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Why are white dwarfs being classified as compact objects instead of black dwarfs? As a black dwarf is the end stage of a white dwarf.

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    $\begingroup$ You seem to imply that a white dwarf being compact excludes a black dwarf being compact or the other way around. Why do you believe it's mutually exclusive? Besides that point: what difference in density do you see between these two objects? $\endgroup$ Aug 3 '21 at 10:20
  • $\begingroup$ @planetmaker I don't believe they are mutually exclusive, as black dwarfs are just white dwarfs that cooled over time, so they should not have a significant density difference. What confused me is that most sites only states that there are three class of compact objects, being white dwarfs, neutron stars and black holes, black dwarfs are not listed. Maybe it is due to the lack of black dwarfs because the universe does not have enough time to produce at the moment? $\endgroup$ Aug 3 '21 at 11:57
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    $\begingroup$ Re Maybe it is due to the lack of black dwarfs because the universe does not have enough time to produce at the moment? That's the answer to your question. $\endgroup$ Aug 3 '21 at 13:18
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Black dwarfs, if they existed now, would be classified as "compact objects".

The only reason that you won't see them on the list is that the the universe hasn't existed long enough to produce them. The compact objects that exist in the universe now are white dwarfs, neutron stars and black holes. though see https://public.nrao.edu/news/cold-white-dwarf/ timing measurements of a pulsar imply the existence of a cool (3000 degree) compact object that may be a white dwarf that is well on the way to being a black dwarf.

Also note that there is a qualitative difference between neutron stars, black holes and white dwarfs. THere is no such difference between white and black dwarfs. A black dwarf is just a white dwarf that has cooled sufficiently.

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