, it can flash brilliantly and change colors rapidly. This is probably why it's often reported as a UFO! So if you hear a report that a bright spacecraft changed colors rapidly as it hovered over the trees, be aware that it almost certainly was not an alien ship but an alien sun.
For astronomers, though, twinkling leads to a very different outcome: light from an object gets spread out over an image's exposure time. Details in a distant galaxy, for example, look out of focus and blurry. Also, faint objects appear even fainter because their light is smeared out. These are serious problems but ones for which we have a solution: adaptive optics. Inside some telescopes are sensors that can detect the amount of twinkling. This information is sent to a computer that rapidly calculates the distortion, then adjusts pistons behind a deformable mirror to reshape the reflective surface in a way to compensate for the twinkling. Most large ground-based telescopes employ this amazing tech, which produces clear, sharp images despite the atmospheric turbulence.
Twinkling has a scientific use as well. The kind of light we see is not the only kind that refracts; radio waves do so as well when they pass through interstellar plasma, the ionized gas between the stars. Pulsars are rapidly spinning neutron stars that send out brief radio wave pulses at rapid intervals. The radio waves scintillate as they pass through plasma on their way to Earth, and astronomers can measure that scintillation to investigate that plasma. Research published in Nature Astronomy in April 2025 used this to look at the material in space close to the sun and map out structures in the Local Bubble, a region of space surrounding the sun where ancient supernovae cleared out much of the gas. Scientists found 21 large arcs of plasma sustained by turbulence inside the bubble, which surprised them because it was previously assumed that the bubble was more smooth.
Personally, I'm of two minds about twinkling. It's lovely, certainly, but it caused me quite a bit of grief when I was using a telescope for my own research. Depending on what you're trying to study, though, it can still be a useful tool. So one might say my own opinion of twinkling is malleable; it can be bent either way.