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Kebsis
2009-Jun-06, 06:03 PM
Hi everyone.

If a white dwarf were to be drowned in a sizable enough cloud of gas, would it be possible for it to gather enough mass that it reignites into a viable, sun-like star again? Perhaps because of a nearby supernovae, or maybe two colliding galaxies could result in this happening?

Thanks.

Siguy
2009-Jun-06, 06:08 PM
White dwarfs and sunlike stars are very different in nature. A white dwarf is extremely hot and dense, if you added mass to it it would not become any cooler or less dense, and I'm pretty sure it would collapse into a neutron star eventually.

antoniseb
2009-Jun-06, 06:48 PM
Going a step beyond Siguy's message, something like this is the model for how type 1a supernovae happen. The only difference is that in a typical sn1a, the matter accumulates from another star in close orbit with the white dwarf. The matter does not just pile on from a nebula. (AFAIK) Type 1a end up exploding completely and don't leave a neutron star behind.

Kebsis
2009-Jun-06, 09:28 PM
That's interesting. What causes the explosion, does fission occur?

Also, would it be possible for a brown dwarf to become a sun-like star in the way I outlined in my original post?

robross
2009-Jun-06, 10:29 PM
That's interesting. What causes the explosion, does fission occur?

Also, would it be possible for a brown dwarf to become a sun-like star in the way I outlined in my original post?

The answer to your second question is yes. For example, say we have a binary system of two low mass stars, one of which is a brown dwarf. Gas from the outer atmosphere of the higher mass star could be drawn off by the brown dwarf and fall onto it. When enough mass has accumulated, the brown dwarf would begin to undergo fusion and become a normal star.

I'm not an expert, but to your first question, what happens is that matter accretes in orbit around the white dwarf. It does not actually "land" on its surface. It orbits like matter around a black hole. Eventually, so much matter is in orbit around the white dwarf that the gravity forces it to collapse to a density sufficient to trigger nuclear fusion. The rotating *disk* actually starts burning like a star. At this point, the system is unstable (because it's not a sphere) and it explodes, casting off all the accumulated material.

Rob

Hornblower
2009-Jun-06, 11:02 PM
That's interesting. What causes the explosion, does fission occur?
No, it would be fusion, like a gargantuan thermonuclear bomb.

loglo
2009-Jun-07, 04:22 AM
That's interesting. What causes the explosion, does fission occur?

Also, would it be possible for a brown dwarf to become a sun-like star in the way I outlined in my original post?

It explodes because the electron degeneracy pressure is exceeded in the core as the mass of the WD approaches the Chandrasekhar limit (http://en.wikipedia.org/wiki/Chandrasekhar_limit). The internal temperature goes up and .... something happens. I have never seen a really good explanation of what happens next though. Anyone?

astromark
2009-Jun-07, 09:10 AM
Sudden and immediate violent expansion. When the energy re least exceeds the gravity force... A Nova event. or have I missed your question ?

antoniseb
2009-Jun-07, 09:15 AM
... I'm not an expert, but to your first question, what happens is that matter accretes in orbit around the white dwarf. ...

I have never heard this before. Do you have a reference?

korjik
2009-Jun-07, 09:16 AM
It explodes because the electron degeneracy pressure is exceeded in the core as the mass of the WD approaches the Chandrasekhar limit (http://en.wikipedia.org/wiki/Chandrasekhar_limit). The internal temperature goes up and .... something happens. I have never seen a really good explanation of what happens next though. Anyone?

Actually, runaway carbon fusion occurs just before the Chandresekar limit is hit.

korjik
2009-Jun-07, 09:20 AM
I have never heard this before. Do you have a reference?

Any mass transfer to a compact object should create some form of accretion disk. Robross is incorrect about no matter reaching the star tho. Friction in the disk causes some material to eventually hit the star. Other than that, he has described one form of recurring novae.

I think I got all that out of my astrophysics text, but I am not around it right now to check.

absael
2009-Jun-07, 05:40 PM
It explodes because the electron degeneracy pressure is exceeded in the core as the mass of the WD approaches the Chandrasekhar limit (http://en.wikipedia.org/wiki/Chandrasekhar_limit). The internal temperature goes up and .... something happens. I have never seen a really good explanation of what happens next though. Anyone?

When the mass approaches the Chandrasekhar limit, carbon fusion begins. The entire white dwarf, being composed mostly of carbon, explodes in a supernova.

[Edit: If I had finished reading the thread before replying, I would have seen that korjik already answered your question.]

chornedsnorkack
2009-Jun-07, 09:38 PM
Any mass transfer to a compact object should create some form of accretion disk

No. Only mass transfer that carries angular momentum.

Then again, precisely zero angular momentum does not have a very large probability. But still some probability, though, if the symmetry of accreting material prevents angular momentum and thus accretion disc.


White dwarfs and sunlike stars are very different in nature. A white dwarf is extremely hot and dense,
Dense yes. Hot, not necessarily. Hot white dwarfs are brighter and easier to see, but cold white dwarfs exist, because they have no heat source. Some are orange, colder than Sun.

if you added mass to it it would not become any cooler or less dense,

White dwarfs consist of metal. Which is high intrinsic density material. Adding hydrogen would decrease the intrinsic density, unless the hydrogen fuses.

How thick could a layer of protium on a white dwarf surface get before it can sustain fusion? The heat from the fall into the white dwarf can be radiated away...

Kebsis
2009-Jun-07, 10:47 PM
Dense yes. Hot, not necessarily. Hot white dwarfs are brighter and easier to see, but cold white dwarfs exist, because they have no heat source. Some are orange, colder than Sun.




Are there any WDs that have cooled completely, to the point of being just black carbon balls? I understand it would probably be difficult to detect such an object, but has the universe been around long enough for that to have happened?

Hornblower
2009-Jun-07, 10:53 PM
Are there any WDs that have cooled completely, to the point of being just black carbon balls? I understand it would probably be difficult to detect such an object, but has the universe been around long enough for that to have happened?

Probably not. The cooling rate is excruciatingly slow, and many billions of years would be required for them to cool off enough to go dark.

Cougar
2009-Jun-07, 11:06 PM
I'm not an expert, but to your first question... Eventually, so much matter is in orbit around the white dwarf that the gravity forces it to collapse to a density sufficient to trigger nuclear fusion. The rotating *disk* actually starts burning like a star. At this point, the system is unstable (because it's not a sphere) and it explodes, casting off all the accumulated material.

This part doesn't sound right either. The accretion disk and localized fusion are sideshows. As loglo explained, "It explodes because the electron degeneracy pressure is exceeded in the core as the mass of the WD approaches the Chandrasekhar limit." That is, the infalling mass accumulates. When the white dwarf exceeds 1.44 solar masses, the force of nature that was holding back the white dwarf from imploding is also overcome, and a complex series of events takes place, the observation of which we call a supernova Ia. I believe there are some fairly sophisticated models of that "complex series of events."

robross
2009-Jun-08, 12:10 AM
I have never heard this before. Do you have a reference?

I was mistaken in my terminology. I was describing a nova event, not a supernova, in which the star is always destroyed. But white dwarfs can accrete enough mass near their surface to cause the hydrogen in the accretion disk to undergo fusion, which then explodes outward.

"When accretion of material does not push a white dwarf close to the Chandrasekhar limit, accreted hydrogen-rich material on the surface may still ignite in a thermonuclear explosion. Since the white dwarf's core remains intact, these surface explosions can be repeated as long as accretion continues. This weaker kind of repetitive cataclysmic phenomenon is called a (classical) nova."

http://en.wikipedia.org/wiki/White_dwarf
http://en.wikipedia.org/wiki/Nova

This is just one type of nova event. There are other mechanics that can cause the accreted matter to cause the white dwarf to explode completely, such as Type 1A supernovas.

http://en.wikipedia.org/wiki/Type_Ia_supernova

Rob

robross
2009-Jun-08, 12:14 AM
Are there any WDs that have cooled completely, to the point of being just black carbon balls? I understand it would probably be difficult to detect such an object, but has the universe been around long enough for that to have happened?

There's an Astrony Cast episode in which they discuss the long term fate of our own Sun. It will eventually become a white dwarf, then cool to a black dwarf, and will be in essence a giant diamond in space!!

Of course it will take hundreds of billions, or even trillions, of years for it to cool down. So we don't expect to find any black dwarfs in the universe at this point in time.

Rob

Cougar
2009-Jun-08, 12:51 AM
...will be in essence a giant diamond in space!!


I wonder if that would be big enough for my wife. :rolleyes:

Tobin Dax
2009-Jun-08, 01:20 AM
I was mistaken in my terminology. I was describing a nova event, not a supernova, in which the star is always destroyed.
As the text you quoted describes, even a nova event which destroys a white dwarf is caused by fusion of hydrogen that has been accreted onto the surface. The material does "land" on the surface.

spseyres
2009-Jun-10, 09:07 AM
Quite a bit of confusion on this one - not all supernovae are the same, and in fact the original question may get closer to the truth than one might think from some of the answers.

There are Born Again Giants where a residual hydrogen layer on a star descending the white dwarf cooling track experiences a Very Late Thermal Pulse, returning it to the red giant phase in a few years (although in fact the internal structure is somewhat different to a first-time red giant and these stars are predicted to reheat in a few decades tops - stellar evolution while you wait). The most recently observed example is V4334 Sgr or Sakurai's object - but you won't be able to find it now even with a very large telescope (I know we have tried with the Southern African Large Telescope) because there is at at least 12 magnitudes of extinction due to a dust shroud. But radio observations show the star is probably starting to heat up as predicted (sorry if this goes beyond your desire for detail, but this is a star I have been studying for 13 years!) :)

The most recently posited candidate for a Supernova Ia, RS Ophiuchi, most likely does accrete matter direct from the nebula it resides in, instead of direct from the companion star via an accretion disk as is the case in more compact systems. (The nebula here is due to the dense wind from the companion which is a red giant.) But it probably still has an accretion disc because it is still necessary to get rid of the angular momentum ifthe gas is to settle on the white dwarf. This star has nova explosions at intervals of a few decades, but due to processes in the accreted material. If the white dwarf mass is increasing due to the accretion, then it will eventually exceed the Chandrasekhar limit and become a SN Ia. Most likely it wil leave behind a neutron star, and possbily the binary will not be disrupted (only seriously disturbed).

There are also a whole slew of white dwarf systems that accrete matter from their companions and undergo so-called dwarf nova and classical nova eruptions. The second of these are particularly interesting as they are generally first spotted by amateur astronomers, and I'd encoruage people with the time and interest to get in to nova searching - Sakurai's object was first thought to be a nova and is named (informally) after the discoverer. Some are even najed eye visible for a few days, although the last one of those that I studied was resolutely hidden by cloud for 2 weeks! Actually V838 Mon (another of "my" stars and the one in my profile picture) was also first found in this way.

spseyres
2009-Jun-10, 09:37 AM
As the text you quoted describes, even a nova event which destroys a white dwarf is caused by fusion of hydrogen that has been accreted onto the surface. The material does "land" on the surface.


But the supernova explosion which destroys such a system is not caused by ignition in the surface material, but inside the white dwarf once the total mass is high enough. Less energetic classical and recurrent nova explosions occur in the surface material, and to be honest it is not definitive that these things become supernovae.