How are the stars in the night sky graded?

Hipparchus was the first to divide the stars visible to the human eye into 6 magnitudes according to their perceived brightness. The 20 brightest stars are designated as 1st magnitude. Brightness

Magnitude

decreases as the number of magnitudes increases. Later J.F. Herschel discovered that 1st magnitude stars are about 100 times brighter than 6th magnitude stars. So Pusen used the formula:

To connect the magnitudes m1 and m2 of the two celestial bodies and their brightness I1 and I2. This definition of the magnitude scale is still used today. The zero point of the magnitude scale is determined by specifying the magnitude value of a star.

The star equivalent obtained directly from the photometric measurement of the celestial body is related to the distance of the celestial body and is called the apparent magnitude, which reflects the apparent brightness of the celestial body. A very bright star can appear very faint (high magnitude) because it is far away; and a star that is actually very dim can appear very bright (low magnitude) because it is close. For point light sources, it represents the illumination of celestial bodies on the earth. Magnitude is often represented by m. The monochromatic magnitude difference measured for a single wavelength is independent of the characteristics of the radiation detector. However, the magnitude difference measured within a certain waveband varies with the selectivity of the detector. Therefore, there are various magnitude systems corresponding to different detectors. For example:

①Visual magnitude mv is the magnitude measured by the human eye. The Harvard University Observatory in the United States stipulates that the mv of the λ Ursa Minor star is +6.55 to determine the zero point of visual magnitude. For example, the visual magnitude of the Sun is -26.74; the visual magnitude of Sirius is -1.6. For a star with visual magnitude 1, the illumination on the ground is approximately equal to 8.3×10-9 lux.

②Photographic magnitude mp is the magnitude measured using blue-sensitive photographic films. The zero point of the international photographic magnitude Ipg is defined as follows: Let the average Ipg of A0-type stars with visual magnitudes between 5.5 and 6.5 be mv.

③The psionic magnitude mpv and the international psionic magnitude Ipv are measured using orthochromatic film and a yellow filter. Its spectral characteristics are similar to those of the human eye and actually replaces visual magnitude.

④ Photoelectric magnitude is the magnitude measured using a photomultiplier tube. The most commonly used photoelectric magnitude system at present is the UBV photometry system. U is the ultraviolet magnitude, B is the blue magnitude, and V is the yellow magnitude (similar to the visual magnitude).

⑤Thermal magnitude mbol is the magnitude that represents the total amount of radiation from a celestial body in the entire electromagnetic band. It cannot be determined directly by observation, but can only be obtained by combining the magnitude of polychromatic photometry with theoretical calculations. With the development of photometry technology in various bands, especially observations outside the atmosphere, the accuracy of determining thermal magnitude

magnitude

is getting higher and higher.

In order to compare the luminous intensity of celestial objects, absolute magnitude is used. The definition of absolute magnitude M is the apparent magnitude obtained by placing a hypothetical celestial object at a distance of 10 parsecs. If the parallax π (in arcseconds) of the celestial body and the interstellar extinction-corrected visual magnitude m are known, the absolute magnitude can be calculated according to the following formula:

M=m+5+5lgπ.

There are different absolute magnitudes corresponding to the apparent magnitudes of different systems.