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Brady Yoon
2004-Mar-24, 08:13 AM
I don't know if this can happen, but what if somewhere in the galaxy there was an iron rich solar nebula and a star that formed was lets say 50% iron. Could such a star form in the first place and if so, could it maintain nuclear fusion?

TriangleMan
2004-Mar-24, 12:23 PM
Iron cannot maintain nuclear fusion, it is a key reason why massive stars explode into supernovas. I googled "iron core, star" and found many links like this one (http://www1.chapman.edu/oca/benet/intro_sn.htm). IIRC, the reason the life of a star ends at iron is because iron is the first element that cannot release energy from fusion, all of the elements that precede it on the periodic table, such as Hydrogen, Oxygen . . . etc. can release energy from fusion.

Lee
2004-Mar-24, 06:47 PM
Right TM... Iron is at the top of the binding energy per nucleon curve. Fusion below Fe and Fission above. As far as Fe stars, I was thinking that there haven't been enough star generations to produce this much Fe in a local area but this is just a guess.

EckJerome
2004-Mar-24, 07:14 PM
The way I read Brady's question is, could a star have an iron core while maintaining fusion in the layers outside the core? It seems an intriguing possibility. Any second-generation star is going to have within it elements heavier than iron. Do those elements exist in continually-mixed suspension, as it were, or do they settle out to the core? Presumably there would be some degree of sorting in the millennia prior to a proto-star initiating fusion.

Brady Yoon
2004-Mar-24, 08:09 PM
The way I read Brady's question is, could a star have an iron core while maintaining fusion in the layers outside the core? It seems an intriguing possibility. Any second-generation star is going to have within it elements heavier than iron. Do those elements exist in continually-mixed suspension, as it were, or do they settle out to the core? Presumably there would be some degree of sorting in the millennia prior to a proto-star initiating fusion.
Yep, that's exactly what i meant. :D Now that I think about it, wouldn't the iron core collapse because it's not burning? The result would then be a supernova... I'm not quite sure though.

wedgebert
2004-Mar-24, 09:15 PM
The Iron would collapse, but it wouldn't cause a supernova. It would actually cool the core down.

Supernovae occur when the outer layers of the star are no longer held up by the pressure from the core's fusion, not when an iron core implodes.

Brady Yoon
2004-Mar-24, 10:57 PM
The Iron would collapse, but it wouldn't cause a supernova. It would actually cool the core down.

Supernovae occur when the outer layers of the star are no longer held up by the pressure from the core's fusion, not when an iron core implodes.
Could someone please explain these two statements? This is how I thought a supernova occurs: A star continues burning its fuel, until iron ash builds up it its core, surrounded by burning silicon, oxygen, neon, magnesium, helium, hydrogen, etc. Then because the burning of iron consumes energy, instead of releasing it, no energy is left to support the star. Then the iron core collapses, along with the rest of the star, and when the outer layers hit, they explode outward...
Could someone clarify those above statements? Thanks.

Andreas
2004-Mar-25, 03:23 AM
Supernovae occur when the outer layers of the star are no longer held up by the pressure from the core's fusion, not when an iron core implodes.
And the outer layer are no longer held up by the pressure of the core's fusion when the iron core implodes. Because a) there is no fusion pressure since fusing iron doesn't release energy and b) the core is already collapsing. Right?

wedgebert
2004-Mar-25, 03:03 PM
My point was that it's not the iron collapsing that causes a supernova. It's the outer shell. The core can be any element that doesn't provide enough fusion pressure to maintain the star's size.

Glom
2004-Mar-25, 04:09 PM
If such a body did exist, can you imagine the magnetic field?

Conor_M
2004-Mar-25, 04:20 PM
Quick point: I realize this is more of a "what if..." question, but I don't think it would even be possible to have a cloud of 50% iron. There's just not that much Iron around.

Slow question: Anyone know when exactly the supernova happens? How long can iron build up before the supernova occurs? Does it build up for a while, and then go boom when the star runs out of silicon? Or does it reach a certain critical mass and then go? Any clues?

etLux
2004-Mar-25, 04:35 PM
Plainly, the answer to all of this is... a little help (http://www.nlm.nih.gov/medlineplus/druginfo/uspdi/202305.html).

Brady Yoon
2004-Mar-25, 06:22 PM
Plainly, the answer to all of this is... a little help.
Nice link, even though it was a little off topic. :lol:

Hamlet
2004-Mar-25, 06:55 PM
Slow question: Anyone know when exactly the supernova happens? How long can iron build up before the supernova occurs? Does it build up for a while, and then go boom when the star runs out of silicon? Or does it reach a certain critical mass and then go? Any clues?

I think it goes something like this. The iron in the core accumulates from the fusion of silicon and sulfur. As the core approaches 1.4 solar masses (Chandrasekhar Limit) and because there is no new fusion energy to resist gravity the iron core will collapse into a neutron star.

The core collapse is very rapid and it takes a while before the surrounding shell layers collapse onto the surface of the core remnant. Since the surface of the neutron star is very stiff the outer layers bounce off the core with a great deal of energy. Think about dropping a rubber ball from a great height onto a hard surface.

Along with the energy from the bounce there are a great many neutrinos that were released when the iron core collapsed. Even though neutrinos are weakly interacting with matter, there are so many that they add energy to the outgoing blast.

Glom
2004-Mar-25, 07:03 PM
I thought the Chandrasekhar limit was to do with the size of black holes?

Hamlet
2004-Mar-25, 07:08 PM
I thought the Chandrasekhar limit was to do with the size of black holes?

This limits the mass of an object to where electron degeneracy pressure can no longer resist gravity and the object will collapse to a neutron star or black hole.

Perhaps your thinking of the Schwarzschild Radius (http://scienceworld.wolfram.com/physics/SchwarzschildRadius.html)?

Glom
2004-Mar-25, 07:09 PM
That would be it. :oops:

Tom Ames
2004-Mar-26, 05:08 PM
Well someone (http://web.umr.edu/~newsinfo/ironsun.html) thinks we have an iron star right here in the solar system.

(It's on account of the Xenon, he says).

Kebsis
2004-Mar-26, 05:38 PM
Does Fission occure anywhere naturally?

Brady Yoon
2004-Mar-26, 10:14 PM
Yes, suprisingly it does. There are radioactive elements in Earth that are always breaking down into lighter and lighter nuclei. This was very important in the early history of the solar system because it kept the Earth molten for differentiation to occur. However, the total energy released isn't very substantial because unlike the sun, which is made 90% of fusible material (hydrogen), the Earth has very few elements that can easily undergo fission, such as uranium, thorium, fission, carbon 14, etc.

Lee
2004-Mar-30, 12:11 AM
Disclaimer: seriously simplified

There have been natural fission reactors.

http://www.curtin.edu.au/curtin/centre/waisrc/OKLO/index.shtml

There are many ways to classify reactors but one way is to compare the types of neutrons that drive the fission. Fast neutrons are high energy and slow neutrons are low energy. You can make a fast neutron into a slow neutron by "moderating" it. Moderation involves the fast neutron bouncing off a low atomic weight material like H2O, D20, Berylium or graphite and loosing energy without being absorbed into the material. I imagine a pool table with spongy rails and few pockets when I want to think of a moderator. The neutron bounces off the rails and slows down without falling into a pocket

Plutonium 239, among other fuels, can use fast neutrons to drive the reactor. Plutonium reactors are fast reactors. But, there is basically no Pl 239 in the environment so... on to:

Uranium 235 uses slow neutrons to fission and there's plenty of U235 in the environment. U235 is a rare isotope of uranium and makes up only about .7% of a natural sample of uranium but this percentage can be higher or lower depending on circumstances.

In Oklo you had a spike in the concentration of U235. Every once in a while the U235 will naturally decay and spit out 2.5 neutrons. These neutrons will be fast neutrons. Normally, this would just be a run of the mill decay but sometimes the Oklo uranium deposit was flooded with water, which acts as a moderator, so the neutrons would get moderated and continue the fission process.... voila natural reactor

Hope that makes sense. Tried to edit for clarity...

iFire
2004-Mar-30, 01:17 AM
Well someone (http://web.umr.edu/~newsinfo/ironsun.html) thinks we have an iron star right here in the solar system.

(It's on account of the Xenon, he says).

My cousin goes to University of Missouri Rolla... I should ask him about that...


I heard that stars die at iron becuase it takes more energy to fuse it than it releases in the fusion. Is that correct?

Hamlet
2004-Mar-30, 01:31 AM
Well someone (http://web.umr.edu/~newsinfo/ironsun.html) thinks we have an iron star right here in the solar system.

(It's on account of the Xenon, he says).

My cousin goes to University of Missouri Rolla... I should ask him about that...


I heard that stars die at iron becuase it takes more energy to fuse it than it releases in the fusion. Is that correct?

Yes that is correct. Fusion of iron and heavier elements absorb energy instead of liberating it. However, only stars with greater than 5 Solar masses are able to fuse elements up to iron and will end their lives as a supernova. Stars like our Sun can only fuse up to carbon and oxygen and will end their lives as white dwarfs.