Why do stars twinkle in the night sky?

Stars twinkle because of atmospheric turbulence. As starlight travels through Earth's atmosphere, it passes through layers of air with varying temperatures, densities, and humidity levels. These layers are constantly moving and mixing, which causes the light to refract, or bend, in slightly different directions from moment to moment. This rapid, random bending makes the star's apparent position and brightness fluctuate, producing the twinkling effect we see, technically called scintillation. Planets generally don't twinkle as noticeably because they are close enough to appear as tiny disks rather than point sources of light. The multiple light paths from across that disk average out the atmospheric distortions, resulting in a steadier glow compared to stars.