Stellar Scintillation

Why stars twinkle, but planets don't.

The Atmosphere as a Lens

To an astronaut in space, stars don't twinkle—they shine with a steady, unwavering light. The twinkling effect, scientifically known as stellar scintillation, is caused entirely by Earth's atmosphere.

As starlight enters our atmosphere, it passes through layers of air with different temperatures and densities. Hot air is less dense than cold air. When light moves between these densities, it bends—a process called refraction.

Since the atmosphere is turbulent (constantly churning and mixing), these pockets of hot and cold air act like moving lenses. They focus and defocus the starlight, causing the amount of light hitting your eye to fluctuate rapidly.

Atmospheric Refraction Simulator

This simulation visualizes a single beam of starlight passing through turbulent atmospheric layers. Watch how the air pockets deflect the beam, causing it to miss the observer's eye momentarily or hit it with concentrated intensity.

The Point Source vs. The Disk

A common question is: "Why don't planets twinkle?"

The answer lies in apparent size.

Stars are so far away they are effectively point sources (a single pixel of light).
Planets are much closer and appear as tiny disks (a cluster of pixels), even if your eye can't resolve the shape.

When the atmosphere bends light from a star, the entire star moves or dims because it's just one point. When the atmosphere bends light from a planet, it bends light from the left side, right side, top, and bottom independently. These random twinkles average each other out, resulting in a steady glow.

Point vs. Disk Interaction

Toggle between a Star and a Planet. The graph tracks the light intensity reaching your eye over time. Notice how the Star's graph is jagged (unstable), while the Planet's graph is smooth (averaged).

Chromatic Scintillation (Color Twinkling)

Have you ever noticed a bright star near the horizon flashing red, green, and blue? This is called chromatic scintillation.

When a star is low in the sky, its light passes through a much thicker layer of atmosphere. The air acts like a prism. Because blue light bends more than red light, the atmosphere splits the starlight into a tiny vertical spectrum.

As turbulence moves across this spectrum, it might block the red part for a split second (leaving the star looking blue), then block the blue (making it look red).

Horizon Prism Effect

Drag the slider to change the star's altitude. Notice how the atmosphere splits the colors more drastically when the star is near the horizon.

"Seeing" Conditions

Astronomers use the term "Seeing" to describe how stable the air is.

Good Seeing: The air is calm. Stars look like sharp points. You can see fine details on planets.
Bad Seeing: The air is turbulent. Stars look like boiling blobs. Planetary details are blurred.

This is why major observatories are built on high mountain peaks (like Mauna Kea in Hawaii) or launched into space (like Hubble and James Webb)—to get above as much of the atmosphere as possible.