why are there upper and lower limits to the masses luminosities and sizes of stars
Main sequence The observed upper limit for a main-sequence star is 120–200 M☉.[36] The theoretical explanation for this limit is that stars above this mass can not radiate energy fast enough to remain stable, so any additional mass will be ejected in a series of pulsations until the star reaches a stable limit.[37] The lower limit for sustained proton–proton nuclear fusion is about 0.08 M☉ or 80 times the mass of Jupiter.[33] Below this threshold are sub-stellar objects that can not sustain hydrogen fusion, known as brown dwarfs.[38]
Red dwarf The term "red dwarf" when used to refer to a star does not have a strict definition. One of the earliest uses of the term was in 1915, used simply to contrast "red" dwarf stars from hotter "blue" dwarf stars.[4] It became established use, although the definition remained vague.[5] In terms of which spectral types qualify as red dwarfs, different researchers picked different limits, for example K8â€“M5[6] or "later than K5".[7] Dwarf M star, abbreviated dM, was also used, but sometimes it also included stars of spectral type K.[8]