How many types of stars can we see with the naked eye? Do they emit light or reflect light? How are their brightness classified?

On a clear night sky, if there is no light pollution, you can see the sky full of stars. We see roughly three types of stars in the sky, namely planets, stars, and galaxies. With a professional large telescope, in addition to the above-mentioned celestial objects, you can also see quasars, black holes, neutron stars, white dwarfs, and various nebulae. Today we will only discuss celestial objects seen with the naked eye. Stars are self-luminous, and stars are the main component of visible matter in the universe.

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

The sun is the leader of our solar system, accounting for 99.86% of the mass of the solar system. All other celestial bodies, including the eight major planets, several dwarf planets, hundreds of satellites, countless asteroids, comets, etc., add up to It has 0.14% of the mass of the solar system. Among them, Jupiter, the largest of the planets, accounts for another 0.1%, which is 2.5 times the total mass of other celestial bodies in the solar system except the sun and itself; while the earth only gets 0.0003% of the remaining 0.04%. Leftovers.

The sun is only a small to medium-sized star among stars, and the largest star is about 300 times the mass of the sun. There are about 400 billion stars in our Milky Way, and the Milky Way is only one of about 10 trillion galaxies in the entire universe. Therefore, the number of stars in the universe should be at least trillions.

A star is a celestial body that emits heat by itself. It is a celestial body that generates huge energy through central thermonuclear fusion and continuously radiates energy into space in the form of electromagnetic radiation. Stars with greater mass have shorter lifespans. The shortest-lived stars will die in just a few million years; the longest-lived stars can reach trillions of years.

The greater the mass of a star, the shorter its lifespan, and the smaller the mass, the longer its lifespan. But the smallest star must reach about 8% of the mass of the sun, otherwise it will not be able to stimulate central nuclear fusion and become a star. More than 99.9% of the stars we see in the sky at night are stars, and most of these stars are more massive than the sun. The brightness of a celestial body is measured in magnitude.

The brightness of celestial objects is divided into magnitudes, which include absolute magnitude and apparent magnitude.

Because stars emit their own light, they are measured by two brightness indicators: absolute magnitude and apparent magnitude; while small celestial bodies such as planets do not emit light themselves, only reflected light, so there is no absolute magnitude, only apparent magnitude. wait.

Absolute magnitude is the brightness seen by the human eye assuming that all stars are placed 10 parsecs (pc) away from us, which is about 32.6 light-years away.

So the absolute magnitude is the true brightness of stars on the same starting line.

The apparent magnitude is the actual brightness of stars seen by human eyes on the earth. These stars may be far or near, and the light entering the retina of the human eye may not necessarily reflect the true brightness of the stars, because their size and brightness are masked by differences in distance. Therefore, the apparent magnitude is not the true brightness of the star, but the brightness perceived by human eyes on the earth.

The brightness of each level of starlight differs by 2.512 times, so it changes at an exponential level of 2.512 times. For example, if the difference between a 1st magnitude star and a 6th magnitude star is 5 magnitudes, the brightness of a 1st magnitude star is 2.512 times that of a 6th magnitude star. ^5 times, a 1st magnitude star is about 100 times brighter than a 6th magnitude star.

The smaller the magnitude value, the greater the brightness, and there are negative numbers. The more negative the number, the brighter it will be. The algorithm of step difference brightness multiples for absolute magnitude and visual magnitude is the same.

The absolute magnitude of the Sun is 4.83, and the absolute magnitude of Betelgeuse is -6.02. The difference in magnitude is 10.85. Therefore, the actual brightness of Betelgeuse is about 22,000 times that of the Sun; and Because the sun is very close to us, its apparent magnitude reaches -26.74, while Betelgeuse's apparent magnitude is only +0.5, a difference of 27.24 magnitude. Therefore, the brightness of the sun we see is 78.8 billion times that of Betelgeuse.

The brightness of the moon when it is full is -13 magnitude, which is 13.74 magnitude different from the sun. Therefore, the sun is 310,000 times brighter than the moon. The human eye can see the faintest 6th magnitude stars.

The human eye can only see the faintest 6th magnitude stars, while the Hubble Telescope can see 28th magnitude stars. In other words, the starlight that Hubble can see is about 16 million times larger than the human eye. But today we only talk about what we can see with the naked eye.

There are only five planets that I can see with my naked eyes, namely gold, wood, water, fire, and earth. Beyond that there is the moon. These planets and the moon do not emit light themselves. Just like the earth and its houses, mountains, rivers, trees, flowers and plants, they rely on reflected light to be accepted by the human eye.

We can only see these celestial bodies and objects under the condition of light.

Therefore, the waxing and waning of the moon is caused by the difference between the position of the sun shining on its surface and the angle we see it from. As the planets move, their distance from us will change, and the angle of the sun's irradiation will also change. changes, so sometimes it's brighter and sometimes it's dimmer.

In fact, the brightest stars in the sky are not stars that emit their own light, but rely on the reflection of the moon and several planets to emit light.

Venus is the closest planet to the Earth, and is about the same size as the Earth. Therefore, it is the brightest star in the sky, with the brightest apparent magnitude reaching -4.6.

Although Jupiter is further away than Mars, it is very bright due to its huge size, with a mass of 318 times that of the Earth and a volume that is more than 1,300 times that of the Earth. It is the second brightest star in the sky, with an apparent magnitude of -2.9 at its brightest; Mars Although it is smaller, with a mass only about 1/9 of the Earth, and a radius only about half of the Earth, it looks very bright because it is much closer than Jupiter. The brightest apparent magnitude is about the same as Jupiter, reaching -2.9. Transit of Mercury

The brightest visual magnitude of Mercury can reach -1.9. It is originally the fourth brightest star in the sky, but because it is too close to the sun, it is often obscured by the radiance of the sun and is difficult to see. It is said that the great astronomer Copernicus never saw Mercury in his life, which he regarded as a great regret; the brightest apparent magnitude of Saturn can be -0.4, and there are several stars brighter than it.

Except for the sun, the top ten brightest stars in the sky are: Sirius is the brightest, with an apparent magnitude of -1.46, and the others are: Canopus -0.72, Nanmen -0.3, and Arcturus -0.04 magnitude, Vega +0.03 magnitude, Capella +0.08 magnitude, Rigel +0.12 magnitude, Procyon +0.38 magnitude, Hydra +0.46 magnitude, Betelgeuse +0.50 magnitude.

The farthest star that can be directly seen by the human eye is Eta Carinae, which is about 6 or 7 thousand light years away from us. This is because it has expanded too much and has a mass of 120 to 200 times that of the sun. times, so we can see it. In fact, most of the thousands of stars that people can see with the naked eye are tens or hundreds of light years away. There are three main galaxies that can be seen with the naked eye.

The Constellation Galaxy M31 is the largest galaxy that can be seen with the naked eye and the closest large galaxy to us.

The Andromeda Galaxy M31 is about 2.54 million light-years away from us, located below the Ursa Major and near the constellation Pegasus. This galaxy is about twice the size of the Milky Way and contains trillions of stars. The apparent area of ??this galaxy is very large to the naked eye, about 7 times that of the full moon, but its brightness is not very high, its apparent magnitude is only 4.8, and because the edges are dim and fuzzy, only the middle is brighter, so it actually looks like a galaxy that is even brighter than the moon. The light spot should be small.

The other two galaxies are the Large and Small Magellanic Clouds. These two galaxies are located in the southern celestial sphere and can only be seen in the southern hemisphere. They can only be found close to the horizon in the Nansha Islands in the southernmost part of my country.

Although these two galaxies are small, they appear very large because they are very close to us.

The Large Magellanic Cloud is about 160,000 light-years away from us, has a diameter of about 1/5 that of the Milky Way, and contains about 10 to 20 billion stars. Viewed from the earth, Damai is located at the junction of the two constellations Doradus and Shantan. It spans the two constellations and occupies an area of ??11? is 0.9 etc.

The Small Ellen Galaxy is about 200,000 light-years away from us, has a diameter of about 1/11 that of the Milky Way, and contains hundreds of millions to billions of stars. Wheat is located in the constellation Tucana, occupying a sky area of ??5.2?x3.5?, which is equivalent to the apparent area of ??30 full moons, and has an apparent magnitude of 2.7.

The two galaxies are not far apart, only about 20 degrees apart. They hang 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. This is a bit like looking at the Big Dipper in our northern hemisphere, which never sets below the horizon.

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

Some people claim to have seen the M33 Triangulum Galaxy with the naked eye. This galaxy is slightly smaller than the Milky Way, 3 million light-years away from us, and has an apparent magnitude of about 5.72. If true, it would be the farthest galaxy that humans can see with the naked eye, but this statement has not yet been confirmed.

The above galaxies all belong to the galaxies in the Local Group of galaxies.

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

Thanks for reading, welcome to discuss.

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