Looking up at the stars in the sky at night is a very wonderful thing. Unfortunately, it is not easy to appreciate the beauty of the universe now. Not only does the heaven beautify it, but you also need to find a suitable place to view.
So in places with good conditions, how many stars are visible to the naked eye? More than 6,000 pieces. Of course, it is impossible for you to see so many stars at once, because they are spread across all directions of the earth, so at most about 2,000 stars can be seen in a fixed location. 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 within Milky Way , but this is not accurate enough.
What are stars?
Usually, the stars we refer to are small highlights visible to the naked eye in the night sky. They can be self-luminous stars, planets that reflect the light of stars, or satellites, , or even a collection of many celestial bodies, such as galaxies. There is only one star in the solar system, that is, the sun. When night comes, some planets and satellites in the solar system rely on reflecting the sun's light to light up themselves. The largest and brightest one is the moon. Secondly, the five planets, gold, wood, water, fire and earth, are also visible to the naked eye, and the brightest one 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 shine and heat can be seen by us.
Of course, not all stars can be directly seen. Whether they are visible to the naked eye depends on the mass and volume of the star itself and the distance to the earth. The larger the size of a star, the easier it is to be seen, which is easy to understand. So why does mass also affect the brightness of stars? Because the larger the mass of a star, the more intense the nuclear fusion inside, the greater the energy released per unit time, and naturally the brighter it will be. Some people have this question: "The universe is almost vacuum, and the propagation of light is not hindered. Why do stars look dimmer the farther away?" The further distance of
is mainly caused by two reasons.
First of all, the universe is almost vacuum, but not completely vacuum. There will still be dust and particles in the universe. When light passes through these matter, some photons and will be absorbed by it. Secondly, light does not propagate point-to-point linearly, and photons will diverge in all directions, so the further 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 wall with a flashlight, a very small and bright aperture will appear on the wall at a close distance, while a large but relatively dim aperture will appear on the wall at a long distance. This is because the farther the distance, the fewer the number of photons that can be received per unit area, and the dimmer it will appear.
There is an upper limit on the mass and volume of a star, which means its brightness is an upper limit, so the distance between stars visible to the naked eye is also an upper limit. At present, the farthest star visible to the naked eye is V762, 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.
In this way, are the stars visible in the night sky within the Milky Way? Not all.
Although the stars outside the Milky Way are too far away and cannot be seen directly with the naked eye, we can see the collection of many stars, namely galaxies. For example, there are two small galaxies around the Milky Way, , and , and the small galaxy. Their distances to the Earth are 160,000 light-years and 200,000 light-years respectively, while the distance between Andromeda galaxy and Earth is 2.56 million light-years.Since the Andromeda galaxy contains hundreds of billions of stars, even if it is millions of light years apart, we can still see it in the night sky with our naked eyes, but in our opinion, it is just a small highlight and is not much different from other stars in the night sky.
