Science

The sky is not just blue – the glow of the air makes it green, yellow and red

This article was originally published in The Conversation. (will open in a new tab) The publication published an article in Expert Voices: Op-Ed & Insights on Space.com.

Michael Jay Brown (will open in a new tab)Associate Professor of Astronomy at Monash University
Matthew Kenworthy (will open in a new tab)Associate Professor of Astronomy at Leiden University

Look up on a clear sunny day and you will see blue skies. But is this the true color of the sky? Or is it the only color of the sky?

The answers are a little complicated, but they concern the nature of light, atoms and molecules, and some bizarre parts of the earth’s atmosphere. And big lasers too – for science!

Related: The world turns sideways in a trippy, glowing photo of Earth from the International Space Station.

Blue skies?

So, first things first: when we see a blue sky on a sunny day, what do we see? Do we see blue nitrogen or blue oxygen? The simple answer is no. Instead, the blue light we see is diffuse sunlight.

The sun emits a wide spectrum of visible light (will open in a new tab), which we see as white, but includes all the colors of the rainbow. When sunlight passes through air, the atoms and molecules in the atmosphere scatter blue light in all directions, much more strongly than red light. This is called Rayleigh scattering and results in a white sun and blue sky on clear days.

At sunset, we can see this effect intensify because the sunlight has to travel through more air to reach us. When the sun is close to the horizon, almost all of the blue light is scattered (or absorbed by dust), so we end up with a red sun with bluer colors around it.

But if all we see is scattered sunlight, what is the true color of the sky? Perhaps we can get an answer at night.

A photo taken from the International Space Station in 2012 shows airglow and auroras. (Image credit: NASA)

dark sky color

If you look at the night sky, it is clearly dark, but not perfectly black. Yes, there are stars, but the night sky itself glows. This is not light pollution, but natural atmospheric glow.

On a dark moonless night in the countryside, away from city lights, you can see the silhouettes of trees and hills against the sky.

This glow called airglow (will open in a new tab), produced by atoms and molecules in the atmosphere. In visible light, oxygen produces green and red light, hydroxyl (OH) molecules produce red light, and sodium produces a sickly yellow. Nitrogen, despite the fact that it is much more in the air than sodium, does not contribute to a strong airglow.

The different colors of air glow are the result of the fact that atoms and molecules release a certain amount of energy (quanta) in the form of light. For example, at high altitudes, ultraviolet light can split oxygen molecules (O₂) into pairs of oxygen atoms, and when these atoms later recombine into oxygen molecules, they produce a pronounced green light. (will open in a new tab).

Yellow light, shooting stars and clear images

Sodium atoms make up a tiny part of our atmosphere, but they make up most of the airglow and come from a very unusual origin – shooting stars.

You can see shooting stars on any clear, dark night if you’re willing to wait. They are tiny meteors made of dust particles that heat up and evaporate in the upper atmosphere as they travel at over 7 miles (11 kilometers) per second.

When shooting stars shine in the sky at an altitude of about 60 miles (100 kilometers), they leave behind a trail of atoms and molecules. Sometimes you can see shooting stars with different colors as a result of the atoms and molecules they contain. Very bright shooting stars can even leave visible trails of smoke. And among these atoms and molecules there is a small amount of sodium.

A shooting star and airglow from the International Space Station. (Image credit: NASA)

This high layer of sodium atoms is actually useful for astronomers. Our atmosphere is in constant motion, it is turbulent and blurs images of planets, stars and galaxies. Think of the shimmer you see when you look down a long road on a summer day.

To compensate for turbulence, astronomers take quick pictures of bright stars and measure how distorted the star images are. A special deformable mirror can be adjusted to eliminate distortion, producing images that can be sharper than images from space telescopes. (Though space telescopes still have the advantage of not looking through airglow.)

This technique, called “adaptive optics”, is powerful, but there is a big problem. There are not enough naturally bright stars for adaptive optics to work across the entire sky. So astronomers create their own artificial stars in the night sky, called “laser guide stars.”

These sodium atoms are high above the turbulent atmosphere, and we can make them glow brightly by pointing a powerful laser at them, tuned to the bright yellow color of sodium. The resulting artificial star can be used for adaptive optics. The shooting star you see at night helps us see the universe with clearer vision.

So the sky is not blue, at least not always. This is also a glow-in-the-dark night sky, painted with a mixture of green, yellow and red. Its colors are the result of scattered sunlight, oxygen and sodium from shooting stars. And with the help of physics and big lasers, we can create artificial yellow stars to get crisp images of our cosmos.

This article is republished from The Conversation (will open in a new tab) under a Creative Commons license. Read original article (will open in a new tab).

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