Core collapse confusion

[billyharris72]

Hi all:

I've been trying to read up a bit more on the more theoretical side of astronomy, to get more insight  on what I am actually observing, and am struggling with supernovae, specifically what happens in what order and why. Apologies for the possibly random seeming questions below, but these are the bits that I'm struggling to get my head around.

1) I've read in some books that the collapse is due to the cessation of fusion once the core has fused to iron and can't fuse any further. However, this seems to assume that the heat released by the fusion is exerting an outward pressure that resists gravitational collapse. How can this be the case if (from the start of Helium fusion) the core is made of degenerate matter which does not increase in pressure when heated? Is the collapse not connected with the cessation of fusion, or does fusion resist the collapse in some other way (e.g. radiation pressure)?

2) On the subject of degenerate matter, if increasing its temperature does not increase its pressure, does this mean that compressing it does not increase its temperature? I'm assuming that  that does (on the basis that temperatures in larger stars' cores are higher and on the idea that each wave of fusion is generated by further collapses of the core) but is that correct? If not, why do the cores of larger stars get so much hotter and fuse heavier elements?

3) Once the core has fused to iron, it can't go any further as fusing iron to heavier elements is highly endothermic and there is no source of energy, so I can get that fusion stops. However, does this mean that even the ongoing collapse (enough to fuse protons and electrons to form a neutron core) is not able to generate enough heat to burn iron? Fair enough if so, but why then does the subsequent shockwave have enough energy to fuse elements heavier than iron? What are the respective temperatures involved? (Presumably the shock wave generates a much higher temperature, but it seems counter intuitive, unless I've misunderstood the relationship between temperature and pressure in degenerate matter and the collapse does not cause the core to get hotter).

4) Is the cooling effect of photo-disintegration critical in this failure to burn iron, and does this not affect the shockwave that causes the supernova? Just a tagged-on thought, but wondering exactly what role it plays.

Apologies for semi-randomness and thanks for any help anyone out there is able to offer.

Billy.

 

 

 

[andrew s]

23 hours ago, billyharris72 said:

However, this seems to assume that the heat released by the fusion is exerting an outward pressure that resists gravitational collapse

This is true the pressure from the plasma and radiation balance the force of gravity.

 

23 hours ago, billyharris72 said:

How can this be the case if (from the start of Helium fusion) the core is made of degenerate matter

This is not true. While normal core fusion up to Fe is going on in the core is not degenerate but a high temperature plasma.  Correction - I was not right here some massive enough stars can have a non fusing core which is supported by electron degeneracy pressure.

 

23 hours ago, billyharris72 said:

However, does this mean that even the ongoing collapse (enough to fuse protons and electrons to form a neutron core) is not able to generate enough heat to burn iron?

It is the release for gravitational potential energy during the collapse that provides the energy for the  fusion of elements beyond Fe and also provides the energy for a degenerate core or black hole if they are formed. (Some star are completely disrupted and no core remains.) 

Hope this helps.

Regards Andrew

[kirkster501]

An interesting note I read in a book (the name of which escapes me) is that the fusion into lead stage of the nuclear "burning" only lasts for a few hours.  The iron accumulates very rapidly and its presence snuff's out the immediately surrounding fusion processes, removing energy from it.  The star is doomed at this stage. 

Would love to find where I read that....

EDIT:

http://abyss.uoregon.edu/~js/ast122/lectures/lec18.html

 

[symesie04]

wheres the confusion? you get older and your core collapses, simples. 

[Paul M]

The confusion is added to by the fact that there are different classes of Supernova which are born of quite different mechanisms.

The best I can do to help is point you towards here: https://en.wikipedia.org/wiki/Supernova

It's a very big subject, good luck!

[ronin]

Going to club talks and other talks it would be reasonable to say that I have yet to recall any 2 talks that explained core collapse the same. So I would suggest that you accept that any 2 or more books and authors will explain it differently. It may come down to how differently, but I very much suspect that differently is the key word. In farness the core collapse depends on the nature of the star also. So this opens up the possibility of a slightly different process.

Sometimes I have the idea that each person has to come up with something just a little different. Suppose it makes a little sense as if you spend 4 years doing a PhD to finish it and say that Bob 10 years ago had it right and I have added nothing to what they found tends not to get you your PhD.

[Tiki]

On ‎7‎/‎4‎/‎2016 at 22:43, billyharris72 said:

 

1) I've read in some books that the collapse is due to the cessation of fusion once the core has fused to iron and can't fuse any further. However, this seems to assume that the heat released by the fusion is exerting an outward pressure that resists gravitational collapse. How can this be the case if (from the start of Helium fusion) the core is made of degenerate matter which does not increase in pressure when heated? Is the collapse not connected with the cessation of fusion, or does fusion resist the collapse in some other way (e.g. radiation pressure)?

2) On the subject of degenerate matter, if increasing its temperature does not increase its pressure, does this mean that compressing it does not increase its temperature? I'm assuming that  that does (on the basis that temperatures in larger stars' cores are higher and on the idea that each wave of fusion is generated by further collapses of the core) but is that correct? If not, why do the cores of larger stars get so much hotter and fuse heavier elements?

3) Once the core has fused to iron, it can't go any further as fusing iron to heavier elements is highly endothermic and there is no source of energy, so I can get that fusion stops. However, does this mean that even the ongoing collapse (enough to fuse protons and electrons to form a neutron core) is not able to generate enough heat to burn iron? Fair enough if so, but why then does the subsequent shockwave have enough energy to fuse elements heavier than iron? What are the respective temperatures involved? (Presumably the shock wave generates a much higher temperature, but it seems counter intuitive, unless I've misunderstood the relationship between temperature and pressure in degenerate matter and the collapse does not cause the core to get hotter).

4) Is the cooling effect of photo-disintegration critical in this failure to burn iron, and does this not affect the shockwave that causes the supernova? Just a tagged-on thought, but wondering exactly what role it plays. Larger stars have relatively small lifetimes as they use their fuel up quickly.

 

1) Fusion stops the gaseous part of the star ( ie. the bit surrounding the core) from falling inwards under the force of gravity. Over time and with the fusion of heavier elements the core gets heavier and more dense. Eventually a point is reached whereupon electron degeneracy pressure is no longer able to support  the core and a collapse ensues.

2) Degenerate matter gets hotter when you compress it.  In a larger star there is more matter to support , therefore greater pressure, and compared to a smaller star, a higher rate of fusion.

3) / 4) As the core collapses,  instead of fusing, iron will either be smashed to bits (protons and neutrons) by thermal photons (at about T=10^10K, density=10^12 kg/m^3) or at even higher subsequent temperatures, a proton residing in an Fe nucleus will capture an electron and will become a Mn nucleus which will in turn capture another electron to become a Cr nucleus etc. This certainly won't go on forever as the timescale of a stellar core collapse is of the order of milliseconds. A massive amount of energy is absorbed in these processes which makes the collapse all the more sudden. It is thought (and debated) that when the  collapsing core reaches a critical density ( something like 5x 10^17 kg/m^3) it becomes suddenly rigid and bounces back to a density of about 2x 10^17 kg/m^3. This is thought to be the progenitor of an outward bound shockwave that meets in-falling stellar material.

Of all the jaw-dropping sights that we are able to marvel at through the eyepieces of our telescopes, it is that barely discernible supernova that I like best. What a pyre. Unfortunately, I have only seen one to date.

[billyharris72]

Thanks all for the input - I think it has helped me get more of a grip on this.

Stop me if I'm way off, but I think a big part of my struggle has been failing to separate the heat and pressure of the core from the expansive force from the fusion reaction. The more I think about they are surely quite different. The model I originally had in my head was something like this (not for core collapse in this case, but transition from hydrogen to helium fusion):

1. A ball of gas, with a super hot, compressed core, which is fusing hydrogen to helium, making it even hotter.
   1. This heat and pressure is what is resisting the weight of the rest of the star bearing down on it.
2. The fusion stops and the core begins to cool.
3. The cooling means the core is not resisting the compression of the gas bearing down on it.
4. The gas bears down, compressing the core, which becomes hotter...
5. Kicking off the next phase of the fusion reaction.

What I was struggling with was that I could not see how that could work - it seemed to fundamentally violate thermodynamic principles, since if the heat and pressure of the core is what resists gravitational compression then surely cooling and losing pressure, leading to gravitational collapse and more compression, can't make the core any hotter than it started out.

That implies some other force resisting the gravitational collapse that is not attributable to, or understandable directly in terms of, the heat and pressure of the core. So off I go looking for another mental picture.

If one stops thinking of the fusion reaction as a heater and starts thinking of it as more akin to a massive hydrogen bomb, that might be a better image. It's not just agitated molecules bouncing randomly about - there is a definite direction to the energy as it radiates away from the core, and this is effectively blasting the rest of the star outwards and away from the core. In that situation, cessation of fusion results in a much greater loss of expansive force than could be attributed to simple cooling, which does away with my thermodynamic objection.

Okay, that's all I've got. Not saying it's great (can you tell I'm not a physicist?) but is it any closer?

Billy.

[Physicist13]

yep pretty much. Fusion is not a thermodynamic effect. Its a v diff way of producing energy and yes its the radiation pressure from this that countering gravity...not the fact that the core is hot. The radiation pressure is a net outward force  as you say. Once the fuel is exhaused fusion stops so the radiation pressure drops dramatically. So the core then contracts under gravity and compresses more and heats akin to normal heating of a compressed gas. Once hot enough for the next fusion process to kick in then rad pressure is active again so further core compression halts....this process repeats though various fusion reactions. other processes also going on but thats main process. 

 

[andrew s]

3 hours ago, Physicist13 said:

its the radiation pressure from this that countering gravity...not the fact that the core is hot

I think it is both. The radiation pressure is felt via the collisions with the free electron and ions which transfers energy to them. The thermal gradient exerts a net presure in addition to the radiation pressure. The balance between them depends on the mass of the star.

In addition some stars have a convective core others a radiative core depending on which mode of energy transport dominates there.

 

Regards Andrew

[Physicist13]

yes it does dep on where the star is in its cycle..mass etc etc.... balance of sev factors.  

[Tiki]

20 hours ago, billyharris72 said:

Okay, that's all I've got. Not saying it's great (can you tell I'm not a physicist?) but is it any closer?

I am not a physicist either, but it might be helpful to think of the core as just a giant ashtray for the fusion processes which goes on above. If the core reaches a certain critical state then fireworks will happen but for most of the time, the core does not influence the rest of the star too much.

IMO, the key idea to appreciate is a star in a 'steady' state: a balancing act which goes on between gravity, the kinetic energy (like gas pressure)  of constituent particles and the rate of fusion reactions. eg. fusion rate decreases-> star contracts ( graviatational potential energy released) and kinetic energy increases, temperature goes up, fusion rate increases-> star expands (gravitational potential energy increases) and kinetic energy decreases, temperature goes down, fusion rate decreases-> star contracts.....