The sky is full of stars, with great differences in brightness. Some stars shine brightly, while others are barely discernible. What factors determine the brightness of stars? The brightness observed by the naked eye is called the luminosity, which is related to the energy emitted by the star and the distance between the observer and the star. The total energy emitted by a star per unit time is its luminosity, which is proportional to the surface area of the star and the fourth power of its surface temperature. The luminosity of different stars varies enormously. The brightest known star is up to one million times brighter than the Sun, while the dimmest star is only one millionth as bright. In astronomy, stars with low luminosity are called dwarf stars, while those with high luminosity are called giant stars, and those with particularly high luminosity are called supergiant stars.
The brightness of a star not only depends on the energy it emits, but also on the square of the distance between the observer and the star. Therefore, the distance between the observer and the star has a greater impact on the brightness. For example, Alpha Cygnus (also known as Tianqin Si in ancient China), located more than 1,740 light-years away, is a blue supergiant star with a luminosity 85,000 times that of the Sun; while Proxima Centauri, the closest star to the Sun (4.22 light-years away), is a red dwarf star with a luminosity less than two-thousandth of the Sun's. Comparing the two, the luminosity difference is about 2 billion times! However, the distance between them and the Sun differs by nearly 400 times, so from the Earth's perspective, their brightness difference is only about 10,000 times.
To compare the true luminosity of stars, astronomers use absolute magnitude to represent the luminosity of stars. That is, different stars are compared at the same distance. It's like a race, where everyone needs to stand on the same starting line. The "starting line" for stars is defined at a distance of 10 parsecs (32.6 light-years) from the observer. The brightness observed at this distance is expressed in absolute magnitude. However, the magnitude we actually observe on Earth is called apparent magnitude. To borrow a common saying, "You can't judge a book by its cover," the same applies to the world of stars. A star may appear bright (with a lower magnitude value), but its actual luminosity may not be strong. Only when its absolute magnitude value is low will its actual luminosity be strong.