Why is the largest number of stars concentrated in the middle part of the main sequence on the Spectrum-Luminosity

Why is the largest number of stars concentrated in the middle part of the main sequence on the Spectrum-Luminosity diagram for a random sample of stars?

The upper part of the main sequence is indeed less populated than the lower one, since stars of high luminosity and, therefore, of greater mass are less than stars of low luminosity. In the lower part of the main sequence, there are low-mass and therefore faint stars, of which only close objects are available for observation.

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