# What does a black dwarf look like?

A black dwarf is a hypothetical object that is the end result of the cooling of a white dwarf. None yet exist, because there hasn't been enough time in the age of the universe for them to cool down sufficiently. The oldest known white dwarfs are at ~3800 K and would glow orange. It will take many billions of years before any are cool enough (~750 K I believe) to not emit light visible to the naked eye.

So, what would a black dwarf look like? I'm assuming it is illuminated by some other source of light such as a star.

The question could be rephrased: What is the visible absorption (and emission) spectrum of electron degenerate matter?

This is my attempt at an answer: Probably not black. A lot would depend on whether electron-degenerate matter behaves as metallic or dielectric, based on its band-structure. My guess would be metallic, considering the electrons are dissociated from their parent nuclei in electron-degenerate matter.

Because the surface gravity of a black dwarf would be so strong, I'd expect the surface to be very smooth and relatively free of Lambertian scattering.

From these two features, I'd make a tentative prediction that a black dwarf would appear as a mirror-like sphere.

• It's possible that you wouldn't see the degenerate matter much through the atmosphere. The surface gravity is obviously crazy high and the cooler temperature might condense the atmosphere even further, but a black dwarf could still have a hydrogen/helium atmosphere. en.wikipedia.org/wiki/White_dwarf#Atmosphere_and_spectra Interesting question what it might look like. Jul 25 '17 at 9:27

Edit: I note that there probably are white dwarfs cooler than 3000K. This is the upper limit to the temperature of an invisible companion of to the pulsar PSR 2222-0137. Kaplan et al. (2014) say that this most likely a massive ($$\sim 1 M_{\odot}$$) old white dwarf that has cooled so much that they cannot detect it at visible wavelengths - so probably you would call this a "black dwarf", though the term is not used in the scientific literature.