Black dwarf art project.
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  chaos

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Posted: 2005-August-29 at 2:02pm | IP Logged Quote chaos

Some more info, From Now Until the End of Time... - gives the timescale for the Sun becoming a black dwarf as 35 billion years, and the shutdown of star formation and the death of the last red dwarfs at 10-100 trillion years.
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  JanL

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Posted: 2005-August-29 at 6:17pm | IP Logged Quote JanL

Chaos, that website has some great theories, thanks for digging out the information.  Jan
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  nkalanaga

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Posted: 2005-August-30 at 12:48am | IP Logged Quote nkalanaga

One possible problem from the artistic side.  Unless it is orbiting a surviving red dwarf, would a black dwarf be visible?  By definition it produces no light, and if the nearest star was several light years away it would be very hard to see.  As a binary it would likely still need to be at least an AU away from the companion, or mass transfer during the red giant phase would probably have increased the companion's mass and shortened its life.  I assume that a black dwarf isn't intrinsically black, and would reflect light, but how much would there be to reflect?

It seems that most of them could be depicted as a black circle obscuring the distant stars, with no other details needed.  Or am I missing something somewhere?

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  chaos

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Posted: 2005-August-30 at 7:21am | IP Logged Quote chaos

Anyone know enough general relativity to figure out whether you'd get noticeable warping around the edge of the white/black dwarf because of gravitational lensing?

-----

Further stuff on the timescale for cooling (this stuff is interesting, honest!) - I've seen estimates ranging from 10 billion years to 10 terayears (10 trillion years). Most sources agree that the time is greater than the current age of the universe.

This kind of range makes me feel slightly uneasy, so I'll do a fairly basic calculation which should give a ballpark estimate:

If we take a starting temperature of 100,000 K, an age for the oldest white dwarfs of 10 billion years, and the temperature of the oldest white dwarfs as 3,500 K, and assume an exponential cooling model, I get a time to cool to background (taken to be 5 K) as about 30 billion years, which fits the estimate I gave in my previous post.

Right, I think I've bored you to death enough with all this. Enough with the physics, this is an art site!


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  JanL

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Posted: 2005-August-30 at 1:08pm | IP Logged Quote JanL

Heck, you are not boring-just one thing some calcuations indicate, the formation of a black dwarf with temperatures close to absolute zero would be in the neighborhood of 100 billion years.  I too would assume an exponential cool rate.  What formula are you using?

Now for some one to answer, what would the composition of a black dwarf be.  I'm thinking carbon perhaps crystalized carbon--diamond maybe--any thoughts.  Jan

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  chaos

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Posted: 2005-August-30 at 2:36pm | IP Logged Quote chaos

JanL: formula is very simple: T(t)=T(0)*exp(-kt), where T(t) means the temperature at time t, and k is a rate constant. Plug in the values for the oldest known white dwarfs and the initial temperature to get k, then solve for t when T(t)=5 K

This article might be useful for the composition of a white/black dwarf - apparently it would be a greenish blue diamond (from oxygen impurities), covered in oxygen snow (solid oxygen is blue IIRC). Extrapolating from observed properties of white dwarfs, which have been observed to have hydrogen or helium atmospheres, there may be some hydrogen frost (white?) on top of the oxygen snow, and perhaps an atmosphere of helium.

If the black dwarf is really cold, you might even get lakes of liquid helium on the surface... now THAT would be interesting, although if you've got a nearby star lighting the black dwarf up, it may not be cold enough. Even the universe's ambient background temperature might be slightly too high for this to happen. Then again, what with the insanely high gravity on the surface of one of these things, you never know.

EDIT

Another thought - the gravity on the black dwarf is so high that the topology will be practically nonexistant. This thing is going to be incredibly smooth. This offers up interesting possibilities, such as the star being covered in a superfluid "ocean" of helium at most millimetres thick (deeper than that the pressure might force the helium into a solid form)... this object is getting stranger by the minute...


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  Asmodeus

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Posted: 2005-August-30 at 4:35pm | IP Logged Quote Asmodeus

one bit of info unless osme one has allready said. but a brown dwarf isn't a type of star it's a super gas giant, but doesn't hold enough mass to go fusion. you've probably get it confused with soemthing else.


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  Asmodeus

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Posted: 2005-August-30 at 4:38pm | IP Logged Quote Asmodeus

if you would like to find it's temperature you could use the black body equation where the temp is just a bit higher than the background temp of the universe. also there is a huge superweight diamond in a white dwarf i think it was.


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  chaos

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Posted: 2005-August-30 at 5:39pm | IP Logged Quote chaos

Asmodeus wrote:
one bit of info unless osme one has allready said. but a brown dwarf isn't a type of star it's a super gas giant, but doesn't hold enough mass to go fusion. you've probably get it confused with soemthing else.


Uh oh... the brown dwarf definition debate rears its ugly head...

What I think most people agree on about brown dwarfs is that they do not undergo protium (hydrogen-1) fusion - the process that powers main sequence stars. However a brown dwarf above ~13 Jupiter masses can fuse deuterium in its core, and a brown dwarf above ~65 Jupiter masses can fuse lithium. Also, most people would say that a brown dwarf is not a planet.

The original definition of the term brown dwarf was theoretical, as these objects were predicted before they were found, and states that a brown dwarf forms like a star from the collapse of a cloud of gas, as opposed to a planet which builds up by accretion. Thus at the low end of the mass range a brown dwarf could have a mass similar to that of a large planet. A brown dwarf is therefore a star which never joins the main sequence.

However when extrasolar planets and brown dwarfs were discovered, it became obvious that working out whether an object had formed by accretion or gas collapse is extremely difficult. There have also been suggestions that Jupiter may have formed by the gas collapse mechanism, perhaps making it a brown dwarf.

An alternative definition is that brown dwarfs must undergo fusion reactions at some point in their evolution, but not protium fusion, as opposed to a planet, which never undergoes fusion, or a star, which undergoes protium fusion. This sets a division in the mass range at about 13 Jupiter masses, below which an object is a planet, and above which an object is a brown dwarf. In this system brown dwarfs may or may not be stars - some low mass brown dwarfs may form (at least in part) by accretion, whereas high mass brown dwarfs form by gas collapse, like stars.

Usually the issue is dodged by simply referring to the objects as "brown dwarfs", which may or may not be taken to be a shortened version of "brown dwarf stars" depending on the position you take.
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  nkalanaga

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Posted: 2005-August-31 at 1:10am | IP Logged Quote nkalanaga

Sounds like they should be fairly reflective, so if there is a light source they would be visible.

As for relativistic effects, we can detect gravitational warping by the Sun during total eclipses, although the effect is to small to see with the naked eye.  This displacement was one of the early tests of relativity.  Planet hunters have detected lensing effects by solar-mass stars, so a white dwarf would be detectable at great distances that way, although not visible itself.  As for visible effects, for a black dwarf the size of Earth, you would have to be less than a million miles from it for it to appear as big as the Moon or Sun from Earth.  At that distance, with its surface gravity, against a bright background, I would think that some distortion, probably a halo effect, would be visible.  Whether there would be enough stars that far in the future for it to be noticed is another question.  The surviving red dwarfs would be most visible in infrared, so that may be the place to start.

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