Astronomy knowledge composition

Magnitude

In 1850, the British astronomer Poussin proposed a unit for measuring the brightness of celestial bodies. A magnitude is defined as 2.512 times the brightness ratio. For example, a 5th-magnitude star is 2.512 times brighter than a 6th-magnitude star. Therefore, a magnitude difference of 5 magnitudes means a 100-fold difference in brightness. Because the magnitude range is too small, negative magnitudes are introduced. A measure of extremely bright objects.

The apparent magnitude is the brightness of the celestial body seen by observers on the earth. The apparent magnitude of the sun is -26.7, the full moon is about -11, and Sirius is -1.5; the absolute magnitude is The brightness seen at a distance of 10 parsecs (32.6 light years) from the celestial body. The absolute magnitude of the sun is 4.8. The thermal magnitude is the magnitude obtained by measuring the entire radiation of the star, rather than only measuring a part of the visible light. Monochromatic stars The magnitude is a magnitude obtained by measuring only a very narrow range of radiation in the electromagnetic spectrum; the narrow-band magnitude is a magnitude obtained by measuring a slightly wider frequency band; the wide-band magnitude has a wider measurement range; the human eye detects yellow The most sensitive, so the visual magnitude can also be called yellow magnitude.

Magnitude is a way of expressing the brightness of stars in astronomy, recorded as m. Astronomy stipulates that the brightness of a star is expressed by magnitude. The smaller the magnitude number, the brighter the star. Every time the magnitude number differs by 1, the brightness of the star differs by about 2.5 times. The dimmest star we can see with the naked eye is 6th magnitude star (6 m). The brightness in the sky is above magnitude 6 (that is, the magnitude is less than 6), which means we can see more than 6,000 stars.

Of course, we can only see half of them, more than 3,000, every night. When the moon is full, the brightness of the moon is equivalent to magnitude -12.6 (written as -12.6m in astronomy); the sun is the brightest celestial body we see, its brightness is -26.7m; and the largest astronomical telescope in the world today can see An object as dark as 24m.

The "magnitude" we are talking about here actually reflects the brightness and darkness of celestial objects "as seen" from the earth, which is called "visual magnitude" in astronomy. The sun looks brighter than all the stars, and its apparent magnitude is much smaller than all the stars. This is just due to the light from its proximity to the earth. What's more, like the moon, it doesn't emit light at all, but only reflects some of the sun's light, making it the second brightest celestial body in people's eyes. There is also a concept of "absolute magnitude" in astronomy. This value can truly reflect the actual luminous ability of stars.