Artists, poets, and philosophers have admired and pondered about the color of the sky for centuries.
Every day, we wake and look out of the window to assess the color above us.
Clear blue skies make for dreamy summer’s days, gray skies in fall, and white skies in winter.
We are even delighted by the fantastic red and pink skies at dusk and dawn.
But with all this variation and splendor in our own sky, why is outer space completely black?
Even the night sky on earth is rarely as pitch black as the darkness of space.
So why, despite being filled with billions of galaxies of shining stars, does outer space appear black? Let’s take a closer look…
How Does Light Work?
The first thing you need to know about light is that there are many different types of light:
- There’s visible light. We are only able to see light that exists on the visible spectrum.
- Infrared light, which we cannot see but which allows us to detect heat.
- Ultraviolet light, which causes sunburn.
- X-ray light, which helps doctors diagnose illnesses by examining internal organs.
- Radio waves, which allow us to communicate via radio and television.
- Microwaves, which help cook food.
- And gamma rays, which cause cancer.
All these forms of radiation travel through space at the speed of light: 300 million meters per second (186,000 miles per second).
This means that if you were standing next to an object traveling at the speed of light, your shadow would move across it at 186,000 miles per second!
That’s fast enough to leave Earth in just over one second.
How Can We See Light?
Human beings can only see light on the visible spectrum. We see visible light when light waves reflet and bounce off particles.
These particles can be atoms in objects, faces, dust particles, mountains, or even the tiny particles suspended in air or water.
The particles scatter the light waves into our eyes, allowing us to see objects and colors.
Visible light waves vary in length, from violet at the short end of the spectrum, through indigo, blue, green, yellow, orange, and finally red at the long end.
Depending on what particles a light wave interacts with, its length and frequency is altered.
Certain particles will absorb more of one type of wavelength and reflect another. This is what gives us color.
Why Is The Sky Blue?
Planet earth is surrounded by an atmosphere made from gasses such as nitrogen, oxygen, argon, carbon dioxide, and other chemicals.
As light travels through the atmosphere, it interacts with the gas molecules and changes its wavelength.
One reason why the sky looks blue on Earth is that the atmosphere absorbs almost all the light that reaches the surface.
When sunlight passes through the atmosphere, some of its colors get absorbed by molecules of nitrogen and oxygen, leaving only the blue light (long wavelengths) behind.
Another reason why the sky appears blue is that clouds absorb more than 99 percent of the light that falls upon them.
Clouds are made up of droplets of liquid water and other substances floating in the air.
Water has a strong absorption band in the blue light part of the spectrum.
As a result, the blue light wavelengths are scattered away from the path of the incoming sunlight.
Because the scattering happens in all directions, the sky appears blue.
Why Is Space Black?
Deep space is an almost perfect vacuum.
Unlike planet earth, with its gas filled atmosphere, there is almost no air in space, and no particles floating about for light to interact with.
When light travels through empty space, it doesn’t interact with anything. It simply goes straight ahead without bouncing off anything.
Therefore, nothing stops the light from reaching Earth. Since no matter is present to stop the light, it continues on its way unimpeded.
At this point, the light is called “free” or “unbounded.” Unbounded light is invisible to us, since it never interacts with anything.
We call unbounded light that which travels in free space.
What About Light From Stars?
Stars are huge balls of hot plasma (gas) that shine brightly because they are extremely hot.
Our own sun is a star, and we know how bright and powerful it is.
Since the universe is filled with billions upon billions of stars and each of these bright stars is surrounded by planets and moons, shouldn’t deep space be lit up like broad daylight?
Well, one of the great complexities of space science is that, despite being hugely bright, the light from stars must travel through miles of space without reflecting or interacting with any particles.
The light from stars therefore reaches our eyes on earth having traveled uninterrupted from its source.
This means that we perceive stars as tiny dots of light surrounded by the vast darkness of space.
It takes the light from distant stars so long to travel to earth that many of the stars we admire in the night’s sky have actually died and no longer exist in reality.
Whilst the source of light may have exploded, the light itself continues to travel until it reaches our eyes.
When you see stars in dark space, you are looking at light from the past!
What Is Dark Matter?
Dark matter is another of the vast complexities of space science. Scientists believe that dark matter makes up 85% of the mass of the universe.
They also think that dark matter is responsible for holding galaxies together. However, scientists don’t really understand what dark matter is.
They just know that it exists. Some people say that dark matter is a type of energy, while others say that it is a particle.
Whatever it is, it is not visible to us.
Is Deep Space Pitch Black?
These days, scientists have started to question the total blackness of space and suggest that it is more of a charcoal than a pitch black.
In fact, scientists at the Rochester Institute of Technology recently proposed that, even if there were no stars or galaxies in existence, space itself would not be completely black because the universe gives off light of its own.
This mysterious light is undetectable to the naked eye, but can be detected as cosmic microwave background light.
It is probably residual light that was first created in the big bang, and it is still vibrating through everything that was created in that epic explosion.
How Do We Know That The Universe Is Expanding?
The expansion of the universe has been observed for quite some time now.
However, it wasn’t until recent years that scientists could accurately measure the distance between objects across the cosmos.
By measuring the distances between galaxies, astronomers can calculate how fast the universe is expanding.
The spectrum of light helped scientists to determine the speed of the universe’s expansion.
Through the Hubble telescope, scientists can detect that the light emitted from stars is redder than expected.
This is because the light is stretched out when traveling through space.
The further away an object is, the more stretched out the light becomes. As a result, the light appears to be redder.
Astronomers use this information to calculate the rate at which the universe is expanding.
If they find that the light is getting redder, then they know that the universe is expanding faster.
In addition, they can also estimate the size of the universe.
So, outer space appears black to the human eye because it is a vacuum in which there are no particles for light to interact with.
The bright light from stars travels uninterrupted through miles of space, reflecting off nothing until it hits our atmosphere and enters our eyes.
However, space is not entirely pitch. The universe gives off its own mysterious light, which does not exist on the visible spectrum of light.
As the universe continues to expand, the distance between stars and objects increases, and the light we can perceive decreases.
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