# Why is there so much iron?

We all know where iron comes from. However, as I am reading up on supernovas, I started to wonder why there is as much iron as there is in the universe.

• Neither brown dwarfs nor white dwarfs deposit iron.

• Type I supernovas leave no remnant so I can see where there would be iron released.

• Type II supernovas leave either a neutron star or a black hole. As I understand it, the iron ash core collapses and the shock wave blows the rest of the star apart. Therefore no iron is released. (I know some would be made in the explosion along with all of the elements up to uranium. But would that account for all of the iron in the universe?)

• Hypernovas will deposit iron, but they seem to be really rare.

Do Type I supernovas happen so frequently that iron is this common? Or am I missing something?

• Therefore no iron is released. are you sure? – Kyle Kanos Mar 17 '19 at 1:47
• This table in Wikipedia's "Nucleosynthesis" article might help, detailed here. – Nat Mar 17 '19 at 2:15
• I would disagree with you... There is a LOT of iron, almost as much as Oxygen and Carbon (as well as silicon)...en.wikipedia.org/wiki/Nucleosynthesis#/media/… – Rick Mar 17 '19 at 22:47
• @Jepsilon Specifically, Ni-62 is the peak. However, iron is easier to produce, so while Ni-62 is (very very slightly) more stable, there's more iron. Binding energy isn't everything - after all, most of the visible matter in the universe is still hydrogen, which is a stable element with (one of?) the highest energy per nucleon. – Luaan Mar 18 '19 at 9:56
• Also, type Ia supernovae leave no remnant. Most type Ib and Ic supernovae do leave a remnant, just like most type II supernovae do. (Exceptions are the rare Ib/Ic/II supernovae resulting from pair-production-triggered instability, which leave no remnants.) – Sean Mar 19 '19 at 2:57

## 4 Answers

The solar abundance of iron is a little bit more than a thousandth by mass. If we assume that all the baryonic mass in the disc of the Galaxy (a few $$10^{10}$$ solar masses) is polluted in the same way, then more than 10 million solar masses of iron must have been produced and distributed by stars.

A type Ia supernova results in something like 0.5-1 solar masses of iron (via decaying Ni 56), thus requiring about 20-50 million type Ia supernovae to explain all the Galactic Fe.

Given the age of the Galaxy of 10 billion years, this requires a type Ia supernova rate of one every 200-500 years.

The rate of type Ia supernovae in our Galaxy is not observationally measured, though there have likely been several in the last 1000 years. The rate above seems entirely plausible and was probably higher in the past.

• On an important side note: Iron has one of the largest nuclear binding energies (See en.wikipedia.org/wiki/…). So eventually, the percentage of iron in the universe will increase with time, as it is a stable end-product of both nuclear fusion and nuclear decay. – Robert Tausig Mar 17 '19 at 15:16
• @RobertTausig doesn't iron have THE largest nuclear binding energy (rather than just "one of the largest")? – N. Steinle Mar 17 '19 at 21:42
• Rob, I like your answer. Perhaps it could be even better if you include an approximate rate of double neutron star mergers (which of course the rate is very uncertain but we know that such mergers produce lots of heavy elements) ? Such a NS-NS rate is expected to be at least on the same order as that of supernovae. – N. Steinle Mar 17 '19 at 21:48
• @N.Steinle The Q asks whether type Ia supernovae can be responsible for all the iron. Neutron star mergers do not produce iron. Iron does have "one of the largest" binding energies per nucleon. It is not the largest. That would be Ni 62. – Rob Jeffries Mar 17 '19 at 22:54
• Thank you very much for clarifying! – N. Steinle Mar 18 '19 at 1:55

Iron comes from exploding white dwarfs and exploding massive stars(Wikipedia).

(One of many amazing images by Cmglee )
Periodic table showing the cosmogenic origin of each element. Elements from carbon up to sulfur may be made in small stars by the alpha process. Elements beyond iron are made in large stars with slow neutron capture (s-process), followed by expulsion to space in gas ejections (see planetary nebulae). Elements heavier than iron may be made in neutron star mergers or supernovae after the r-process, involving a dense burst of neutrons and rapid capture by the element.

• While this may answer the question, it is preferable to have the content of the link copied into the post to avoid issues such s link rot, going off-site, etc. – Kyle Kanos Mar 17 '19 at 21:12

Iron is at the minimum point for energy release from fusion. For all atomic numbers less than that of iron, there is a net release of energy as additional protons and neutrons are added. Beyond iron, it's the reverse; energy must be input to fuse protons and neutrons into larger nuclei, which is why larger nuclei are only formed in supernova-type events and larger nuclei release energy on fission. As long as there are conditions to drive these processes, the tendency will be to build smaller nuclei up to iron and split larger nuclei down toward iron.

• True, but not what the Rick is asking about. He's not concerned with how iron is produced, but how it's distributed - that is, how it gets into interstellar space and (eventually) other stars and planets. – Luaan Mar 18 '19 at 9:51
• "which is why larger nuclei are only formed in supernova-type events". Not true. – Rob Jeffries Mar 18 '19 at 13:34

The nucleosynthesis in the inner of the stars generates energy: The huge amounts of energy form Helium from hydrogen. The star then start generating carbon from helium and so an. This finishes with iron. To generate with larger atomic numbers the star needs more energy. Most of them are generated in supernovae, where there is a lot more energy.