What causes a supernova to occur?

[frankhawes]

Hi everyone, I'm sitting my A-Level physics this year and am currently doing a bit of revision.
I was reading about Neutron Stars and Supernovas when I came across this:

"when the outer layers hit the surface of the neurtron star they rebound, setting up huge schockwaves, ripping the star apart and causing a supernova"

What I don't understand is how the outer layers "rebound". In my head the neutron star is attracting the outer layers too strongly for them to get away at the speed with which they approach.

Please could someone explain this to me and/or give me a better way to think about it!

Thanks in advance!

 

[Jonathan Scott]

Hi everyone, I'm sitting my A-Level physics this year and am currently doing a bit of revision.
I was reading about Neutron Stars and Supernovas when I came across this:

"when the outer layers hit the surface of the neurtron star they rebound, setting up huge schockwaves, ripping the star apart and causing a supernova"

What I don't understand is how the outer layers "rebound". In my head the neutron star is attracting the outer layers too strongly for them to get away at the speed with which they approach.

Please could someone explain this to me and/or give me a better way to think about it!

Thanks in advance!

If they merely rebounded, then you are correct that they could only bounce back to where they started from.

What happens is that the huge temperatures and pressures caused by the collapse trigger massive amounts of nuclear fusion, where smaller atoms fuse together to form larger ones and larger ones may fuse together to become part of a neutron star. This fusion process releases huge amounts of heat and energy which power the rebound process.

 

[frankhawes]

If they merely rebounded, then you are correct that they could only bounce back to where they started from.

What happens is that the huge temperatures and pressures caused by the collapse trigger massive amounts of nuclear fusion, where smaller atoms fuse together to form larger ones and larger ones may fuse together to become part of a neutron star. This fusion process releases huge amounts of heat and energy which power the rebound process.

Ahh thanks that kind of makes sense - so by coming in close to the massive, hot core they get 'new energy' from further fusion possible because of the high temperatures?
Thanks for your reply that really did turn a lightbulb on haha :)

 

[Matterwave]

Ahh thanks that kind of makes sense - so by coming in close to the massive, hot core they get 'new energy' from further fusion possible because of the high temperatures?
Thanks for your reply that really did turn a lightbulb on haha :)

A supernova is actually QUITE a complicated process! It is no wonder that even today they are not FULLY understood. Computer models basically attribute the supernova explosion to two effects. The initial rebound shock wave, an the shock re-heating by neutrinos.

When a stellar core collapses to form a Neutron star, this reaction happens in a matter of milliseconds (essentially the free-fall time). As all the nuclei are undergoing reverse beta decay, they emit a massive amount of neutrinos, which, due to the high nuclear densities found inside the neutron star are actually TRAPPED inside the neutron star (for a few hundredth or tenths of a second, but for a neutrino, this is a long time to be trapped by anything). All of these reactions happen VERY FAST, and the equation of state stiffens (pressure increases) very very rapidly. So, while the outer layers of the core is still collapsing, the inner layer of the core has basically over-collapsed a little bit, and then rebounded a bit and just gone completely stiff. (Like a piston).

The outer layers of the core then hit this inner core, and the piston effect imparts ~10^51 ergs of energy to the outer layers of the core, sending a shock wave to disrupt the star.

During the 1980's, it was expected that this sudden rise in pressure of the inner core as it reached nuclear densities would be enough to drive the shock wave and destroy the star. But computer simulations found that endothermic nuclear reactions happen at the edge of the shock wave and basically steal the energy away so that the star is not able to be disrupted in this manner. The key, it was found, was in those neutrinos that were produced. Neutrinos actually carry away a good percentage of the rest mass of the core in energy. These neutrinos actually do work on the shock front, and reheat it! This reheating is what is currently thought to be the mechanism for driving the star apart.

 

[frankhawes]

A supernova is actually QUITE a complicated process! It is no wonder that even today they are not FULLY understood. Computer models basically attribute the supernova explosion to two effects. The initial rebound shock wave, an the shock re-heating by neutrinos.

When a stellar core collapses to form a Neutron star, this reaction happens in a matter of milliseconds (essentially the free-fall time). As all the nuclei are undergoing reverse beta decay, they emit a massive amount of neutrinos, which, due to the high nuclear densities found inside the neutron star are actually TRAPPED inside the neutron star (for a few hundredth or tenths of a second, but for a neutrino, this is a long time to be trapped by anything). All of these reactions happen VERY FAST, and the equation of state stiffens (pressure increases) very very rapidly. So, while the outer layers of the core is still collapsing, the inner layer of the core has basically over-collapsed a little bit, and then rebounded a bit and just gone completely stiff. (Like a piston).

The outer layers of the core then hit this inner core, and the piston effect imparts ~10^51 ergs of energy to the outer layers of the core, sending a shock wave to disrupt the star.

During the 1980's, it was expected that this sudden rise in pressure of the inner core as it reached nuclear densities would be enough to drive the shock wave and destroy the star. But computer simulations found that endothermic nuclear reactions happen at the edge of the shock wave and basically steal the energy away so that the star is not able to be disrupted in this manner. The key, it was found, was in those neutrinos that were produced. Neutrinos actually carry away a good percentage of the rest mass of the core in energy. These neutrinos actually do work on the shock front, and reheat it! This reheating is what is currently thought to be the mechanism for driving the star apart.

Wow thank you that was a brilliant reply with great detail. There really is a lot going on! The bit about the core over-collapsing slightly and the springing back is great to visualise. I have gone from this being one of the more shakey areas of the course to one I feel I understand better than required and I'm interested to learn more about. Thank you again for your reply.

 

[Chronos]

Actually, a neutron star is the end result of a core collapse supernova. The core of a massive star runs out of fuel and becomes cool. Without the immense heat of an active stellar core, there is not enough outward pressure to prevent the outer layers from collapsing. During collapse, the core gets compressed causing it to become degenerate. When the outer layers hit this now degenerate core they violently recoil. This further compresses the already compressed core into a neutron star [which, aside from a black hole, is the densest known state of matter in nature] and violently expels the outer layers in a herculean explosion. The particulars are not well understood, as already noted, but, that is the basic explanation. In low mass stars this reaction is much less violent. The core becomes a white dwarf and the outer layers rebound at a more leisurely pace forming a planetary nebula around the newborn white dwarf [e.g., the ring nebula]. For supermassive stars, it is believed the core can even become a black hole. Such a star would apparently just disappear. This, however, has never been observed. Some believe, under the right conditions, such a star can instead become a GRB, the most violent known explosion in the universe. Supermassive stars are, however, exceedingly rare and probably only existed in significant numbers when the universe was much younger and metal poor. That would explain why GRB's are usually billions of light years distant [which is a good thing].

 

[frankhawes]

When the outer layers hit this now degenerate core they violently recoil.

For supermassive stars, it is believed the core can even become a black hole. Such a star would apparently just disappear. This, however, has never been observed. Some believe, under the right conditions, such a star can instead become a GRB, the most violent known explosion in the universe. Supermassive stars are, however, exceedingly rare and probably only existed in significant numbers when the universe was much younger and metal poor. That would explain why GRB's are usually billions of light years distant [which is a good thing].

Thank you for your reply - This is my first time using the site and the quality of all the responses I've had is incredible!

When you say the outer layers "violently recoil" is this because when they strike the degenerate core it causes further contraction and produces the extra energy needed for the layers to rebound far further than they 'fell' from?

Also reading your response brought another thought to me - Do all supernovas create all elements - for example are there elements that are not made from supernovas? (like the ones synthesised in labs in return for nobel prizes) or are these only made by some extreme supernovas and decay quickly?
This was quite an ill conceived question and I'm sorry if didn't make much sense but I would be thrilled to know your thoughts on it!

Thanks again for your reply. (...and also for reminding me about that Douglas Adams quotation)

 

[Matterwave]

Actually, a neutron star is the end result of a core collapse supernova. The core of a massive star runs out of fuel and becomes cool. Without the immense heat of an active stellar core, there is not enough outward pressure to prevent the outer layers from collapsing. During collapse, the core gets compressed causing it to become degenerate. When the outer layers hit this now degenerate core they violently recoil. This further compresses the already compressed core into a neutron star [which, aside from a black hole, is the densest known state of matter in nature] and violently expels the outer layers in a herculean explosion. The particulars are not well understood, as already noted, but, that is the basic explanation. In low mass stars this reaction is much less violent. The core becomes a white dwarf and the outer layers rebound at a more leisurely pace forming a planetary nebula around the newborn white dwarf [e.g., the ring nebula]. For supermassive stars, it is believed the core can even become a black hole. Such a star would apparently just disappear. This, however, has never been observed. Some believe, under the right conditions, such a star can instead become a GRB, the most violent known explosion in the universe. Supermassive stars are, however, exceedingly rare and probably only existed in significant numbers when the universe was much younger and metal poor. That would explain why GRB's are usually billions of light years distant [which is a good thing].

Well, neutronization is happening at basically all the stages during core collapse. I don't think it's fair to imply it all happens at once after the outer core rebounds from the inner core. In fact, the neutronization is one of the reasons for a drop in the pressure, due to loss of electrons (=loss of electron degeneracy pressure). I don't believe electron degeneracy pressure is strong enough to produce a rebound shock wave, by the time the core rebounds, it should be nearly entirely neutrons (or other exotic materials).

 

[Matterwave]

Thank you for your reply - This is my first time using the site and the quality of all the responses I've had is incredible!

When you say the outer layers "violently recoil" is this because when they strike the degenerate core it causes further contraction and produces the extra energy needed for the layers to rebound far further than they 'fell' from?

Also reading your response brought another thought to me - Do all supernovas create all elements - for example are there elements that are not made from supernovas? (like the ones synthesised in labs in return for nobel prizes) or are these only made by some extreme supernovas and decay quickly?
This was quite an ill conceived question and I'm sorry if didn't make much sense but I would be thrilled to know your thoughts on it!

Thanks again for your reply. (...and also for reminding me about that Douglas Adams quotation)

MOST of the elements in the universe came from the big bang itself. The ~75% Hydrogen and ~24% helium were actually the result of big bang nucleosynthesis.

The 1% of other stuff comes from stars fusion during their main sequence life-times. Stars burn and produce all the elements up to iron through standard stellar nuclear fusion. The stuff produced by supernovae account for only a fraction of a percent of elements, and these are the elements heavier than iron (like gold, or uranium, etc.).

 

[frankhawes]

MOST of the elements in the universe came from the big bang itself. The ~75% Hydrogen and ~24% helium were actually the result of big bang nucleosynthesis.

The 1% of other stuff comes from stars fusion during their main sequence life-times. Stars burn and produce all the elements up to iron through standard stellar nuclear fusion. The stuff produced by supernovae account for only a fraction of a percent of elements, and these are the elements heavier than iron (like gold, or uranium, etc.).

Thanks again - and for you reply about the neutronisation.
I was also wondering though if the elements that we can't find on Earth (and resort to manufacturing in labs) are ever the product of supernova? for example Livermorium

 

[Matterwave]

Thanks again - and for you reply about the neutronisation.
I was also wondering though if the elements that we can't find on Earth (and resort to manufacturing in labs) are ever the product of supernova? for example Livermorium

Those elements tend to have very short half-lives which is probably why we don't find any of them here. They are probably produced in the supernova environment, but do not survive very long.

In fact, in a supernova environment, such as from the r-process, very exotic isotopes might be produced since a large neutron flux accompanies the supernova. I'm not familiar with all the chains of the r-process however.

 

[Chronos]

As I recall, Californium [98] is the heaviest element observed in core collapse supernovae. The isotope Cf 254 is regularly present in their spectra, with a half life of about 2 months. Another suspected source of heavy elements is neutron star mergers. The core of a dying star is much denser than the collapsing layers. The recoil is like a sonar wave bouncing off the sea floor, which propels infalling material away from the core. The core also gets superheated, which further powers the explosion.

 

[Jonathan Scott]

Ahh thanks that kind of makes sense - so by coming in close to the massive, hot core they get 'new energy' from further fusion possible because of the high temperatures?
Thanks for your reply that really did turn a lightbulb on haha :)

Note that the high temperatures arise from the energy of the collapse and from the subsequent additional nuclear reactions, not from being close to the core. The fact that collapse occurred at all usually indicates that the previous core temperature and radiation pressure was insufficient to hold up the outer layers.

 

[Chronos]

Here is a relatively uncomplicated description of a core collapse event - http://abyss.uoregon.edu/~js/ast122/lectures/lec18.html