The "black dwarf" that Bishop refers to is what's left when the white dwarf has lost all residual heat; a cold cinder. The "cool white dwarf" definition of black dwarf you are quoting is unknown to me; I suggest that the lecturer you are referring to may have made some generalizations or used a term loosely. Certainly by the time a white dwarf has cooled that much, it no longer has a planetary nebula. White dwarfs are the product of stars up to about 1.4 solar masses and generally Population I stars- "metal rich", second-generation stars. Again, I'm no expert but if it's possible that a white dwarf can evolve from a Pop. II star then there very well may have elapsed enough time since the formation of the first generation of stars for a black dwarf to exist today. And yes, there probably is no way to detect them at astronomical distances except by the effect of their gravity on a companion, if any. On Sun, Jan 25, 2009 at 8:05 PM, <zaurak@digis.net> wrote:
Chuck,
I imagine a black dwarf would be very difficult to observe, he must think the Universe is very young. I think there was mention that some planetary nebula gases are excited enough by a "very cool white dwarf" (they have a wide range of temperatures) that can't be observed, hence its a "black" dwarf. In essence some nebulas outlive (or at least cease to exist at the same time of) the white dwarf. Her explanation was that it is the heat from the white dwarf that illuminates the nebula. As Dan has pointed out amateurs commonly have the need equipment to see white dwarfs in nebula, but to see difficult companion stars not so much.