ELI5: Why do stars twinkle?

Stars twinkle because their light has to travel through the moving air of Earth's atmosphere to reach our eyes.

Imagine looking down at a shiny coin at the bottom of a swimming pool. If the water is splashing, the coin looks like it is wobbling, wiggling, and flashing. The coin is not actually moving at all, but the shifting water bends the light before it reaches you.

The air all around our planet acts just like that swimming pool:

• Moving air: Even when it feels calm outside, the air high up in the sky is constantly dancing, swirling, and mixing hot and cold breezes together.
• Bending light: As the star's thin beam of light shoots through this moving blanket of air, the light gets pushed and bent in different directions.
• The blink effect: By the time that bent light reaches your eyes, it quickly jumps back and forth. Sometimes a little more light hits your eye, and sometimes a little less. This rapid flickering makes the star look like it is blinking or twinkling.

You might wonder why planets like Mars or Venus do not twinkle in the night sky. That is because stars are super far away, making them look like tiny, sharp pinpricks of light that are easily pushed around by the wind. Planets are much closer to Earth, so they appear as larger disks of light. The wiggling air cannot bend the entire beam of a planet at once, so its light stays steady and bright.

If you were an astronaut floating in outer space, stars would not twinkle at all because there is no air there to disturb the light.