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. You also mention that he is talking about white dwarfs from cataclysmic double stars. Perhaps in the case of planetary nebula (I believe most of these are not from cataclysmic doubles), that there is nothing to see until the "life" of White Dwarf is turned on, to illuminate the gases expelled from the red giant. Simply the intermediate period is before the nebula is visible. Again, I imagine most white dwarfs are not the product of double star systems gone wrong. Erik --- chuck.hards@gmail.com wrote: From: Chuck Hards <chuck.hards@gmail.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Subject: Re: [Utah-astronomy] White dwarfs and YOU Date: Sun, 25 Jan 2009 19:17:50 -0700 Yes, the pressure remains constant, independant of temperature, as does gravity, which is dependent on mass; the dwarf stays the same size as it cools. As to why the star continues to shine post-fission, here is Bishop's explanation: "Because of their immense thermal energy and small surface area, white dwarfs cool extremely slowly. The universe is not yet old enough for any white dwarf to have cooled sufficiently to become a "black dwarf"." He continues, "Also, white dwarfs are intrinsically very faint; thus only those close to the solar system can be seen. *Only one white dwarf is easily observable with a small telescope*." This supports the idea that for distant white dwarfs that are part of cataclysmic binary systems, it's the infalling matter that is seen and not the dwarf itself. But, like Kim I'm curious as to the details of the intermediate stage between the formation of the planetary nebula (the asymptotic giant star throwing-off its outer atmosphere), and true "white dwarf" status, after the nebula has faded. It could be that fusion doesn't turn off quickly, but slowly peters-out, in fits and starts, during the same time-frame as the lifespan of the nebula itself. I wonder... All the argument about the definition of "easily observable" and "small telescope" is superfluous and unnecessary, especially when we begin bringing things like averted vision into the argument. On Sun, Jan 25, 2009 at 6:14 PM, <zaurak@digis.net> wrote:
Kim, I am quoting from the article I posted earlier.
"Under the extreme conditions which prevail within a white dwarf, the laws of quantum mechanics become important. Quantum mechanics is the study of how subatomic particles (such as electrons, protons, and neutrons) behave. Subatomic particles do not always obey the same laws as large objects. Hence, the laws of quantum mechanics sometimes seem contrary to common sense.
One rule of quantum mechanics (known as the Pauli exclusion principle) is this: Two identical electrons, located in the same region of space, cannot have the same velocity. In a dense white dwarf, where the electrons are packed close to each other, some of the electrons are forced to have high velocities, and hence provide a high pressure. In a degenerate object such as a white dwarf, the fast-moving high-energy electrons provide a pressure which is independent of temperature. Even as the temperature of a white dwarf falls toward absolute zero, the Pauli exclusion principle demands that the high-velocity electrons keep moving at the same speed. Hence, the pressure exerted by the electrons remains constant as the temperature falls."
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