There are so many stars in the universe, why is the sky dark at night?

On a clear, cloudless night, if you come to the countryside or mountains far away from city lights, when you look up at the night sky, you will see a sky full of stars and a brilliant Milky Way. The stars are blinking tirelessly, emitting a poetic and soft light.

Under very good observation conditions, the human eye can see about 6,000 stars. But because we live in the northern hemisphere, we cannot see the stars in the southern half of the sky below us, so we can only see about 3,000 stars. However, in fact, there are many times more stars in the sky than 6,000, but those stars are very dim and can only be seen with the help of an astronomical telescope.

Facing the starry sky, have you ever wondered why the night sky is dark when there are so many stars in the universe?

Olbers' paradox

Here, we have to mention a person, Olbers. Olbers was a German who was born in 1758, during the reign of Emperor Qianlong of the Qing Dynasty in China. Olbers studied medicine at university and became a doctor after graduation. However, he particularly liked astronomy. So he worked as a doctor to treat patients during the day and observed the sky at night. He once discovered several asteroids and comets.

Oberth

Olbers was a man who loved to think. Looking at the starry night sky, Olbers raised such a question: if the universe is infinite, then there will be an infinite number of stars like the sun in the sky, and they are constantly emitting a large amount of light and heat into the surrounding space. In this way, they should illuminate our night sky very brightly, but why is the starry sky we see still dark?

In fact, earlier than Olbers, similar questions had puzzled the German astronomer Kepler.

He said that if the universe is infinite and the number of stars is infinite, then our Earth is like being in a dense forest of stars, surrounded by trees without any gaps. These stars surrounding the Earth are radiating light and heat, so instead of having a dark night sky, the Earth should become a scorching hot hell.

Olbers' paradox states that the night sky in a steady-state universe should be bright.

Olbers came up with an answer to this question. He thought that although there are many stars in the distance, their starlight is absorbed by the matter in space before reaching us, so the starry sky is black.

But someone stood up to oppose him, saying that your answer is wrong. The matter in the space can absorb starlight, but after absorbing the starlight, they should emit light again, so the total amount of light will not decrease!

If we look closely, the question raised by Olbers seemed to make sense at first, but it led to an incorrect conclusion, because the night sky was not very brightly illuminated by the stars, but was still dark. This situation is called a "paradox." Later, people called the question raised by Olbers the "Olbers paradox."

The Olbers paradox sounds like a simple question, but its answer is actually not simple. Many famous scientists are also puzzled by it.

Distant stars

To answer this question, many people would first consider the stars around the Earth. The closest star to our Earth is of course the Sun, which always illuminates half of the Earth, making that hemisphere in the bright daytime, but cannot illuminate the other half of the Earth in the night, which can only see starlight.

How far is the distance between the sun and the earth? It is about 150 million kilometers. If you fly from the earth to the sun on a jet plane, it will take about 20 years.

Wide-angle photo of the sky area near Alpha Centauri

Apart from the sun, the closest star to the earth is Alpha Centauri in the southern sky, also known as "Nanmen 2" in Chinese. It is about 4.3 light years away from our earth. This distance is about 40 trillion kilometers, equivalent to 270,000 times the distance from the earth to the sun! If you take a jet plane to fly to Alpha Centauri without stopping, it will take about 4.5 million years!

As for other stars, they are extremely numerous, but they are too far away from us.

We know that there is a law in optics that states that the intensity of light received at a point is inversely proportional to the square of the distance from that point to the light source. This means that if there is a lamp, the light it emits will spread forward, backward, left, right, and in all directions. We can imagine that these lights form a sphere. The farther the light travels, the bigger the sphere becomes. As the sphere gets bigger, the surface area of ​​the sphere becomes larger and larger, and the light on it becomes more and more dispersed, so the light intensity becomes weaker and weaker.

Therefore, the light from stars farther away will become dimmer when it reaches our earth, so that some stars can still be seen barely, while more stars cannot be seen at all. Of course, they cannot illuminate our night sky.

This so-called "inverse square law" of light is one of the explanations given for the "Olbers' paradox".

The result of the expansion of the universe

In recent years, with the development of science, some people have proposed another explanation for the "Oberth's paradox". It turns out that a large number of celestial bodies in the universe, mainly galaxies composed of stars that are too weak to be seen by the naked eye, are moving away from us rapidly. This is the so-called "cosmic expansion".

The result of "cosmic expansion" is that those extremely distant stars and galaxies are moving away from us at such a high speed that when the light they emit reaches the earth, not only is the intensity extremely weak, but the energy of the photons that make up the light is also very low, so we can no longer see them! Therefore, the universe is "dark" in our eyes.

Schematic diagram of the Big Bang

At this point, do you understand why the night sky is dark?

Until now, people still have different interpretations of it, and some are very different. It is difficult to say which answer is the only correct one. Therefore, this question is still "open" and people are sometimes still discussing and even arguing about it.

In fact, for many major scientific problems, there is often no answer that everyone agrees is correct, and they are all in the process of exploration.

Usually, scientists will make observations first, then propose hypotheses to explain the observations, and then conduct observations to test the hypotheses and find evidence to support or deny the hypotheses. The exploration of the famous scientific problem of Olbers' paradox actually reflects this process.

Another point to emphasize is that our night sky is not absolutely dark. We can receive a variety of high-energy or low-energy radiation on Earth, but we cannot see them with our naked eyes. This is exactly the subject and object that scientists are studying!

Author: Fu Yuan (retired astronomical scientist)

Planning: Zhang Chao, Li Peiyuan, Yang Liu