Is Iron the Ultimate Killer of Stars?

[vkash]

How Iron is poison for a star. I have not read this thing in my course book. Today I was watching Discovery channel. there was a show on stars which describe this.
I think this is related to binding energy per nucleon of Iron which is highest of all.(I am not sure it is just an Idea)

 

[Drakkith]

You could call it that I guess. A small amount probably wouldn't have a DRASTIC effect, however in high mass stars the core actually fuses elements together to get nickel/iron. This results in no net gain in energy for the core and after a certain amount of mass has accrued in the core as iron it collapses on itself and causes a supernova. You are correct in that it is the binding energy of Iron and Nickel being the highest of all elements that causes this.

 

[vkash]

You could call it that I guess. A small amount probably wouldn't have a DRASTIC effect, however in high mass stars the core actually fuses elements together to get nickel/iron. This results in no net gain in energy for the core and after a certain amount of mass has accrued in the core as iron it collapses on itself and causes a supernova.[/color] You are correct in that it is the binding energy of Iron and Nickel being the highest of all elements that causes this.

I did not understand it fully.Please try to explain once again. specially red[/color] region.
Do you want to say that energy is converted to make Iron Nickel due to which supernova occur if yes then how loosing energy is related to supernova.

What you try to say by these words.
It's not about what's possible, it's about what's probable[/color]

 

[Drakkith]

No. The accumulation of Iron in the core of a star results in a mass of non fuseable material due to the binding energy of iron being the highest of all the elements. (Iron and Nickel that is) This means that the fusion of iron requires an input of energy and nets none. This is the point during the life of a star that it is on it's "final leg". Unable to generate any energy, the core builds up with iron until the mass is so high that it cannot hold itself up against the force of gravity.

At this point, depending on the exact mass of the core, it collapses either into a Neutron Star or a Black Hole. This collapse causes a huge release of energy. The exact mechanisms and such are explained here: http://en.wikipedia.org/wiki/Type_II_supernova

 

[Drakkith]

Read that wikipedia article and then click on the Silicon Burning Process link in the "Formation" section to see exactly what happens when the core runs out of fuel.
Also, don't edit one of your current posts. Instead create a new post, as that will cause the thread to be marked in people's control panel as having a new reply.

 

[Borek]

in high mass stars the core actually fuses elements together to get nickel/iron. This results in no net gain in energy

I must say I am at loss as well. While I understand that you need to input energy to fuse iron into something heavier, fusing lighter elements to get iron still produces energy, so I don't understand the "no net gain" statement.

The only thing I can think of is that at the conditions (temp/density) required to produce iron/nickel further reactions are also possible, and they consume the energy produced together with iron - is that the case?

 

[turbo]

Once you have a star with iron accumulating in its core, it produces less and less energy (via fusion) to keep itself supported against gravitational collapse. At least, that's the stellar model that we have now, and it is probably pretty accurate.

 

[Borek]

it produces less and less energy

How does it translate to "no net gain"? Net gain is produced minus consumed. As long as energy is produced and not consumed by other processes, there is a net gain, isn't it?

I guess I am missing something simple, but I don't see how energy production (diminishing, but still existing) translates into "no net gain".

 

[turbo]

How does it translate to "no net gain"? Net gain is produced minus consumed. As long as energy is produced and not consumed by other processes, there is a net gain, isn't it?

I guess I am missing something simple, but I don't see how energy production (diminishing, but still existing) translates into "no net gain".

I won't argue for "no net gain". That is a loser. I will argue that a star that is actively fusing lighter elements into iron is on the losing end when it comes to maintaining the radiative pressure that protects it from gravitational collapse. That is where the "poison" (bad term, IMO) comes in. A star that is fusing elements into more and more stable elements can't produce enough energy to support itself against collapse.

 

[marcus]

..."poison" (bad term, IMO) comes in...

Turbo is right. It is a really bad choice of word.

It is normal to find iron in stars. The sun's mass is about 0.1% iron
http://en.wikipedia.org/wiki/Abundance_of_the_chemical_elements
and it does no harm to the sun.

Look at the table for mass abundances of chemical elements in the solar system. Pretty much the same as that of the sun, since the sun is most of the mass of the solar system.
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To get straight at the outset, why not give a direct answer to the question
Is Iron poison for a star? NO!

If you ate 1/10 of a percent of your body weight of a typical poison you would die.
For a fairly usual body weight that might be 1/6 of a pound. Imagine eating 1/6 of a pound of something poisonous!

Iron is not harmful to stars in the way that poisons are harmful. The point of iron is that it has no FOOD VALUE. In the case of very massive stars, not having any fuel in its core that it can get energy from is fatal because the star needs a constant supply of heat to prevent violent collapse.

Hydrogen is the best fusion fuel---the easiest to get energy out of. Heavier elements, like helium, carbon, oxygen, silicon...will work but are progressively harder and harder to fuse. It takes a more massive star, with more pressure at the core, to get energy out of them. And iron is impossible to get energy from (by fusing it to something higher) no matter how massive the star is.

How far up the scale a star can fuse depends on how massive it is. A small star might only be able to fuse H to He and then, when its core fills up with helium, it just stops fusing and gradually cools off. Small stars don't collapse when they cool. They just quietly cool down. They eventually shrink a bit. But nothing violent or dramatic happens.

A somewhat heavier star might get up to carbon and oxygen, go through some changes, eventually stop fusing, and then it too would gradually cool off.

The heaviest stars are able to fuse all the way up to iron, and then fusion necessarily stops for them too. In the case of a very heavy star this can lead to a supernova explosion because the star is so massive that it NEEDS to be constantly producing energy in its core just to prevent itself from collapsing.

I don't like thinking of iron as a poison. It is more like something with no food value, like chopped straw or hay, roughage, cellulose BRAN, like bulk that your body can't digest and get energy from.
It would be bad for you if you only had that to eat, because you'd starve.

 

[Drakkith]

What I was trying to explain was that in the core of a star which fuses elements into Nickel, any further fusion beyond Nickel requires a net input of energy since the fusion process yields less energy than it took to cause them to fuse together. Because of this, Nickel builds up in the core and once it is massive enough it can no longer hold itself up against gravity and collapses.

The process of fusion up to and including the production of Nickel-56 does release energy. Past that it does not. Hence then "no net gain" once you get nickel.

I apologize, I have a hard time trying to explain things in terms someone new can understand and also be technically accurate.

 

[Gliese123]

I assume that smaller stars, such as the Sun relative to the mass and size, iron has a drastic effect for its core and stabilization. That's the time when the fusion isn't available anymore, the star collapses. However, much bigger stars has the ability to create layers without making it's whole structure to collapse, that's when heavier elements than iron creates. I think. Correct me if I'm mistaken.

 

[qraal]

I assume that smaller stars, such as the Sun relative to the mass and size, iron has a drastic effect for its core and stabilization. That's the time when the fusion isn't available anymore, the star collapses. However, much bigger stars has the ability to create layers without making it's whole structure to collapse, that's when heavier elements than iron creates. I think. Correct me if I'm mistaken.

Iron can't be created inside smaller stars - it has to be there from the beginning, formed in larger stars that came before. Iron also collapses more than lighter elements - it forms denser degenerate matter for the same pressure - and potentially adding a lot of iron suddenly to a star core could cause a localized pressure increase and potential explosion. But it would have to be a LOT of iron, more than inside the Earth.

Elements heavier than iron are generally created in the detonation of a supernova, through a variety of processes. Look up "chemical evolution" and the "r-process" and "s-process" for more details.

 

[Gliese123]

Okay. Thank you. :)