Causing a supernova

[Duane]

Ok, odd question. On Stargate SG1 there was a recent episode where they created a supernova in a solar-like star by suddenly removing mass from (I assume) the star's centre. My intial reaction was to guffaw, however when asked by a friend to explain why, I couldn't provide a satisfactory answer.

My initial thought was that removing the mass would only cause fusion in the star's centre to stop, it would then collapse under the weight of it's own gravity, then re-settle into a dwarf.

Ignoring all the reasons of why removing the mass would be difficult (if anyone is interested, I'll outline how they did it in the show) my question is: if one could suddenly remove a significant amount of mass from a solar-like star, is it possible it would explode as a supernova?

[trinitree88]

Ok, odd question. On Stargate SG1 there was a recent episode where they created a supernova in a solar-like star by suddenly removing mass from (I assume) the star's centre. My intial reaction was to guffaw, however when asked by a friend to explain why, I couldn't provide a satisfactory answer.

My initial thought was that removing the mass would only cause fusion in the star's centre to stop, in would collapse under the weight of it's own gravity, then re-settle into a dwarf.

Ignoring all the reasons of why removing the mass would be difficult (if anyone is interested, I'll outline how they did it in the show) my question is: if one could suddenly remove a significant amount of mass from a solar-like star, is it possible it would explode a a supernova?

Duane....Not likely. Two routes to supernova. White dwarf accumulates mass from a near companion....once past ~1.2-1.44 solar masses...it ignites. Core collapse supernovae are ~ 8-20 solar masses...far from a solar-like star. Critical to the white dwarf ignition is it's high density, and commensurate gravitational field compressing the accumulated gas. For fusion, the three big parameters are time, temperature, density....for confinement and ignition. Pete.

[Van Rijn]

Supernova, I don't know, but if you removed a good chunk of a star's core in an instant there would be an implosion, and I would expect a serious rebound (explosion). I doubt you could determine the end result without modelling it. Would the star settle down eventually back to a normal (but somewhat smaller) main sequence star? Maybe, but you wouldn't want to be around for a while.

On SG-1, it was a little more complex - they had dialed to a black hole to do the matter collection, and I was assuming that the gravitational effects were also involved (which still wouldn't make much sense, but gives them a little more magic to play with).

The silliest one for me was the episode where a small change to a star's core was instantly changing the star's spectrum. Not a chance.

[Tim Thompson]

If one could suddenly remove a significant amount of mass from a solar-like star, is it possible it would explode a a supernova?

I should think not.

The standard "big picture" for a core collapse supernova looks like this:

Core Collapse
The core of a massive star is made mostly of iron atoms, which are held in mechanical equilibrium, against the collapsing force of gravity, by the pressure of the electron clouds around the nuclei (the Pauli exclusion principle). That resistance to collapse works because the electrons are in bound orbitals around the nuclei. As the core increases in mass it also gets hotter. Once it is hot enough, the bath of thermal photons rips the iron nuclei apart, converting them into free alpha particles (helium-4 nuclei). The electrons formerly bound to the nuclei are now free electrons, and no longer are subject to the Pauli exclusion principle, so no longer produce a pressure to resist collapse. The unopposed force of gravity now forces the core to collapse. The process is sudden, and virtually instantaneous; the entire stellar core is destroyed in approximately 0.1 second.

Outer shell collapse
The outer layers of the star are held up by the dense core. When it vanishes under them, they too are suddenly subjected to the unopposed force of gravity. Following the collapse of the core, the outer layers of the star also fall, chasing the core into collapse. But it's a game of catch-up, the core is well ahead of the outer layers chasing it.

Neutrino emission
As the core collapses it emits an enormous number of neutrinos. The energy that comes out of a supernova, that we see as an optical flash, is probably no more than a few percent of the total energy released, the rest is released as neutrinos.

Neutrino energy deposit
The infalling outer layers of the star are so dense that there is a significant interaction between them and the neutrinos emitted by the collapsing core. A significant amount of energy is deposited into the infalling layers by the neutrinos.

Core rebound
The core collapses very fast, about 10 seconds is all that is required for it to reach maximum collapse. It collapses with so much inertia that it goes too far, collapses to too small a size to reach mechanical equilibrium. It rebounds rapidly outwards, seeking an equilibrium configuration.

Collision
As the core rebounds outward, it collides with the rapidly infalling outer layers. Both the core & infalling material are extremely dense, extremely hot, and moving extremely fast (a few percent of the speed of light). The result is an enormus transfer of energy & momentum to the infalling material, forming a supersonic shock wave, also fed by ongoing neutrino emission, which races outwards from the core.

Race to the surface
The surface of the star is so far from the action that it is as yet unware of the exciting events going on in its own core. The shock wave races outwards towards the surface. But, as it does so, it has to pass through successive layers of unsupported, infalling material. The shock wave loses energy heating the infalling material that passes through it.

Supernova explosion
If the shock wave is energetic enough, it will make it to the surface, where it will break out into an observable supernova explosion. If it is not energetic enough, it will be snuffed out by losing energy to the infalling layers that pass through it. In that case, there will be no supernova explosion, even if there was a core collapse. Until fairly recently, computer models of core collapse supernovae never produced supernova explosions, because the shock waves were invariably snuffed out. It is still the case that the vast majority of models do not explode, but there have now been a few successful models in that regard.

Now, with that narrative in mind, what happens if you suddenly "beam out" the core of any star, of any mass? The unsupported outer layers collapse, as the story above describes. But in this case, there is no neutrino emission, and no core rebound, both of which combine to be the driving force behind the supernova explosion. Therefore I think, in your scenario, that a supernova explosion would not happen for a star of any mass, let alone a star so light as a solar mass.

[DALeffler]

...but if you removed a good chunk of a star's core in an instant there would be an implosion...

Implosion? Why? The mass causing the outer shells of the star to accelerate inward is no longer there. And if the core is gone, there's no longer anything for the in-falling material to rebound against.

I'd think one would have to remove all or some of the layers above the densest part of the core already burning elements heavier than hydrogen, allowing the hydrogen in the outer shells to in-fall where pressures & temps fuse all the hydrogen at once - ba-BOOM!.

But if we do that, is there enough temp and pressure left to ignite all the hydrogen?

Maybe we shouldn't export anything out of the core: Import iron - lots and lots of it; all at once.

Anybody figure out them door frames in Atlantis when the ancients are all human form?

[trinitree88]

I should think not.

The standard "big picture" for a core collapse supernova looks like this:

Core Collapse
The core of a massive star is made mostly of iron atoms, which are held in mechanical equilibrium, against the collapsing force of gravity, by the pressure of the electron clouds around the nuclei (the Pauli exclusion principle). That resistance to collapse works because the electrons are in bound orbitals around the nuclei. As the core increases in mass it also gets hotter. Once it is hot enough, the bath of thermal photons rips the iron nuclei apart, converting them into free alpha particles (helium-4 nuclei). The electrons formerly bound to the nuclei are now free electrons, and no longer are subject to the Pauli exclusion principle, so no longer produce a pressure to resist collapse. The unopposed force of gravity now forces the core to collapse. The process is sudden, and virtually instantaneous; the entire stellar core is destroyed in approximately 0.1 second.

Tim. The core collapse model usually shows an accumulation of ~ a solar mass of iron. ~...2 times 10³⁰ kg. Density of iron ~ 7,874 kg/ m³. Radius of core ~ 4 times 10⁸meters. For that to collapse in 0.1 seconds would require an infalling velocity of ~ 40 times 10⁸m/sec...or roughly 13 times the speed of light. Not likely to happen. At ~ c/10 it would take about 13 seconds (near the spread of SN1987A's neutrino burst...11 seconds) More likely. Pete.

[Tim Thompson]

I should think not.

The standard "big picture" for a core collapse supernova looks like this:

Core Collapse
The core of a massive star is made mostly of iron atoms, which are held in mechanical equilibrium, against the collapsing force of gravity, by the pressure of the electron clouds around the nuclei (the Pauli exclusion principle). That resistance to collapse works because the electrons are in bound orbitals around the nuclei. As the core increases in mass it also gets hotter. Once it is hot enough, the bath of thermal photons rips the iron nuclei apart, converting them into free alpha particles (helium-4 nuclei). The electrons formerly bound to the nuclei are now free electrons, and no longer are subject to the Pauli exclusion principle, so no longer produce a pressure to resist collapse. The unopposed force of gravity now forces the core to collapse. The process is sudden, and virtually instantaneous; the entire stellar core is destroyed in approximately 0.1 second.

Tim. The core collapse model usually shows an accumulation of ~ a solar mass of iron. ~...2 times 10³⁰ kg. Density of iron ~ 7,874 kg/ m³. Radius of core ~ 4 times 10⁸meters. For that to collapse in 0.1 seconds would require an infalling velocity of ~ 40 times 10⁸m/sec...or roughly 13 times the speed of light. Not likely to happen. At ~ c/10 it would take about 13 seconds (near the spread of SN1987A's neutrino burst...11 seconds) More likely. Pete.

You didn't notice this part ...

Core rebound
The core collapses very fast, about 10 seconds is all that is required for it to reach maximum collapse. It collapses with so much inertia that it goes too far, collapses to too small a size to reach mechanical equilibrium. It rebounds rapidly outwards, seeking an equilibrium configuration.

The flash of destruction is what takes about 0.1 seconds, which is what I thought I said. But the collapse takes about 10 seconds, which is what I did say.

[Jeff Root]

What is the speed of light in the core as it is "destroyed" just
before it collapses? I would think the core would be much more
than 0.1 light-second in radius.

-- Jeff, in Minneapolis

[Ken G]

The electrons formerly bound to the nuclei are now free electrons, and no longer are subject to the Pauli exclusion principle, so no longer produce a pressure to resist collapse.

Hang on Pete, free electrons do experience the Pauli exclusion principle, and this does hold up the core of red giant stars, and white dwarfs, for example. What happens in core collapse is that when the core mass exceeds 1.4 solar masses, the gravity simply exceeds the pressure due to the Pauli exclusion principle, it is not needed for the latter to go away. Just thought I'd clear that up.

[Van Rijn]

Now, with that narrative in mind, what happens if you suddenly "beam out" the core of any star, of any mass? The unsupported outer layers collapse, as the story above describes. But in this case, there is no neutrino emission, and no core rebound, both of which combine to be the driving force behind the supernova explosion. Therefore I think, in your scenario, that a supernova explosion would not happen for a star of any mass, let alone a star so light as a solar mass.

As I said, I don't expect a supernova, and I'm probably using the wrong terminology by sayind "rebound," but wouldn't the collapse of the infalling mass temporarily cause a large increase in temperature and pressure, including temporary increased fusion at the (new) core? And might that not cause a "rebound" or at least a very obvious increase in the radius of the star, at least temporarily?

(As a note, I'll admit I've gone right into "intuition" and "guessing" mode, so I wouldn't defend anything around this idea for a second. It just seems that there is a lot of potential gravitational energy released quickly.)

[Duane]

Thanks all!

On SG-1, it was a little more complex - they had dialed to a black hole to do the matter collection, and I was assuming that the gravitational effects were also involved (which still wouldn't make much sense, but gives them a little more magic to play with).

Heh, yea there are a number of problems with the whole scenerio, not the least of which is the gate getting to the centre of the star in the first place. Like I intimated though, suspend your disbelief while considering the problem!

Now, with that narrative in mind, what happens if you suddenly "beam out" the core of any star, of any mass? The unsupported outer layers collapse, as the story above describes. But in this case, there is no neutrino emission, and no core rebound, both of which combine to be the driving force behind the supernova explosion. Therefore I think, in your scenario, that a supernova explosion would not happen for a star of any mass, let alone a star so light as a solar mass.

That was my thought too. Thinking along the lines of Van Rijn's idea (and the reason I asked the question in the first place) if enough mass was removed from a star, such that the outter layers could collapse, what wold we see?

I think that we would see the star contract and grow very dim, then begin to brighten again as core fusion was reinitiated (assuming enough mass for fusion) thus becoming an K or even M type dwarf, or maybe even become a "hot" Jupitor type object. I would think that the dampening effect you mention is going to prevent any type of "explosion" even if the collapse creates a shock wave.

[Duane]

What is the speed of light in the core as it is "destroyed" just
before it collapses? I would think the core would be much more
than 0.1 light-second in radius.

-- Jeff, in Minneapolis

Well, the speed of light would be the same (it's a constant) but the core would shrink significantly before electron degeneracy kicked in and it rebounded. So 0.01ls is probably pretty accurate.

[eburacum45]

Putting a 'hole into the heart of a sunlike star would move the core somewhere else- the core would emerge like water out of a fire hose at an initial temperature of fifteen million degrees. Now that would make a good weapon.

[Duane]

Heh good one eburacum, I never thought of it that way. A good weapon is a bit of an understatement

[Relmuis]

I once wrote a short story set in the Traveller 2300 universe, where humans destroyed a star to end the menace of the so-called Kafers. I will now translate a few passages from this story:

On january the 31st, 2303, humanity (that is, the 90 % of humanity who were still lving on Earth) decided in a free, fair and secret ballot that the Geneva Convention did not apply to Kafers. At long last general Nitokri had the authorization he needed.

Two months later, the War was finished.

"We had a ship ready in orbit around Mars. It was a prototype, but it had traveled. With four aligned warp generators we could make 15 lightyear jumps. So that we didn't have to run a gauntlet through all the systems which the Kafers had conquered. We could use stars which were outside the reach of the normal stutterwarp. By an outflanking route, using never-explored systems, the ship penetrated their empire. From an unexpected direction it reached Gamma Serpentis. Their home system."

His hand trembled as he brought the teacup to his lips.

"It was of course a one-way trip. There were four of them, the absolute minimum necessary to run the ship, and to rebuff Kafer attacks. They had just enough space to lie; the rest of the ship was filled with machinery. Even the radiation-shields had been dispensed with; the four crew were deathbound, anyway, and they had drugs with them against the nausea. According to our calculations they should be able to reach Gamma Serpentis before succumbing."

"What kind of weapon did they carry?"

"A Hadamard Quantum Rectifier, but that wouldn't mean anything to you. The device ensures that normal matter comes into thermal contact with Dark Matter. Usually, Dark Matter has a temperature close to absolute zero; the normal matter will therefore cool down. The Dark Matter warms up, and evaporates like a cloud of harmless WIMPS and neutrinos. Of course, it doesn't work, unless Dark Matter is present. Not all stars contain large crystals of Dark Matter, but Gamma Serpentis did. We knew that from comparing its spectrum, its temperature and its mass. The Kafer prisoners did not expect harm from telling us the length of their year."

"So you lot cooled their sun? So that their world would become very cold?"

"Two worlds. In addition to their homeworld there was a terraformed world. Once, we had plans to terraform Mars, but then the stutterwarp was invented, and we could go to the stars. The Kafer developed more slowly. By now, we know that there were two and a half billion Kafer living on Gamma Serpentis III and half a billion on Gamma Sepentis IV".

"And those are all dead?"

"I should think so. Look, if you suddenly and drastically cool down a star's core, the nuclear reactions will stop. The star's upper layers are no longer being held aloft by the radiation pressure, and so they collapse, fall upon the core. Pressure and temperature increase once more, but too swift for nuclear reactions to stabilize. The star implodes to the point where a neutronium core is formed. This happens in nature with very heavy stars when they have used up their fuel, or with binaries which start to swallow their companion. Fortunately, life never develops in those systems; the heavy stars don't live long enough, and the binaries of that kind have no stable planetary orbits in their ecospheres..."

He emptied his cup and dropped it rather than putting it down.

"Suddenly, the neutronium core withstands the infalling matter. By now, it has one third of lightspeed. It rebounds. There is a shock wave, blowing away the outer layers of the star. It's called a supernova. Type I for a binary, and type II for a very heavy star. We created a third type. The majority of the Kafer is no longer in existence. Their worlds were vaporized."

He sighed. This time she saw his eyes wander to the teapot, she poured the tea before he could do it himself.

"Thanks... On march the 31st, Gamma Serpentis was blown up. During the following week, all eight of the Kafer warlords surrendered to us. Sometimes, in my most perverted moments, I can even be proud of it; they did want an impressive opponent after all... Yes, I am a monster, too, just like the rest of us."

[trinitree88]

Hang on Pete, free electrons do experience the Pauli exclusion principle, and this does hold up the core of red giant stars, and white dwarfs, for example. What happens in core collapse is that when the core mass exceeds 1.4 solar masses, the gravity simply exceeds the pressure due to the Pauli exclusion principle, it is not needed for the latter to go away. Just thought I'd clear that up.

Hi Ken. Actually...that's not my quote, ...it's Tim's. Pete

[trinitree88]

You didn't notice this part ...



The flash of destruction is what takes about 0.1 seconds, which is what I thought I said. But the collapse takes about 10 seconds, which is what I did say.

Tim , I read the entire post. I guess the issue for me is geometry. You could presuppose a non-rotating supernova precursor in a static universe. But, that doesn't seem to correlate too well with real supernovae remnants. In a non-rotating precursor, the core would be homogeneous, and isotropic...the same viewed from every angle.
Then, you might model a temperature being reached at which photo-dissociation of iron nuclei occurs by high energy gammas...part of the energy spectrum in the core being gammas capable of supplying the activation energy necessary for the dissociations. This is an endothermic ...energy consuming... process, and cools the core temperature. The infalling adjacent layers would find nuclei lower than iron in the curve of binding energy.
But, the remnants are rich in nuclei heavier than iron....all of which sap energy from the star as they form in endothermic reactions. In addition, most stars do rotate. That means they're not isotropic. They have radial symmetry, but not spherical symmetry. So, you'd think that a collapse would not proceed spherically symmetric, but radially symmetric. Thinking this, you'd expect that the equatorial regions of the star, with higher angular velocities...would collapse later than the polar regions with smaller angular velocities. That would lead to lower equatorial gas densities...and a window for neutrinos to escape as a burst formed. Such a belt-shaped window of reduced neutrino opacity would lead to an oblate spheroid as the shape for the ejecta cloud....as is indicated in the TV NOVA program's prelude animation. They show an equatorial burst in a "nova".
Not true. Just the opposite occurs. The ejecta show prolate (football-shaped) spheroids. (Manchester, Kesteven...Australian Journal of Physics). That means that the neutrinos jet out the poles at higher levels...and a pair of torii are created, stretching out the cloud (Duane's avatar). The equator is more opaque.
This all suggests that the explosion is not spherically symmetric...as does the ejection of nascent pulsars. That suggests that it does not occur in an instant....but initiates, and propagates throughout the core. It initiates radially, with a polar asymmetry....most likely due to parity effects which are universal in weak interactions. No effect may propagate faster than c...so, I think the 0.1 second core collapse for a core ~ 4 times 10⁸meters in radius is off by at least an order of magnitude. It begins along the rotation axis, not throughout the entire core, and one pole always dominates. It proceeds to maximum density, rebounds, and births the pulsar. I'm not arguing the outer layers kinematics...the core itself cannot collapse in 0.1 seconds, and produce the geometries seen in the remnants. Pete.

[ArgoNavis]

but wouldn't the collapse of the infalling mass temporarily cause a large increase in temperature and pressure, including temporary increased fusion at the (new) core?

If a substantial amount of matter was removed from the core (for use as a weapon or otherwise) then less mass = less gravity. If the outer layers had any angular momentum at all (which they would) then they would be taking a trip by flying off into space, not collapsing into what is left of the core.

[Ken G]

Hi Ken. Actually...that's not my quote, ...it's Tim's. Pete

Yikes, my bad Pete, you were quoting Tim. Tim, the 1.4 solar-mass Chandrasekhar limit comes from the Pauli exclusion principle applied to free electrons, that have become highly relativistic by the exclusion from all the non-relativistic states.

[Grey]

And I believe that when we talk about the gravitational pressure being larger than the outward pressure from the exclusion principle, it's a little more complicated. The exclusion principle cannot really be violated, no matter how high the pressure. However, if you increase the pressure enough, the typical energy of an electron becomes high enough that it can combine with a proton and form a neutron. That releases a neutrino (which is the primary source of neutrinos in a supernova).

[trinitree88]

And I believe that when we talk about the gravitational pressure being larger than the outward pressure from the exclusion principle, it's a little more complicated. The exclusion principle cannot really be violated, no matter how high the pressure. However, if you increase the pressure enough, the typical energy of an electron becomes high enough that it can combine with a proton and form a neutron. That releases a neutrino (which is the primary source of neutrinos in a supernova).

Grey. I think that's a little too simplified for accuracy. The decay of a free neutron is exothermic, as it forms a proton, electron and an electron-type antineutrino. If you put the energy back (endothermic) into the trio (assuming they arrive together as you do) you can recreate the neutron.
But the quantum picture of the weak current is that the neutron emits a W^-, as a down quark flavor shifts to an up...the ddu neutron is now duu proton..plus the W^-. The W^- then with a slight time delay, decays into an electron, and an electron-type antineutrino. The electron is captured by the proton to make a hydrogen atom..or not,..the antineutrino zips away pretty much unfettered.
To reverse the reaction, you could hope for the unlikely cross-section in my first paragraph, or as you said you could use the energy of the electron. But, when that happens, it doesn't combine directly with the proton...annihilating electric charge, and releasing the neutrino. The high speed collision between the electron and the proton's constituent quark/gluon bag creates a W⁺, as a down quark flavor shifts to an up quark....with a slight time delay, the W⁺ decays to a positron and a neutrino which slips away unfettered. The new positron annihilates a nearby electron in a few gamma rays (2 or three). The picture some might hold of the electron directly annihilating the charge on the proton is somewhat misleading, since the W's and the Z⁰, have been experimentally verified. Busy little place, the high temperature nucleus. Pete

[Van Rijn]

If a substantial amount of matter was removed from the core (for use as a weapon or otherwise) then less mass = less gravity. If the outer layers had any angular momentum at all (which they would) then they would be taking a trip by flying off into space, not collapsing into what is left of the core.

"Any angular momentum at all"? Uhm, no. It would have to exceed self-gravity, and I'm assuming you're still leaving most of the mass of the star in place. Yes, there is less mass, but there would be a period of time where the surrounding mass of the star falls inward. Long term, you may well end up with a smaller main sequence star, but I suspect there would be a spike in output due to the implosion.

Even without that, there would probably be some immense solar storms. Not a supernova, but not good for anything nearby.

[Grey]

Grey. I think that's a little too simplified for accuracy.

Actually, I'd disagree. I think it's just the right level of simplification for this situation. Yes, it's true that this is a weak interaction, and to properly model it you'd need to look at the consituent quarks and the W particle that mediates the interaction. However, the net result is that we start with a proton and an electron and end up with a neutron and an electron-type neutrino, and any excess energy in the interaction is carried off by the resulting particles and a few photons. We mostly need to model it with more detail if we're concerned abouot the interaction cross section. But in the case of a supernova and the creation of a neutron star, nearly all of the electrons and protons combine in this manner in a very short time period. Actually, you can do some pretty good first order modeling of a supernova and the energy released just by assuming exactly that.

[artmso]

Tim you said, on 28-April-2006 12:36 PM, that "The core of a massive star is made mostly of iron atoms,..."
This is not correct, since the temperatures are (by far) high enough to ionize iron. Iron is single ionized at about 10,000 K (one electron kicked out) and completely ionized at about 100,000,000 K (26 electrons kicked out). No atoms are present in stellar cores - only ions.
Also the latent heat in ionization proccesses is about a million times less than the latent heat involved in nuclear reactions - the latter rule in these invirons.

Tim. The core collapse model usually shows an accumulation of ~ a solar mass of iron. ~...2 times 10³⁰ kg. Density of iron ~ 7,874 kg/ m³.

As I explained above, the iron in stellar cores is a plasma (a "gas" of positive ions/nuclei and negative electrons) which means the density is not constant, but obeys the ideal gas law (approximately). When the density is high enough the degenerate pressure of the free electrons kick in and quickly dominate the pressure (by far! - no more ideal gas). At this stage your stellar core is a white dwarf precursor and the density can be 100,000 to 100,000,000 kg/ m³ depending on the temperature and pressure.
At high enough temperatures, the thermal gamma-rays will split the iron nuclei and reduce them to a rubble of the constituent protons and neutrons as happens in supernovae. In supernovae the high pressure then makes it favourable for the electrons and protons to combine and form neutrons (releasing a neutrino) turning the core into a precursor neutron star.... and so on...

Regner Trampedach

[Tim Thompson]

Tim, the 1.4 solar-mass Chandrasekhar limit comes from the Pauli exclusion principle applied to free electrons, that have become highly relativistic by the exclusion from all the non-relativistic states.

Yes, this came as quite a surprise to me; none of my old textbooks even mention the Pauli Exclusion Principle (PEP) except with reference to bound states. I have until now thought that collapse was caused by the destruction of the nuclei to which the electrons were bound. I always thought it was peculiar, the nuclei should be ionized completely at such temperatures, but I just assumed that the pressure would prevent that. Now it is obvious that I need to educate myself on this subject.

Here is my primary reference:Explosion mechanism, neutrino burst and gravitational wave in core-collapse supernovae; Kotake, Sato & Takahashi; Reports on Progress in Physics 69(4): 971-1143, April 2006. The paper is 173 pages long, and a very thorough study of the subject. I also read through Chandrasekhar's derivation of degeneracy (An Introduction to the Study of Stellar Structure, S. Chandrasekhar, University of Chicago Press, 1939, chapter 10, The Quantum Statistics; the book is still in print through Dover). He is quite explicit about his use of the PEP in his derivation. This is why I have not responded sooner, reading Chandrasekhar takes time!

My description of the collapse process is also slightly outdated. It is indeed the case that the iron nuclei are photodissociated, in an endothermic reaction that sucks energy out of the core (gamma + ⁵⁶Fe -> 13 alpha + 4 neutrons - 124.4 MeV). That weakens the thermal pressure, but the rest of the story is electron capture by the iron nuclei (⁵⁶Fe + e^- -> ⁵⁶Mn + electron neutrino), and that's what relieves the pressure from the PEP on the free electrons, by eliminating electrons. Those two processes together are what eliminates the support & brings on core collapse.

However, the rest of the story appears accurate. Rebound & the shock are generally as I described them, although there is a lot more detail to be had regarding the nature of the rebound event.

Radius of core ~ 4 times 10⁸meters

Off by a factor of about 100, the size of the collapsing core is roughly 2x10⁸ centimeters, not meters. See the Kotake, et al. paper, linked above, page 981, equation 19. It is only the core of the core that actually dissociates, the rest of the core is destroyed in the subsequent collapse. The dissociation of the inner core only takes about 0.01 seconds.

EDITED 5/2/06 to change my response to trinitree88 & remove my own erroneous conclusion.

[Duane]

Speaking as a gnat among giants--once the Chandra limit of ~1.44 solar masses is reached in a Type II precurser, the core overcomes the Paulii Principle and collapses to the point where it just exceeds the equlibrium level of electron degeneracy. At that point, the core rebounds seeking equilibrium, thus saving the matter from the ultimate "lost" of falling into a black hole. The rebound of the giant star core meets the infalling layers, which combined with the sudden neutrino flux causes the outer layers to "explode" outwards, leading in a very short time the the visual display we see as a supernova.

Ok, we all seem to agree to this part.

The thing is, the core condenses from a roughly solar mass-sized object to a city sized object then back to an Earth-sized object (roughly) in the space of a few nanoseconds. The question is not about the fact of neutrino emmision and core rebound, but rather how efffective neutrino emmision is in affecting the infalling layers and how, in combination with the core rebound, these two factors will impart energy to the infalling outer layers.

In the case where a solar-mass star has it's core removed, will the infalling layers create enough pressure to reinitiate fusion, or will it do more--such as create a condition where a small core of material can rebound.

I don't think this can occur, simply because I dont think the amount of mass is enough to overcome electron degeneracy in the absense of a 1.44 solar mass core. As such, the core of such an object will not rebound as is considered in a core collapse, but rather will condense and ignite in a fusion reaction.

The biggest question then ( as it relates to my OP), is whether or not such a reaction would be enough to throw off the outer layers of the star. None of you (as far as I can see) have addressed this, in the context of the question I've asked.

Put differently, considering the role of overlying layers of material present in a stellar nursery in dampening the effect arising from sudden ignitian of stellar fusion, if a main-sequence star whose core was suddenly removed collapsed, would the resulting reinitiation of fusion exert enough outward pressure to cause the outer layers of the star to "suddenly expand" in a manner that would throw the material outward and through it's own system, thus creating a "supernova-like" affect?

[Tim Thompson]

The thing is, the core condenses from a roughly solar mass-sized object to a city sized object then back to an Earth-sized object (roughly) in the space of a few nanoseconds.

No, it condenses from a roughly Earth sized object to a roughly county sized object. White dwarfs, which are roughly Earth sized, are not formed in supernova events. In the case of supernovae, we get either neutron stars or black holes, the latter probably coming only after an intermedite neutron star has formed. The neutron star will not be larger than a radius about 50 km.

The question is not about the fact of neutrino emmision and core rebound, but rather how effective neutrino emmision is in affecting the infalling layers and how, in combination with the core rebound, these two factors will impart energy to the infalling outer layers.

This is handled in far more detail that I can relate here, in the Kotake, et al. paper, section 2.3. Neutrinos are probably the dominate source of energy feeding the shock, and it seems clear that the shock would invariably stall without them, and there would be no supernova explosion. But the neutrinos don't come from the dissociation of the iron nuclei, they come from copious electron capture (e^- + p > neutron + neutrino), generated right behind the shock, which itself has dissociated nuclei into free nucleons. Those neutrinos are followed by an intense shower of thermal neutrinos from the exceptionally hot core.

In the case where a solar-mass star has it's core removed, will the infalling layers create enough pressure to reinitiate fusion, or will it do more--such as create a condition where a small core of material can rebound.

I don't think this can occur, simply because I dont think the amount of mass is enough to overcome electron degeneracy in the absense of a 1.44 solar mass core. As such, the core of such an object will not rebound as is considered in a core collapse, but rather will condense and ignite in a fusion reaction.

I think the case of removing the core will result in an end product that is just a lower mass star, depending on how much mass is removed (no "star" if the final mass is less than ~0.08 solar masses). There will be some excitement along the way, but if only a small, low mass core is "removed", the excitement might not show up as much at the surface of the star, the body of which should be dense enough & large enough to thermalize radiation generated by events around the core. No reboundable core will form because the star is not massive enough to provide enough pressure to overcome the thermal excitement that would prevent the formation of anything arguably "condensed", such as a core.

The biggest question then ( as it relates to my OP), is whether or not such a reaction would be enough to throw off the outer layers of the star. None of you (as far as I can see) have addressed this, in the context of the question I've asked.

Put differently, considering the role of overlying layers of material present in a stellar nursery in dampening the effect arising from sudden ignitian of stellar fusion, if a main-sequence star whose core was suddenly removed collapsed, would the resulting reinitiation of fusion exert enough outward pressure to cause the outer layers of the star to "suddenly expand" in a manner that would throw the material outward and through it's own system, thus creating a "supernova-like" affect?

My money is on "No" as an answer. The energy involved in a real supernova is extraordinary, but even there, the only reason an explosion of any kind breaks the surface of the star appears to be the copious flood of neutrinos from the core. And those neutrinos are not generated by any "normal" nuclear reactions, they are produced either by electron capture on dissociated nuclei, or are thermal neutrinos from the untra hot core. In the "core removal" scenario at work here, the starting core temperature is orders of magnitude lower (solar core temperature ~1.5x10⁷ K, SN pre-collapse core temperature about 10⁹ K goes way up during collapse). The collapse of the outer layers will not provide enough heat to produce the flood of thermal neutrinos. Furthermore, the shock produced by the collapse is unlikely to produce enough electron capture neutrinos to fuel a breakout shock & explosion, because it is not as hot & therefore not as efficient at dissociating nuclei.

And I think the driving factor here is mass. In a SN explosion, the whole core weighs in at about 1.44 solar masses. While only the core of the core is initially dissociated, the whole core goes through collapse, and that's where all the electrons come from for the electron capture generation of neutrinos. And that's where the mass comes from to hold the collapsed core together with gravity, despite being over 10⁹ K hot, so it has time to emit lots of thermal neutrinos.

But in your "core removal" scenario, for a star of total mass ~1 solar mass, how much mass will be directly involved in the collapse of the core? I should think 0.1 solar masses is a pretty reasonable limit, not having a mass model of the sun handy. That's a lot less of everything. So if a real SN can barely make a bang, I don't see how anything this wimpy can measure up. There will be no bang.

I think that final result of the "core removal" scenario you describe will be something akin to a blue straggler star in a globular cluster. The sudden collapse at the center will certainly convert gravitational potential energy into kinetic energy, which will invigorate fusion at the core. The mechanical shock will not be able to break the surface, but there would likely be an "unexplained" neutrino burst, from the sudden burst of fusion in the core, and perhaps some asteroseismological clues. The radiation from the fusion burst would propagate no faster than does radiation in our own sun, say about 1,000,000 years for the trip to the surface. This would make the star look rejuvinated, brighter than it should be, at least for some time. But it will also result in a final star that is lower in mass, and will therefore eventually dim to a value befitting its new lower mass.

[mugaliens]

Very little of a star's mass is actually undergoing fusion at any given time. Something like 1x10-(double-digit number). In fact, there's not enough temp/pressure to cause fusion, except for on a localized (atomic) scale, and certainly not enough to cause a chain reaction (else all suns would have gone nova long ago.

If one were to remove a sizeable portion, say, an amount equivalent to our Moon, the remainder would slam back to the center and collide with enough of a temp/pressure spike to cause widescale fusion. Think of a water hammer and it's this heightened compression that's mirrored in a thermonuclear weapon which does the trick. If sufficiently large, this reaction would be enough to sustain continued fusion for a short time.

Nova? Sure, if the initial mass removed were large enough in diameter, but obviously only to a point - you have to leave enough mass behind to go boom.

[mugaliens]

On second thought, Tim, I think your conclusion is probably the correct one, despite the temps reached when the collapsing material stops suddenly at the center.

http://www.bautforum.com/showpost.ph...11&postcount=4

[trinitree88]

Tim quote;

Off by a factor of about 100, the size of the collapsing core is roughly 2x10⁸ centimeters, not meters. See the Kotake, et al. paper, linked above, page 981, equation 19. It is only the core of the core that actually dissociates, the rest of the core is destroyed in the subsequent collapse. The dissociation of the inner core only takes about 0.01 seconds.

Tim..OK, I've not read Chandra or Kotake...if they're modeling iron at greater densities, or a smaller core at that radius...it's in the ball park, and I stand corrected. Kudos Pete.