system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does the sun not burn up all of its hydrogen and subsequent fused elements almost instantaneously? The sun is literally on fire 100% of the time, fusing hydrogen to helium, helium to carbon,etc.. How does it not burn up all at once?
gpt
There is no scale we have that can sufficiently explain just how big the sun is. It fuses 400 million metric tons of hydrogen per second. However, there is *so much* sun that even 400 million metric tons per second means that it only runs out after ~9 billion years of constant burning. Only then does it move on to higher order fusions, which then gives it another billion years.

As for "why" it doesn't all fuse at once, the same reason why we're not fusing right now. Fusion requires insanely high temperature and pressure, something the outer layers of the sun do not reach, so only the core is ever actually fusing.