system
Given the following post, respond as if you're commenting on Reddit:
human
How is hydrogen stored in the sun?

Basically what im asking is how un-fused hydrogen is kept in the sun without it just being fused immediately. Is there a max amount of hydrogen that can be fused at a time?
gpt
A better way of looking at this topic is asking why wouldn't the hydrogen on earth fuse to helium but instead form the chemical compound water together with oxygen? And why wouldn't water exist on the sun, although oxygen is the third most abundant element on the sun? 

The both intuitive and correct answer is, that the conditions aren't right. Earth is to cold to allow fusion, but cool enough to allow water; the sun is too hot to allow water, everything is in the plasma state and basically no chemical bounds can exist there. 

The logical conclusion is then, that the conditions for fusion aren't met everywhere in the sun, because otherwise the hydrogen would fuse the star away in an instant. Fusion depends on temperature, density and time. If you increase each factor, the fusion rate will increase as well. 

That means fusion can in fact occur everywhere, even in extremely cold and thin gas. Though, the timescales involved here are unimaginable. If you want to create a lot of energy, such as needed to maintain a star stable, you would have to increase both temperature and density, because you obviously can't increase time for that matter. 

The right conditions are only met in the very core of the star. The vast majority of the star's matter does nothing but contributing mass, to increase the pressure and thus density to the core. Almost all energy of the sun is produced within 1/4 of its radius, the rest is just heated up to enormous temperatures from a human standpoint, but not enough for any significant fusion reaction. 

The fusion rate increases squared  with density and exponentially with the temperature. Since increasing mass eventually leads to higher densities and temperatures, this causes the interesting effect that higher mass star fuse faster than small stars. Although being more massive, larger stars have a shorter lifetime than smaller ones. 

Small stars with 0.1 M^solar can have lifetimes up to trillions of years, because of their low reaction rate and other effects like convection. They can fuse almost 100% of their hydrogen in that lifetime. Large stars with 100 M^solar live only some million years. In that time they fuse only about 3% of their hydrogen. Small stars are are more efficient, while large ones are more effective.