There are more than 6,000 stars visible to the naked eye. Are they all in the Milky Way? Not all of them

It is a wonderful thing to look up at the starry sky at night. Unfortunately, it is not easy to appreciate the beauty of the universe now. Not only does it require good weather, but you also need to find a suitable place to view it.

So how many stars are visible to the naked eye in places with good conditions? As many as 6,000. Of course, you can't see so many stars at once, because they are scattered all over the earth, so at most 2,000 stars can be seen in a fixed place. So where do these stars visible to the naked eye come from? Some people say that all the stars visible to the naked eye are in the Milky Way, but this is not accurate enough.

What are stars?

Usually, the stars we refer to are those small bright spots in the night sky that are visible to the naked eye. They can be self-luminous stars, planets or satellites that reflect the light of stars, or even a collection of many celestial bodies, such as galaxies. There is only one star in the solar system, the sun. When night falls, some planets and satellites in the solar system rely on reflecting the sun's light to light themselves. The largest and brightest one among them is the moon. Secondly, the five planets, gold, wood, water, fire and earth, are also visible to the naked eye, and the brightest one among them is Venus.

The planets visible to the naked eye in the night sky are all within the solar system, and outside the solar system, only stars that can emit light and heat themselves can be seen by us.

Of course, not all stars can be seen directly. Whether they are visible to the naked eye depends on the mass and volume of the star itself and its distance from the earth. The larger the volume of a star, the easier it is to see, which is easy to understand. So why does mass also affect the brightness of a star? Because the more massive a star is, the more intense the nuclear fusion inside it is, the more energy is released per unit time, and naturally the brighter it is. Some people may have this question: "The universe is almost a vacuum, and there is no obstacle to the propagation of light. Why do stars that are farther away look dimmer?"

The reason why the farther the distance, the dimmer the star is is mainly due to two reasons.

First, the universe is nearly a vacuum, but not completely. There are still dust and particles in the universe, and when light passes through these substances, some photons will be absorbed by them. Secondly, light does not propagate linearly from point to point. Photons will diverge in all directions, so the farther the distance, the more photons will diverge in all directions, and the fewer photons will eventually reach the target position.

For example, when we shine a flashlight on a wall, a small and bright circle of light will appear on the wall at a close distance, while a large but relatively dim circle of light will appear on the wall at a long distance. This is because the greater the distance, the fewer photons can be received per unit area, and the dimmer it appears.

The mass and volume of a star have an upper limit, which means that its brightness has an upper limit, so the distance of stars visible to the naked eye also has an upper limit. At present, the farthest star visible to the naked eye is V762 Cassiopeiae, which is about 16,000 light-years away from the Earth. Obviously, it is within the Milky Way, because the diameter of the Milky Way is more than 100,000 light-years.

So, are all the stars visible in the night sky in the Milky Way? Not entirely.

Although we cannot see the stars outside the Milky Way directly with our naked eyes because they are too far away, we can see the collection of many stars, that is, galaxies. For example, there are two small galaxies next to the Milky Way, the Large Magellanic Cloud and the Small Magellanic Cloud, which are 160,000 light-years and 200,000 light-years away from the Earth respectively, while the Andromeda Galaxy, which is farther away, is 2.56 million light-years away from the Earth. Since the Andromeda Galaxy contains hundreds of billions of stars, even if it is millions of light-years away, we can still see it with our naked eyes in the night sky, but in our eyes, it is just a small bright spot, not much different from other stars in the night sky.

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