# Are white dwarfs like(Sirius B) Cool or hot stars?

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#### remcot

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And how big are white dwarfs? Question 2 ,What happens with the climate and life on Earth when we have a white dwarf (like Sirius B) at the same distance like the moon is from the Earth (320.000) kilometers? Question 2 ,What happens when a white dwarf is at the place of our Sun? (the Sun removed of course) Wil it be warm or cold then? Will somebody explain this stuff? Big thanks. remcot

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#### mamakos

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I think that white dwarfs are generally remains of a star's core that are still glowing, and that brown dwarfs are either failed stars (never get the process of fusion going) or faded white dwarfs. I could be wrong, but that is my understanding.

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#### harmonicaman

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This Hertzsprung-Russell diagram shows the temperature changes our Sun will go through as it consumes all of it's fuel.

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#### doubletruncation

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<blockquote><font class="small">In reply to:</font><hr /><p>And how big are white dwarfs? <p><hr /></p></p></blockquote><br /><br />The radius of a white dwarf is about 1% the radius of the Sun, or similar to the radius of the Earth. White dwarfs are supported by electron degeneracy pressure rather than gas pressure, as a result their radii do not change very much depending on the mass - in fact they have the odd behavior where more massive white dwarfs have slightly smaller radii than less massive ones.<br /><br />As to whether the Earth would be warm or cold if a white dwarf were to replace it, note that the amount of energy per second that a white dwarf puts out drops over time. Since white dwarfs are not supported against gravity by their gas pressure (temperature), they will cool as they radiate away the energy that they had when they were stars, so an old white dwarf will be very much cooler than a young white dwarf. Just to give you some numbers, when the white dwarf is only a 1000 years old or so (still during the planetary nebula stage) the white dwarf can be quite a bit brighter than the Sun, but once the white dwarf is ~30 million years old (still very young) it will put out about 10% the energy per unit time that the Sun does (so the Earth at it's present location would probably be fairly cold. Also note that a good amount of that energy comes out in the ultraviolet rather than the visible). When the white dwarf is about a billion years old the amount of energy per second that it puts out is about 0.1% that of the Sun, and when it's about 10 billion years old it puts out 0.001% the energy per second that the Sun does. For the specific case of Sirius B, the star has about 0.3% the luminosity of the Sun. The surface temperature of Sirius B is still quite a bit hotter than the Sun (~10000 K rather than ~5600 K), but its much smaller size means that it puts out much less total energy per second than the Sun does. (Must have got my numbers wrong - looking at the HR diag <div class="Discussion_UserSignature"> </div>

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#### doubletruncation

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<blockquote><font class="small">In reply to:</font><hr /><p>I think that white dwarfs are generally remains of a star's core that are still glowing, and that brown dwarfs are either failed stars (never get the process of fusion going) or faded white dwarfs. I could be wrong, but that is my understanding.<p><hr /></p></p></blockquote><br /><br />That's right. The term brown dwarf usually refers to the failed stars rather than the faded white dwarfs. <div class="Discussion_UserSignature"> </div>

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#### remcot

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1 ,i have read some stories that Sirius B has 1 solarmass(1 times the mass of the Sun) is that true? 2 What if Sirius B is at the same distance from us as the Moon? Wil t be warm or cold then? 3 Is the radiation of a white dwarf dangerous when you are close to it or not? Can you answer these last questions please? Thank you.

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#### doubletruncation

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Sirius B is very nearly 1 solar mass. (see for example 1980, ARA&A, 18, 363 if you wanted the reference - the value quoted there is 1.053 +- 0.028 Msun). From http://www.solstation.com/stars/sirius2.htm Sirius B is 360 times fainter than the Sun. If you put it at the distance of the moon away from the Earth, you'd expect the flux that we receive from Sirius B to be:<br /><br />(1/360)*(1 AU / dmoon)^2 = (1/360)*400^2 ~ 400 times the flux received from the Sun. <br /><br />So we would receive about 400 times the energy per second per square meter from Sirius B at the distance of the moon than we currently receive from the Sun (in terms of flux received, that would be equivalent to us being about 20 times closer to the Sun than we are now). Also note that since the surface temperature of Sirius B is so much hotter than the surface temperature of the Sun, Sirius B would put out a lot more of its energy in the ultraviolet - so I imagine the radiation would more dangerous in that sense. <div class="Discussion_UserSignature"> </div>

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#### doubletruncation

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<blockquote><font class="small">In reply to:</font><hr /><p>Any planets in close orbit at the time the star began to expand into a red giant would be incinerated and absorbed into the star's atmosphere.<p><hr /></p></p></blockquote><br /><br />Good point, also note that the distance of the Earth to the moon is ~4x10^10 cm whereas the radius of the Sun is 7x10^10 cm - the planet would have had to have been orbiting inside the star's atmosphere during it's main sequence stage even to end up at that distance to the white dwarf. I think the only possibilities would be if either planets are somehow formed out of the ejecta of the stellar atmosphere (though the cometary knots in the planetary nebula that have been suggested, though I think doubtfully, as planet formation sites are way away from the white dwarf), or if the white dwarf had a close encounter with another system and came close enough to one of its planets to steal it. The latter situation might have a chance of occuring in a dense environment like the core of a globular cluster. <div class="Discussion_UserSignature"> </div>

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#### remcot

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Will we survive or die when Sirius B is as close to us as the Moon? Will it be hot then? And why is there no planetary nebulae around Sirius B? Can somebody explain these things? Thanks.

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#### qso1

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I don't think it can be known for certain but IMO, yes we would all die from extreme radiation and effects of the star on our atmosphere at such short distance. Sirius "B" probably never got the chance to form one or it was slowly blown off by Sirius "A". <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>

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#### nexium

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Extrapolating the earlier post of double: Earth would be about the same temperature as now if a 300 million year old white dwarf at moon distance, replaced our Sun. The white dwarf (yellow-orange) would appear about 3 times wider than our present moon and sun, but would be a bit less bright as the Sun in visable light. Earth would likely disintigrate from the tides = inside the roache limit for one solar mass. It is hard to imagine any combination of events that could impliment the switch. Neil

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#### Saiph

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well, a few comments here. First a white dwarf is very hot, thus the name. Second, the time required for a white dwarf to cool down is very, very long, like 10+ billions of years, so I don't know if <i>any</i> are cool enough to deserve a different designation. <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

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#### remcot

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I can remember that somebody have said that it is possible (when we have the technology of course) to go to the surface of a white dwarf like Sirius B. Is that really possible ,or not. The person have said that (i dont know who it was) that there will be no danger when you are descend through the surface of a white dwarf. Can somebody try to explain this (strange) story? Will it be possible? And is it dangerous or just not.

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#### qso1

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IMO, I don't think we know enough about white dwarfs to know we could descend to the surface without danger.<br /><br />With the right sort of technology, anything may be possible but so far into the future the question cannot be accurately answered without knowing more about the star and the technology to do what your suggesting. <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>

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#### doubletruncation

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Can humans survive 300000g?<br /> <div class="Discussion_UserSignature"> </div>

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#### qso1

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No.<br /><br />Imagine someone weighing 300,000 times their weight. For a 200 pounder, thats 60,000,000 pounds. Humans can withstand no more than 20 Gs for a very short period of time. <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>

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#### doubletruncation

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Then I think it's pretty clear that we wouldn't last long on the surface of a white dwarf. <div class="Discussion_UserSignature"> </div>

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#### qso1

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Thats for sure. <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>

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#### Saiph

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well, there is a solid surface to stand upon (or one dense enough you'd float really, really well). But no known technology would allow a person or craft to withstand the 25,000 K plus temperature, the tidal forces, or the intense gravity, let allone all three at once. <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

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#### nexium

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Using the numbers from double. There are likely some white dwarfs almost 10 billion years old that produce about 0.001 solar luminosity, likely yellow colored light, but the disk is 0.0001 of the area of our sun's disk. Earth at a distance of 92,000,000 miles would cool to near absolute zero. Only very advance beings could live on Earth without supplies from a warmer world.<br />A planet would need to be about a million kilometers from a ten billion year old white dwarf to have liquid water. Such a system might support even primitive colonists for several billion years when the dwarf would have cooled to orange instead of yellow. Neil

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#### alokmohan

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No doubt it is cool.

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#### alokmohan

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Cambridge, Ma - Astronomers at the Harvard-Smithsonian Center for Astrophysics (CfA) have found that a supernova discovered last year was caused by two colliding white dwarf stars. The white dwarfs were siblings orbiting each other. They slowly spiraled inward until they merged, touching off a titanic explosion. CfA observations show the strongest evidence yet of what was, until now, a purely theoretical mechanism for creating a supernova. <br /><br />"This finding shows that nature may be richer than we suspected<br />http://www.spaceref.com/news/viewpr.nl.html?pid=23933

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