# Fusion and the Sun **URL:** \ **Category:** Factual Questions\ **Created:** [July 24, 2002, 10:05am UTC](https://boards.straightdope.com/t/fusion-and-the-sun/120538 "2002-07-24T10:05:47Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![Tuckerfan](https://avatars.discourse-cdn.com/v4/letter/t/eada6e/32.png) [@Tuckerfan](https://boards.straightdope.com/u/Tuckerfan)\ **Post date:** [July 24, 2002, 10:05am UTC](https://boards.straightdope.com/t/fusion-and-the-sun/120538/1 "2002-07-24T10:05:47Z")
Okay, I’ve always had difficulty grasping part of the sun runs on fusion thing. I know how fusion works (sans the mathematics behind it), but what I don’t get is why the sun doesn’t just go “phoof!” (Yes, I know there’s no sound in space.) and burn all its fuel at once. Is it simply because there’s so much freaking much matter in the sun or is there something else going on (say, interaction between the sun’s massive gravity and the reactions which slows them down somehow and prolong’s the sun’s life)? ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [July 24, 2002, 10:15am UTC](https://boards.straightdope.com/t/fusion-and-the-sun/120538/2 "2002-07-24T10:15:10Z")
Fusion reaction requires a certain pressure and density. If the reaction goes a bit too fast, this produces more heat, which causes the sun to expand. So the density becomes lower, and the rate of fusion reaction goes down, putting the system back to equilibrium. So it’s a _stable_, self-regulating equilibrium. Even if you dump more mass (say, another small star) onto the sun, the density will increase and the rate of fusion reaction with it, until it reaches a new equilibrium. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [July 24, 2002, 10:40am UTC](https://boards.straightdope.com/t/fusion-and-the-sun/120538/3 "2002-07-24T10:40:02Z")
Plus, of course, it isn’t actually _burning_ the fuel in the everyday sense with which we are familiar. ---
**Author:** ![Popup](https://avatars.discourse-cdn.com/v4/letter/p/94ad74/32.png) [@Popup](https://boards.straightdope.com/u/Popup)\ **Post date:** [July 24, 2002, 11:04am UTC](https://boards.straightdope.com/t/fusion-and-the-sun/120538/4 "2002-07-24T11:04:46Z")
It’s also worth noting that the fusion going on in the sun is not particularly intensive! Assuming (incorrectly) that the fusion is homogeneous throughout the volume of the sun, the energy generated is about 1W/m[sup]3[/su (according to [G. Cornelius](http://boards.straightdope.com/sdmb/showthread.php?&postid=2277880#post2277880)) Quite far from your everyday H-bomb… ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [July 24, 2002, 3:29pm UTC](https://boards.straightdope.com/t/fusion-and-the-sun/120538/5 "2002-07-24T15:29:48Z")
The sun is trying to go _phoof_. Gravity is all that stops it. If by some magical means you turned off gravity the sun would explode quite violently. Fusion only occurs in the core of the sun. The rest of the sun (the vast majority of its volume) is the _phoof_ part of the fusion reaction at the core. FWIW the sun fuses around 5 million tons of hydrogen per second.