The flickering of a star, often called “twinkling,” is a common astronomical observation. This effect is not an intrinsic property of the distant star, but rather a visual consequence of the light’s journey through Earth’s atmosphere. This phenomenon is technically known as astronomical scintillation, and it is most noticeable when viewing bright stars low on the horizon.
The Atmospheric Cause of the Flicker
The primary reason stars appear to flicker is atmospheric turbulence between the observer and the celestial object. Starlight travels as a straight, consistent beam until it encounters the Earth’s atmosphere, which is a dynamic, swirling mix of air pockets with changing temperatures and densities.
These pockets of air act like tiny, unstable lenses, rapidly bending and redirecting the light’s path. Warm air is less dense and refracts light differently than cooler, denser air, creating thermal variations, eddies, and currents. As these air cells move across our line of sight, the starlight is refracted multiple times in slightly different directions.
This continuous bending of the light beam causes the star’s apparent position and brightness to change many times per second. When a turbulent air pocket redirects the light away from the observer, the star dims; when the light is focused toward the observer, the star brightens. This rapid change in intensity is what the human eye perceives as a flicker or twinkle.
How the Atmosphere Creates Red and Blue Shifts
The color shifts, ranging from red to blue, are caused by chromatic dispersion. The Earth’s atmosphere acts like a weak, constantly moving prism because different colors of light are different wavelengths. Each wavelength is refracted, or bent, at a slightly different angle when passing through the atmosphere.
Blue light, which has a shorter wavelength, is bent more significantly than red light, which has a longer wavelength. The turbulent air pockets momentarily separate these colors, directing the red and blue components of the starlight along slightly different paths.
As the air pockets rapidly shift, the observer’s eye catches these separated colors in quick succession, creating the flashing red and blue display. This effect is pronounced for bright stars whose light has traveled through a thick section of the lower atmosphere.
Why the Effect Varies Between Celestial Objects
The intensity of flickering depends heavily on two main factors: the object’s apparent size and its altitude in the sky. Stars appear to us as “point sources” of light because they are so distant. Since all the star’s light comes from this single, tiny point, the light beam is narrow and highly susceptible to distortion.
A single turbulent air pocket passing through the path of the starlight can easily deflect the entire beam, causing a noticeable twinkle. Conversely, planets do not generally flicker because they are much closer to Earth and appear as small discs, not pinpoint sources. The light from a planet is a wider beam, essentially a collection of many point sources across its diameter.
Atmospheric turbulence still affects the light from a planet, but different parts of the beam are deflected in different ways. This averaging effect means that while one part of the disc’s light might be momentarily dimmed, another part is still reaching the observer, canceling out the twinkle and resulting in a steady glow. A star low on the horizon also flickers more intensely because its light must travel through a significantly longer, thicker column of the atmosphere than a star directly overhead.