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Thread: White Dwarf Structure

  1. #1
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    White Dwarf Structure

    Can someone explain to me why a white dwarf is so dense? Are the electrons somehow compacted to make the atoms smaller? Are there even atoms in a white dwarf, or is it just plasma? I have searched the internet (and this site) for answers and not found anything I understand, so please tell it to me in high school physics terms.
    Many thanks.

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    From WIKI,

    white dwarf material is not composed of atoms joined by chemical bonds, but rather consists of a plasma of unbound nuclei and electrons.

  3. 2020-Sep-01, 10:21 AM
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    Quote Originally Posted by hush36 View Post
    From WIKI,

    white dwarf material is not composed of atoms joined by chemical bonds, but rather consists of a plasma of unbound nuclei and electrons.
    For this it appears that white dwarfs are comprised of the same plasma as main sequence stars. I still do not understand why they are so much denser.

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    Quote Originally Posted by sdsperth View Post
    For this it appears that white dwarfs are comprised of the same plasma as main sequence stars. I still do not understand why they are so much denser.
    Normal stars generate energy through fusion. This heats the centre up and can be thought of as a kind of outwards force trying to push the star apart (stars are basically a trapped nuclear explosion). Their gravity holds them together so it is a tug of war between these two forces that results in a normal star with quite a large radius. In a White Dwarf there is no fusion so gravity has things much more its own way. They contract and contract until they reach the limit of how much the plasma can be compressed (at this point something called electron degeneracy replaces fusion as the 'outwards force'). So basically White Dwarfs are dense because they don't create the large amount of fusion energy a normal star does.

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    Quote Originally Posted by sdsperth View Post
    For this it appears that white dwarfs are comprised of the same plasma as main sequence stars. I still do not understand why they are so much denser.
    Normal cool atoms like we are used to on Earth have a nucleus surrounded by one or more electrons. The actual atom is almost all empty space, but resists compression. Compression can occur however when enough force is applied. One example we see is Jupiter versus Saturn. They have fairly similar diameters, but Jupiter is significantly more massive because its core is more compressed. Further, reasonably cool worlds much more massive than Jupiter are about the same diameter as Jupiter (hot worlds can be larger). In the case of white dwarfs, more massive white dwarfs are smaller than less massive ones because they are more highly compressed.

    If we delve much further into this, we start to get into subjects that take us out of typical high school level science, especially if you haven’t taken some chemistry and physics. Electrons around nuclei don’t really work like planets around stars, though that’s often used as an analogy. Rather, they tend to occupy regions around nuclei, stratified in certain ways, and there are limits to how many there can be for a certain nucleus. If you’re interested, you might want to web search on the terms “electron orbitals” “electron shells” and “fermi exclusion principle”.

    One of the things this leads to is something called “electron degeneracy pressure” that provides pressure to resist compression. The matter in the interior of a white dwarf is highly compressed. If you could somehow extract it and release it on Earth, you would end up with a massive explosion as it expanded.

    In a star like the sun, as Shaula said, heat keeps the star from compressing and shrinking, but once the energy source ends, that stops and it shrinks to a white dwarf as long as its mass is no more than about 1.4 times the sun’s mass. Then it is too much for electron degeneracy pressure and the star collapses to a much more compact neutron star which is itself subject to “neutron degeneracy pressure.” If there is too much mass than even that will collapse further to a black hole.
    Last edited by Van Rijn; 2020-Sep-02 at 06:48 AM.

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    Quote Originally Posted by Van Rijn View Post
    One of the things this leads to is something called “electron degeneracy pressure” that provides pressure to resist compression. The matter in the interior of a white dwarf is highly compressed. If you could somehow extract it and release it on Earth, you would end up with a massive explosion as it expanded.
    Thanks Van Rijn. I'm familiar with the term electron degeneracy pressure, but I can't find an explanation for what it is. I know of the Pauli exclusion principle, but was led to understand that this is apparent in regular atoms. How does it allow atoms to be smaller in white dwarfs?
    I am also confused about orbitals. I wasn't aware that electrons were in such arrangements in a plasma.

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    Atoms are mainly empty space, because the electrons are spaced into orbitals, and those orbitals exist because the Pauli exclusion principle prevents two electrons occupying the same energy/momentum state. But if you crush matter together into very high densities, the electrons stop moving in orbitals, and instead turn into a sort of electron gas. At which point, the nuclei of the atoms can get a lot closer together. There are no atoms left--just positively charged nuclei surrounded by negatively charged electron gas.
    But the Pauli exclusion principle still applies to the electron gas, except now it's spread through the volume of the gas. Only one electron is allowed to occupy the lowest available energy/momentum state; only one in the next higher state, and so on. So the electron gas has a high pressure, sufficient to oppose the force of gravity (up to a limit beyond which the white dwarf will collapse).
    So conventional matter looks like a big empty space, dotted with nuclei which are held apart by electron orbitals with lots of space between them; electron-degenerate matter has all those orbitals collapsed into a high-pressure electron gas, threading between closely packed nuclei.

    Grant Hutchison

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    The volume of everyday atoms is largely occupied by probabilistic electron clouds. But, no chunk of Minkowski spacetime in this universe, is "empty". It contains three things. 1 The microwave radiation left over from our putative Big Bang. 2. The zero point radiation due to the Heisenberg Uncertainty Principle. 3 The neutrino sea consisting of stellar fusion neutrinos, Diffuse Supernova Background Neutrinos, left over cosmological neutrinos from the Big Bang, created along with the CMB, and the unknown neutrino "floor".
    You can build a refrigerator to remove the CMB photons, but nobody can remove the Zero Point Radiation, or the neutrinos flowing through your volume of space. Space is not empty.
    pete

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    "Putative" Big Bang?

    ...That's an argument for another thread.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

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    Quote Originally Posted by trinitree88 View Post
    The volume of everyday atoms is largely occupied by probabilistic electron clouds. But, no chunk of Minkowski spacetime in this universe, is "empty". It contains three things.
    Well, yes. But, oddly enough, none of these invalidates the use of the word "empty", any more than the presence of digestive juices means my stomach isn't empty, or the presence of air, dust and a couple of paperclips means my desk drawer isn't empty, or the presence of words on paper doesn't mean that I haven't written an empty phrase.
    "Empty" always focuses on the things that are important by their presence to the subject under discussion. Just saying.

    Grant Hutchison

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    Quote Originally Posted by grant hutchison View Post
    Atoms are mainly empty space, because the electrons are spaced into orbitals, and those orbitals exist because the Pauli exclusion principle prevents two electrons occupying the same energy/momentum state. But if you crush matter together into very high densities, the electrons stop moving in orbitals, and instead turn into a sort of electron gas. At which point, the nuclei of the atoms can get a lot closer together. There are no atoms left--just positively charged nuclei surrounded by negatively charged electron gas.
    But the Pauli exclusion principle still applies to the electron gas, except now it's spread through the volume of the gas. Only one electron is allowed to occupy the lowest available energy/momentum state; only one in the next higher state, and so on. So the electron gas has a high pressure, sufficient to oppose the force of gravity (up to a limit beyond which the white dwarf will collapse).
    So conventional matter looks like a big empty space, dotted with nuclei which are held apart by electron orbitals with lots of space between them; electron-degenerate matter has all those orbitals collapsed into a high-pressure electron gas, threading between closely packed nuclei.

    Grant Hutchison
    Thanks Grant. This is the exact level of explanation I was looking for. While I understand trinitree88's empty objection, I am very grateful for your choice of words, to aid my understanding.

  13. #12
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    Quote Originally Posted by hush36 View Post
    From WIKI,

    white dwarf material is not composed of atoms joined by chemical bonds, but rather consists of a plasma of unbound nuclei and electrons.
    Is that actually true, though?
    Metals are not usually regarded as plasma.
    Melting point of most substances rises with pressure (and those that don´t will adopt different crystal structures at high pressure).

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