On a dark, clear night away from the city, thousands of stars can be seen in the sky. The first thing that strikes the eye is that they all differ in brightness or, as astronomers say, brilliance. Astronomers have long ago learned to accurately determine the shine of stars and other celestial lights.
A stellar value is one of the oldest measurement standards we use.
For the first time this term was used by the great astronomer of antiquity Hipparkh (130 BC). Hipparch decided to divide all the stars in the sky by their brightness into six groups or "values". He gave the brightest stars the 1st size. They became as if the best, the first among others. Slightly less bright stars received the 2nd value. Even less bright - the 3rd. Finally, the dullest stars, visible to the naked eye, were given the 6th size by Hipparkh.
According to this division, stars such as Vega, Aldebaran or Sirius were classified as first consequence stars, and the Big Bear bucket stars as second consequence stars. Today, it seems a little strange that the stars of greater luster are smaller. Today, we would act in the opposite direction: if a star is brighter, its brightness is greater!... But in general, the logic of Hipparch is clear.
The system of stellar values was very convenient, although very subjective. The scale depended heavily on the observer. For example, if one astronomer by his perception seemed that this star is the second consequence, the other astronomer - that it is the first consequence. What is the real shine of the star? It was necessary to give some strict definitions for the stellar value to move to objective estimates.
Scale of stellar values
That's the definition given in the 19th astronomer Norman Poghson. He noticed that the difference in one-star value corresponds to a change in the luminous flux of approximately 2.5 times. That is, a star of 0m illuminates our eyes 2.5 times stronger than a star of 1m. It turns out that a 1-star is 100 times brighter than a 6-th star.
For someone, this moment may seem strange. The subjective feeling suggests that stars of the 6th consequence are only 6-10 times weaker than stars of the 1st consequence. Guided by this feeling, Hippard actually developed a scale of star values.
But our vision, as well as our hearing, are arranged differently. When the power of a light source changes geometrically, we take it for an arithmetic progression! It seems to us that two stars of the 6th consequence will give us a star of 3m, and two stars of 3m will give a star of 1m. But if we really bring two stars of the same luster to each other (flashlights can play a role in them), then this ratio will not work!
Pogson has offered a logarithmic scale of sizes - the difference in 5 units on a scale of star sizes precisely corresponds to a 100-fold difference of a light stream. Namely, a star of the 1st size is exactly 100 times brighter than a star of the 6th size and 100 × 100 = 10000 times brighter than a star of the 11th size. This rule is exactly as it should be.
What remains to be determined is the standard against which the stellar values of all other stars can be measured. For a long time, this standard was considered to be the star Vega, whose luster was taken as a zero point of stellar values (0m).
In practice, the luster of stars is measured by photovoltaic photometers. Consequently, the stellar values should be related to the generally accepted physical value of the radiation flux. In physics, light intensity is measured in lux. The relationship between the stellar value (m) and the lux (J) is expressed by the formula: m = -14 - 2.5lgJ. Thus, the Sun has a stellar value of -26.75m or 125000 lux. The luster of the full Moon is 12.74m, which corresponds to 0.3 lux.
Why the stellar values?
So, in physics, there is an analog of the stellar value - luxury. Why do astronomers still use the scale of stellar values? The answer is simple: it is extremely convenient.
The illumination created by celestial objects such as the Sun, Moon, planets, and stars differs millions of times when measured in suites. These are huge numbers, which are quite difficult to operate.
Stellar values, on the other hand, are very convenient. The sun is only 25 times brighter than Sirius. And Sirius himself is about as much brighter than the weakest stars that can be photographed in the telescope. Hubble (their shine is about 30m). The entire range of shine of celestial objects, thus, falls within 60 stellar values. Very convenient!