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]
Milky Way The Milky Way contains between 200 and 400 billion stars[61][62] and at least 100 billion planets.[63] The exact figure depends on the number of very-low-mass stars, which are hard to detect, especially at distances of more than 300 ly (90 pc) from the Sun. As a comparison, the neighboring Andromeda Galaxy contains an estimated one trillion (1012) stars.[64] Filling the space between the stars is a disk of gas and dust called the interstellar medium. This disk has at least a comparable extent in radius to the stars,[65] whereas the thickness of the gas layer ranges from hundreds of light years for the colder gas to thousands of light years for warmer gas.[66][67]