<p>Apart from the good reasons given in the other answers as to why a ball of iron in excess of about $1.4$M$_\odot$ cannot be stable, there is another reason. Namely, there is no way to form that much pure iron. Iron is produced in supernovae, but only a fraction of the matter expelled by a supernova is actually iron and there is no natural way to select is. The remaining matter is locked in the stellar remnant, either a white dwarf, neutron star, or (stellar-mass) black hole. The latter two are already to massive for the material to be in the form of iron, while the a white dwarf may only contain some iron, but never is a ball of iron.</p>

CC BY-SA 3.0
2014-08-20T08:20:56.587Z
2014-08-20T08:20:56.587Z
Walter
<h2>Iron spheres</h2>

<p>There is a hypothesis that after about $10^{1500}$ years (many star generations) iron stars (iron sphere star remnants) will form. This would require all the elements lighter than Fe to fuse into Fe and all the elements heavier than Fe to decay into Fe. Also proton is required not to decay for this hypothesis to work.</p>

<h2>Mass limit</h2>

<p>Only stars under certain mass limit can possibly form a sphere of iron. As others pointed out after the nuclear fusion ceases stars heavier than 1.44 solar masses will collapse into a <a href="https://en.wikipedia.org/wiki/Neutron_star" rel="nofollow">neutron star</a> and above several solar masses they will further collapse into a <a href="https://en.wikipedia.org/wiki/Black_hole" rel="nofollow">black hole</a>.</p>

<p>This means that iron spheres considerably heavier than the solar mass cannot form because the iron nuclei of heavier objects will be crushed to mostly neutrons or some other state of matter not consisting of iron.</p>

<h2>References for the iron star hypothesis</h2>

<ul>
<li><a href="https://en.wikipedia.org/wiki/Iron_star" rel="nofollow">iron star</a> on Wikipedia</li>
<li>Dyson, Freeman J. (1979). "Time without end: Physics and biology in an open universe". Reviews of Modern Physics 51 (3): 447–460. Bibcode:1979RvMP...51..447D. doi:10.1103/RevModPhys.51.447.</li>
<li>Dyson, Freeman J. (1996). "<a href="http://books.google.cz/books?id=nnyNUidX1OMC&amp;lpg=PA534&amp;ots=T-eVa_TpyS&amp;dq=%22iron%20star%22%20fusion&amp;pg=PA534#v=onepage&amp;q=%22iron%20star%22%20fusion&amp;f=false" rel="nofollow">Selected Papers of Freeman Dyson with Commentary</a>", American Mathematical Society</li>
</ul>

CC BY-SA 3.0
2015-05-03T13:06:52.553Z
The maximum mass of a ball of fully ionised iron, supported by ideal electron degeneracy pressure, is around $1.2 M_{\odot}$. The limit of 1.44$M_{\odot}$ you quote is too high even for a carbon/oxygen white dwarf - it is more like 1.38$M_{\odot}$. The outcome of a C/O WD exceeding this is almost certainly a type Ia supernova, not a neutron star. However, I concur that an iron WD that exceeds its Chandrasekhar mass will likely end up as a neutron star.
ProfRob
CC BY-SA 3.0
2014-10-08T00:56:58.630Z
The last paragraph is really only true for stars that cannot undergo nuclear fusion any longer. Thanks for citing sources! I've been hoping for an uptick in citations.
HDE 226868
CC BY-SA 3.0
2014-10-08T01:11:05.787Z
@HDE226868 thanks for the note. I thought that it was obvious (also from the first paragraph) that the collapse happens when the fusion activity decreases. I have edited the answer to explicitly mention the end of the fusion. --- The second paragraph is only to mention the star mass limit for the Fe nuclei not to be destroyed by the star collapse.
pabouk - Ukraine stay strong
CC BY-SA 3.0
2014-10-08T00:47:26.190Z
2014-10-08T17:11:05.043Z
2014-10-08T17:11:05.043Z
pabouk - Ukraine stay strong
<p>If you had a galaxy sized ball of iron the gravitational force emitted would be immense, and without any force pushing out from it the ball would collapse into itself to form a black hole. In stars such as the sun (main-sequence stars) the outward force of nuclear reactions taking place in its core keep it from collapsing into itself. However, with a ball of iron the only force acting on it would be the pull of gravity, eventually crushing the ball into itself. </p>

<p>The ball of iron wouldn't even be able to reach the size of a galaxy. There is a limit known as the Chandrasekhar limit, which says anything that is about 1.44 solar masses would collapse. This would most likely form a neutron star, but this also has a limit to the amount of mass it can hold. Eventually, even this will collapse upon itself to form a black hole. </p>

CC BY-SA 3.0
2016-10-01T15:59:15.070Z
The "Chandrasekhar mass" for a ball of iron is more like 1.2 solar masses.
ProfRob
CC BY-SA 3.0
2014-08-19T08:29:04.577Z
Isn't this essentially what [@Warrick said about an hour ago](http://astronomy.stackexchange.com/a/6167/373)?
John Dvorak
CC BY-SA 3.0
2014-08-19T08:34:46.523Z
It is similar to his answer. We both had the same ideas
Nirvik Baruah
CC BY-SA 3.0
2014-08-19T08:25:03.340Z
2014-08-19T08:25:03.340Z
Nirvik Baruah
<p>Our sun will eventually become a white dwarf. A star 10 times it mass will become a neutron star. A star 100 times the mass of the sun will become a black hole. So if 100 stars came together into one mass, it would collapse into a black hole. It is currently believed that a super massive black hole (at least 1000 times the mass of our sun) is at the center of our galaxy and also in the Andromeda galaxy. This is where all the iron might be found from this region of space.</p>

CC BY-SA 3.0
2014-08-19T16:50:41.090Z
You wouldn't need 100 stars to come together to make a black hole. Also, the masses of the two black holes mentioned are way more than 1,000 solar masses. And would an atom of iron really survive deep inside a black hole?
HDE 226868
CC BY-SA 3.0
2014-08-20T00:30:59.363Z
@HDE226868 I knew that. I just didn't want to take the time to look up the correct amounts.
LDC3
CC BY-SA 3.0
2014-08-19T02:47:00.307Z
2014-08-19T02:47:00.307Z
LDC3
<p>In short, you can't make a galaxy-mass ball of iron because there's no way to support the ball against the inward crush of its own gravity. First, it would collapse into a neutron star, but even neutron stars can't hold themselves up beyond about 2-3 solar masses.  Your galaxy-sized object would collapse into a black hole.  But we do see these!  All massive galaxies are believed to have supermassive black holes at their centres. These are up to 10 billion solar masses in size, which is comparable to smaller galaxies.</p>

<p>In any star, be it a main-sequence star like the Sun, a giant, or even a white dwarf or neutron star, the inward force of gravity must be balanced by some outward force or pressure.  (In fact, we basically assume this when we construct stellar models, although it's a very good assumption.) In main-sequence stars, this pressure is provided by the nuclear reactions happening inside them but as a star evolves, it gets a bit more complicated. In some situations, the nuclei get so close together that they their electrons begin to share quantum states. And because no two electrons can be in the same state, they have to occupy higher energy states. This in effect exerts a kind of pressure (called <em>degeneracy pressure</em>), because the electrons are forced to move around faster than they otherwise would. This is what supports white dwarfs and the cores of some evolved stars. e.g. low-mass red giants.</p>

<p>Degeneracy pressure is fine up to a limit known as the <em>Chandrasekhar limit</em>, which is about 1.44 solar masses (depending on the composition of the object in question). Anything above this would collapse. First, this collapse would yield a neutron star, where things are a bit different. But a similar principle holds, and there's a degeneracy pressure because the neutrons themselves also occupy one quantum state each.  The details are <em>much</em> more difficult here, but the overall consensus is that neutron degeneracy pressure can support objects up to about 2-3 solar masses. After that, there's nothing left to balance gravity, and the star collapses into a black hole.</p>

CC BY-SA 3.0
2014-08-19T06:49:58.697Z
2014-08-19T08:55:22.900Z
2014-08-19T08:55:22.900Z
Warrick
<p>Since we know that stars burn out, collapse, or blow up when too many of their atoms fuse into iron, then how come, with all the ancient galaxies we've been observing, there isn't one galaxy whose stars have become giant balls of iron, which would eventually collide into one galaxy-sized iron globule in space?</p>

CC BY-SA 3.0
2014-08-19T02:04:03.647Z
_meta:_ I also recommend tags like `star-death` and `star-lifecycle`
Ky -
CC BY-SA 3.0
2014-08-19T06:10:29.227Z
Your hypothesis is not correct: not all stars produce iron. Conversely, the most of them have not enough mass to do it.
Py-ser
<star><galaxy>
Why aren't there galaxy-sized balls of iron out there?
Ky -
# Why aren't there galaxy-sized balls of iron out there?

(Asked by: username_0 on 2014-08-19)

Since we know that stars burn out, collapse, or blow up when too many of their atoms fuse into iron, then how come, with all the ancient galaxies we've been observing, there isn't one galaxy whose stars have become giant balls of iron, which would eventually collide into one galaxy-sized iron globule in space?

**Tags:** star, galaxy
> **Comments:**
> **username_0**: _meta:_ I also recommend tags like `star-death` and `star-lifecycle`  
> **username_1**: Your hypothesis is not correct: not all stars produce iron. Conversely, the most of them have not enough mass to do it.  

---

## 5 answers

### Answer by username_9 on 2014-08-19. Score: 20 | Accepted answer

In short, you can't make a galaxy-mass ball of iron because there's no way to support the ball against the inward crush of its own gravity. First, it would collapse into a neutron star, but even neutron stars can't hold themselves up beyond about 2-3 solar masses. Your galaxy-sized object would collapse into a black hole. But we do see these! All massive galaxies are believed to have supermassive black holes at their centres. These are up to 10 billion solar masses in size, which is comparable to smaller galaxies.

In any star, be it a main-sequence star like the Sun, a giant, or even a white dwarf or neutron star, the inward force of gravity must be balanced by some outward force or pressure. (In fact, we basically assume this when we construct stellar models, although it's a very good assumption.) In main-sequence stars, this pressure is provided by the nuclear reactions happening inside them but as a star evolves, it gets a bit more complicated. In some situations, the nuclei get so close together that they their electrons begin to share quantum states. And because no two electrons can be in the same state, they have to occupy higher energy states. This in effect exerts a kind of pressure (called *degeneracy pressure*), because the electrons are forced to move around faster than they otherwise would. This is what supports white dwarfs and the cores of some evolved stars. e.g. low-mass red giants.

Degeneracy pressure is fine up to a limit known as the *Chandrasekhar limit*, which is about 1.44 solar masses (depending on the composition of the object in question). Anything above this would collapse. First, this collapse would yield a neutron star, where things are a bit different. But a similar principle holds, and there's a degeneracy pressure because the neutrons themselves also occupy one quantum state each. The details are *much* more difficult here, but the overall consensus is that neutron degeneracy pressure can support objects up to about 2-3 solar masses. After that, there's nothing left to balance gravity, and the star collapses into a black hole.


---

### Answer by username_2 on 2014-08-20. Score: 5

Apart from the good reasons given in the other answers as to why a ball of iron in excess of about $1.4$M$\_\odot$ cannot be stable, there is another reason. Namely, there is no way to form that much pure iron. Iron is produced in supernovae, but only a fraction of the matter expelled by a supernova is actually iron and there is no natural way to select is. The remaining matter is locked in the stellar remnant, either a white dwarf, neutron star, or (stellar-mass) black hole. The latter two are already to massive for the material to be in the form of iron, while the a white dwarf may only contain some iron, but never is a ball of iron.


---

### Answer by username_3 on 2014-10-08. Score: 2

## Iron spheres

There is a hypothesis that after about $10^{1500}$ years (many star generations) iron stars (iron sphere star remnants) will form. This would require all the elements lighter than Fe to fuse into Fe and all the elements heavier than Fe to decay into Fe. Also proton is required not to decay for this hypothesis to work.

## Mass limit

Only stars under certain mass limit can possibly form a sphere of iron. As others pointed out after the nuclear fusion ceases stars heavier than 1.44 solar masses will collapse into a [neutron star](https://en.wikipedia.org/wiki/Neutron_star) and above several solar masses they will further collapse into a [black hole](https://en.wikipedia.org/wiki/Black_hole).

This means that iron spheres considerably heavier than the solar mass cannot form because the iron nuclei of heavier objects will be crushed to mostly neutrons or some other state of matter not consisting of iron.

## References for the iron star hypothesis

* [iron star](https://en.wikipedia.org/wiki/Iron_star) on Wikipedia
* Dyson, Freeman J. (1979). "Time without end: Physics and biology in an open universe". Reviews of Modern Physics 51 (3): 447–460. Bibcode:1979RvMP...51..447D. doi:10.1103/RevModPhys.51.447.
* Dyson, Freeman J. (1996). "[Selected Papers of Freeman Dyson with Commentary](http://books.google.cz/books?id=nnyNUidX1OMC&lpg=PA534&ots=T-eVa_TpyS&dq=%22iron%20star%22%20fusion&pg=PA534#v=onepage&q=%22iron%20star%22%20fusion&f=false)", American Mathematical Society

> **Comments:**
> **username_5**: The last paragraph is really only true for stars that cannot undergo nuclear fusion any longer. Thanks for citing sources! I've been hoping for an uptick in citations.  
> **username_3**: @HDE226868 thanks for the note. I thought that it was obvious (also from the first paragraph) that the collapse happens when the fusion activity decreases. I have edited the answer to explicitly mention the end of the fusion. --- The second paragraph is only to mention the star mass limit for the Fe nuclei not to be destroyed by the star collapse.  
> **username_4**: The maximum mass of a ball of fully ionised iron, supported by ideal electron degeneracy pressure, is around $1.2 M_{\odot}$. The limit of 1.44$M_{\odot}$ you quote is too high even for a carbon/oxygen white dwarf - it is more like 1.38$M_{\odot}$. The outcome of a C/O WD exceeding this is almost certainly a type Ia supernova, not a neutron star. However, I concur that an iron WD that exceeds its Chandrasekhar mass will likely end up as a neutron star.  

---

### Answer by username_6 on 2014-08-19. Score: 2

If you had a galaxy sized ball of iron the gravitational force emitted would be immense, and without any force pushing out from it the ball would collapse into itself to form a black hole. In stars such as the sun (main-sequence stars) the outward force of nuclear reactions taking place in its core keep it from collapsing into itself. However, with a ball of iron the only force acting on it would be the pull of gravity, eventually crushing the ball into itself.

The ball of iron wouldn't even be able to reach the size of a galaxy. There is a limit known as the Chandrasekhar limit, which says anything that is about 1.44 solar masses would collapse. This would most likely form a neutron star, but this also has a limit to the amount of mass it can hold. Eventually, even this will collapse upon itself to form a black hole.

> **Comments:**
> **username_7**: Isn't this essentially what [@Warrick said about an hour ago](http://astronomy.stackexchange.com/a/6167/373)?  
> **username_6**: It is similar to his answer. We both had the same ideas  
> **username_4**: The "Chandrasekhar mass" for a ball of iron is more like 1.2 solar masses.  

---

### Answer by username_8 on 2014-08-19. Score: -2

Our sun will eventually become a white dwarf. A star 10 times it mass will become a neutron star. A star 100 times the mass of the sun will become a black hole. So if 100 stars came together into one mass, it would collapse into a black hole. It is currently believed that a super massive black hole (at least 1000 times the mass of our sun) is at the center of our galaxy and also in the Andromeda galaxy. This is where all the iron might be found from this region of space.

> **Comments:**
> **username_5**: You wouldn't need 100 stars to come together to make a black hole. Also, the masses of the two black holes mentioned are way more than 1,000 solar masses. And would an atom of iron really survive deep inside a black hole?  
> **username_8**: @HDE226868 I knew that. I just didn't want to take the time to look up the correct amounts.  

---

