by Curious Kids is a series for children of all ages. If you have a question you want to answer from an expert, send it to curiouskidsus@theconversation.com.
Why is the room so dark despite all the stars in the universe? – Nikhil, 15 years, Neu -Delhi
People asked why the room is dark, although it is filled with stars for so long that this question has a special name – Olbers’ paradox.
Astronomers estimate that there are around 200 billion trillion stars in the observable universe. And many of these stars are so bright or even brighter than our sun. Why is the room not filled with shimmering light?
I am an astronomer who studies stars and planets – including those outside of our solar system – and their movement in space. Studying distant stars and planets helps astronomers like me to understand why the room is so dark.
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You could suspect that it is the case that many stars in the universe are very far from the earth. Of course it is true that the further a star is removed, the less bright – a star that is 10 times further away looks 100 times dimmer. But it turns out that this is not the whole answer.
Imagine a bubble
For a moment, stand so that the universe is so old that the light had the most distant stars from the most distant stars to reach the earth. In this imaginary scenario, all stars in the universe do not move at all.
Imagine a big bladder with the earth in the middle. If the bladder were about 10 light years, it would contain about a dozen stars. Of course, many of these stars would look quite weak from the earth in several light years.
If you always enlarge the bladder to 1,000 light years, then to 1 million light years and then 1 billion light years, the most distant stars in the bladder will look even weak. But there would also be more and more stars in the ever larger bladder that all contribute to light. Although the most widespread stars and dimmers look, there would be much more of them, and the whole night sky should look very bright.
It seems that I am again where I started, but I am actually a bit closer to the answer.
Age is important
In the imaginary Bubble illustration, I asked her to imagine that the stars do not move and that the universe is very old. But the universe is only about 13 billion years old.
Although this is a surprisingly long time in human terms, it is short in astronomical terms. It is short enough that the light of stars, which are more than about 13 billion light years away, have not yet reached the earth. And so the actual bladder around the earth, which contains all the stars we can see, only extends from earth to about 13 billion light years.
There is simply not enough stars in the bladder to fill every line of sight. If you look in some directions in the sky, you can of course see stars. If you look at other parts of the sky, you cannot see stars. And this is because the stars that could block their line of sight are so far away in these dark areas that their light has not yet reached the earth. Over time, light from these ever distant stars have time to reach us.
The Doppler shift
You could ask yourself whether the night sky will eventually light up. But that brings me back to the other, which I told you that I should imagine that all the stars will not move. The universe actually expands, and the most distant galaxies moves from the earth at almost the speed of light.
Since the galaxies move away so quickly, the light is pressed from their stars in colors that the human eye cannot see. This effect is called Doppler shift. Even if it had enough time to reach them, they still couldn’t see the light with their eyes from the distant stars. And the night sky would not be fully illuminated.
If you wait longer, the stars will burn out at some point – stars like the sun only last about 10 billion years. Astronomers assume that the universe will become dark in the distant future – in a thousand trillion years – inhabited by only outstanding remains such as white dwarfs and black holes.
Although our night sky is not completely filled with stars, we live in a very special time in the life of the universe when we are lucky enough to enjoy a rich and complex night sky that is filled with light and dark.
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This article will be released from the conversation, a non -profit, independent news organization that brings you facts and trustworthy analyzes to help you understand our complex world. It was written by: Brian Jackson, Boise State University
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Brian Jackson receives research grants financed by NASA.
