"One sparkle and one sparkle, the sky is full of small stars..." More than 6,000 stars can be seen in the entire celestial sphere, without the help of any observation instruments and only naked eye observation. In a particular location, if the air quality is good, there is no light pollution, and there is no obstruction of tall giants, more than 2,000 stars can be seen. When you look up at the stars on a clear night, you will find that the brightness of the stars is different, some stars are very dazzling, and some stars look dull. The difference in the brightness of the stars also reflects other related conditions: the distance from the Earth, the internal activity of the star, the mass of the star, the albedo (of the object itself does not emit light), and so on.
As early as two thousand years ago, the astronomer Hipparchus drew a star map, classified the stars according to the brightness of the stars, and proposed the concept of apparent magnitude, the brightest 20 stars were divided into first-class stars, and the later ones were slightly dimmer than second-magnitude stars, and the fainter ones were third-magnitude stars... The lowest brightness visible to the naked eye is a sixth-magnitude star. A first-magnitude star is 100 times brighter than a sixth-magnitude star. That is to say, the brightness of a first-magnitude star is 2.512 times the brightness of a second-magnitude star, the brightness of a second-magnitude star is 2.512 times the brightness of a third-magnitude star, and in short, the brightness difference between adjacent integer-valued stars is 2.512 times.
With the advancement and development of observational technology, only 1 to 6 magnitude stars were not enough to divide the brightness of more celestial bodies, so by 1850 negative magnitudes were introduced. The smaller the magnitude, the brighter the celestial body; the larger the magnitude, the fainter the celestial body.
For example, the nearest star, the Sun, has an apparent magnitude of -26.74; the full moon has a brightness of -12.8; Sirius in the constellation Canis Major is the brightest star of the day besides the Sun, 8.6 light-years away from Earth, and its apparent magnitude is -1.46; Betelgeuse in the constellation of Orion, 640 light-years away, has an apparent magnitude of 0.5;
However, apparent magnitude sometimes does not reflect the true brightness of celestial bodies, because an important factor cannot be ignored, that is, the actual distance between celestial bodies and the Earth. A star looks dim, not necessarily because it is really weak, but more likely because it is too far away from the Earth; on the contrary, if a star that looks bright is moved farther away from the Earth, its apparent star equivalent will certainly increase as the distance from the Earth increases.
Therefore, there is also a concept of absolute magnitude. The apparent magnitude is the brightness of the celestial body seen by the observer, ignoring the distance between the celestial body and the Earth. The absolute magnitude is the brightness measured by placing the celestial body at 10 parsecs from Earth, which is 32.6 light years. In this way, the distances are unified, so that the absolute magnitude of the celestial body reflects its true brightness. For example, the apparent magnitude of the Sun is -26.74, but if the Sun is 32.6 light-years away from Earth, that is, the absolute magnitude of the Sun, it is 4.83 magnitude, which is a relatively faint-looking star. Betelgeuse 's apparent magnitude is 0.5 , and if it is 10 parsecs from Earth , its brightness is -5.85 , or absolute magnitude.
When the actual distance between an object and the Earth is less than 10 parsecs (32.6 light years), then the apparent magnitude of the object is smaller than the absolute magnitude; if the distance between the object and the Earth is greater than the absolute magnitude, then the apparent magnitude of the object is greater than the absolute magnitude. It is worth noting that absolute magnitude does not apply to planets, moons, and other celestial bodies that do not emit light themselves.