In a clear night sky, if there is no light pollution, you can see the stars all over the sky. We see that there are roughly three kinds of stars in the sky, namely, planets, stars and galaxies; With professional large telescopes, quasars, black holes, neutron stars, white dwarfs and various nebulae can be seen in addition to the above celestial bodies.

Today, we only discuss the celestial bodies seen by the naked eye. Self-luminous is the star, which is the main body of visible matter in the universe.

Stars are the most important existence in our universe, accounting for more than 99% of the visible matter in the universe. A star is a star that shines by itself, and the nearest star is the sun.

The sun is the oldest in our solar system, accounting for 99.86% of the mass of the solar system. All the other celestial bodies, including eight planets, several dwarf planets, hundreds of satellites and countless asteroid comets, add up to .14% of the mass of the solar system. Among them, the mobile phone in Jupiter takes up .1%, which is 2.5 times the total mass of other celestial bodies in the solar system except the sun and itself; The earth only got .3% of the remaining .4%.

The sun is only a small and medium-sized star among the stars, and the largest star has about 3 times the mass of the sun. There are about 4 billion stars in our Milky Way, and the Milky Way is only one of about 1 trillion galaxies in the whole universe, so there should be at least 1 billion stars in the universe.

A star is a celestial body that glows and gives off heat by itself. It is a celestial body that forms huge energy by central thermonuclear fusion and continuously radiates energy into space in the form of electromagnetic radiation. The greater the mass, the shorter the life span of the star, and the star with the shortest life span will die in just a few million years; The longest-lived star can reach trillions of years.

the greater the mass of a star, the shorter its life span, and the smaller its mass, the longer its life span. But the smallest star must reach about 8% of the mass of the sun, otherwise it will not stimulate the central nuclear fusion and become a star. When we see stars in the sky at night, more than 99.9% of them are stars, and most of them are heavier than the sun. The brightness of celestial bodies is measured by magnitude.

the brightness of celestial bodies is divided into magnitude, which includes absolute magnitude and apparent magnitude.

Because a star can emit light, it is measured by two brightness indicators, such as absolute magnitude and apparent magnitude. However, small celestial bodies such as planets do not emit light, but only reflect light, so there is no absolute magnitude, only apparent magnitude.

Absolute magnitude is the brightness seen by human eyes when all the stars are assumed to be 1 parsec (pc) away from us, which is about 32.6 light years away.

Therefore, absolute magnitude is the true brightness of stars at the same starting line.

The apparent magnitude is the brightness of the stars actually seen by human eyes on the earth. The distance between these stars is far or near, so the light entering the retina of human eyes is not necessarily the true brightness of starlight, because their size and brightness are covered up because of the different distances.

Therefore, the apparent magnitude is not the true brightness of the stars, but the brightness perceived by human eyes on the earth.

the brightness of starlight varies by 2.512 times, so it changes exponentially by 2.512 times. For example, the brightness of the first-class star is 2.512 5 times that of the sixth-class star, and the first-class star is about 1 times brighter than the sixth-class star.

the smaller the magnitude value, the greater the brightness, and there is a negative number, the brighter the more negative it is. Absolute magnitude and visual magnitude have the same gradient brightness multiple algorithm.

The absolute magnitude of the sun is 4.83, and that of Betelgeuse is -6.2, with a magnitude difference of 1.85, so the actual brightness of Betelgeuse is about 22, times that of the sun. Because the sun is very close to us, the apparent magnitude reaches -26.74, and the apparent magnitude of Betelgeuse is only +.5, with a difference of 27.24, so the brightness of the sun we see is 78.8 billion times that of Betelgeuse.

The brightness of the moon at the full moon is -13, and the gap with the sun is 13.74, so the sun is 31, times brighter than the moon. The darkest human eye can see a 6-star.

human eyes can only see 6 stars at the darkest, and Hubble telescope can see 28 stars. That is to say, the starlight that Hubble telescope can see is about 16 million times larger than that of human eyes. But today we only talk about what we see with the naked eye.

There are only five planets that I can see with the naked eye, that is, water, fire and earth in Jin Mu. Besides, there is the moon. These planets and the moon themselves don't shine, just like the earth and the houses, mountains, rivers, trees and flowers on the earth, they all rely on reflected light to be accepted by the human eye.

these celestial bodies and objects can only be seen under the condition of illumination.

Therefore, the moon is full or full, because the position where the sun shines on its surface is different from the viewing angle we see. As the planet moves, its distance will change, and the angle of the sun will also change, so sometimes it will be brighter and sometimes it will be dim.

In fact, the brightest stars in the sky are not the stars that glow by themselves, but the moon and several planets that glow by reflection.

Venus is the closest planet to the Earth, and it is about the same size as the Earth, so it is the brightest star in the sky, and its apparent magnitude can reach -4.6. Although Jupiter is farther away than Mars, because it is huge, its mass is 318 times that of the Earth, and its volume is more than 1,3 times that of the Earth, so it is very bright, and it is the second brightest star in the sky, with an apparent magnitude of -2.9 at its brightest. Although Mars is small, its mass is only about 1/9 of that of the Earth and its radius is only about half of that of the Earth, it looks very bright because it is much closer to Jupiter. At its brightest, its apparent magnitude is similar to Jupiter's, reaching -2.9.

Mercury's apparent magnitude can be as bright as -1.9. It was originally the fourth brightest star in the sky, but because it is too close to the sun, it is often hidden under the sun's brilliance, so it is difficult to see it. It is said that Copernicus, a great astronomer, never saw Mercury in his life, which is regarded as a great pity. Saturn's apparent magnitude can be as bright as -.4, and several stars are brighter than it.

Except for the sun, Sirius is the brightest star in the top ten, with an apparent magnitude of -1.46, followed by Laoge -.72, Nanmen -.3, arcturus -.4, Vega +.3, Capella +.8, and Betelgeuse +.12.

The farthest star that can be directly seen by human eyes is Haishan II, which is about 6, to 7, light years away. This is because it is so inflated that its mass is 12 to 2 times that of the sun, so it can be seen. In fact, thousands of stars that people can see with the naked eye are mostly in dozens of light years and hundreds of light years. There are three main galaxies that can be seen by the naked eye.

A Andromeda galaxy M31 is the largest galaxy that can be seen by human naked eyes, and it is also the largest galaxy closest to us.

Andromeda galaxy M31 is about 2.54 million light years away, located below Ursa Major and near Pegasus. This galaxy is about twice as large as the Milky Way and has trillions of stars. To the naked eye, this galaxy has a large viewing area, about 7 times that of the full moon, but its brightness is not high, and its apparent magnitude is only 4.8. Moreover, because the edge is dim and fuzzy, only the middle is brighter, so it actually looks like a spot smaller than the moon.

There are also two large and small Magellanic galaxies, both of which are located in the southern hemisphere. Only the southern hemisphere can see them, and they can be found near the horizon in the southernmost Nansha Islands in China.

Although these two galaxies are small, they look very big and obvious because they are very close to us.

The Large Magellanic Cloud is about 16, light years away, about 1/5 of the diameter of the Milky Way, and contains about 1-2 billion stars. On the earth, barley is located at the junction of the two constellations, Swordfish and Yama, spanning the two constellations, occupying a sky area of 11 x 9, which is equivalent to the visual area of more than 2 full moons, and the apparent magnitude is .9.

The Mailun galaxy is about 2, light years away, about 1/11 of the diameter of the Milky Way, and contains hundreds of millions to billions of stars. Wheat is located in Rhododendron, occupying 5.2°x3.5°, which is equivalent to the visual area of 3 full moons, and the apparent magnitude is 2.7.

The two galaxies are not far apart, only about 2 degrees. They are hung high in the sky near the southern zenith, and complement each other. They can be seen in the southern hemisphere all year round and never fall below the horizon. It's a bit like watching the Big Dipper in the northern hemisphere, and it will never fall below the horizon.

These two galaxies are the satellite galaxies of our Milky Way, and they move around the Milky Way. Due to the close proximity, both galaxies become irregular galaxies due to the gravitational pull of the Milky Way. According to the modeling analysis of NASA, both galaxies will be annexed by the Milky Way in the next billions of years.

Some people claim to have seen M33 Triangle galaxy with naked eyes. This galaxy is slightly smaller than the Milky Way, 3 million light years away from us, and its apparent magnitude is about 5.72. If so, it will be the farthest galaxy that human eyes can see, but this statement has not been confirmed.

the above galaxies all belong to the galaxies in the local galaxy group.

This is the kind of starry sky stars seen by human naked eyes on the earth, and the division of their luminosity levels.