system
Given the following post, respond as if you're commenting on Reddit:
human
How big does a hydrogen gas ball have to be before its own gravity effect starts self sustaining fusion like the Sun?

Assume it's a "universe sandbox" where there's no other objects to exert a gravity field on that hydrogen ball.
gpt
It needs to have about 80 times the mass of Jupiter, or about 0.08 times the mass of the Sun, before it can sustain proton-proton chain fusion in its core. Probably a little bit more mass if you're using pure hydrogen rather than the 75/25 H/He mix (by mass) that actual stars are comprised of, but it won't make a huge difference.