While stargazing, you may have noticed that stars twinkle, but other sky objects shine steadily. Why does it happen, and how does it affect astronomical observations? Let’s figure it out.
Why do stars appear to twinkle?
In reality, the stars do not twinkle; they only seem to do so when observed from the Earth. We see space objects not directly but through the atmospheric layer, which is the air around and above us. To the unaided eye, it seems clear and steady, but in fact, it rather resembles a wavy glass. This affects our perception of the night sky. There are three factors that make us see twinkling stars.
The air is not standing still: it moves chaotically due to various forces. For instance, warmer air rises and mixes up with cooler air when the Sun heats the Earth’s surface. The air also bumps into mountains, breaking up into many swirling currents. These conditions cause eddies in the atmosphere, known as air turbulence. Due to this effect, the light emitted and reflected by space objects has to pierce through the uneven layer of air.
The atmosphere acts like a lens. As the “lens” is turbulent and unsteady, the starlight is refracted multiple times while passing through it. As a result, we get a distorted image of a star. To the naked eye, it looks like twinkling. Astronomers call the effect atmospheric scintillation, describing changes in a star’s apparent brightness and color caused by atmospheric irregularities. By the way, scintillation can obstruct astronomical observations: the star that seems blinking to the naked-eyed observer may seem bouncing to those using a telescope. It’s terribly inconvenient to look at a star jumping back and forth. Therefore, astronomers seek to minimize the effect of scintillation.
Stargazers from locations with humid weather see more flickering stars. Humidity makes the air thicker, therefore, more wobbly. To get a clearer image, astronomers prefer dry, high places with thin, calm air. These conditions are referred to as “good seeing”: the less air between the telescope and the observed star, the better. Dense and unstable humid air is called “bad seeing”. The best way to obtain an undistorted image is to observe stars directly from outer space: out there, the sky objects don’t seem to blink at all. That’s why we need space telescopes like Hubble and James Webb.
Why do planets not twinkle?
Generally, planets shine more steadily than stars because they are closer to the Earth. The stars still look like dots when observed through a telescope, while planets appear as tiny discs. The thick beam of sunlight they reflect is less affected by atmospheric refraction than the light from distant stars. However, planets hanging low above the horizon may seem flickering as the light has to travel through more air.
Why do stars twinkle more on the horizon?
The density of the atmosphere is uneven: it increases with decreasing altitude, which causes an optical effect known as atmospheric (or astronomical) refraction. When a star is hanging low, it tends to flicker more intensely because its light has to travel through a thicker layer of air before reaching our eyes.
Why do stars change colors when they twinkle?
The stars seem to twinkle red and blue due to scintillation — distortion of the starlight caused by irregularities in the atmosphere. The air is unsteady; it moves and swirls, refracting different light wavelengths. As a result, stars appear to change colors.
Why don’t stars twinkle green or purple?
In fact, stars emit various wavelengths representing different color spectrum parts, including green and purple. The human eye just can’t see that. We perceive the light from mid-temperature green stars as white, while hot purple stars appear blue.
Why does the Sun not twinkle?
The Sun is closer to the Earth than the other stars. Therefore, it looks like a large shiny disc, not a dot. The beam of sunlight is big enough to pass through the atmosphere unaffected by refraction.
If a sky object doesn’t twinkle, can it be assumed as a planet?
Many people believe that a blinking sky object is a star, but a steadily glowing one is a planet or a satellite. However, the flickering effect depends not on the type of sky object but the observation conditions and air vibrations. Planets may also twinkle while hanging low above the horizon. To know for sure if you see a star or a planet, use Sky Tonight: point your device at the object in question, and the app will tell you its name.
Bottom line: the stars appear to twinkle since we observe them through the layer of the atmosphere. Its disturbance causes multiple refractions of light, which seem like flickering. The Sun and planets shine more steadily because they are closer to the Earth, so their light is less distorted by the air.