Spin prevent collapse?

  • #1

Main Question or Discussion Point

If a star is spinning very fast before it collapses, then as it collapses it's going to spin faster, like a skater bringing his or her arms in to speed up rotation.

Intuitively I'd imagine that there's no way a star could ever be spun up to the point where its outer edge speed would exceed c.

So is it possible that a collapsing star could be prevented from collapsing, regardless of the gravitational forces trying to crush it, due to the fact that it was spinning too fast before the collapse began for collapse to continue beyond a certain point?
 

Answers and Replies

  • #2
I'm not totally sure (and correct me if i'm wrong), but i think that as the matter tightens, the tangential speed of the material (in other words the "outer speed") stays the same. However, since the radius decreases, the angular speed increases. That is why it look like they are going faster. So, the outer material would stay at exactly the same speed.

or am i totally off-base?
 
  • #3
xantox
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So is it possible that a collapsing star could be prevented from collapsing, regardless of the gravitational forces trying to crush it, due to the fact that it was spinning too fast before the collapse began for collapse to continue beyond a certain point?
Yes, it is possible that spin halts a full collapse until such star (termed 'fizzler') loses the excess angular momentum in some way (eg by electromagnetic coupling, emission of gravitational waves because of axisymmetry loss, etc). The same applies for collapse to a black hole, which must be stopped when the initial angular momentum exceeds the limit of GM²/c as defined by the extremal Kerr geometry.
 
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  • #4
pervect
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Yes, it is possible that spin halts a full collapse until such star (termed 'fizzler') loses the excess angular momentum in some way (eg by electromagnetic coupling, emission of gravitational waves because of axisymmetry loss, etc). The same applies for collapse to a black hole, which must be stopped when the initial angular momentum exceeds the limit of GM²/c as defined by the extremal Kerr geometry.
Do you have a reference for this usage of "fizzler"? I don't think I've ever seen the term before. (It's an interesting enough term that I might want to use it, but I wouldn't want to use it without seeing it in a reference first).

Also, while I believe that the cosmic censorship hypothesis is generally accepted, I think there is a small amount of dissent. For instance there are the numerical simulations in

http://adsabs.harvard.edu/abs/1991PhRvL..66..994S

which however have been criticized for assuming spherical symmetry.

I guess I should add that the cosmic censorship hypothesis states that no naked singularity will ever form - as xantox mentions in an earlier post, a black hole that spins "too fast" would become a naked singularity.
 
  • #5
xantox
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Do you have a reference for this usage of "fizzler"? I don't think I've ever seen the term before. (It's an interesting enough term that I might want to use it, but I wouldn't want to use it without seeing it in a reference first).
I believe the term was introduced in 1974 by Thomas Gold. One of the first uses was in Shapiro and Lightman in "Rapidly rotating, post-Newtonian neutron stars", The Astrophysical Journal, 207:263 (1976). For a recent reference, check J. Imamura, B. K. Pickett, R. H. Durisen, "The nonlinear evolution of massive stellar core collapses that 'fizzle'", The Astrophysical Journal, 587:341 (2003).
 
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  • #6
pervect
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OK, thanks! Fortunately for me, the Shapiro and Lightman reference is online.

http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?letter=.&classic=YES&bibcode=1976ApJ...207..263S&page=&type=SCREEN_VIEW&data_type=PDF_HIGH&send=GET&filetype=.pdf [Broken]

If I'm understanding the reference correctly, all "fizzlers" should be a source of gravitational radiation. Some fizzlers do collapse to black holes, others do not. The ones that do not collapse are basically a neutron star that is rotating so fast that it isn't stable in a spherical configuration. I'm not quite sure what configuration such a neutron star has (does it vibrate? does it form a stable ellipsoid?), but as long as it's not stable as a sphere, it has some other shape, which means it radiates gravitational radiation.
 
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  • #7
xantox
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I'm not quite sure what configuration such a neutron star has (does it vibrate? does it form a stable ellipsoid?), but as long as it's not stable as a sphere, it has some other shape, which means it radiates gravitational radiation.
For very high angular momentum, such partially collapsed star (less dense than a neutron star) should assume a flattened disk configuration, but nonlinear instability would evolve it into a barlike shape with spiral arms, and then settle into a stable approximate Dedekind ellipsoid (with a lower energy content than the axisymmetric flattened disk). Or it could also break up into two pieces. In both cases it would be indeed a strong gravitational radiator, and has been considered a possible target for GW experiments.
 
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