Coolest white dwarf

[Tom Mazanec]

I would like to see a black dwarf but the universe is not that old. How cool would a white dwarf “nearly fourteen billion years old” be?
Would it be red yet?

[eburacum45]

This white dwarf has an expected temperature of around 3000K, about the middle of the range for a red dwarf. It is supposed to be roughly 11 billion years old. So a white dwarf which was even older would presumably be even cooler, although it would probably look orangeish rather than red.

A 1.05 M⊙ Companion to PSR J2222-0137: The Coolest Known White Dwarf?
https://arxiv.org/abs/1406.0488

[kpatz]

If a black dwarf did exist how could it be detected and identified as such, as opposed to a neutron star, black hole, brown dwarf, or a large rogue planet? Probably by mass estimates if it's in a binary pair, but what if it's by itself?

If it's hundreds or thousands of light years away could it be detected at all? Perhaps if it gravitationally lenses an object behind it?

I wonder how many of the first generation of stars would have produced white dwarfs at all, compared to later generations.

[antoniseb]

The one that eburacum45 points to is also well above the minimum mass for a white dwarf. If there were a 0.5 solar mass white dwarf created 12 billion years ago, it seems plausible that it might have cooled enough to have a temperature of about 2000K.

[kpatz]

A star born 12 billion years ago that forms a 0.5 solar mass WD would have lived a lot longer than the aforementioned 1.05 M⊙ WD progenitor, and thus it wouldn't have had as much time to cool down yet.