Why is there so much iron?

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.