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Does anyone know

  1. May 26, 2003 #1
    exactly how gravity in a star can generate temperatures high enough for thermonuclear reactions? i need a straight answer this time.
     
  2. jcsd
  3. May 26, 2003 #2
    It was the pressure origionally, and then the reactions which also produce heat.
     
  4. May 26, 2003 #3
    I'm pretty sure

    that most new stars contain some Lithium and Deuterium which would fuse at much lower temperatures, and help "kindle" the reaction. Does anyone more knowledgeable about this have any comments?
     
  5. May 26, 2003 #4

    Integral

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    All that is necessary is gravitation. When the mass of Hydrogen in a region is great enough, gravitation draws it together. If the mass is great enough the force of gravity is sufficient to over come the electronic forces which sperate atoms. When this happens the nucleons are crushed close enough together to allow nuclear reactions to occur. These nuclear reactions release heat. It is not temperature which starts the reactions it is gravitational forces. The temperature is a result of the nuclear reactions.

    Haven't I already answered this question?
     
  6. May 26, 2003 #5
    Nuclear rections inside Sun have nothing to do with pressure. Indeed, protons in Sun's interior (there are no atoms there) are about 2-3 times closer to each other than in liquid or solid H, still way way far for any nuclesr interaction between them to take place. What makes protons fuse is thermal motion. As H protostar shrinks its cold H atom are losing gravitational energy (because they are descending closer) thus gaining kinetic energy. While cloud is still transparent, this energy is radiated away by infrared radiation and the protostar cloud keeps collapsing (=falling on itself). But sooner or later the cloud becomes dense enough to trap most of its IR radiation inside. Since this moment temperature inside cloud starts gradually rizing as a protostar condenses under its own weight futher. When temperature reaches about 0.5-1 KeV (~10 7 K) then proton's velocities are high enough to allow to some protons occasionally gain in collisions high enough speed to roll over tall Coulomb barrier of repulsion and to fuse. At that low temperature (10 7 K fusion is still very and very seldom - once in few billion years per proton. Thus, slow (very and very slow) lazy occasional fusion takes place in proton plasma. Released energy then is trapped inside and makes long long way outside via billions of meters (usually it takes hundreds of thousand years for a photon inside Sun like star to reach the surface despite that it always moves with the speed of light), and then shines from a surface out. We then call such object "a star", because it becomes visible. Futher collapse of star is halted by the balance of thermal pressure (due to flow of fusion energy) with gravitational (hydrostatic) pressure of star's weight.

    Production of energy inside stars is quite slow. Ordinary candela releases about million times more energy (per same volume) than Sun's core.
     
    Last edited by a moderator: May 26, 2003
  7. May 26, 2003 #6

    chroot

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    The gravitational potential energy is transferred into translational kinetic energy of the falling particles. Another name for 'translational kinetic energy' of microscopic particles is 'heat.'

    If you drop a bowling ball, it gains kinetic energy as it falls. As the hydrogen atoms fall in towards a protostar, they also gain kinetic energy.

    - Warren
     
  8. May 26, 2003 #7

    marcus

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    Integral, your answer makes it seem that if it were not for fusion a contracting self-gravitating mass would not heat up.

    But you know it would and that the story is a bit more complicated.

    The core of Jupiter would be hot even without a single event of radioactive decay or fusion ever having occured there, essentially by the release of gravitational potential and its conversion into kinetic energy (heat, temperature).


    Maximus if you drop a brick onto the ground then the brick and the ground are both slightly warmer afterwards.

    Self-gravitation of a star involves enormous quantities of mass coming down vast distances. Even if it is gradual the kinetic energy is going to reach very high temps---even without fusion occurring---because so much gravitational potential energy is being given up by the matter as it moves in towards the center.

    this might be a case where one could calculate a rough estimate of the temperature achieved by a self gravitating mass of some size coming together---there is a related (not directly applicable but suggestive) formula called the Virial Theorem asking to be tried here.
     
  9. May 26, 2003 #8
    Might I suggest that some of you read the pages in the Astronomy and Cosmology forum, Proof of the cause of gravity, near the end pages 11, 12, 13, this particular topic is, in a sense of it, being 'Hotly' debated, just that it is presently about the earths core....such fun.

    In a star the pressure is generated by gravity, generation of pressure is a heating event, pressure alone is not, pressurization is, hence it is the stars gravity that initiates the event, and the resultant increases in heat values, further drives it, as heat, and gravity, play a dance of expansion and contraction, in opposition to each other.

    The nuclear reaction (fusion) simply adds to the heating side of the event, and slowly to the mass side, through the fusion of higher mass elements. Gravity builds!
     
  10. May 26, 2003 #9
    Exactly. It is not pressure, but change of position (protons come closer) in mutual attraction force (gravity) which makes protons move faster. Make Earth and ball come closer, and both will move faster (=gain "temperature").
     
  11. May 27, 2003 #10
    So the idea that temperature is actually 'Ambient Energy Pressure" (AEP) holds to the idea that it is actually pressure that is active, just that we are calling it heat, as it is a pressure of energy/EMR, AEP.

    Used that term in the "Proof of Gravity" pages, AEP.
     
  12. May 27, 2003 #11
    Well, what we call pressure is just a change of momentum of moving particles in collisions with each other, with walls of a container (if any) and with the device measuring pressure (pressure gauge, barometer, etc). What we call temperature is average kinetic energy per particle. So, still these are different (though closely related as momentum and energy are closely related) concepts.
     
  13. May 27, 2003 #12
    I agree, but note that both arise from the same source, matter, and the energy that surrounds all matter.

    Hence an interactive viewpoint of the two, as the same, (from the same source) enables the understanding of the function of the two, as a function of one thing, matter.

    Is that OK?
     
  14. May 28, 2003 #13
    Yes, this is OK. Everything in physics is related.
     
  15. May 31, 2003 #14
    Mr. Robin Parsons

    can you explain the 1/r-1/2 ? at first glance, it just looks like another way to write [squ]r , I suspect theres more to it than that.
     
  16. Jun 1, 2003 #15
    The 1/r-1/2 is the version of "one over the square root of r"
     
  17. Jun 1, 2003 #16

    FZ+

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    Actually, Mr. Parsons, there is an error in your maths.

    1/r-0.5 == (r-0.5)-1

    == r0.5, which is the square root of r.
     
  18. Jun 1, 2003 #17
    Insert disclaimer ¤HERE¤

    Thank you, I appreciate the help!

    From this site....

    Cal Tech

    this statement........

    Comments anyone??
     
  19. Jun 1, 2003 #18

    Integral

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    Appearently, the original topic of this thread is done.

    Mr. Parsons if you wish to start a new topic concerning your last post, please do.

    I am locking this one.
     
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