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Dead stars

  1. May 9, 2007 #1
    I have a question about dead stars. I have only read about 2 ways a star can die. Either explode into a supernova or collapse into a black hole. It seems strange to me that when a star consumes all its energy that it could not simply become a huge lump of slag out there. Is it possible that there are some dead solar systems out there?

    I am a casual observer of astronomy and not at all studied up on it, I am just wondering.

  2. jcsd
  3. May 9, 2007 #2
    Hi Frozenoak,
    I have my own theory about why dead stars explode:

    When you drop a rock, what happens to the stored potential energy? It gets converted into heat.

    In a star, there are 2 opposing forces, heat generated from fusion causing the star to expand outward. Gravitational forces causing it to compress.

    When a star runs out of fuel, the some layers fall inward. Now, it is no more heat generated from fusion, but heat generated because potential energy is being converted to heat. This causes the outermost layers to expand outward, while the inner layers are falling inward.

    This is just speculation from me... I don't know how to prove my speculation.
  4. May 9, 2007 #3


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    Most stars, when they run out of fuel, simply shrink into being a white dwarf or a neutron star. To become a black hole, it has to have a lot of mass after a supernova explosion.

    Our sun is expected (a few billions from now) to first become a red giant, after running out of hydrogen, and ultimately collapsing into being a white dwarf.
  5. May 9, 2007 #4
    Does the white dwarf then burn forever? The fire has to die at some point I would think.
  6. May 9, 2007 #5


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    What you're looking for is the white dwarf. This is just the dead core of the star; it generates no energy. It is extremely hot, though, and very small, so it takes a long time to cool. Most stars end their lives as white dwarves. Only very massive stars are capable of becoming supernovae or black holes.

    - Warren
  7. May 9, 2007 #6
    Ohhh, Thant makes a lot of sense then. Thanks for the explanation.
  8. May 17, 2007 #7
    After the supernova explosion, if the remaining (core of star) mass is 1.4 solar mass, the remnant becomes neutron star, if mass is less than this value, it becomes white dwarf. finally, if the mass of remnant is approximately 3 solar masses, the degenerate neutron pressure can not prevent to collapse due to gravitation, then it becomes black hole.

  9. May 19, 2007 #8


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    Main sequence stars, like our sun, fade into obscurity. A typical white dwarf can simmer for much longer than the age of the universe.
  10. May 23, 2007 #9
    I have another question regarding black holes.

    Before stars collapse, they undergo supernova, which means the envelope or outer layer blows up. Will the total mass of the star decrease such that instead of being a black hole, at the very last minute it becomes a dwarf?
  11. May 23, 2007 #10
    This is very interesting to me I never knew white dwarfs were not undergoing some kind of fusion.Some questions though

    You say the white dwarf generates no energy. What causes it to shine?

    You said that white dwarfs are extremely hot. Isn't heat a form of energy?
  12. May 23, 2007 #11


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    I think the key word in chroot's post is generates. For example, one can heat a metal bar, say, to a certain temperature, then remove it from the heat source. The bar will still be hot, and if hot enough, will glow whilst it is cooling. It is not generating heat, but is losing heat.

    As chroot says, the white dwarf takes a long time to cool, therefore whilst cooling still shines. But the star is not generating this heat, it is in fact slowly losing it.
  13. May 23, 2007 #12

    George Jones

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    If a star is massive enough to go supernova during collapse, calculations show that either a neutron star or a black hole will be left as remnant. So, collapse supernovae don't always form black holer.

    A new type of collapse supernova (predicetd theoretically 40 years ago) has been observed for the first time. This type of supernova leaves nothing behind. See https://www.physicsforums.com/showpost.php?p=1338296&postcount=16" and links.
    Last edited by a moderator: Apr 22, 2017
  14. May 23, 2007 #13


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    Yes! Have a look at the discussions on Eta Carinae, the coming supernova 7000 ly away. Right now it is undergoing a very complex series of expansions and contraction, periodically blowing-off parts of its outer layers. Stars that are near the threshold size may throw off enough to reduce their size below the threshold.
    Last edited: May 23, 2007
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