# Why don't stars explode? What holds them together?

by alancj
Tags: explode, holds, stars
 P: 59 I do know that stars explode on occasion... but I don't understand how gravity could possibly have enough force (attraction or whatever you want to call it) to hold trillions and trillions and trillions of tons of hydrogen undergoing nuclear fusion. It seems to me that there would be vastly more pressure to expand outward than to hold it together. Can anybody point me towards some math that would show that a Sun's estimated mass would have enough gravity to keep things together? Also according to my high school science book's explanation, stars form from vast clouds of gas that eventually compress enough (do to mutual attraction between particles) to initiate nuclear fusion. It would seem that clouds of gas would form (brought together by gravity) but the gas would resist further compression, and remain as simple clouds (or one big giant cloud). Can anyone help my confusion here? Thanks, Alan
 P: 428 only a little bit of the gas is undergoing nuclear fusion in the very center of the star that why our star the sun will last for billions of years while the sun looks like a big ball of fire the real burning is only only happening in a small part of it gravity is a hard thing to understand and even the best minds in physics are just starting to try to understand it thats why some think 95% of the mass in the univerce is missing [dark matter] or maybe we just donot understand gravity very well YET!!
 Sci Advisor HW Helper PF Gold P: 562 Our Sun has a mass about 1 million times greater than that of the Earth, which translates into BIG gravity. The sun is stable precisely because the outward pressure produced by the fusion process is exactly balanced by the inward gravitational force.
P: 59

## Why don't stars explode? What holds them together?

 Quote by James R Our Sun has a mass about 1 million times greater than that of the Earth, which translates into BIG gravity. The sun is stable precisely because the outward pressure produced by the fusion process is exactly balanced by the inward gravitational force.
Well I know that things are obviously in equilibrium, I'm looking more for an accounting of all the forces. Because if gravity equals 10 and nuclear reaction and heat equals 1000 then you know that some things are amiss.
 P: 1,373 convection... look for a textbook by Ostlie and Carroll...it will explain it all in there There's like 4 pressure equations(keeps teh star togeterh like how max eq'n are used for E&M) think the book is called intro to astrophysics.
P: 2,163
 Quote by alancj if gravity equals 10 and nuclear reaction and heat equals 1000 then you know that some things are amiss.
I don't know the answer to your question. But I do know that a = b for the sun. I've been told that when the forces get out of balance, as in a nova or supernova explosion, it's gravity and not the heat that is the larger of the two. It's easy to see why. Nuclear reactions may die out over time, but gravity is forever.
 P: 24 According to Einstein gravity is caused by a number of things, pressure, mass and density energy, energy flux, etc, curving space-time. The Sun curves so much space that there is enough gravity to keep it together. The nuclear reaction is at the center of the star and then the rest of it will be held together with gravity. The energy escapes out the star and the extra mass goes to the mass of a shell of a star.
Emeritus
PF Gold
P: 2,352
 Quote by alancj I do know that stars explode on occasion... but I don't understand how gravity could possibly have enough force (attraction or whatever you want to call it) to hold trillions and trillions and trillions of tons of hydrogen undergoing nuclear fusion. It seems to me that there would be vastly more pressure to expand outward than to hold it together. Alan
One way of lokkoing at this is comparing the Sun's gravitational binding energy(the amount of energy needed to pull the sun completely apart) to the amount of energy it produces through fusion.
The Sun's gravitational binding energy is equal to about 2.24 x 1043 This is about equal to the amount of energy it produces through fusion in 150 million years.
 Emeritus Sci Advisor PF Gold P: 2,352 Hmm, the edit feature doesn't seem to be working for me right now, but that 150 million year figure should be 15 million years.
P: 2,163
 Quote by Janus that 150 million year figure should be 15 million years.
That reminds me of a joke:

A. The sun will die out in 5 billion years.
B. Did you say 5 billion years?
A. Yes.
B. Thank goodness, I though I heard you say 5 million years.
 Emeritus Sci Advisor PF Gold P: 16,101 Here's one way to think about it: Gravity likes to pull things together Hydrogen doesn't like to fuse into helium... it would rather just sit around being hydrogen. But, hydrogen dislikes being packed very tightly even more than it dislikes fusing into helium. So, under considerable duress, hydrogen will fuse to relieve the packing problem. So that's why a cloud of gas might erupt into a star. But, remember that hydrogen doesn't like to fuse. So, only enough fusion will happen to combat the problem of being excessively packed together. (roughly) So that's why the star generally doesn't explode.
 P: 152 Why should nuclear fusion produce a repulsive force?
 PF Gold P: 8,961 To grossly oversimplify that, the energy produced seeks equillibrium with its environment... violently.
 P: 459 The nuclear fussion reactions dosn't fuse hydergon and helium togther; it converts the hydrogen into helium.
 PF Gold P: 8,961 True at our sun's level of activity. Farther down the line, the helium-3 will fuse into helium-4, then lithium, etc.. I can't recall the exact sequence right now.
Emeritus
PF Gold
P: 16,101
 Why should nuclear fusion produce a repulsive force?
Because it emits a lot of energy.
 P: 459 When the photons form at the Sun's core it take it about 1,000,000 years to get to the surface(so the sun's core is 1millon light-years away).I think whatever takes light to get 1 millon years to get to the surface is probally whould the thing that's holding it togther.
Emeritus