Nuclear Fusion process in the Sun (or generally, any star)

[Positron137]

Why can't the Sun (or any star) fuse elements higher than iron? Could anyone provide a technical answer? Thanks!

 

[Chronos]

Not enough mass to generate the temperatures required to synthesize heavy elements is the short answer.

 

[Positron137]

Does the quantity of iron in the star not provide enough mass for fusion to occur?

 

[russ_watters]

It isn't the quantity of iron that matters, it is the total mass of the star (and therefore the pressure and temperature).

 

[Positron137]

Ah ok. Thanks!

 

[SteamKing]

Up until Fe is produced, the various stellar fusion reactions produce a net excess of energy, which serves to overcome the force of gravity tending to collapse the star. When the stellar core is composed entirely of iron, any further fusion reaction requires an external source of energy. Since the only available source of this energy is from other parts of the star which are net producers of energy, the stellar core is no longer able to support itself against gravity, and it begins to collapse into either a neutron star or a black hole, if the core is massive enough.

 

[Positron137]

Ah ok. So there are no other parts of the star capable of producing external energy besides internal fusion, because the star has used it up? But can't the net excess of energy produced from each fusion be used for iron? Why can't that happen? Once the star reaches Fe, has it used up all of its available net energy through fusion, so it can't get its energy from elsewhere?

 

[SteamKing]

In stars, the region which actively fuses elements is the core. The surrounding layers of gas, although very hot, serve to help carry heat away from the core. If energy is needed in the core to promote the fusion of Fe nuclei, it can't be called back once it has escaped. As more and more of the core material is fused into Fe, there is a drop in total energy production until the core is almost entirely iron. This occurs very rapidly. Once a star's innermost core region is composed of Sulfur and Silicon, the time for complete fusion of these elements into iron has been estimated to take less than 24 hours.

In stars that have reached this point in their lives, the core has also become stratified, with the heavier elements, such as iron, closest to the center, while the lighter elements are located further away from the center, and different fusion reactions are occurring in each layer. The hottest region is the deepest, and as depth decreases, so does the temperature. Since energy in the form of heat flows from a hotter region to a colder one, once the heat has left the core, it is unlikely that it will flow back into the core to promote the fusion of iron nucleai.

 

[Positron137]

Ah ok. So the energy needed to fuse heavy non-iron elements near the core, into iron, would have escaped through the upper atmosphere of the sun (as solar wind, perhaps), thereby not allowing for more fusion into iron?

 

[SteamKing]

Well, perhaps not that far, but once the energy leaves the core, it's gone. With the sun, most of the energy which we perceive today as heat and light was generated between 20,000 and 100,000 years ago. On its way out of the core, the energy keeps getting absorbed and re-emitted by the hot gases of the sun's interior. The path from the core is never a straight one.

 

[Positron137]

Ah ok. That makes sense. Thanks!

 

[Drakkith]

Ah ok. So there are no other parts of the star capable of producing external energy besides internal fusion, because the star has used it up? But can't the net excess of energy produced from each fusion be used for iron? Why can't that happen? Once the star reaches Fe, has it used up all of its available net energy through fusion, so it can't get its energy from elsewhere?

In addition to the answers above, there is also the fact that iron fusion requires even higher temperatures than silicon. In order for the star's core to reach this higher temperature is must contract. (That's how the core is heated progressively higher and higher. After each round of fusion is over, contraction takes place and heats it up until the new elements can begin to fuse and equilibrium is reached once more) However, since iron fusion doesn't release energy the contraction simply never ends, resulting in a supernova as the core collapses in on itself.

Edit: Just to be clear, silicon fusion results in Nickel-56, not iron. Nickel-56 decays with a half-life of about 6 days, turning into Cobalt-56 and then into Iron-56. However, the Nickel-56 never has time to decay, as the collapse takes place after about a single day.
From Here: http://en.wikipedia.org/wiki/Silicon_burning_process#Nuclear_fusion_sequence_and_the_alpha_process

After Nickel-56 is created the next step in the alpha process results in the creation of Zinc-60. But since Zinc-60 has a higher mass per nucleon than Nickel-56, thus consuming energy instead of releasing it.

 

[Positron137]

Why doesn't iron fusion release energy? (Sorry if that's a redundant question which has already been answered previously by someone's response.)

 

[Drakkith]

Why doesn't iron fusion release energy? (Sorry if that's a redundant question which has already been answered previously by someone's response.)

Well, to understand that you need to understand how fusion releases energy in the first place. Fusing two atoms together releases energy because the MASS of the new atom is LESS than the combined mass of the two atoms prior to fusing. This missing mass is released as energy.

There's something known as Binding Energy. Basically, it's the energy required to completely take apart an object. That is, take every single particle and remove them from their bonds and throw them waaaaaay out away from each other. For fusion, what we look at is called the Nuclear Binding Energy.

This nuclear binding energy is typically described as "binding energy per nucleon" because it allows us to compare nuclei with different amounts of nucleons. (Particles in a nucleus)

Because of the way the protons and neutrons attract and repel inside the nucleus, different configurations and numbers of these nucleons results in different amounts of binding energy per nucleon. The MORE binding energy per nucleon, the more energy is required to pull them apart.

Now think about this. If we have to ADD energy to pull them apart, then they must RELEASE energy when they come together! (If you don't understand that, just trust me. They do.) So a more tightly bound nucleus, aka one with more binding energy per nucleon, gave up more energy per particle than a nucleus with less binding energy per nucleon.

So once you get to Nickel-56, the very next step in the chain, Zinc-60, has LESS binding energy per nucleon. What does this mean? This means that going from a nucleus with MORE binding energy per nucleon to a nucleus with LESS cannot release any net energy! It's not as tightly bound, so it can't release as much energy as the Nickel-56 did.

Or, to look at it a different way, look at the mass of all the particles before and after the fusion. A single nucleus of Nickel-56 + an alpha particle has LESS mass than a single Zinc-60 nucleus. So when the Nickel-56 and alpha particle fuse the process gobbles some of the energy up and turns it back into mass instead of releasing some of the mass as energy.

So, once the core of a star reaches this point and starts to collapse, fusion does start to occur once the temperature reaches the ignition point. But since Zinc-60 doesn't release energy, this reaction eats up thermal energy that would have opposed the collapse. So there's nothing to oppose the collapse and it accelerates, eventually turning the core into a neutron star or black hole.

 

[Positron137]

Ah ok. Brilliant explanation Drakkith. Thanks a lot! This will definitely help :)

 

[SteamKing]

It has to do with the binding energy of the nucleus. Once Fe is reached, it takes additional energy to fuse two nuclei. Without excess energy being produced by fusion, the core is unable to counteract the gravitational force which wants to compress it.

See: http://en.wikipedia.org/wiki/Silicon_burning_process
http://en.wikipedia.org/wiki/Nuclear_binding_energy

 

[Positron137]

Thanks! So it basically takes more energy to fuse Fe into heavier elements than the energy available from previous fusion events. Ok, that makes sense. Thanks!

 

[Drakkith]

Thanks! So it basically takes more energy to fuse Fe into heavier elements than the energy available from previous fusion events. Ok, that makes sense. Thanks!

Hmmm. Just to be clear, the energy it takes to fuse Fe into heavier elements is simply more than it releases. How much energy the fusion chain released prior to Fe is irrelevant.

 

[Positron137]

Right. Ok. Sorry, I got a bit confused there. I'm just studying quantum mechanics so I'm not familiar with nuclear binding energy, the mechanics of fusion, and stellar stuff in general. Thanks for clarifying!