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I know black dwarf stars do not exist yet, and that they are what white dwarfs end up after a long time. The question is really simple: would it be possible to have a binary system in which one of the two is a black dwarf, and the other companion star is dumping mass on the black dwarf, causing the black dwarf to supernova? I guess based on the timescales, the maximal initial mass of the second object could be calculated to see if it is possible that one object is at the black dwarf stage and the other still a giant. Or is the timescale on which the star goes from white dwarf to black dwarf big enough that it will never reach the black dwarf stage while the other one is still an AGB star?

Second part: If not possible that one is black dwarf and the other AGB star, is it possible if the companion is a brown dwarf, that they are close binary, and that a brown dwarf would dump mass on a black dwarf? This would need the binary system to be a very close binary system at this stage of the system.

Third part of the question: would the spectrum of the supernova be any different from the normal one with a white dwarf and a companion AGB star? I guess the spectral lines would be different, since the companion would have to be a very low-mass AGB star, since that companion would be about 20billion years or so, when the black dwarf star becomes a black dwarf.

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Technically, we don't know that black dwarf stars don't exist yet; we just know they shouldn't exist yet based on our current theories. Of course, for anything theoretically possible, it is impossible to know that it doesn't exist for sure--you can just gain better confidence that it probably doesn't exist. –  called2voyage Mar 14 at 12:27
@called2voyage "Technically, we theorize that such objects might one day exist", is the phrase you are searching for? –  usethedeathstar Mar 14 at 12:42
Succinctly, yes, but that phrasing misses the nuance that there is a possibility that we are incorrect and such objects already exist--we just haven't observed them yet. –  called2voyage Mar 14 at 13:44
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