Star Death and supernova

[spacetrace]

Hello,

I was wondering whilst waiting for the dinner to cook.... when a star dies it runs out of fuel for nuclear fusion and the star collapses, why does it then explode into a supernova, what is the driving force that provides the energy for the explosion. Is it the actual collapse that drives the explosion by the immense "pressure" also what causes the collapse to happen, why this inward force?

[arad85]

Oh dear... that's a hard one to explain concisely. You may be better off googling it and reading through a few hits to get your head around it. Having said that, what happens to a star when it dies depends on the mass it is when it's formed. The smaller it is, the more peaceful death it will have, the larger it is, the more violent it's ending will be....

[kev 102 42]

Oh dear... that's a hard one to explain concisely. You may be better off googling it and reading through a few hits to get your head around it. Having said that, what happens to a star when it dies depends on the mass it is when it's formed. The smaller it is, the more peaceful death it will have, the larger it is, the more violent it's ending will be....

Yeah and the Nutrinoes go a bit mental BOOM:headbang:..........KEV

[brianb]

why does it then explode into a supernova, what is the driving force that provides the energy for the explosion.

A star about to go supernova contains an iron core, a shell buring silicon to iron, a shell burning carbon to silicon, a shell burning helium to carbon, a shell burning hydrogen to helium and an envelope consisting (mostly) of hydrogen & helium - in layers like an onion. When the core collapses, the support for the other layers is removed, they fall inward causing a large increase in pressure & temperature, which causes the infalling material to start to "burn" very fast. Normally this would just make the material expand again, but it's contained by the momentum of the inrushing material, so a substantial fraction of the material undergoes nuclear fission in a very few seconds ... providing the huge surge of neutrinos characteristic of a SN blast. The light actually comes mostly from the fission decay of short-lived isotopes of heavy elements created incidentally in the few seconds of the event proper, material which is ejected along with the unreacted remains of the outer part of the star and the fusion products formed at the time of the event and which will eventually form the nebula characteristic of supernova remnants when it's expanded enough to be visible as such.

[spacetrace]

A star about to go supernova contains an iron core, a shell buring silicon to iron, a shell burning carbon to silicon, a shell burning helium to carbon, a shell burning hydrogen to helium and an envelope consisting (mostly) of hydrogen & helium - in layers like an onion. When the core collapses, the support for the other layers is removed, they fall inward causing a large increase in pressure & temperature, which causes the infalling material to start to "burn" very fast. Normally this would just make the material expand again, but it's contained by the momentum of the inrushing material, so a substantial fraction of the material undergoes nuclear fission in a very few seconds ... providing the huge surge of neutrinos characteristic of a SN blast. The light actually comes mostly from the fission decay of short-lived isotopes of heavy elements created incidentally in the few seconds of the event proper, material which is ejected along with the unreacted remains of the outer part of the star and the fusion products formed at the time of the event and which will eventually form the nebula characteristic of supernova remnants when it's expanded enough to be visible as such.

Thanks, That makes sense. I thought that the actuall action of collapse must play an important role in assisting in driving the reaction towards an explosion. Thanks that has helped me stop thinking a little tonight, could do with a break my mind has been in over drive all day!

[spacetrace]

Just been reading up about it all in further detail, its amazing, not only that this has all been worked out but also how intricate the systems are in continuing the cycle and the importance when you think about the bigger picture of how it all relates to each other.

[ngc2403]

The inward force driving the collapse is gravity, from the stars own mass.

the star runs out of fuel and so the inside cools this reduces the preasure, and so the star begins to collapse.

all of the elements heavier than iron are made in SN and they are all made in the last split second of a stars life. it can take as little as quarter of a second, for a star to go from full size, to the size of a large town. As the matter free falls almost without limit, reaching speeds close to that of the speed of light.

The explosion of the star takes years (in mainy cases 4 years) for it to complete. bombs completely detonate in factions of a second! a star is so large and massive that it takes years.

The neutions carry way most of the energy and pass through the outer layers of the star, out into the universe.

but even with a the remaining energy there is enough to eject up to 90% of the mass in some cases!

[narrowbandpaul]

effectively the outer layers bounce of the collapsing core and are propelled at a much higher speed than that which it fell with....

neutrinos.....lots of neutrinos...i would guess maybe more than 10? haha

[brianb]

effectively the outer layers bounce of the collapsing core and are propelled at a much higher speed than that which it fell with....

No, the infall rate becomes relativistic (close to the speed of light), the expelled matter which forms the supernova remnant has a much lower speed of "only" a few thousand km/s. The neutrino flux is so great that even that has an effect in propelling the ejecta, though most of it is direction reversed & accelerated by a mixture of degeneracy pressure & radiation (photonic) pressure.

Though we do need to distinguish at this point between a type 1 supernova - where essentially the whole star is degenerate, and the main energy source is from the gravitational collapse - and a type 2 SN, where much of the energy is derived from runaway fusion of the unburned fuel outside the collapsing core.

[ngc2403]

effectively the outer layers bounce of the collapsing core and are propelled at a much higher speed than that which it fell with....

tut tut tut

school boy error lol

[narrowbandpaul]

i do know what radiation pressure is.

ally i was poorly explaining (hence the lack of any scientific words) the demo we got from diver....remember the stack of 5 rubber balls...when droped, the top one shoots up way further than expected?...i was using that analogy, but in a way that made me look foolish!.....

i think I am conserving energy (always a good idea I feel), the inner layers transferring their energy to the energy of the outer layers. By energy I mean momentum...interchangeable in newtonian dynamics....p^2/2m

this isnt my area of expertise though....im sure that was obvious

[brianb]

Momentum is not conserved any more than it is on a snooker table when you cue a ball. But much of the momentum (and energy) of a SN explosion is carried away in the flood of neutrinos resulting from the runaway nuclear reactions.

[ngc2403]

Momentum is not conserved any more than it is on a snooker table when you cue a ball

???? what does this mean ????

[ollypenrice]

Is there not a supernova scenario in which the initial infalling gas produces a neutron star, totally inelastic, from which more still infalling gasses rebound and collide with more still infalling gasses triggering the more exotic reactions? The 'superelastic bounce' I thought it was called?

Put me out of my misery.

Olly

[brianb]

At the time the neutron core is forming (electron degeneracy is setting in) the particle count is decreasing, resulting in a loss of pressure and lowering of temperature, which accellerates the gravitational collapse. This continues until the pressure is sufficient to maintain the balance again, or neutron degeneracy sets in (in which case a collapse into a black hole is inevitable).

There is no surface to bounce off - the gravitational energy of the infalling gases not involved in the core is converted adiabatically into heat, which eventually results in runaway nuclear fusion, converting some of the material into cobalt-56 and ejecting all the material which is not in the core into space (whether or not the reaction is completed). Radiation pressure is what provides the energy for the ejection, though at extreme pressure even neutrino pressure plays a part. The cobalt-56 decays to nickel-56 then iron-56, this nuclear decay process providing most of the visible light received from the explosion.

You can think of the infall & subsequent high energy outflow as a super-efficient "bounce" but it is important to understand the energy source.

[ollypenrice]

Thanks Brian. Excellent, I will go back to my text books.

Olly

[astoc]

Very interesting read, this thread.

John

[ollypenrice]

For a stunningly good read you could try The Supernova Story by Laurence A Marschall.

Olly

[waccoo]

Very good thread indeed!

What I find amazing is that not only do you get other elements from the explosion, but, you get lumpy groups. Instead of there being a gold atom here and a lead atom there, we get lumps and blocks of it

I can understand an iron meteor. That is just a chunk of star core. But a gold vein is truly amazing.