The sun travels 7 billion kilometers every year, so why do the stars in the Milky Way remain in the same position? You need to wait patiently~

Current observations and research by the astronomical community suggest that the Milky Way is a large disk with a diameter of about 200,000 light years. This disk is spiral-shaped, thick in the middle and thin at the edges, a bit like a discus used in a stadium (see picture below).

Introduction to the Milky Way

There are about 200-400 billion stars in the Milky Way, and our sun is one of them, a yellow dwarf. Although yellow dwarfs are not very large, about 80% of the stars in the entire Milky Way are red dwarfs, which are the smallest stars and are much smaller than the sun. Therefore, the sun is considered to have some weight in the Milky Way.

But the largest star has a mass more than 200 times that of the Sun. Compared with such stars, the Sun is just a little brother.

The Milky Way is a big spiral with two large arms and two smaller arms. The sun is located on one of the arms called Orion, about 26,000 light-years away from the center of the Milky Way.

The Milky Way is not motionless. It is like a huge whirlpool in the water, constantly rotating. Therefore, the sun also moves with the arm of the Milky Way, rotating around the center of the Milky Way at a speed of about 220 kilometers per second, and revolving around the Milky Way about 250 million times (pictured above).

If this were on Earth, 220 kilometers per second would be an astonishing speed. But in space, in a huge system measured in tens of thousands of light years, this speed is nothing. Not only the sun, but almost all the stars in the Milky Way are moving, and the speed of movement is tens to hundreds of kilometers per second.

The sun is not just running wildly by itself, but is also pulling the eight planets, as well as a variety of dwarf planets, satellites, asteroids, and various debris and dust together. Our Earth is of course among them, but the sun appears to be stationary relative to us, which is why we are confused (picture above).

There are 3600 seconds in an hour and 31557600 seconds in a year. The sun keeps running around the Milky Way, and it travels about 7 billion kilometers in a year. So why do the stars in the sky seem to be motionless? Are they also running at the same speed as the sun?

My answer is: Yes, but no

What does this mean? It means that the stars we see in the sky are moving around the center of the Milky Way as a whole. This is beyond doubt. This is what "yes" means; but the speed of these stars is not the same as that of the sun. Some are fast, some are slow; and the directions of movement of different stars are not completely consistent. Relative to the sun, some are moving along, some are moving sideways, some are moving towards or away from it, making apparent motion.

Since there are so many different kinds of motion, if there are such a group of people around us, and they are also moving irregularly, their positions relative to us cannot be fixed. But how can the positions of these stars remain unchanged? Not only will they remain the same after a year, but they will also remain the same after thousands of years.

A special note: Except for a few stars from other galaxies, all the stars in the night sky seen by human eyes belong to the Milky Way.

The difference between stars and planets

In the night sky, the human eye can only see five planets, namely Venus, Jupiter, Mercury, Mars, and Saturn. The rest are stars. Stars are celestial bodies that emit light and heat by themselves. Planets do not emit light by themselves, but emit light by reflecting the radiation of stars. The moon is also a planetary body, but it revolves around the earth and is classified as a satellite.

The only things that don't move are the fixed stars, while the positions of the planets relative to each other change every day. In a person's lifetime, or even for thousands of years, except for a few moving planets, the positions of the other stars don't seem to change much, so our ancestors called these stars fixed stars, to distinguish them from the planets whose positions change every day.

The fixed positions of stars mean that the positional relationship between the stars does not seem to change, not that they do not move in the sky. Relative to the Earth, the positions of the stars in the sky also move every day. This is because the Earth rotates, and it seems that all the stars are rotating around the Earth.

There is only one star that is basically motionless, and that is the North Star. Because it is above the line connecting the North Pole of the Earth's rotation axis, the Earth rotates around this invisible rotation axis. People call this huge sphere that rotates around the Earth, like a sphere that wraps around the Earth, the celestial sphere.

Although the positions of stars on this celestial sphere are different every day relative to the Earth, their positions relative to each other do not seem to change. For example, from ancient times to the present, the Big Dipper has always been shaped like a spoon, and the North Star has always been a guide for humans? Are they all running around the Milky Way at the same speed and direction as the sun?

The reason is simple: they are too far away from us.

We all have this experience in life: when we are walking or riding, the scenery nearby changes and moves rapidly relative to us, while the mountains far away or the moon in the sky hardly move. After walking for a long time, their relationship with us seems to remain the same. The lyrics of the song "The moon moves, I move too" describe this scene.

The reason why the relative positions of planets change every day is that they are too close to us compared to stars. The farthest planet from us, Saturn, is less than 1.5 billion kilometers away on average, and the visible planets all revolve around the sun, so we can see that their relative positions change every day; and the closest star is 4.3 light years away from us, and a light year is 9.46 trillion kilometers, which is more than 6,300 times the distance of Saturn.

So even if the star moves, the distance relative to us is very small, and it is difficult to distinguish with the naked eye. It's like looking at a person 1 kilometer away, if he moves half a meter in a year, can you tell?

The sun travels about 7 billion kilometers per year in the Milky Way, which seems a long distance, but in terms of space, it is only 1/1351 light-year. Even if the stars we see with our naked eyes move at 100 kilometers per second relative to the sun, they are only about 3.2 billion kilometers per year, which is only about 1/3000 of a light-year.

The stars we see are tens or hundreds of light years away from us, and the farthest one, Eta Carinae, is 7,500 light years away.

Why has the Big Dipper remained in the same position since ancient times?

The Big Dipper has nothing to do with each other, but their positions in the sky are a little strange to humans, which gave rise to many stories. Each of the Big Dipper is at a different distance from us. The farthest Tianshu is 124 light years away from us, and the closest Kaiyang is 78.2 light years away from us. The distance between them is even much greater than this distance.

If these seven stars move towards or away from each other at a speed of 100 kilometers per second, one year would be equivalent to 1/3000 light-year, and 2000 years would be equivalent to 0.6 light-year. A distance of 100 light-years is equivalent to 0.6 light-years closer or farther away. And if they are viewed from a distance of 100 light-years, it would be difficult to distinguish them with the naked eye.

Of course, it can be measured with precision instruments, but the change is very subtle. This is why the Big Dipper has remained the same as seen by the naked eye for thousands of years. In fact, the position changes between the Big Dipper are not that big.

According to observations and research by astronomers, Tianxuan, Yaoguang and Yuheng are all approaching us, with apparent velocities of 12, 11 and 9.3 kilometers per second respectively. Let's take Tianxuan as an example. This star is 79.4 light years away from us. If it approaches us at 12 kilometers per second, it will be about 380 million kilometers per year, and 378.7 billion kilometers in 1,000 years, about 0.04 light years, or 4% of 1 light year.

So, compared to the distance of Tianxuan from us, which is 79.4 light years, the difference is 1/1985 in 1000 years. This star is 1/1985 closer to us, can we recognize it with naked eyes? Moreover, human life span is only several decades or hundreds of years. Even if you watch it for 100 years from birth to old age, the change of this star from us is only 1/19850, so of course there is no change.

But over time, the positions of these stars change significantly.

For example, the Big Dipper. Based on the interactive changes in the positions of the seven stars, scientists used computer modeling to obtain their positional relationships 100,000 years ago and 100,000 years later: the Big Dipper was a bit like an arrowhead 100,000 years ago, but now it is shaped like a spoon, and in another 100,000 years, it will become the shape of a bucket in front of a forklift (see the picture above).

Scientists have also discovered a star system numbered WISE J072003.20-084651.2, which was named "Scholz Star" because it was discovered by astronomer Scholz. This is a binary star system consisting of two very small red dwarf stars, about 20 light-years away from us.

The mass of the main star is about 86 times that of Jupiter, which is 0.083 times the mass of the sun. Incredibly, the other star is smaller. This mass is already the minimum required mass for star formation. If it is smaller, it cannot become a star, so these two stars are very dim.

Based on its orbit, scientists have modeled and surprisingly found that this binary star system passed by the solar system 70,000 years ago, and was only 0.8 light-years away from the sun at its closest point. It should be noted that the gravitational radius of the sun is believed to be about 1 light-year, and at the edge of the gravitational radius there is an Oort cloud belt, where trillions of comets gather.

The distance of 0.8 light years means that they have entered the Oort Cloud. 70,000 years ago, it was the most active time for the ancestors of mankind, the late Homo sapiens, to migrate from Africa to all parts of the world. How come there was no major disaster? Scientists analyzed that it might be because the mass of these two stars was too small and they passed by very quickly. After all, there was a distance of 0.8 light years, so they did not cause major harm to the creatures on Earth.

If this star was a little bigger, the seeds of humanity might have been destroyed long ago, and we wouldn't be sitting here talking about it.

In fact, all celestial bodies interact with the Sun.

For example, Proxima Centauri, the closest star to us, is only 4.22 light-years away and is approaching us. According to scientific modeling, in about 3,000 years, Proxima Centauri will be only 3 light-years away from us; and in another 80,000 years, Proxima Centauri is likely to enter the solar system and form a binary star system with the sun.

Going further, the Milky Way and the Andromeda Galaxy are moving closer at a speed of about 300 kilometers per second due to the huge gravitational attraction between the two galaxies. They will collide with each other within the next 3 to 4 billion years. What kind of spectacle will it be then? Let us wait and see.

Therefore, we cannot say that the sun travels 7 billion kilometers a year and the stars around it do not change. We can only say that the change is very small and difficult to detect with our naked eyes in the short term. The vast space is measured in light years, and any object moving is very slow compared to a light year.

But as long as time goes by, these position changes will be seen. Therefore, as long as everyone lives long enough, they can see the sea of ​​stars constantly changing. Let us wait together, watching the Big Dipper gradually become a bucket shape, waiting for the solar system to become a binary star system, and appreciating the magnificence and splendor of the collision between the Milky Way and the Andromeda Galaxy (pictured above).

Are you willing to look forward to it together? Welcome to discuss, thank you for reading.

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