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The properties of a black dwarf

  1. Jan 18, 2016 #1
    I understand that a black dwarf is a white dwarf which reached thermal equilibrium with the surrounding space, so it's very cold and do not radiates anymore (at least, not in the visible wavelengths).
    But I'm curious about its other properties. From there, some questions:
    1. If it is so cold, is its matter still degenerate (i.e, it supports its own weight through the electron degeneracy pressure) or it is an ordinary solidified matter?
    2. Would it be more like a massive (in terms of mass) rogue planet?

  2. jcsd
  3. Jan 18, 2016 #2
    It's kind of a tricky question, because "ordinary solidified matter" is supported by electron degeneracy pressure. The reason you don't fall through the floor is largely because the chemical bonds in your body and in the floor holds their electrons in compact enough configurations that the electron degeneracy pressure between the electrons in your body and the electrons in the floor is greater than the gravitational force between your body and the Earth.

    This is in contrast to the dominant form of matter in the universe: plasma. In plasma (e.g., every ordinary star everywhere), the kinetic energy of the particles, observed by us as heat, is what resists the inward pull of gravity.

    The difference between ordinary planetary matter and white-dwarf degenerate matter is that in a white dwarf, no molecules exist; all chemical bonds have been ripped apart and it is nothing more than a ball of atoms, supported only by degeneracy pressure rather than by chemical bonds or heat circulation or anything else. In a rogue planet or gas giant, the gravity is far lower and so the majority of the body is under less gravitational force and can still form chemical bonds and undergo convection.
  4. Jan 18, 2016 #3


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    One other comment is that no black dwarves exist in our universe, because the universe has not existed long enough for any white dwarves to cool into thermal equilibrium.
  5. Jan 19, 2016 #4
    I know, but I thought that there are some models for black dwarfs.

    Yes, but the question was about white dwarf matter vs black dwarf matter. I guess I did not clearly formulated the question, so I rephrase: if the degenerate matter cools enough, will it turn back into normal matter, i.e matter with molecules and chemical bonds?
    I want to know this: if the cooled degenerate matter becomes normal matter, how does it supports its own gravity, in the case of a black dwarf? We are speaking about an object with a radius similar to Earth's, but with the mass of a star.
    Basically a rogue planet, in this respect.

    That's my curiosity.
  6. Jan 19, 2016 #5
    Ah, I understand the question better now.

    No, the black dwarf will not return to a state of non-degenerate matter. The equations of state for white dwarf electron-degenerate matter are temperature-independent.
  7. Jan 19, 2016 #6

    Ken G

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    What does happen, though, is the ions crystallize. So it ends up being a lot like a metal, where the ions are like bricks, and experience electrostatic repulsion with very little kinetic energy, and the electrons are like a gas, with a huge kinetic energy that resists the gravity of the star without a lot of help from the ions.
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