The sky is not only blue: atmospheric emissions also make it green, yellow and red

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

The answer is somewhat complicated, but involves the twisted properties of light, atoms, molecules and some parts of the earth’s atmosphere. And big lasers too – for science!

blue sky

First things first: when we look at the blue sky on a clear day, what do we see? Are we seeing blue nitrogen or blue oxygen? The simple answer is no. Instead, the blue light we see is diffuse sunlight.

The sun produces a wide variety spectrum of visible lightWhat we see is white, but that includes all the colors of the rainbow. When sunlight passes through the air, atoms and molecules in the atmosphere scatter blue light in all directions, much more than red light. This is called Riley was distractedproduces a white sun and blue sky on a clear day.

As the sun sets, we can see this effect turn on, because sunlight has to go through more air to reach us. When the sun is near the horizon, the blue light is almost completely scattered (or absorbed by the dust), so we end up with a bright red sun surrounding it.

But if all we see are the sun’s rays, what is the true color of the sky? Maybe we can get some answers tonight.

dark sky color

If you look at the night sky, it’s definitely dark, but not completely black. Yes, there are stars, but the night sky itself lights up. It’s not light pollution, but the atmosphere glows naturally.

On a dark night in the countryside, away from city lights, silhouettes of trees and hills can be glimpsed in the sky.

The trees cast shadows against the glowing night sky.
Rodney Campbell/Flickr

This light is called antenna radius, 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 yellow light. Even though there is more nitrogen in the air than sodium, it contributes little to luminescence.

The characteristic color of atmospheric light is caused by atoms and molecules releasing 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 are They combine into oxygen molecules and produce the characteristic green light.

You can see the atmospheric glow in dark places, such as the European Southern Observatory in Chile.

Shooting star yellow light and sharp image

Sodium atoms make up a small part of our atmosphere, but they make up most of the atmospheric glow and come from a very unusual shooting star.

You can see shooting stars on a clear dark night, if you are willing to wait. They are very small meteors, resulting from the heating and evaporation of dust grains in the upper atmosphere as they travel at speeds in excess of 11 kilometers per second.

When shooting stars shine brightly in the sky, almost 100 kilometers high, they leave behind specks of atoms and molecules. Sometimes you can see shooting stars with different colors, which are caused by the atoms and molecules they contain. Very bright stars can leave visible smoke trails. Between these atoms and molecules there is very little sodium.

Shooting stars and atmospheric flares as seen from the International Space Station.
NASA

This high layer of sodium atoms is actually useful for astronomers. Our atmosphere is in constant motion, churning and blurring the images of planets, stars and galaxies. Think of the twinkling lights you see as you gaze down a long road on a summer afternoon.

To compensate for the turbulence, astronomers quickly photograph bright stars and measure how distorted the star’s image is. A special deformable mirror can be adjusted to eliminate distortion, resulting in sharper images than those found in space telescopes. (Although space telescopes still have the advantage of not peering through the atmospheric glow.)

This technology — called “adaptive optics” — is powerful, but there’s a big problem. There aren’t enough naturally bright stars for adaptive optics to work in the sky. So astronomers create their own artificial stars in the night sky, which are called “laser guide stars”.

Sodium atoms are far above the turbulent atmosphere, and we can make them glow brightly by firing energetic lasers at them that are tuned to sodium’s distinctive yellow color. The resulting artificial stars can then be used in adaptive optics. The rising stars you see at night help us see the universe with a clearer view.

So the sky isn’t blue, at least not always. This is also the glow-in-the-dark night sky, colored with a mixture of green, yellow, and red. The color comes from scattered sunlight and the oxygen and sodium from colliding stars. And with a little physics and some big lasers, we can artificially create yellow stars for sharp images of our universe.