Recently watching a documentary about stars (how the universe works), I was wondering about two things. The first one is: how is it possible to have stars with such a wide range of sizes? As far as I understand, stars are created in swirling gas clouds, nebulae. When the accumulation of gas reaches a certain density and heat, with the help of gravity, a star is "ignited". Yet, why do some "gas clouds" ignite as soon as they reach, say, the size of the sun, while others need to be ten or a hundred time bigger? Why isn't there some kind of tipping point at which ignition should automatically occur, making all stars pretty much similar in size? is this related to the composition of the gas cloud, or something else? The second question is: how is it possible that, despite vast differences regarding gravity and fusion conditions inside small and big stars, they all manage to maintain equilibrium between attraction and repulsion forces? It looks like the increase of the gravitational force, from a small to a big star, is compensated by a "fine tuning" of the fusion process, so that the repulsion force is increased just enough, so that equilibrium is always preserved, no matter the size of the star. How likely is that?