Video narration
Have you ever wondered whether a star is really switching on and off? A distant object above a hot road can seem to shimmer because moving air bends its light differently. The air above us is not uniform: its density changes from place to place and moment to moment. As starlight passes through these changing layers, atmospheric refraction-the bending of light through air-keeps changing its path. So the star's apparent position shifts, and the light reaching your eye seems to grow brighter or dimmer. The star itself is not switching its light on and off. A star is so distant it looks like a point source, a tiny point of light. A planet has a wider image, so its fluctuations mostly average out; planets usually twinkle much less. So no: changing air makes a steady star seem to flicker.