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EI2GYB > ASTRO 02.01.23 13:30l 123 Lines 5994 Bytes #999 (0) @ WW
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Subj: The sky isn't just blue-airglow makes it green, ye
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Sent: 230102/1107Z @:EI2GYB.DGL.IRL.EURO #:8743 BPQK6.0.23
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The sky isn't just blue-airglow makes it green, yellow and red too
by Michael J. I. Brown and Matthew Kenworthy, The Conversation
Look up on a clear sunny 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 answers are a little complicated, but they involve the nature of light,
atoms and molecules and some quirky parts of Earth's atmosphere. And big lasers
too-for science!
Blue skies?
So first things first: when we see a blue sky on a sunny day, what are we
seeing? Are we seeing blue nitrogen or blue oxygen? The simple answer is no.
Instead the blue light we see is scattered sunlight.
The Sun produces a broad spectrum of visible light, which we see as white but
it 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, far more than red light. This is called Rayleigh scattering, and
results in a white Sun and blue skies on clear days.
At sunset we can see this effect dialed up, because sunlight has to pass
through more air to reach us. When the Sun is close to the horizon, almost all
the blue light is scattered (or absorbed by dust), so we end up with a red Sun
with bluer colors surrounding it.
But if all we are seeing is scattered sunlight, what is the true color of the
sky? Perhaps we can get an answer at night.
The color of dark skies
If you look at the night sky, it is obviously dark, but it isn't perfectly
black. Yes, there are the stars, but the night sky itself glows. This isn't
light pollution, but the atmosphere glowing naturally.
On a dark moonless night in the countryside, away from city lights, you can see
the trees and hills silhouetted against the sky.
This glow, called airglow, is 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, while far more abundant in the air than sodium, does not contribute
much to airglow.
The distinct colors of airglow are the result of atoms and molecules releasing
particular amounts 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 distinct green light.
Yellow light, shooting stars and sharp images
Sodium atoms make up a minuscule fraction of our atmosphere, but they make up a
big part of airglow, and have a very unusual origin-shooting stars.
You can see shooting stars on any clear dark night, if you're willing to wait.
They are teensy tiny meteors, produced by grains of dust heating up and
vaporizing in the upper atmosphere as they travel at over 11 kilometers per
second.
As shooting stars blaze across the sky, at roughly 100 kilometers altitude,
they leave behind a trail of atoms and molecules. Sometimes you can see
shooting stars with distinct colors, resulting from the atoms and molecules
they contain. Very bright shooting stars can even leave visible smoke trails.
And among those atoms and molecules is a smattering of sodium.
This high layer of sodium atoms is actually useful to astronomers. Our
atmosphere is perpetually in motion, it's turbulent, and it blurs images of
planets, stars and galaxies. Think of the shimmering you see when you look
along a long road on a summer's afternoon.
To compensate for the turbulence, astronomers take quick images of bright stars
and measure how the stars' images are distorted. A special deformable mirror
can be adjusted to remove the distortion, producing images that can be sharper
than the ones from space telescopes. (Although space telescopes still have the
advantage of not peering through airglow.)
This technique-called "adaptive optics"-is powerful, but there's a big problem.
There are not enough natural bright stars for adaptive optics to work over the
whole sky. So astronomers make their own artificial stars in the night sky,
called "laser guide stars".
Those sodium atoms are high above the turbulent atmosphere, and we can make
them glow brightly by firing a power laser at them tuned to the distinct yellow
of sodium. The resulting artificial star can then be used for adaptive optics.
The shooting star you see at night helps us see the Universe with sharper
vision.
So the sky isn't blue, at least not always. It is a glow-in-the-dark night sky
too, colored a mix of green, yellow and red. Its colors result from scattered
sunlight, oxygen, and sodium from shooting stars. And with a little bit of
physics, and some big lasers, we can make artificial yellow stars to get sharp
images of our cosmos.
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