Research of radiation of stars in different beams allows to receive a number of the important physical characteristics of stars. For this purpose also define shine of stars in different beams for what before photo cells put optical filters of different colour.
Now, when we have got acquainted with measurement of shine of stars, it is curious to notice that not too it is a lot of very bright stars of zero and first star size, only 24 in all sky, but weak - billions! This results from the fact that shine of stars depends not only on their valid luminosity, but also from distances: the further from us there are stars, the more poorly they look. Colour of stars depends on their superficial temperature.
In total in sky northern hemisphere is about 2900 stars, visible with the naked eye, i. e. To 6.