Why is space still pitch black even though the universe is filled with stars?

1. Why is the night sky of Earth never illuminated by stars?

2. A universe filled with bright stars vs. a dark space

Galaxy

(Photo credit: Luke Burton/Getty Images)

People have always had a question: Why is space still dark when there are stars all over the universe? This question has existed for a long time and has a special name - Olbers' paradox.

Astronomers estimate that there are about 200 trillion stars in the universe that humans have observed. Most of the stars are as bright as the sun, and some are even brighter than the sun. But why is the space not illuminated by the light of stars?

I am an astronomer who studies the stars and planets in and around our solar system and their paths through the universe. Studying distant stars and planets helps us astronomers understand why space is so dark.

You might think this is because many stars in the universe are very far away from Earth. Indeed, the farther away a star is, the dimmer it appears to us - a star that is 10 times farther away from Earth will appear 100 times dimmer. But this is not the full answer.

Imagine a bubble

We will assume for the moment that the universe is very old and that it takes time for the light from the most distant stars to reach us. In this hypothetical scenario, all the stars in the universe are no longer moving.

Imagine the Earth is in the middle of a big bubble. If the bubble is about 10 light years in diameter, then there are more than 10 stars in the bubble. Obviously, the Earth is several light years away from these stars, and the stars appear very dim from Earth.

If you expand the diameter of this bubble to 1,000 light-years, 1,000,000 light-years, or 1,000,000 light-years, the most distant stars in the bubble will appear dim. But the larger the bubble, the more stars it will contain, and the stars will all shine. Even if the most distant stars appear dim, there are so many of them that the night sky should appear very bright.

It seems like I'm back to the answer at the beginning of the article, but I'm actually closer to a complete answer.

The age of the universe matters

In the imagination bubble section, I asked you to imagine that the stars are no longer moving and the universe is very old. But the universe is only about 13 billion years old.

The galaxy was born about 13.1 billion years ago. The picture shows the galaxy captured by the James Webb Space Telescope. (Image source: NASA/Space Agency/Canadian Space Agency/Space Telescope Institute/

(Xinhua News Agency via Getty Images)

In the human dimension, 13 billion years is very long, but in the astronomical dimension, 13 billion years is actually very short - the light from the stars about 13 billion light-years away from the Earth has not yet reached the Earth. The real "bubble" surrounding the Earth contains all the stars we can see, but the diameter of this "bubble" is only 13 billion light-years long.

There just aren't enough stars to fill every part of human vision. If you look in one direction of the sky, you can see lots of stars. But if you look in another direction, you can't see any stars. That's because in the dark, starless direction, the light of the stars that can be captured by our vision has not yet reached Earth. As time goes by, the light of more and more distant stars will reach Earth.

Doppler shift

You might be wondering if the night sky will eventually be completely illuminated. Recall what I told you earlier: Imagine that all the stars stopped moving. In fact, the Universe is constantly expanding, with the most distant galaxies moving away from Earth at speeds of nearly light years.

Because galaxies move so fast, the light from the stars in the galaxies will be squeezed into colors that the human eye cannot see. This phenomenon is called "Doppler shift." So, even if the light from the stars can reach our Earth, we still can't see the light from the most distant galaxies. As a result, the night sky will not be fully illuminated.

If you wait long enough, stars will eventually burn out - stars like our sun only last about 10 billion years. Astronomers assume that in the distant future, about 100 billion years from now, the universe will go dark, with only stellar remnants such as white dwarfs and black holes remaining.

Even though the night sky won’t be completely filled with stars, we are lucky enough to enjoy the alternation of light and dark in the night sky. We live in a very special time in the life of the universe.

Related knowledge

A star is a celestial body, a spherical mass of luminous plasma held together by gravity. The Sun is the closest star to Earth. Almost all other stars visible on Earth's night side are in the Milky Way galaxy, but they are so far away that they appear to be fixed points of light. Historically, the more prominent stars have been grouped into constellations and groups, and the brightest stars have traditional names. Astronomers have compiled catalogs of stars that provide standards for naming many different types of stars. Stars undergo nuclear reactions in their cores that generate heavy elements, transmitting energy from the star's interior outwards over a long path and then radiating from the surface into outer space. Once the nuclear reactions in the core are exhausted, the star's life is over.

Stars undergo nuclear fusion at their cores throughout their lifetimes, and at the end of their lives, they will contain degenerate matter. Astronomers determine a star's mass, age, metallicity (the abundance of chemical elements), and many other properties by observing its motion in space, brightness, and spectrum. A star's total mass is the main factor in its evolution and ultimate fate: during its lifetime, the diameter, temperature, and other characteristics of a star change at different stages of its life, and the environment around the star affects its rotation and motion. A graph of the temperature of many stars relative to their brightness, the Hertzsprung-Russell (HR) diagram, allows us to measure a star's age and evolutionary state.

BY: Brian Jackson

FY: Haily Zhu

If there is any infringement of related content, please contact the author to delete it after the work is published.

Please obtain authorization for reprinting, and pay attention to maintaining integrity and indicating the source