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Why do stars collapse in on itself ?

  1. Apr 2, 2013 #1
    Phase of the star's death ,when star's fuel ended,we know that it will collapse into itself
    Why do stars collapse in on itself ?
    How to collapse in on itself ?

    Although the end of the fuel in the star,how to collapse in on itself? Because inside the star there is no mass so there is no force to pull star's top layer.

    Thanks!
     
  2. jcsd
  3. Apr 2, 2013 #2

    A.T.

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    If there was no mass, there would be no star to collapse anymore. When stars burn their fuel the matter doesn't disappear, and only a part is thrown into space.
     
  4. Apr 2, 2013 #3
    You are right, there would be some matter, but it doesn't give an answer my question.
    but thanks
     
  5. Apr 2, 2013 #4

    sophiecentaur

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    Every part of the star is attracting every other part. Whether or not there is anything actually present at the centre of mass of an object (say, a ring shape, to start with or even a giant, very diffuse nebula), all the component parts will have a net attraction towards the centre.
    Of course, inside the star, there is plenty of mass.
     
  6. Apr 2, 2013 #5
    thanks
     
  7. Apr 2, 2013 #6
    There are two things that are really at work within a star, gravity trying to collapse it, and outward pressure which is preventing the collapse, this act of being held in balance is what is referred to as hydrostatic equilibrium.

    No more fuel to use, no more pressure, so it collapses on itself. It causes atoms to get closer together, which increases density and temperature, if the stars mass is small enough eventually the matter will become what is referred to as "degenerate matter" which prevents particles from getting too close to one another which in itself is another type of pressure which prevents further collapse, this occurs in a white dwarf, or neutron star.

    If the mass is large enough, then gravity can overcome this degeneracy pressure and squeeze everything down to a singularity, and you get a black hole.
     
  8. Apr 2, 2013 #7

    Drakkith

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    P.Bo's description was good, but I wanted to add a few things.

    When you compress a gas, that gas heats up. When it expands, it cools down. This has been known for a long time and is very widely used in all kinds of things such as air conditioners. In space it is no different. When a cloud of gas undergoes collapse, it must heat up. This increases the pressure of the gas cloud on itself because the atoms and molecules are moving faster when they heat up and exerting said pressure. Now, for the cloud to collapse, it must get rid of that heat. How does it do this? It radiates it away in the form of light, IR, or other electromagnetic radiation. This reduces the temperature, and thus the outward pressure of the gas, allowing it to collapse.

    How does this relate to a star? Well, a star is basically a giant ball of compressed gases. The core is compressed to extreme temperatures and pressures because the star's outer layers, which have more mass than thousands of Earths, are pressing down upon it. So what happens? The core, thanks to the temperature, generates an extreme amount of high energy EM radiation. (In addition to the normal gamma rays and such from the fusion events) This EM radiation carries energy out from the core and into the rest of the star, eventually escaping through the photosphere of the star.

    Now, remember what I said up above. When that gas cloud loses energy through EM radiation, the pressure inside is reduced, right? So what happens to the core? If we DIDN'T have nuclear fusion the core would constantly shrink in size as energy is radiated out from the star. But the fusion replaces this energy lost at an equal rate, which means the core doesn't shrink. Eventually, when the fuel supply for fusion runs out, the core can no longer maintain its size and starts to collapse.

    Uh oh, its a gas and its collapsing. That means that the temperature is increasing again! If the star is large enough the collapse can increase the temperature so high that it can use the next element up from Hydrogen, which is Helium, as a fuel source for yet another round of nuclear fusion. At least until it runs out too. Very massive stars go through many cycles of this and build up heavier and heavier elements in their core. Once fusion in each round starts, the core stops collapsing as the energy lost through radiation is replaced by fusion, only to start collapsing again once the fuel runs out and fusion ceases.

    Once we reach nickel as products of fusion, it's the death sentence for the star. Attempting to fuse nickel with itself does NOT give you energy. It TAKES energy. So once you have nothing but nickel inside a star's core, there is nothing the star can do to replace the energy lost. So again, the star runs out of fuel once nickel is built up and starts to collapse, heating the core up again.

    Now, at a certain point, the core simply cannot collapse any further. The atoms cannot get any closer together because all the electrons repel each other and cannot occupy the same spot. But as the core of nickel builds up, the pressure increases to the point where even this 'degeneracy pressure' can't withstand the crushing weight of the star upon itself. The electrons combine with protons and form neutrons, a process with TAKES energy away from the core. This only causes further collapse and a chain reaction starts that results in the creation of a core collapse supernova.

    http://en.wikipedia.org/wiki/Type_II_supernova

    I hope that helps.
     
  9. Apr 2, 2013 #8
    A little correction for Drakkith, not all stars go through the process, it needs sufficient mass to compress further. Otherwise the degenerate state of matter, whether it's helium, carbon or neutrons will prevent further collapse, and degenerate matter doesn't have pressure change with temperature.

    Also this was sent via private message, and I thought answering here would be best since it's relevant to the topic.
    sophiecentaur basically answered the why gravity pulls inward. As long as there is some symmetry to the object, even though the pull of 1 atom might not be in the direction of the center, there is another atom at a similar angle but opposite direction which is pulling, so the average of those two pulls is towards the center. Then extrapolate that over every part of the object and you can see how the pull is towards the center. (FYI things like asteroids are not symmetrical, so if you ever walk on one watch out!)

    I'm not sure what you mean by the second part of your message, pressure pushes outward, gravity pulls inward. Don't over think it by imagining gravity as starting from the center, it's simply the average direction gravity pulls when you add up all the little pieces of a star.
     
  10. Apr 2, 2013 #9

    Drakkith

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    Isn't that in my post?

    Absolutely. Until the pressure becomes so great that degeneracy pressure can't hold it up the core won't collapse any more.

    Don't think 'inward', think 'attractive'. Gravitation attracts all objects towards all other objects. In a very massive compact object like a star, this attraction between all atoms and molecules pulls everything together. Think about standing on the surface of the Earth. You weigh a certain amount, let's say 150 pounds. Well, the ground weighs a certain amount too. Every cubic foot weighs, oh I dunno, let's say 50 pounds. There's about 5 quadrillion of these little cubic foot sections of earth at the surface. Imagine the immense pressure that everything underneath has to support. And all material underneath also weighs a certain amount. And ALL of it is supported by the core of the Earth. For a star, it's far more.

    The 'source' of the gravity is the matter itself, which is everywhere in the star. The outward pressure is caused by the ionized atoms bouncing around off of each other. The faster they go, the more pressure they exert, so very hot gas exerts more outward pressure than cooler gas does.
     
  11. Apr 2, 2013 #10
    Also the increased pressure increases the energy and thus the gravitational force.

    And interesting question is what is the probability of quantum tunneling of particles through the event horizon during collapse.
     
  12. Apr 2, 2013 #11

    Drakkith

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    If we're talking about just the core I'm not sure. Wouldn't it be same energy, smaller volume? Or have I overlooked something?
     
  13. Apr 2, 2013 #12
    The stress will positively influence the EM tensor and thus increase curvature.
     
  14. Apr 2, 2013 #13

    Drakkith

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    Of course, but...well, I suppose I'll keep my questions for another thread, as they're probably not helpful to the OP.
     
  15. Apr 3, 2013 #14
    The star is in a state of equilibrium. You've gravity concentrating the nuclei. Then you've got fusion going on in the core that produces radiation. That radiation carries momentum away from the core into the outer layers and this momentum opposes gravity. (More precisely, it carries momentum in all directions but the geometry of the star is such that on the average it carries momentum away from the core.) When the fuel for fusion runs out then the radiation stops and the star collapses.

    Almost all of the mass of the star is inside the star.
     
  16. Apr 3, 2013 #15
    This is significant in a neutron star. I've gotten two estimates. One was "most of the gravity comes from pressure," the other was that it was a few percent. I'd like to see some serious results
    on this, if anyone has them.
     
  17. Apr 3, 2013 #16

    sophiecentaur

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    How can it be this way round? Doesn't 'gravity' come from the presence of mass. producing pressure inside an object? Increased pressure can result in increased density, which can cause a deeper Gravitational Potential well but that is hardly the same thing.
     
  18. Apr 3, 2013 #17
    You are mistaken, gravity comes from many things: mass (which is basically energy at rest), energy, stress as pressure or shear and that even includes stress from the 'pressured' electromagnetic field in a neutron star.
     
  19. Apr 3, 2013 #18

    sophiecentaur

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    Fair enough. It makes sense in the GR world.
     
  20. Apr 10, 2013 #19
    first they break past the electron degeneracy pressure then the neutron degeneracy pressure. the gravity keeps collapsing so the space curves to a "point" with a deep potential
     
  21. Apr 16, 2013 #20
    Does anyone have a source on degenerate matter other than wikipedia?
     
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