Why do stars twinkle?

The scientific name for twinkling is stellar scintillation. Starlight travels light-years to reach us as an almost perfect point of light, but in the final fraction of a second it crosses tens of kilometers of turbulent air with pockets of different temperatures and densities. These layers refract the light in shifting directions, making the star seem to flicker and change brightness. Planets, being much closer, appear as tiny disks rather than points; the many light rays from across the disk average out the atmospheric distortion, so planets shine with a steadier glow. Above the atmosphere, stars do not twinkle at all.