Some stars can change their brightness. This is ignored, and a mean is used for their magnitude.
For a bit of background, star brightness is measured with "apparent magnitude". The lower the number, the brighter the star, confusingly. A star of mag 0 is brighter than one of mag 6. The human eye can see stars down to mag 6, and readily down to mag 4. Brighter stars are mag 2 and below.