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How long will the sun remain a white dwarf?
Hi:
When the sun becomes a white dwarf, approximately how long will it take for it to cool off to a cold black dwarf that does not emit anymore heat or light [or other energies] than the surrounding space? Thanks, Radium |
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How long will the sun remain a white dwarf?
On Fri, 13 Jul 2007 19:45:36 -0000, Radium wrote:
Hi: When the sun becomes a white dwarf, approximately how long will it take for it to cool off to a cold black dwarf that does not emit anymore heat or light [or other energies] than the surrounding space? That would take forever. But a few tens of billions of years should be long enough that the temperature drops low enough that no photons in the visible range are produced, which is generally what defines a black dwarf. _________________________________________________ Chris L Peterson Cloudbait Observatory http://www.cloudbait.com |
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How long will the sun remain a white dwarf?
On Jul 13, 1:16 pm, Chris L Peterson wrote:
That would take forever. But a few tens of billions of years should be long enough that the temperature drops low enough that no photons in the visible range are produced, which is generally what defines a black dwarf. Approximately how long until this white drawf [which used to be the sun] cools to around 70 degrees Fahrenheit? Will the universe exist long enough for this to happen? |
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How long will the sun remain a white dwarf?
On Fri, 13 Jul 2007 21:42:41 -0700, Radium wrote:
Approximately how long until this white drawf [which used to be the sun] cools to around 70 degrees Fahrenheit? Will the universe exist long enough for this to happen? The temperature falls off somewhat exponentially... looks like it would take hundreds of billions of years to get that cool, maybe trillions. But it has all the time in the world (or the Universe), since it seems likely the Universe will not ever cease to exist, but will just keep cooling down and expanding forever. _________________________________________________ Chris L Peterson Cloudbait Observatory http://www.cloudbait.com |
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How long will the sun remain a white dwarf?
On Fri, 13 Jul 2007 19:45:36 -0000, Radium
wrote: When the sun becomes a white dwarf, approximately how long will it take for it to cool off to a cold black dwarf that does not emit anymore heat or light [or other energies] than the surrounding space? It takes a solar type star about 100 billion years, which is more than the presently estimated age of the universe. Bud -- The night is just the shadow of the Earth. |
#6
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How long will the sun remain a white dwarf?
In article .com,
Radium wrote: When the sun becomes a white dwarf, approximately how long will it take for it to cool off to a cold black dwarf that does not emit anymore heat or light [or other energies] than the surrounding space? SInce the cooloff is pretty much exponential, it will take forever and the rediated energy will never drop to exactly zero. SO you should rephrase your question as: # When the sun becomes a white dwarf, approximately how long will it # take for it to cool off to a cold black dwarf that does not emit # any significant anounts of heat or light [or other energies] than # the surrounding space? That time span will depend a lot on your precise definition of "significant". But it will probably take hundreds of billion years, or more. Let me give you one concrete example: suppose the outside temperature is exactly freezing, i.e. 0 deg C. You boil a pot of water, then bring it outside. How long time will it take until the pot has cooled down so there's no temperature difference between the pot of water and the surroundings? At the start, the temperature difference is 100 deg C of course. Let's say after one hour the temperature difference is 10 degrees. After two hours it's 1 degree, after three hours 0.1 degrees, after four hours 1E-2 degrees. After a day+night the temperature difference is 1E-22 degrees, after a week 1E-166 degrees, after a year 1E-8764 degrees, and after 15 billion years 1E-131490000000000 degrees. The temperature difference will never ever go all the way to zero! Of course a temperature difference of 1E-6 degrees or smaller is indistinguishable from zero - we call such a difference insignificant. But where do you draw the line between "significant" and "insignificant" here? In the example above, do you consider the temperature of the pot of water to be "the same" as the surroundings when the temperature difference drops below one degree? If so, it will happen after two hours. Or do you require a temperature difference of less than 0.01 degrees to consider the temperatures "the same"? If so, the cooldown will require four hours. The situation is similar with a white dwarf: no internal fusion gives any energy anymore, so the white dwarf just cools down. OK, there's one internal energy source remaining: gravitational contraction: the white dwarf can contract somewhat under its own gravitation, and that will release some energy, keeping it warmer for some time(*). But still the white dwarf cools off more or less exponentially. Now, since the white dwarfs are faint stars, they don't radiate that much energy out into space, and that's one major reason they can "live" for so long. (*) Before astronomer's realized that the Sun's energy source was nuclear fusion, other sources of energy were suggested. If the Sun had been a huge coal fire, it would be able to shine only for a few centuries, maybe a millennium. A more efficient energy source was gravitational contraction, which would be able to keep the Sun hot for approximately a million years. Back then, geologists suggesting that the Earth is some 5 billion years old had an obvious difficulty: what energy source could have kept the Sun shining for those five billion years? The answer is of course nuclear fusion - but it wasn't until well into the 1900's that astronomers realized this. -- ---------------------------------------------------------------- Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN e-mail: pausch at stockholm dot bostream dot se WWW: http://stjarnhimlen.se/ |
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How long will the sun remain a white dwarf?
In article ,
Chris L Peterson wrote: On Fri, 13 Jul 2007 19:45:36 -0000, Radium wrote: Hi: When the sun becomes a white dwarf, approximately how long will it take for it to cool off to a cold black dwarf that does not emit anymore heat or light [or other energies] than the surrounding space? That would take forever. But a few tens of billions of years should be long enough that the temperature drops low enough that no photons in the visible range are produced, which is generally what defines a black dwarf. That would take forever too!!! According to the Planck blackbody radiation curve, any blackbody hotter than absolute zero radiates photons over all wavelengths. Yes, you and I radiate visible light, X-rays and even gamma rays, although at an extremely low rate - but still at a rate slightly above zero. And if a cool body would emit, say, one gamma ray photon over a billion years, is that "no gamma radiation" or not? So if you say "that would take forever" about cooling down until no more energy is radiated, you must say the same about radiation at visible wavelengths, or any other wavelength rate. Only if you set some level near but above sero as "insignificant" you can assign a finite cooldown time span. Chris L Peterson Cloudbait Observatory http://www.cloudbait.com -- ---------------------------------------------------------------- Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN e-mail: pausch at stockholm dot bostream dot se WWW: http://stjarnhimlen.se/ |
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How long will the sun remain a white dwarf?
On Sat, 14 Jul 2007 09:42:19 GMT, (Paul Schlyter) wrote:
That would take forever too!!! That's true. I meant until the level of visible photons drops below detectability (which is, of course, a moving target). As you say, all that can be done is to set a threshold, or a specific target temperature. _________________________________________________ Chris L Peterson Cloudbait Observatory http://www.cloudbait.com |
#9
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How long will the sun remain a white dwarf?
Don'tforget about fluctuation theory, sometimes known as Statistical
Mechanics. You will et a time (in that time range) where the probability is that once heated water is colder than the ice. On Jul 14, 5:42?am, (Paul Schlyter) wrote: In article .com, Radium wrote: When the sun becomes a white dwarf, approximately how long will it take for it to cool off to a cold black dwarf that does not emit anymore heat or light [or other energies] than the surrounding space? SInce the cooloff is pretty much exponential, it will take forever and the rediated energy will never drop to exactly zero. SO you should rephrase your question as: # When the sun becomes a white dwarf, approximately how long will it # take for it to cool off to a cold black dwarf that does not emit # any significant anounts of heat or light [or other energies] than # the surrounding space? That time span will depend a lot on your precise definition of "significant". But it will probably take hundreds of billion years, or more. Let me give you one concrete example: suppose the outside temperature is exactly freezing, i.e. 0 deg C. You boil a pot of water, then bring it outside. How long time will it take until the pot has cooled down so there's no temperature difference between the pot of water and the surroundings? At the start, the temperature difference is 100 deg C of course. Let's say after one hour the temperature difference is 10 degrees. After two hours it's 1 degree, after three hours 0.1 degrees, after four hours 1E-2 degrees. After a day+night the temperature difference is 1E-22 degrees, after a week 1E-166 degrees, after a year 1E-8764 degrees, and after 15 billion years 1E-131490000000000 degrees. The temperature difference will never ever go all the way to zero! Of course a temperature difference of 1E-6 degrees or smaller is indistinguishable from zero - we call such a difference insignificant. But where do you draw the line between "significant" and "insignificant" here? In the example above, do you consider the temperature of the pot of water to be "the same" as the surroundings when the temperature difference drops below one degree? If so, it will happen after two hours. Or do you require a temperature difference of less than 0.01 degrees to consider the temperatures "the same"? If so, the cooldown will require four hours. The situation is similar with a white dwarf: no internal fusion gives any energy anymore, so the white dwarf just cools down. OK, there's one internal energy source remaining: gravitational contraction: the white dwarf can contract somewhat under its own gravitation, and that will release some energy, keeping it warmer for some time(*). But still the white dwarf cools off more or less exponentially. Now, since the white dwarfs are faint stars, they don't radiate that much energy out into space, and that's one major reason they can "live" for so long. (*) Before astronomer's realized that the Sun's energy source was nuclear fusion, other sources of energy were suggested. If the Sun had been a huge coal fire, it would be able to shine only for a few centuries, maybe a millennium. A more efficient energy source was gravitational contraction, which would be able to keep the Sun hot for approximately a million years. Back then, geologists suggesting that the Earth is some 5 billion years old had an obvious difficulty: what energy source could have kept the Sun shining for those five billion years? The answer is of course nuclear fusion - but it wasn't until well into the 1900's that astronomers realized this. -- ---------------------------------------------------------------- Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN e-mail: pausch at stockholm dot bostream dot se WWW: http://stjarnhimlen.se/ |
#10
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How long will the sun remain a white dwarf?
"Chris L Peterson" wrote in message ... On Sat, 14 Jul 2007 09:42:19 GMT, (Paul Schlyter) wrote: That would take forever too!!! That's true. I meant until the level of visible photons drops below detectability (which is, of course, a moving target). As you say, all that can be done is to set a threshold, or a specific target temperature. I think the OP asked about 70F with the idea of having some humans around. His question is really whether there could be habitable planets around at a time when the galaxies were evaporating rather than how long it would take for a white dwarf to cool to 290K. One can imagine a time when gathering hydrogen to keep their brown dwarf running was a civilisation's main occupation, but I guess that's another story. George |
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