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

1. Oct 28, 2005

### 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.
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

2. Oct 28, 2005

### ray b

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!!

3. Oct 28, 2005

### 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.

4. Oct 28, 2005

### alancj

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.

5. Oct 28, 2005

### neurocomp2003

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.

6. Oct 28, 2005

### Jimmy Snyder

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.

7. Oct 28, 2005

### JCCol

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.

8. Oct 28, 2005

### Janus

Staff Emeritus
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.

9. Oct 28, 2005

### Janus

Staff Emeritus
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.

10. Oct 28, 2005

### Jimmy Snyder

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.

11. Oct 28, 2005

### Hurkyl

Staff Emeritus
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.

12. Nov 11, 2005

### X-43D

Why should nuclear fusion produce a repulsive force?

13. Nov 11, 2005

### Danger

To grossly oversimplify that, the energy produced seeks equillibrium with its environment... violently.

14. Nov 11, 2005

### scott1

The nuclear fussion reactions dosn't fuse hydergon and helium togther; it converts the hydrogen into helium.

15. Nov 11, 2005

### Danger

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.

16. Nov 11, 2005

### Hurkyl

Staff Emeritus
Because it emits a lot of energy.

17. Nov 11, 2005

### scott1

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.

18. Nov 11, 2005

### SpaceTiger

Staff Emeritus
Nuclear fusion doesn't itself provide a repulsive force, it provides energy to maintain the pressure balance inside the star. The "force" that holds up a star like the sun is the pressure of the gas of which the star is made (which, ultimately, is electromagnetic) and the magnitude of this pressure is dependent on temperature. A balance can be maintained as long as the gas is hot, but if the star is radiating energy away, then it needs an energy source to keep from cooling. In most cases, fusion is this energy source.

Last edited: Nov 11, 2005
19. Nov 11, 2005

### SpaceTiger

Staff Emeritus
Let's not confuse diffusion time with a measure of distance. "Light year" refers to the distance that light travels in a year while freely propagating in a vacuum. The distance from the sun's core to its surface is actually around one ten millionth of a light year.

You could look at it that way. The random walk of a photon inside a star is a direct result of its interactions with the gas. The gas is also the ultimate source of the pressure for stars like the sun. However, for heavier stars, the pressure of the light itself can be important in supporting the star against gravity.

20. Nov 11, 2005

### scott1

That was just a joke lol

21. Nov 11, 2005

### SpaceTiger

Staff Emeritus
Most of the lithium in the universe was produced by the Big Bang and cosmic ray spallation. After the hydrogen in its core is exhausted, a star will, if it's heavy enough, then begin the triple-alpha process, which converts helium into carbon.

22. Nov 12, 2005

### Danger

Thanks, Tiger. I knew that there was something wrong with my post. Those sequential thingies always mess me up.

23. Nov 12, 2005

### Labguy

Some Lithium could be produced within about three minutes of the big bang, but most is the cosmic ray-to carbon collisions after carbon is formed many years after the big bang from reactions in the cores of stars.
From: http://www.astro.ucla.edu/~wright/BBNS.html" [Broken] we get that:
And from http://astron.berkeley.edu/~mwhite/darkmatter/bbn.html" [Broken] we find that formation of Lithium first requires tritium and deuterium:
and the neat chart near the top of that page shows one H isotope, two He isotopes and Li.
So, if that's the case and order of formation, when do we consider the "big bang" to be in process? One second, one minute, three minutes (as in the book name) ? This is just a general (and pickey) question that I haven't seen brought up before; how much time after zero is the BB still considered to be in process?.. Anyone answer.

Last edited by a moderator: May 2, 2017
24. Nov 12, 2005

### SpaceTiger

Staff Emeritus
How are you distinguishing this from the cosmic ray spallation I referred to? Or are you just expanding on what I said?

I think it's pretty arbitrary. In this case, I suspect that they were trying to distinguish it from stellar nucleosynthesis. Since it was one of the primary predictions of the Big Bang Theory, they just called it "Big Bang Nucleosynthesis".

25. Nov 12, 2005

### Astronuc

Staff Emeritus
Well those trillions and trillions and trillions of tons (~1.9891×1030 kg (332 950 Earth masses)) produce a strong gravitational field. Remember, it is strong enough to keep planets in orbit, which are millions of miles away.

The pressure comes from the intense heat/temperature which comes from the fusion reactions.

Fusion reactions produce energy on the order of 1 MeV and at 11605 K/ev, that's about 11 billion K. However, only a fraction of mass is fusing at a given time, so that kinetic energy of the fusion reactants is dissipated quickly to the various atoms and electrons nearby. Temperature of the sun's core is estimated to be ~13.6 MK, and the corona temperature is about 5 MK, while the photosphere 'surface' temperature is about 5800 K.

For information on pp and CNO chain reactions see -
http://csep10.phys.utk.edu/astr162/lect/energy/cno-pp.html

http://www.shef.ac.uk/physics/people/vdhillon/teaching/phy213/phy213_fusion3.html [Broken]

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Re: Li production (is it necessary that most(?) Li was formed around time of BB as opposed to stars later on or novae?)

p + 9Be -> 4He + 6Li

d + 4He -> 6Li

t + 6Li -> 7Li + d (or p +n)

There is a paper "Influence of Gravity Waves on the Internal Rotation and Li Abundance of Solar-Type Stars" by Corinne Charbonnel and Suzanne Talon in Science mag, but I can't access it since I am not a member. Anyone read this?

Ann Merchant Boesgaard. (http://www.ifa.hawaii.edu/~boes/) is looking at Li abundance in stars.

Last edited by a moderator: May 2, 2017