The observation of a star twinkling and flashing with multiple colors is an optical phenomenon caused entirely by the Earth’s atmosphere. Starlight travels through the near-vacuum of space for light-years before encountering the turbulent, layered air surrounding our planet. This journey through the dynamic atmospheric layer distorts the light just before it reaches the observer’s eye, creating the shimmering and color-shifting effect.
The Atmospheric Cause of Flickering
The flickering, or twinkling, of a star is known as astronomical scintillation. This occurs because a star is an extremely distant point source of light, appearing as a tiny, concentrated pinpoint. This highly focused beam is susceptible to any disturbance it encounters on its path to Earth.
Earth’s atmosphere is a chaotic mixture of air currents, temperature variations, and differing densities, referred to as atmospheric turbulence. These turbulent pockets act like constantly shifting lenses and prisms, continuously bending, or refracting, the path of the incoming starlight. A light ray from a star is refracted many times as it passes through these layers of fluctuating density.
Since starlight is a tiny point source, even a slight shift in its path due to turbulence can cause the beam to momentarily move off the observer’s eye. This rapid redirection of light results in a perceived drop in brightness, which the human eye interprets as a flicker or twinkle. Stars lower on the horizon flicker more intensely because their light travels through a thicker layer of the turbulent atmosphere.
How the Atmosphere Creates Color Changes
The multicolored flashes seen in bright, low-hanging stars are known as atmospheric dispersion. This occurs because the Earth’s atmosphere acts like a weak prism, separating the constituent colors of the starlight. Light is composed of different wavelengths, with blue light having a shorter wavelength and red light having a longer wavelength.
When starlight passes through the air, the atmosphere refracts the shorter blue wavelengths slightly more than the longer red wavelengths. As turbulent air currents rapidly move, they cause the light’s path to shift, momentarily separating the colors. This rapid separation makes the star appear to flash red, green, or blue as the dispersed colors hit the eye. The effect is most pronounced near the horizon where the light traverses a greater amount of air, intensifying the prismatic effect.
Distinguishing Actual Star Color from Illusion
The rapid color changes caused by atmospheric dispersion are an optical illusion, distinct from the star’s actual, stable color. A star’s inherent color is determined by its surface temperature, which is a constant physical property. Hottest stars, with surface temperatures exceeding 30,000 Kelvin, appear blue or blue-white.
As surface temperature decreases, the color shifts along the spectrum. Stars around 6,000 Kelvin, like our Sun, appear yellow-white, and the coolest stars, below 3,500 Kelvin, appear red. Astronomers classify these stable colors using spectral types (O, B, A, F, G, K, and M), with O-type stars being the hottest and M-type stars being the coolest.
Why Planets Appear Steady
Planets generally do not twinkle or exhibit dramatic color shifts because of their relative proximity to Earth. Planets are not perceived as point sources of light but rather as objects with a measurable angular size, appearing as tiny disks. Light from a distant star arrives as a single, narrow beam, which is easily deflected by atmospheric turbulence.
The light from a planet, however, arrives as a much wider column of parallel light rays. When turbulence shifts the light coming from one edge of the planet’s visible disk, light from the opposite edge often fills the gap. This causes the light rays from the entire disk to average out, canceling the rapid fluctuations in intensity and color, and making the planet appear to shine with a steady glow.