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

  1. 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. jcsd
  3. 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!!
     
  4. James R

    James R 562
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    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.
     
  5. 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.
     
  6. 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.
     
  7. 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.
     
  8. 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.
     
  9. Janus

    Janus 2,367
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    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.
     
  10. Janus

    Janus 2,367
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    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.
     
  11. 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.
     
  12. Hurkyl

    Hurkyl 16,090
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    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.
     
  13. Why should nuclear fusion produce a repulsive force?
     
  14. Danger

    Danger 9,879
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    To grossly oversimplify that, the energy produced seeks equillibrium with its environment... violently.
     
  15. The nuclear fussion reactions dosn't fuse hydergon and helium togther; it converts the hydrogen into helium.
     
  16. Danger

    Danger 9,879
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    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.
     
  17. Hurkyl

    Hurkyl 16,090
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    Because it emits a lot of energy.
     
  18. 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.
     
  19. SpaceTiger

    SpaceTiger 2,977
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    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
  20. SpaceTiger

    SpaceTiger 2,977
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    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.
     
  21. That was just a joke lol
     
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