Journal Information
Universe:
Cosmos
Focus:
Starlight, Earth’s Atmosphere, Colour and the Night Sky
Reading Time:
7 Minutes
" When we look at a bright star in the night sky, its light can sometimes appear to be constantly changing. One moment it may seem brighter, then slightly dimmer, and particularly near the horizon it can even appear to flicker between red, blue or green tones. This effect has become one of the most familiar characteristics we associate with stars, yet much of the twinkling we see does not begin with the star itself. It is produced during the final part of a journey that has carried starlight not for kilometres, but across light-years, before it encounters Earth’s atmosphere."
Why Do Stars Twinkle?
What Actually Causes Stars to Twinkle?
The appearance of a star flickering or changing in brightness over short intervals is known in astronomy as scintillation, or atmospheric twinkling. Most of these rapid changes we see in the night sky are not caused by the star itself becoming brighter and dimmer within seconds. Before starlight reaches us, it has to travel through the constantly moving layers of Earth’s atmosphere.
The atmosphere is not a completely still layer with the same density throughout. Regions of air with different temperatures and densities are constantly moving, rising, falling and passing through one another. As starlight travels through these changing layers, its direction is repeatedly bent by very small amounts. The apparent position and intensity of the light reaching our eyes can therefore change slightly over very short periods.
Instead of noticing each of these tiny changes individually, we perceive the star as a flickering point of light.
The star may continue shining in much the same way far away, while Earth’s atmosphere adds the final touch to the light we see.
There is also an important distinction here. Variable stars really do change in brightness over time, but this is different from the ordinary twinkling we see happening within seconds in the night sky. In most cases, the effect we observe is not a rapid change in the star’s physical brightness, but a distortion of its light as it passes through our atmosphere.
What Does Earth’s Atmosphere Do to Starlight?
When a ray of light passes through materials of different densities, its direction can change. Within Earth’s atmosphere, temperature, pressure and density are not exactly the same at every altitude or location. When starlight enters our atmosphere from space, it therefore travels not through one perfectly uniform layer, but through many constantly changing regions of air.
Each of these regions can bend the light by a slightly different amount. Because stars appear to us as extremely small points of light, even tiny deviations can become noticeable. At one moment slightly more light from the star may reach our eyes, while a moment later slightly less may arrive, and its apparent position can shift by an extremely small amount.
The more turbulent the atmosphere is, the more noticeable this effect can become. Air currents and temperature differences influence the stability of the image in the sky, which is why astronomers also consider how calm the atmosphere is when assessing observing conditions.
This is also one of the important advantages of space telescopes. A telescope positioned above Earth’s atmosphere can observe starlight without that final layer of atmospheric distortion. Images taken beyond the atmosphere therefore do not experience the same twinkling effect faced by telescopes on the ground.
When we watch a star twinkle, then, we are not only seeing the star.
We are also indirectly seeing the atmosphere of our own planet.
Why Do Planets Usually Twinkle Less Than Stars?
When you look at a bright point in the night sky, one way to judge whether it might be a star or a planet is to notice how steady its light appears. Planets are also viewed through our atmosphere, so their light passes through the same layers of air. Even so, they usually do not twinkle as noticeably as stars.
The reason is largely connected to the way they appear in the sky. Stars are so extraordinarily distant that, without powerful telescopes, they appear to the human eye as almost perfect points of light. When the atmosphere alters the light coming from that tiny source, the effect can influence the entire apparent image of the star.
Planets, however, are much closer and occupy a very small disc in the sky rather than behaving as perfect point sources. Our eyes may not be able to resolve that disc clearly, but light still reaches us from different parts of its surface. The atmosphere may slightly intensify the light from one area while affecting another differently, allowing some of these small variations to partially balance one another.
As a result, planetary light usually appears steadier.
This is not an absolute rule. If the atmosphere is particularly turbulent or a planet is very close to the horizon, it can also show a degree of flickering. In general, however, when bright stars and bright planets are compared, stellar twinkling is usually much more noticeable.
Two points of light may look similar in the sky, yet the light reaching us from them does not have to behave in the same way.
Why Do Stars Twinkle More and Change Colour Near the Horizon?
Light from a star high in the sky takes a more direct path through the atmosphere. When the same star is close to the horizon, however, its light must travel through a much longer section of the atmosphere before reaching our eyes. As that path becomes longer, the light encounters more regions of air with different temperatures and densities.
For this reason, twinkling can become much stronger when a star is close to the horizon. The star may not only appear to brighten and dim, but can also seem to change colour rapidly. With particularly bright stars, brief flashes of red, blue, white or even greenish light can sometimes be noticed.
The star itself is not repeatedly changing colour within a few seconds. Because the atmosphere does not affect every wavelength of light in exactly the same way, the colour components of the starlight can reach our eyes at very slightly different angles. This effect can become more noticeable near the horizon, where the atmosphere is turbulent and the light follows a longer path.
That is why the same star can appear relatively steady when it is higher in the sky, then seem to flicker dramatically as it approaches the horizon.
The star has not changed.
What has changed is the amount of atmosphere its light must travel through to reach us.
This also shows why position matters when observing the night sky. The way we see a celestial object depends not only on its physical properties, but also on where we are and what lies between us and the object.
Why Do We Remember Twinkling as a Property of the Stars Themselves?
Although much of the everyday twinkling of stars is scientifically caused by our atmosphere, culturally it has become almost inseparable from the way we imagine stars. From children’s drawings and paintings to graphic design and poetry, stars are often shown not as perfectly steady points of light, but as small symbols that flicker or send light out around them.
This is understandable. We have observed stars from the surface of Earth for thousands of years and usually see them through the atmosphere. Our visual memory of stars is therefore shaped not only by the stars themselves, but also by the atmosphere between us and them.
This distinction is particularly interesting within cosmic visual language. When we add delicate rays, small bursts of light or subtle colour shifts around a star in a design, we often recognise it immediately as a star. Yet when viewed directly from space, the familiar atmospheric twinkling we associate with stars would not appear in the same way.
Part of the cultural image of the star, then, actually belongs to Earth.
A star in the night sky is more than a distant source of light. After travelling the immense distance between the star and our planet, its light encounters the atmosphere during the final moments of its journey. The movement of the air bends it, changes its apparent brightness by tiny amounts and can sometimes make its colours appear to flicker.
The twinkling of stars can therefore be understood as a small but continuous meeting between the distant cosmos and the planet on which we stand.
Starlight begins its journey in the depths of space.
But the final form we see is shaped here on Earth.
Perhaps that is exactly what makes the twinkle of the stars feel so familiar.
The light comes from the stars, but its flickering appearance is, in part, a story written by our own atmosphere...
Continue the Journey
If the contrast between darkness and light is what draws you to the cosmos, continue with Within the Darkness. A radiant white star burns within a dark blue cloud of gas and dust, revealing hidden layers and scattered stars without removing the darkness around them.
Within the Darkness carries the same visual tension explored throughout this Journal into a cosmic graphic composition. The light becomes more visible because of the darkness surrounding it, while a deeper region remains beyond its reach. The design turns darkness from an empty background into an active part of the image, shaping what can be seen and what still remains unknown.
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