An educational article with references listed below.
On a clear night, stars can appear to twinkle or change brightness rapidly. This gives the impression that their light is constantly changing. The rapid twinkling we see from Earth mainly happens because starlight must pass through the atmosphere before reaching our eyes.
Light passes through moving air
The air in the atmosphere is always moving. This motion, also called turbulence, bends starlight slightly as it travels towards the observer. At one moment, some of the light reaches the eye directly. A moment later, changes in the air can bend some of it in another direction. These small, continuous shifts make the star appear to flicker.
Why is twinkling more noticeable near the horizon?
The strength of the effect also depends on where the star appears in the sky. Stars close to the horizon usually twinkle more noticeably than stars higher up. Light from a star near the horizon must pass through more of the atmosphere before reaching us. That longer path gives turbulent air more opportunities to bend the light. This is why the viewing position affects how strongly the twinkling appears.
Try observing the night sky
You can observe this difference yourself on a clear, dark night. Compare a star low near the horizon with another star higher in the sky. The lower star tends to show more noticeable changes in brightness because its light passes through more atmosphere.
Rapid twinkling therefore need not be understood as a change taking place in the star itself. The effect mainly arises as the light travels through layers of air around Earth. The moving atmosphere subtly changes the direction from which the light arrives, and our eyes perceive those changes as a shimmer that rises and falls over time.
