# Does any iron fuse in stars before they go supernova?

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I understand that iron and all heavier elements consume more energy to produce than they make, and that is what eventually leads to a supernova. I also understand that a lot of the heavier elements are produced during that supernova. However, what I'm wondering is, before the star goes supernova, does any of the iron fuse with other elements? Yes, there would be a net energy loss, but if there is only small amount of iron in the star, it would probably be able to handle that.

Yes, but it's slow. (I'm not an expert, so feel free to correct if I miss something important), but once the star is into the later stages, past the helium stage, up to iron, fusion mostly takes place by fusing a helium onto a heavier element, raising each atomic number by 2. That's not the only method but it's the most common.

Iron can also fuse into nickel in this way inside a star and it does in small amounts, but mostly beyond iron, and certainly beyond nickel, heavier elements are created through the S-Process. (short for slow neutron capture process). This happens when a free neutron binds to the atomic nucleus and over time, the addition of neutrons can lead to beta decay, where an electron is ejected and a proton remains - adding to the atomic number.

but if there is only small amount of iron in the star, it would probably be able to handle that.

This is undoubtedly true. The stars that go super-nova are incredibly large and the iron doesn't exactly sink to the core right away. It takes some time. For a star to go kablooie (supernova), it needs an iron core of both enough purity where it's no longer undergoing expansion from nearby fusion, and enough size for it to undergo rapid collapse in a way that effects the star around it almost instantaneously. I'm not clear on the exact process, but it requires way more than just a little iron. As a layman's guess, it might require a Jupiter sized ball of iron. Perhaps a fair bit more than that.

The "iron core" in a supernova is actually the end product of a nuclear statistical equilibrium that begins when the silicon core begins to fuse with alpha particles (helium nuclei). Exothermic reactions are possible right up to Nickel-62 (which is actually the nucleus with the highest binding energy per nucleon). In fact, successive, rapid alpha captures produce nuclei with the same number of protons and neutrons, but at the same time, the competing processes of photodisintegration and radioactive decay work in the other direction. The process is thought to mostly stop at Nickel-56 which, because heavier nuclei are more stable with $n/p>1$, then undergoes a couple of $eta^{+}$ decays via Cobalt-56 to Iron-56. However, the core of a supernova just before it explodes is likely to contain a bit of a mixture of iron-peak isotopes.

Before all this happens it is possible for iron and nickel to undergo nuclear reactions if there is an appropriate source of free neutrons. The elements beyond iron in our universe are predominantly created by neutron-capture in either the r-process or the s-process.

The r-process is thought to occur after a core-collapse supernova (or a type Ia supernova) has been initiated. The neutron flux is created by the neutronisation of protons by a dense, degenerate electron gas in the collapsing core.

However, the s-process can occur outside the core of a massive star before it explodes. It is a secondary process because it needs iron nuclei to be present already - that is, the iron that is used for the seed nuclei is not produced inside the star, it was already present in the gas from which the star formed. The s-process in massive stars uses free neutrons produced during neon burning (so at advanced nuclear burning stages beyond helium, carbon and oxygen burning) and results in the addition of neutrons to iron nuclei. This builds up heavy isotopes, which may either be stable or undergo $eta$ decay and/or further neutron captures to build up a chain of "s-process elements" (e.g. Sr, Y, Ba) all the way up to lead. The overall process is endothermic, but the yields and reaction rates are so small that it has no major influence on the overall energetics of the star. The newly-minted s-process elements are easily blasted into the interstellar medium shortly afterwards when the supernova explodes.

## Why can't stars fuse Iron?

Well, stars, if they are large enough, CAN fuse iron, as long as it's an iron isotope of 55 or lower. Our star can't fuse iron at all, because it's not big enough either.

The reason is that fusion up until nuclei of 55 protons/neutrons or less will release energy. At nickel and Iron 56 and higher, it TAKES energy to create fusion to higher isotopes.

This is why Andreas Rossi's claim about the E-cat is wrong because he claims energy release from fusing nickel to copper. That's contrary to the laws of physics as most all supernovae have shown.

This is essentially why giant stars SuperNova. Soon as they've fused up to Iron 56, the nuclear fusion energy processes quickly shut down. The star begins to contract because fusion was keeping it large, it compresses to a very tight size, and then it supernovas!! Spewing out much of its mass in neutrinos, light, & mass ejections to spread newly formed metallic elements upwards of well beyond uranium and Pu to seed the local interstellar region. This will eventually create some metal containing stars (& planets) as children many megayears down the line.

The iron in our hemoglobin and cytochromes, and the Cobalt in our B12 vitamins mostly came from supernovae. We are composed of star stuff, of at least one supernova's matter. & We are here, living right now, reading this because of the fact that Fe-56 can't fuse thus releasing energy. Cool, huh.

## Supernova

• topic starter

hello all.
i'm new (to this forum, not to astronomy ) and i have a question.
in the process of a supernova, what is the sequence of elements that the star's core is fusing? i mean, from hydrogen to iron.

and another question that just popped up in my head.
last week i saw spiderman 2 ( good movie) and in the movie there is a "sun" that is working on tritium (an isotope of hydrogen). my question is this: is this possible? a star that it's fuel is deuterium or tritium?
raz

### #3 bierbelly

hello all.
i'm new (to this forum, not to astronomy ) and i have a question.
in the process of a supernova, what is the sequence of elements that the star's core is fusing? i mean, from hydrogen to iron.

and another question that just popped up in my head.
last week i saw spiderman 2 ( good movie) and in the movie there is a "sun" that is working on tritium (an isotope of hydrogen). my question is this: is this possible? a star that it's fuel is deuterium or tritium?
raz

As I recall, the first step towards a nova is the helium flash, which is where the hydrogen fuel of a star is essentially exhausted, the star collapses due to it's mass no longer being supported by the forces of hydrogen fusion inside the core, and the heat generated being high enough to 'ignite' a helium fusion reaction. at least that was the theory when I learned it. So the helium fusion would result in two helium atoms being fused into a beryllium atom (atomic number 4), or three He's fused into a carbon atom (atomic number 6), etc. I don't know if there's specific theory as to whether or not multiple helium atoms can be just added like that.

In any event, my understanding of these events would sort of mitigate against the idea of a 'tritium' star, since the vast majority of naturally occuring hydrogen is H1 (single proton, no neutrons). A conventional fusion reaction would be formation of 4 hydrogen atoms (all single protons) into a helium atom (two protons, 2 neutrons). I can't fathom how so much tritium would be formed in nature. but then again, I'm no nuclear engineer.

### #4 matt

I don't have the list at hand, but, in no particular order:
- oxygen is turned into silicium
- silicium is turned into iron
- lithium is turned into carbon

I don't know if there are different "paths" leading to one element or if it always follows the same path. As for the Helium "adding", I think it's hard to fuse more than two atoms at a time.
Interesting questions, we should search for an authoritative document on the matter.

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### #8 Jim Svetlikov

Much of the way a star undergoes fussion depends on the stars initial composition, its mass and the temperatures required for the fussion of one element to another.

Therefore, we will see fussion of the heaviest elements only in stars with a very high mass and will follow the CNO Cycle (Carbon, Nitrogen, Oxygen) as catalysts for the production of helium, while smaller main sequence stars like the Sun will follow the proton-proton chain.

Although a variety of isotopes are formed in the CNO cycle, as carbon, nitrogen and oxygen are mcuh larger atoms compared with hydrogen and helium, the pathway to heavier elements can simply be followed using the periodic table:

### #9 Guest_**DONOTDELETE**_*

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### #11 Jim Svetlikov

We can't say that there is a constant sequence in supernovae due to the composition of mass, but there is a sequence with fusion. So, its a matter of knowing the composition and mass of a star in order to know what elements it will produce.

### #12 Guest_**DONOTDELETE**_*

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### #14 Guest_**DONOTDELETE**_*

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To the tritium question: there ARE stars which burn isotopes of hydrogen and not hyrogen itself. These are brown dwarfs, stars born with too little mass to support 15 million degrees inside the core but enough mass to support the less needy Deutirium->Helium interaction. They burn up all the Deutirium, leaving the regular Hydrogen as it is, and die when this relatively rare fule runs out.

A small remark to what was written above:

A supernova does not occur as a result of energy released from fast nuclear fusion. A supernova occurs because the electron degenarate force preventing the iron core from collapsing breaks down suddenly. Quantom forces, unlike "conventional forces", are not continuous: once gravity over-comes this sub-atomic force, every atom inside the core loses it's ability to support itself, almost at once.

This results in the immediate collapse of the core: an earth-size iron core will collapse into a city-sized Neutron star in less than a millisecond. The result is that the outer portions of the core are moving at a significant fraction of the speed of light. When collapse is over a huge amount of kinetic energy from the infalling matter must go somewhere, and so an outer layer of matter bounces back at a great speed carrying all this energy back up. This is the shockwave of supernova. Colliding with the outer layers of the stars, it exerts it's energy upon this matter in many ways: usually causing fusion of more massive elements, and blowing the entire mantle away from the core.

In a type I supernova a white dwarf collapses in exactly the same way, but this time there is no star mantle to dampen the shock wave. This is why much more energy is released.

### #15 Guest_**DONOTDELETE**_*

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### #16 JamesBaud

Probably nothing new, but I'll add my 2 cents.

The exact process will depend upon the mass of the star in question. Smaller stars wont be able to "finish" the series.

When hydrogen gets rare enough, the rate of reaction in the sun's core slows down. The core will shrink. The shrinking will increase the temperature, gently increasing the size of the outer layers. (red giant)

Eventually, the temperature at the core will get hot enough to fuse helium.

Two helium atoms fuse into beryllium(spelling?), but this is only an intermediate step. beryllium fuses with helium to produce carbon. This is the "main" way at this point. Sometimes carbon will fuse with helium to produce oxygen.
The sun actually shrinks during this, and becomes a yellow giant. The energy produced by this reaction is only a fraction of hydrogen fusion, so it lasts a much shorter time.

Suffeciently massive stars will skip the red giant phase and go straight to yellow giants as they have enough pressure in the core to fuse helium without the intermediate, core shrinking phase.

At this point, our own sun will stop. It has not enough mass to fuse the carbon and oxygen it's creating. If I understand it right, the core will essentially be a white dwarf, and it'll blow off the shell of gasses surrounding it, turning us into a nice pretty planetary nebula.

A more massive star will be able to fuse the carbon and oxygen into silicon. I don't know exactly how this works, and there may be intermediate steps producing other elements. This only produces a fraction of the energy produced by the previous fusion, so it lasts an even shorter time.

After that, silicon is fused into Iron. This produces very little energy comparitively, and this step lasts only a couple days I think.

Iron fusion always TAKES energy rather than releasing it, so fusion chain reactions stop there. The outer layers of the star start falling inwards towards the superdense iron core. In the case of a supernova, all kinds of weird stuff happens due to the obscene pressure. The core starts freaking out, photons destroying atoms and some such. My physics is far too meager to understand whats happening at this point. It collapses to a certain point and then suddenly stops as it reaches the density around that of an atomic nucleus. There's a shockwave produced when it stops contracting which blows the rest of the inwart falling material all over the place. I think thats a type II supernova.

Then there are stars that don't fall in the "main sequence" and they do all kinds of weird stuff. One of the stars in Cassiopeia is like this.

So I don't think there's really a perfect sequence they follow. If I remember right, helium->carbon produces hydrogen atoms, which I assume will fuse into helium. So they're happening at the same times. But in general hydrogen -> helium -> carbon -> oxygen -> silicon -> iron. A certain percentage will probably form other weird combinations just due to chance conditions.

I suppose it isn't random chance that our planet is made up mostly of hydrogen, carbon, oxygen, silicon, and iron I'm sure there's a good explanation for all the nitrogen too, though I don't know what it is.

## Ask Ethan: Can Normal Stars Make Elements Heavier (And Less Stable) Than Iron?

“Comrades, this man has a nice smile, but he’s got iron teeth.”
-
Andrei A. Gromyko

There are some 90+ elements of the periodic table that occur naturally in the Universe, but of them all, iron is the most stable. If you fuse lighter elements together to get closer to iron, you gain energy the same is true if you split heavier elements apart. Iron represents the most stable configuration of protons and neutrons, combined, of any atomic nucleus yet discovered. At only element 26, however, it represents the end-of-the-line for most fusion reactions in even the most massive stars. Or does it? That’s what James Beall wants to know:

Iron has been called stuff like solar fusion ash that collects inside stars, as the last of the elements that fuse w/o consuming more energy than the fusion creates. I have read about the r-process and others that lead to heavier elements in novas and supernovas. My Q is if any elements heavier than iron fuse anyway in normal stars, even if it does consume more energy then it generates.

The answer, as you might expect, is a little complicated: you do make heavier elements than iron in normal stars, but only a very small amount comes from fusion.

All stars begin by fusing hydrogen into helium, from the tiny red dwarfs as little as 8% the mass of our Sun, up to the largest, most massive stars in the Universe that weigh in at hundreds of times the mass of our own. For about 75% of these stars, helium is the end of the line, but the more massive ones (like our Sun) will develop a red giant phase, where they fuse helium into carbon. But a very tiny percentage of stars — just over 0.1% — are among the most massive of all, and can initiate carbon fusion and beyond. These are the stars destined for supernovae, as they fuse carbon into oxygen, oxygen into silicon and sulphur, and then enter the final burning phase (silicon-burning) before going supernova.

That’s the normal life-cycle of the most massive stars in the Universe, but “silicon-burning” doesn’t work by smashing two silicon nuclei together to build something heavier. Instead, it’s just a chain reaction of the addition of helium nuclei to a silicon nucleus, occurring at temperatures in excess of 3,000,000,000 K, or more than 200 times the temperature at the center of the Sun. The chain reaction proceeds as follows:

• silicon-28 plus helium-4 yields sulphur-32,
• sulphur-32 plus helium-4 yields argon-36,
• argon-36 plus helium-4 yields calcium-40,
• calcium-40 plus helium-4 yields titanium-44,
• titanium-44 plus helium-4 yields chromium-48,
• chromium-48 plus helium-4 yields iron-52,
• iron-52 plus helium-4 yields nickel-56, and
• nickel-56 plus helium-4 yields zinc-60.

You’ll notice there’s no iron-56 produced, and there are two reasons why.

One is that, if we look at this portion of the periodic table, we can see that there are too few neutrons for the number of protons in these nuclei. Iron-52, for instance, is unstable it emits a positron and decays to manganese-52, moving down the periodic table. (The manganese then emits another positron and decays to chromium-52, which is stable.) The nickel-56 is also unstable, decaying to cobalt-56, which then decays to iron-56, and is how we arrive at the periodic table’s most stable element. And zinc-60 decays first to copper-60, which then decays again to nickel-60. All of these end products are stable, so yes, these stars — even before going supernova — can produce cobalt, nickel, copper, and zinc, all of which are heavier than iron.

If this isn’t energetically favorable, though, how is this possible? I want you to look at the chart above, which details the binding energy per nucleon in each of the atomic nuclei. I want you to notice how flat the chart is near iron-56 many elements on either side have almost the exact same binding energy per nucleon. Now look all the way over on the left side to helium-4. What do you notice?

Helium-4 is not as tightly bound as any of the nuclei around iron-56. So even though, for example, zinc-60 might have less binding energy per nucleon than nickel-56, it still has more binding energy per nucleon than nickel-56 combined with helium-4. Overall, the net reaction is positive. What we wind up with, therefore, in the last moments before a supernova, is a mix of elements all the way up to zinc: a full four elements heavier than iron.

You might wonder about even heavier elements, then. Would it be possible to, say, add another helium-4 nucleus to zinc-60, producing germanium-64? In trace amounts, probably, but not in any significant quantities. The simple reason? In part, it’s that the energy difference is now almost exactly zero between the two states. But more significantly, you run out of time. For an extremely massive star, the lifetime of the various stages are approximately:

• Hydrogen fusion: millions of years
• Helium fusion: hundreds of thousands of years
• Carbon fusion: hundreds to one thousand of years
• Oxygen fusion: months to one year
• Silicon fusion: hours to one-or-two days.

In other words, that final stage — the one that produces iron and the iron-like elements — doesn’t last long enough to go beyond that.

But if you’re willing to consider what takes place inside a massive star that already has iron and iron-like elements, you can build your way all the way up to lead and bismuth. You see, once you’ve had supernovae in the Universe, you have significant amounts of iron, cobalt, nickel, etc., and these heavy elements wind up in new generations of stars that form. In stars that are between 60–1000% as massive than the Sun (but not usually massive enough for supernovae), you can fuse carbon-13 with helium-4, you can produce oxygen-16 and a free neutron, while stars that will go supernova will fuse neon-22 with helium-4, producing magnesium-25 and a free neutron. Both of these processes can build up heavier and heavier elements, reaching all the way up to lead, bismuth, and even (temporarily) polonium.

Perhaps ironically, it’s the higher-mass stars that produce large amounts of the lighter elements (up to rubidium and strontium or so: elements 37 and 38), while the lower-mass (non-supernova) stars will take you the rest of the way up to lead and bismuth. It isn’t technically a fusion reaction it’s neutron capture, but it’s how you build up the heavier and heavier elements. The biggest reason why the lower-mass stars can get you to such great heights, metaphorically?

The lower-mass stars remain in this neutron-producing state for tens or even hundreds of thousands of years, while the stars destined for supernovae produce neutrons for only hundreds of years, or even fewer. The energy concerns are a really big deal when it comes to fusion even at temperatures of billions of degrees, reactions still proceed in the direction that’s more energetically favorable. But precious time is the biggest constraint for building up heavier and heavier elements. Incredibly, with the right combination of neutron capture and nuclear fusion, about half of all the elements beyond iron are produced inside stars, without supernovae or merging neutron stars at all.

## As a star begins to die, what are the effects on orbiting planets before the supernova?

Say an Earth-like planet (read: Earth) is orbiting a star of 1 solar mass that is nearing it's death. At what point does life on this planet cease to exist (read: no longer habitable)? What exactly happens to scour all possibilities of survival? Or would all be well and good until the supernova begins?

All life on Earth will cease to exist when the sun is still clearly on mainastage. Despite the fact that sun will not become a red giant before 4billion more years, the sun is steadily becomming more luminous. After about 1billion years from now, the sun is so hot that all life on earth is compleatly impossible. After that the sun becomes a red giant and possibly destroyes the entire planet. After that the sun goes through some up and downs in power (so severe that life at any distance orbiting the sun is impossible.), and then finally shrinks to a white dwarf.

A 1 solar mass star will never explode.

Stars that do go supernova go through phase transitions aswell but MUCH faster, at an increasing rate. One a year before explosion, one a week before and a final one 24h before. So no, one would not be well and good before the supernova. A super nova is just the grand finale of a destructive dance the star performs.

Lone 1 solar mass stars do not go supernova (or nova). Eventually they enter a period of Helium burning and become red giants. During this phase the star gets brighter and much, much larger. So much larger that it's outer layers will eventually reach Earth's orbit. Though long before then Earth would have had its atmosphere stripped by increasingly strong solar heating and winds. Over a period of millions of years Earth would lose its atmosphere, have its surface baked by the Sun, and then would eventually be vaporized (or, more accurately, eroded piece by piece by a hot plasma) as the Sun's photosphere reached it. During this red giant phase the Sun would lose a considerable amount of mass from incredibly strong solar winds. Eventually the the Sun would run out of elements it could fuse given the pressures and temperatures possible for its mass, the remainder of the outer envelope of the Sun would be stripped off and a small (Earth-sized) white dwarf would be left, which would slow cool down over a period of trillions of years.

Planets with life around stars that go supernova would be extraordinarily rare. Solitary stars do not go supernova unless they start off very massive (more than 8x the Sun's mass) and such stars have very short lifetimes (less than 60 million years), likely too short for life to even begin evolving on a planet, even should one be favorable for such. It might also be possible for a lone white dwarf to go supernova if it was spinning incredibly rapidly when it was born, and then when it slows down it could undergo a Type Ia supernova. However, that too is problematic because the only stars that would be candidates for such would have already gone through a red giant phase and they would also have to have been originally fairly massive (again, around 8 solar masses or more). It's unlikely that life would have been able to evolve on a planet in only a few tens of millions of years, somehow survive a red giant phase, and also survive the billions of years of existing around a white dwarf.

Another possibility is for a planet to exist in a multiple star system with one of the stars being a white dwarf with a close companion of a more massive star that it feeds from, eventually resulting in a Type Ia supernova. Again, this raises the question of how the planet managed to survive the original red giant phase. Potentially I could see something like a planet around a Sun-like star which distantly orbits a pair of stars that are close together (a trinary system) with one of the pair being a lower mass star and the other having been a higher mass star that became a white dwarf and then started taking mass from the close companion.

## Is Iron poison for a star?

I did not understand it fully.Please try to explain once again. specially red 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.

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

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?

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".

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 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.
==============================

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.

## Does any iron fuse in stars before they go supernova? - Astronomy

Why will fusion wait until hydrogen is gone before starting to fuse heavier atoms? Why will endothermic fusions wait until iron is everywhere?

For the fusion of any element you have to have sufficiently high temperature. This high temperature comes along with sufficiently high density and pressure. Inside a star, only the core (innermost 10% or so of the star) has sufficient density and pressure to start fusing Hydrogen.

When stars switch from fusing one element to another they have a sort of a hiccup. I'll explain. As a star burns the hydrogen in its core, the Helium produced in the reaction sinks to the center because it is heavier. Over time you have a successfully larger Helium core with a hydrogen shell. Hydrogen burning is not energetic enough to start off the Helium burning on its own. Instead what happens is that once all the burnable Hydrogen (only the hydrogen within the innermost 10% or so of the star) is used up fusion temporarily ceases, the core cools and contracts (the contraction is primarily due to the fact that when you convert H to He you have a fewer number of atoms left over thus decreasing the pressure), and the core begins to collapse in on itself. The collapse quickly increases the temperature, pressure, and density in the core. IF the star is massive enough to produce sufficient pressure, Helium will start burning. The heat released from the reaction re-expands the core and is sufficient to increase the temperature in the core to the point where helium burning can be sustained. However, only a certain portion of the Helium will be burnable (an even smaller region than that for the burnable hydrogen), and once that it burnt up, if the star is massive enough, the process will repeat itself with successively higher elements.

Due to the violence of each hiccup the star will lose some of its outer hydrogen envelope. We've actually seen stars surrounded by several expanding shells of gas which correspond to each hiccup.

As for iron, the fusion burning of lighter elements does not produce not nearly enough energy to start fusing iron in any appreciable quantities. For that you need a much more energetic event such as a supernova explosion. The difference between the energies involved in fusion and a supernova is several orders of magnitude.

#### Marko Krco

Marko has worked in many fields of astronomy and physics including planetary astronomy, high energy astrophysics, quantum information theory, and supernova collapse simulations. Currently he studies the dark nebulae which form stars.

## Supernova explosions

13.8 billion years ago caused primal matter to only form those two atoms, plus a tiny amount of lithium and a few other elements.

So, all stars consist of about that same proportion of hydrogen and helium. [I am ignoring how stars produce the other elements.]

Stars spend most their life in what is known the Main Sequence and understanding what happens during this period serves better to understand the other periods. So what happens.

1) Stars come from clouds, really big ones. Thousands of stars can emerge from a cloud that has gone through its phase of collapsing into thousands, sometimes hundreds of thousands, of regions.

2) Each collapse will have its own amount of mass that forms the central body.

3) If the mass formed exceeds about 1% that of the Sun's mass, the core density and temperature that has steadily gotten hotter and hotter during formation, will smoothly, IIRC, transition to fusing hydrogen. [Deuterium and lithium will likely fuse earlier but this is a story for protostars and pre-mainsequence stars, not main sequence stars that are in an equilibrium state.]

4) A very low mass star will take fuse hydrogen slowly. They are small and very dim. They use up their hydrogen and they're done.

Red dwarfs. Guaranteed to be a low mass star, some can be less than

1% the mass of the Sun. Being a lightweight, it will last for perhaps a trillion years. So none have expired naturally. [I guess that’s a pun. ]

5) More collapsed mass means the core will be more dense and it will have a higher temperature. Fusion will be far more productive. They are bigger and brighter.

As these more massive protostars form, their centers get hotter and hotter, much more so than the little red dwarfs. Eventually, the core conditions, as always, collapse to the point they will allow a smooth transition to hydrogen fusion.

But stars, like our Sun, have enough mass that as the hydrogen fuses to helium, the core itself will contract. Eventually, this shrinkage will have higher and higher temperatures and other conditions that will reach a point that helium fusion takes place.

But, surrounding this helium burning core is what? Hydrogen, right? So circumstances cab be that the inner core is fusing helium while the outer core is fusing hydrogen. I think that's true, eventually, for Sun-mass stars. [I know its true for the more massive ones.]

6) The really massive stars also go through this same process, but their greater mass causes a much faster rate of fusion. This makes them bigger and brighter.

These massive stars will slip off the Main Sequence, far sooner than the less massive stars, as they have layers of fusion, the top layer being hydrogen. They will continue to shrink as they consume their fuel until the point where the next element to burn is iron. When you fuse iron it absorbs energy. So if the star is massive enough to reach this point, the core will collapse as it fuses more and more iron, absorbing more and more energy, cooling the core, which triggers a super fast collapse. This triggers a supernova (Type II).

## Ask Ethan: Can Normal Stars Make Elements Heavier (And Less Stable) Than Iron?

The cluster Terzan 5 has many older, lower-mass stars present within (faint, and in red), but also . [+] hotter, younger, higher-mass stars, some of which will generate iron and even heavier elements.

There are some 90+ elements of the periodic table that occur naturally in the Universe, but of them all, iron is the most stable. If you fuse lighter elements together to get closer to iron, you gain energy the same is true if you split heavier elements apart. Iron represents the most stable configuration of protons and neutrons, combined, of any atomic nucleus yet discovered. At only element 26, however, it represents the end-of-the-line for most fusion reactions in even the most massive stars. Or does it? That's what James Beall wants to know:

Iron has been called stuff like solar fusion ash that collects inside stars, as the last of the elements that fuse w/o consuming more energy than the fusion creates. I have read about the r-process and others that lead to heavier elements in novas and supernovas. My Q is if any elements heavier than iron fuse anyway in normal stars, even if it does consume more energy then it generates.

The answer, as you might expect, is a little complicated: you do make heavier elements than iron in normal stars, but only a very small amount comes from fusion.

A young star cluster in a star forming region, consisting of stars of a huge variety of masses. Some . [+] of them will someday undergo silicon-burning, producing iron and many other elements in the process.

All stars begin by fusing hydrogen into helium, from the tiny red dwarfs as little as 8% the mass of our Sun, up to the largest, most massive stars in the Universe that weigh in at hundreds of times the mass of our own. For about 75% of these stars, helium is the end of the line, but the more massive ones (like our Sun) will develop a red giant phase, where they fuse helium into carbon. But a very tiny percentage of stars — just over 0.1% — are among the most massive of all, and can initiate carbon fusion and beyond. These are the stars destined for supernovae, as they fuse carbon into oxygen, oxygen into silicon and sulphur, and then enter the final burning phase (silicon-burning) before going supernova.

The anatomy of a very massive star throughout its life, culminating in a Type II Supernova when the . [+] core runs out of nuclear fuel. The final stage of fusion is silicon-burning, producing iron and iron-like elements in the core for only a brief while before a supernova ensues.

That's the normal life-cycle of the most massive stars in the Universe, but "silicon-burning" doesn't work by smashing two silicon nuclei together to build something heavier. Instead, it's just a chain reaction of the addition of helium nuclei to a silicon nucleus, occurring at temperatures in excess of 3,000,000,000 K, or more than 200 times the temperature at the center of the Sun. The chain reaction proceeds as follows:

• silicon-28 plus helium-4 yields sulphur-32,
• sulphur-32 plus helium-4 yields argon-36,
• argon-36 plus helium-4 yields calcium-40,
• calcium-40 plus helium-4 yields titanium-44,
• titanium-44 plus helium-4 yields chromium-48,
• chromium-48 plus helium-4 yields iron-52,
• iron-52 plus helium-4 yields nickel-56, and
• nickel-56 plus helium-4 yields zinc-60.

You'll notice there's no iron-56 produced, and there are two reasons why.

Iron and the iron-like elements (highlighted here) surrounding it are primarily produced in the . [+] final moments of an ultra-massive star's life, shortly before it goes supernova, in the processes that ensue during the silicon-burning stage.

Michael Dayah / https://ptable.com/

One is that, if we look at this portion of the periodic table, we can see that there are too few neutrons for the number of protons in these nuclei. Iron-52, for instance, is unstable it emits a positron and decays to manganese-52, moving down the periodic table. (The manganese then emits another positron and decays to chromium-52, which is stable.) The nickel-56 is also unstable, decaying to cobalt-56, which then decays to iron-56, and is how we arrive at the periodic table's most stable element. And zinc-60 decays first to copper-60, which then decays again to nickel-60. All of these end products are stable, so yes, these stars — even before going supernova — can produce cobalt, nickel, copper, and zinc, all of which are heavier than iron.

Iron-56 may be the most tightly-bound nucleus, with the greatest amount of binding energy per . [+] nucleon. However, slightly lighter and heavier elements are almost exactly as stable and tightly bound, with only minuscule differences.

If this isn't energetically favorable, though, how is this possible? I want you to look at the chart above, which details the binding energy per nucleon in each of the atomic nuclei. I want you to notice how flat the chart is near iron-56 many elements on either side have almost the exact same binding energy per nucleon. Now look all the way over on the left side to helium-4. What do you notice?

Helium-4 is not as tightly bound as any of the nuclei around iron-56. So even though, for example, zinc-60 might have less binding energy per nucleon than nickel-56, it still has more binding energy per nucleon than nickel-56 combined with helium-4. Overall, the net reaction is positive. What we wind up with, therefore, in the last moments before a supernova, is a mix of elements all the way up to zinc: a full four elements heavier than iron.

Artists illustration (left) of the interior of a massive star in the final stages, pre-supernova, of . [+] silicon-burning. A Chandra image (right) of the Cassiopeia A supernova remnant today shows elements like Iron (in blue), sulphur (green), and magnesium (red).

NASA/CXC/M.Weiss X-ray: NASA/CXC/GSFC/U.Hwang & J.Laming

You might wonder about even heavier elements, then. Would it be possible to, say, add another helium-4 nucleus to zinc-60, producing germanium-64? In trace amounts, probably, but not in any significant quantities. The simple reason? In part, it's that the energy difference is now almost exactly zero between the two states. But more significantly, you run out of time. For an extremely massive star, the lifetime of the various stages are approximately:

• Hydrogen fusion: millions of years
• Helium fusion: hundreds of thousands of years
• Carbon fusion: hundreds to one thousand of years
• Oxygen fusion: months to one year
• Silicon fusion: hours to one-or-two days.

In other words, that final stage — the one that produces iron and the iron-like elements — doesn't last long enough to go beyond that.

The spiral structure around the old, giant star R Sculptoris is due to winds blowing off outer . [+] layers of the star as it undergoes its AGB phase, where copious amounts of neutrons (from carbon-13 + helium-4 fusion) are produced and captured.

ALMA (ESO/NAOJ/NRAO)/M. Maercker et al.

But if you're willing to consider what takes place inside a massive star that already has iron and iron-like elements, you can build your way all the way up to lead and bismuth. You see, once you've had supernovae in the Universe, you have significant amounts of iron, cobalt, nickel, etc., and these heavy elements wind up in new generations of stars that form. In stars that are between 60-1000% as massive than the Sun (but not usually massive enough for supernovae), you can fuse carbon-13 with helium-4, you can produce oxygen-16 and a free neutron, while stars that will go supernova will fuse neon-22 with helium-4, producing magnesium-25 and a free neutron. Both of these processes can build up heavier and heavier elements, reaching all the way up to lead, bismuth, and even (temporarily) polonium.

Chart representing the final part of the s-process. Red horizontal lines with a circle in their . [+] right ends represent neutron captures blue arrows pointing up-left represent beta decays green arrow pointing down-left represents an alpha decay cyan arrows pointing down-right represent electron captures.

R8R Gtrs / Wikimedia Commons

Perhaps ironically, it's the higher-mass stars that produce large amounts of the lighter elements (up to rubidium and strontium or so: elements 37 and 38), while the lower-mass (non-supernova) stars will take you the rest of the way up to lead and bismuth. It isn't technically a fusion reaction it's neutron capture, but it's how you build up the heavier and heavier elements. The biggest reason why the lower-mass stars can get you to such great heights, metaphorically?

Periodic table showing origin of elements in the Solar System, based on data by Jennifer Johnson at . [+] Ohio State University.

Cmglee at Wikimedia Commons

The lower-mass stars remain in this neutron-producing state for tens or even hundreds of thousands of years, while the stars destined for supernovae produce neutrons for only hundreds of years, or even fewer. The energy concerns are a really big deal when it comes to fusion even at temperatures of billions of degrees, reactions still proceed in the direction that's more energetically favorable. But precious time is the biggest constraint for building up heavier and heavier elements. Incredibly, with the right combination of neutron capture and nuclear fusion, about half of all the elements beyond iron are produced inside stars, without supernovae or merging neutron stars at all.

## Does any iron fuse in stars before they go supernova? - Astronomy

Posted on 05/14/2021 9:31:37 AM PDT by Red Badger

L2 Puppis, a red giant star like SPLUS J2104-0049. (ESO/Digitized Sky Survey 2)

A red giant star 16,000 light-years away appears to be a bona fide member of just the second generation of stars in the Universe.

According to an analysis of its chemical abundances, it seems to contain elements produced in the life and death of just a single first-generation star. Therefore, with its help, we might even find the first generation of stars ever born - none of which have yet been discovered.

Additionally, the researchers performed their analysis using photometry, a technique that measures the intensity of light, thus offering a new way to find such ancient objects.

"We report the discovery of SPLUS J210428.01−004934.2 (hereafter SPLUS J2104−0049), an ultra-metal-poor star selected from its narrow-band S-PLUS photometry and confirmed by medium- and high-resolution spectroscopy," the researchers wrote in their paper.

"These proof-of-concept observations are part of an ongoing effort to spectroscopically confirm low-metallicity candidates identified from narrow-band photometry."

Although we feel like we have a pretty good grasp of how the Universe grew from the Big Bang to the star-studded glory we know and love today, the first stars to turn on their blinking lights in the primordial darkness, known as Population III stars, remain something of a mystery.

Current day star-formation processes give us some clues about how these early stars came together, but until we find them, we're basing our understanding on incomplete information.

One trail of breadcrumbs are the Population II stars - the next few generations following Population III. Of those, the generation immediately succeeding Population III are perhaps the most exciting, since they are the closest in composition to Population III.

We can identify them by their extremely low abundance of elements like carbon, iron, oxygen, magnesium and lithium, detected by analysing the spectrum of light emitted by the star, which contains the chemical fingerprints of the elements therein.

That's because, before stars came into existence, there were no heavy elements - the Universe was a sort of cloudy soup of mostly hydrogen and helium. When the first stars formed, this is what they ought to have been made of, too - it's via the process of thermonuclear fusion in their cores that the heavier elements were formed.

First, hydrogen is fused into helium, then helium into carbon, and so forth all the way down to iron, depending on the mass of the star (the smallest ones don't have enough energy to fuse helium into carbon, and end their lives when they reach that point). Even the most massive stars don't have enough energy to fuse iron when their core is entirely iron, they go supernova.

These colossal cosmic explosions spew all that fused material out into nearby space in addition, the explosions are so energetic, they generate a series of nuclear reactions that forge even heavier elements, such as gold, silver, thorium and uranium. Baby stars then forming from clouds that contain these materials have higher metallicity than the stars that came before.

Today's stars - Population I - have the highest metallicity. (By-the-by, this does mean that eventually no new stars will be able to form, since the Universe's hydrogen supply is finite - good times.) And stars that were born when the Universe was very young have very low metallicity, with the earliest stars known as ultra-metal-poor stars or UMP stars.

These UMPs are considered bona fide Population II stars, enriched by material from just a single Population III supernova.

Using a photometric survey called S-PLUS, a team of astronomers led by the National Science Foundation's NOIRLab identified SPLUS J210428-004934, and although it doesn't have the lowest metallicity we've detected yet (that honor belongs to SMSS J0313-6708), it has an average metallicity for a UMP star.

It also has the lowest carbon abundance astronomers have ever seen in an ultra-metal-poor star. This could give us an important new constraint on the progenitor star and stellar evolution models for very low metallicities, the researchers said.

To figure out how the star could have formed, they performed theoretical modeling. They found the chemical abundances observed in SPLUS J210428-004934, including the low carbon and the more normal UMP star abundances of other elements, could best be reproduced by a high-energy supernova of a single Population III star 29.5 times the mass of the Sun.

However, the closest fits from the modeling still were unable to produce enough silicon to exactly replicate SPLUS J210428-004934. They recommended looking for more ancient stars with similar chemical properties to try to resolve this strange discrepancy.

"Additional UMP stars identified from S-PLUS photometry will greatly improve our understanding of Pop III stars and enable the possibility of finding a metal-free low-mass star still living in our Galaxy today," the researchers wrote.

Their paper has been published in The Astrophysical Journal Letters.

TOPICS: Astronomy History Science