Talk:Black dwarf

Black body radiation

This entry used to say: "only emits black-body radiation". But black body radiation can be bright white; the "black-body" refers to the fact that the body doesn't reflect light--all the light it emits it generates itself. A white dwarf is as much (or as little) of a black body as a black dwarf. -- — Preceding unsigned comment added by 24.6.67.123 (talk • contribs)

Black dwarfs in other universes

Removed "It is possible, though, that some black dwarfs currently exist in other universes that are old enough to have them, if they exist." Pretty much anything is possible when you invoke other universes, so this doesn't add anything to the reader's understanding of black dwarfs. -- JustSayin 18:13, 7 April 2006 (UTC)

"Other Universe"?O.o —The preceding unsigned comment was added by 68.237.238.121 (talk • contribs) on 02:29, 1 May 2006.

Depending on what assumptions you make about how cosmic inflation works, or how m theory works, you can get laws of physics that allow "other universes" to exist (universe-like spaces that are not in causal contact with our own). Under some situations, these may have existed for far longer than our own universe, raising the possibility of burned-out stars cooling to become black dwarfs (which hasn't yet occurred in our universe). As was pointed out above, you can postulate just about any scenario you like existing in other universes, so it isn't usually a terribly useful exercise. --Christopher Thomas 22:52, 2 May 2006 (UTC)

Colonizing a black dwarf

Something I've been vaguely curious about for a while: whenever the time comes that we do get black dwarfs, would it be "possible" to land future astronauts on them? What would the ground be like? Could we actually colonize these... dead stars as planets? —The preceding unsigned comment was added by 72.60.109.138 (talk • contribs) on 00:30, 11 June 2006.

A black dwarf would be a ball of mostly carbon, nitrogen, and oxygen. If I understand correctly, most of the nuclei would be in a spherical close-pack crystalline lattice (most space-efficient possible), with the degenerate electron gas permeating through the lattice. This would form a metallic core of the star. Close to the surface, you still get a close-packed lattice, but some of the electrons are bound to individual nuclei. Very close to the surface, you get something resembling normal matter (carbon, nitrogen, and oxygen compressed enough to become metallic and share valence electrons, but not compressed enough to lose core electrons). Above this would be a thin layer of carbon nitride, diamond, and other compounds with more normal crystal structures. The atmosphere would be hydrogen and helium with trace amounts of water, methane, and ammonia.

Colonizing a world like this would be difficult, mostly due to the extremely strong surface gravity. A Newtonian approximation gives something like 200,000 times Earth's gravity, but the actual result will be very different. The gravity well is deep enough to require General Relativity for a full description; Newtonian gravity gives an escape velocity greater than the speed of light. Humans would be instantly crushed, and any kind of structure would have to be very small (less than a millimetre high gives the kinds of stresses found in the tallest existing buildings). A more viable approach with today's technology would be to find a white dwarf that hadn't quite cooled to the black dwarf stage, and build a dyson swarm of habitats around it.

It would make a nifty science fiction story, though. See Dragon's Egg for a similar story about a neutron star. --Christopher Thomas 19:42, 11 June 2006 (UTC)

Thanks a lot, Mr. Thomas. You've answered a question I've wondered about since I was a young lad, nose deep in outdated astronomy books. :) Nerva 16:34, 16 June 2006 (UTC)

Just caught a mistake I'd made: The black dwarf would be almost completely carbon. The CNO cycle transmutes existing carbon into nitrogen and oxygen (and then carbon again), but that would only cycle carbon that was present when the star formed, during hydrogen burning. Most of the white dwarf's carbon would be formed later via the triple-alpha process during helium burning. --Christopher Thomas 16:25, 27 June 2006 (UTC)

drunk scientest = black dwarf

ok who ever came up with the blacck dwarf idea must have been drunk or something because seriouly who comes up with such a dumb idea i mean seriously think about it "it takes longer than our universe to form" ok right sir drunkscientest what gives this any proof and if there are such things and they exist now (we dont know that yet so dont give me the takes longer than our universe thing)how the heck would we find them they give off no radiation because from what i know white dwarfs just go to a little ash in the universe or what so if you can give me any thing that gives this theory any solidity that makes it actually seem possile please inform me but from reading this i think the dude who came up with this was Drunk --209.159.197.82 (talk) 23:37, 13 January 2008 (UTC)

We know white dwarf stars exist. White dwarfs do not "turn into ash", they just cool, becoming dimmer and dimmer. At some point, they become so cool and dim that they are categorized as black dwarfs. This would take an exceedingly long period of time, and the universe is not old enough for this to have happened yet. It would be difficult, but not impossible to detect them, however, as none are expected to exist now, I doubt there is a lot of effort spent in looking for them. --RLent (talk) 18:40, 18 January 2008 (UTC)

drunk scientest= black dwarf

ok so from reading this article i think the guy who came up with the black dwarf theory was seriously drunk what give us any evidence that this is possible —Preceding unsigned comment added by 209.159.197.82 (talk) 23:38, 13 January 2008 (UTC)

When does a white dwarf turn into a black dwarf?

I'd really like to know. The article isn't clear on this. Is a white dwarf a black dwarf when it has become so cool that only radiates infrared light? Or is it truly black when not even infrared telescopes [theoretically] can detect it? --Harald Khan Ճ 17:20, 7 May 2008 (UTC)

There probably isn't a strict definition, because I don't think there is some sharp change in the nature of the star as it cools to low temperatures. The name "black dwarf" seems to just be an acknowledgment that eventually white dwarfs, which are quite hot and bright (for their size), will become cool and dim. I talk to a lot of astronomers and have never heard one use this term. They would probably just say something like "cold white dwarf". 146.139.199.27 (talk) 20:58, 5 June 2008 (UTC)

Albedo

What would the albedo of a black dwarf be like? Would it be a perfect mirror surface or would the skin of compressed normal matter described dull the finish? Wnt (talk) 06:07, 26 June 2008 (UTC)

Colliding black dwarfs

If two black dwarfs collide, does that reignite carbon burning and abruptly create a gigantic blue star apparently from nowhere which lasts perhaps a thousand years? Strange to imagine a late universe of darkness where stars sparkle like fireflies in the night. Wnt (talk) 06:07, 26 June 2008 (UTC)

It depends on the mass. If two CO black dwarfs of sufficient mass collide, a carbon star or a type IA supernova could be created. Another possibility is that colliding helium black dwarfs could create a helium star. See [1], §IIIC. Spacepotato (talk) 06:41, 26 June 2008 (UTC)

Dark matter?

Assuming black dwarfs are invisible to our standard means of detection (visible light, X-Ray, infrared, and radio telescopes), could they be the "missing" dark matter postulated by cosmological math? If not, why not? --BlueNight (talk) 05:00, 25 June 2009 (UTC)

There are two reasons why this is very unlikely. The first is that our universe isn't old enough for white dwarf stars to have cooled to become black dwarfs. The cooling process takes quite a while, even at the temperatures the dwarfs are at, due to low surface area. The second reason is that most dark matter isn't baryonic. The evidence for this is described at dark matter, but the short version is that the ratios of elements produced by the big bang nucleosynthesis is very sensitive to the amount and type of dark matter present. If the dark matter was "normal" matter, such as the protons, electrons, and neutrons found in white dwarf and black dwarf stars, the ratios of elements produced would be very different from what we observe. Instead, the production ratios are consistent with most matter in the universe being of some exotic type that doesn't interact much with normal matter (interacting by gravity and possibly the weak nuclear force, but not by electromagnetism or the strong nuclear force).

There have been proposals of some of the dark matter being baryonic (something like 4% of the universe's mass is baryonic, but only about 1% of this is visible as stars). The closest candidate to what you're proposing would be brown dwarf objects (intermediate between gas giants and stars).

I hope this answer is useful to you. --Christopher Thomas (talk) 05:13, 25 June 2009 (UTC)

"white dwarf stars haven't cooled enough to become black dwarfs"? Excuse me, but what is your definition of a black dwarf. White dwarfs emit little light to begin with; wouldn't you say that by the time they emit no visible or infrared light they are extremely hard if not impossible to detect, even with an array of radio telescopes? I would even put a sharp criterion to the formation of black dwarfs; by the time the white dwarf has a solid surface and no ions in the atmosphere it is a black dwarf, though still far warmer than the cosmic microwave background. Would it then look like a diamond? And could THAT form within the timeline of the universe?24.184.234.24 (talk) 22:14, 30 June 2010 (UTC)LeucineZipper —Preceding unsigned comment added by 24.184.234.24 (talk) 22:13, 30 June 2010 (UTC)

If the proton is not stable

Ouch. "If the proton is not stable, white dwarfs will also be kept warm by energy released from proton decay". Would someone care to clarify what the heck this sentance is supposed to be saying? WHAT proton is not stable? Great reading up to that point. 98.208.102.195 (talk) 06:34, 25 June 2009 (UTC)

BTW, came to this article via the main page, Today's Featured Article, linked from White Dwarf. 98.208.102.195 (talk) 06:55, 25 June 2009 (UTC)

This refers to the idea of proton decay. Protons are normally considered stable particles, but some models of particle physics predict that they are unstable, and will decay into lighter particles after a very, very long time (much longer than the current age of the universe). The paragraph you're quoting from is talking about sources of heat that would eventually stop a black dwarf from cooling. Proton decay, if it happens, is one such source (it'd generate a small amount of internal heat, much as Earth generates internal heat from radioactive decay). --Christopher Thomas (talk) 16:43, 25 June 2009 (UTC)

Current minimum temperature

What would the current minimum temperature of a black dwarf be? This should be calculable, given that the oldest ones have been cooling for 13bn years. Fig (talk) 00:04, 16 December 2009 (UTC)