[jeffreyH (OP)]

The article highlights a disagreement about the timing and location of the supernova as the source for the Iron-60. I do believe the source was a supernova. I think overall neutron star mergers, rather than simply supernovae, are a more likely source for the majority of the heavier elements in planets.

[evan_au]

I think overall neutron star mergers, rather than simply supernovae, are a more likely source for the majority of the heavier elements in planets.

It comes down to mass and frequency.

It is thought that about 3 supernovae should occur per century in our galaxy (although the clouds of dust probably hide many of them). This happens to any massive star, including some that are truly enormous. They dump lots of matter into interstellar space.

I saw an estimate that there is about 1 neutron star merger every 800 centuries in our galaxy. This requires 2 massive stars in close proximity to go supernova (but not too heavy, or one or both will turn into a black hole). Then you must wait billions of years while they slowly radiate away their angular momentum as gravitational waves, before they finally merge.

That’s why the elements from helium to iron (produced by fusion and released in supernovae) or nickel and cobalt or so (mainly produced from iron by neutron capture during a supernova) are far more abundant than the really heavy elements like gold and above (mainly produced by neutron star mergers).

[puppypower]

Another concern I have with conventional thinking of star formation, is many of the limits we assume of our sun is based on its internal temperature, but not on its internal temperature and pressure. In the real world, temperature and pressure can result in new phases of matter, that temperature alone cannot define.

As an example, at 5000C water is an ionized gas of dissociated radials. If we add sufficient pressure, like that of the core of the earth, water at that same temperature will change into a solid metal. This metallic phase allows a whole new range of properties, that one would not expect, if we assume temperature properties only.

As another example, the core of Jupiter is thought to be metallic hydrogen. One is not dealing with high pressure hydrogen gas with random motion following a bell curve  Rather we are dealing with an orderly solid, that conducts electricity very well. There is room for huge voltages and huge magnetic affects. 

One might even assume our early forming sun, went through a metallic hydrogen core phase, similar to that of Jupiter, capable of generating huge voltages within a rigid matrix. As the sun added more and more material, which added work and pressure, the sun's core may not necessarily have evolved as hydrogen plasma gas phase core. It may have ended up with an evolving plasma metallic hydrogen phase. This core would approach fusion in a different way.

[jeffreyH (OP)]

@evan_au Then it looks like the supernova explanation wins out over neutron star mergers.

[jeffreyH (OP)]

There is still one thing that puzzles me. Why isn't the earth just a uniform mix of all the elements. We have pockets of iron, silver and such like. We cannot mine for particular metals just anywhere.

[chiralSPO]

There is still one thing that puzzles me. Why isn't the earth just a uniform mix of all the elements. We have pockets of iron, silver and such like. We cannot mine for particular metals just anywhere.

This has to do with the history of the planet as well as the densities, solubilities, and chemical reactivities of the different elements. When the earth was formed, it was in a molten state for a long enough time for most of the really heavy stuff to sink to the core (mostly iron, with some nickel and cobalt and traces of other dense metals that are soluble in molten iron, like platinum, iridium, osmium, gold... Actually, I believe that one of the reasons bismuth is fairly common in the crust despite its high atomic number (83, compared to gold at 79), is that it is not soluble in iron at all.

Mineral deposits can be formed by tectonic motions bringing elements from deep down up towards the surface (think of sulfur from volcanos and gemstones from mountains), or from bodies of water that have dried up over the centuries (thus concentrating compounds that are soluble in water, like borax.)

Some mineral deposits are due to life itself. Almost all of the limestone (and marble) around the world is composed of the exoskeletons of marine organisms, which were able to extract ezymatically extract CO₂ from the atmosphere and combine it with calcium to make calcium carbonate.

[evan_au]

many of the limits we assume of our sun is based on its internal temperature, but not on its internal temperature and pressure.

You underestimate scientists.

Helioseismology studies temperature, pressure and convection speeds within the sun.

The Lawson criterion for nuclear fusion includes temperature, pressure and time.
See:https://en.m.wikipedia.org/wiki/Lawson_criterion

plasma metallic hydrogen phase.

This is partly a self-contradiction.

A plasma is so hot that the electrons are ripped off the nuclei, and the electrons and nuclei form a gas.
A conductive metal may be a solid or liquid. The atoms are in contact, so the outer electrons form a conduction band, where they act a bit like a gas. The inner electrons are still locked to a particular nucleus.

As an example, at 5000C water is an ionized gas of dissociated radials. If we add sufficient pressure, like that of the core of the earth, water at that same temperature will change into a solid metal.

Much of the Earth’s core is iron, so there won’t be much water there.
It is true that hydrogen will form a metallic solid at Jupiter pressures, but Jupiter is mostly hydrogen, without enough oxygen to make much water.

the core of Jupiter is thought to be metallic hydrogen... we are dealing with an orderly solid, that conducts electricity very well. There is room for huge voltages and huge magnetic affects. 

I agree. Scientists on Earth study these phase changes in diamond anvils. The Juno spacecraft is currently studying the internal density and magnetic field of Jupiter.

Scientists expect Jupiter’s magnetic field to originate in convection in Jupiter’s liquid hydrogen metallic outer core, rather than the solid hydrogen metallic inner core.
See: https://en.m.wikipedia.org/wiki/Magnetosphere_of_Jupiter

One might even assume our early forming sun, went through a metallic hydrogen core phase, similar to that of Jupiter,

That is possible.
However, Jupiter has had around 4 billion years to cool down from the heat of its formation, and form orderly layers.
In contrast, the early Sun was in the center of a maelstrom, continually bombarded by planetesimals. It is thought that stars ignite fusion in 10s of millions of years, so there was probably little time for the center of the Sun to spend in a solid hydrogen state.

[evan_au]

Neutron star collisions and gold production featured in a recent episode of Brian Cox’s Infinite Monkey Cage podcast.
See: http://www.bbc.co.uk/programmes/b09kxt28

[puppypower]

Much of the Earth’s core is iron, so there won’t be much water there.
It is true that hydrogen will form a metallic solid at Jupiter pressures, but Jupiter is mostly hydrogen, without enough oxygen to make much water.

The existence of metallic water, in the earth's core, is a result of chemical potential and not density or entrainment.   The layering of the earth from surface, to the mantle, to the outer core and then to the core, correspond with a phase diagram of water, based on the estimated conditions of the inner earth. Much of this understanding and data for water is relatively new, while the current models are based on old data and old assumptions such as density differences.

For example, water in the earth's crust, not even very far down, exists in a hydrothermal state, which is water above its critical point. This phase of water can dissolve most minerals, as well decompose most organics. The solubility of minerals in super critical water increases with temperature and pressure. There is a chemical potential for super critical water to eat downward in the direction of the core, since this is the place of highest temperature and pressure; water is driven by free energy.

Water is well known for its astonishing range of unusual properties, and now Thomas Mattsson and Michael Desjarlais of Sandia National Laboratories in New Mexico have suggested yet another one. They found that water should have a metallic phase at temperatures of 4000 K and pressures of 100 Gpa, which are a good deal more accessible than earlier calculations had indicated.


Metallic water
The two researchers used density functional theory to calculate from first principles the ionic and electronic conductivity of water across a temperature range of 2000–70,000 K and a density range of 1–3.7 g/cm3. Their calculations showed that as the pressure increases, molecular water turns into an ionic liquid, which at higher temperatures is electronically conducting, in particular above 4000 K and 100 GPa. This is in contrast to previous studies that indicated a transition to a metallic fluid above 7000 K and 250 GPa. Interestingly, this metallic phase is predicted to lie just next to insulating "superionic" ice, in which the oxygen atoms are locked into place but all the hydrogen atoms are free to move around.

The temperature of the earth core is about 5,700 K (5,400 °C; 9,800 °F). The pressure in the Earth's inner core is slightly higher than it is at the boundary between the outer and inner cores: it ranges from about 330 to 360 gigapascal (3,300,000 to 3,600,000 atm). This is in the range of the metallic water phase. The superionic ice phase, just outsider the core, with its hydrogen proton currents is very corrosive to metallic iron; super acid. The water is rusting the iron core and releasing energy. This is driven by the continuity of water from surface to core and solar evaporation. Solar  evaporation adds the positive charge to the atmosphere, which is felt all the way to the core, over time. The net flux of electrons upward is reflected in the slight negative charge of the oceans.


Relative to Jupiter, if it has a metallic hydrogen core and is made mostly of hydrogen, then it follows that the answer to the original topic, did or could the sun form from a cloud of hydrogen, is possible. This is because the less massive Jupiter, can consolidate hydrogen, just short of the hydrogen phases needed for fusion.

In terms of a metallic hydrogen plasma, hydrogen is unique in the sense the each atom of hydrogen only has one electron. The mobility of the electrons in a metallic phase, essentially means no hydrogen atom has its own personal election in a hydrogen metal. The ionized electrons are being shared by the solid metallic matrix, due to the application of temperature and pressure.

If we add more and more pressure and temperature, the dwell time for any electron, on any given hydrogen proton gets shorter and shorter. To make this less repulsive, due to the extreme pressure restrictions, the elections and protons will attempt magnetic addition, sort of similar to orbitals. A simple spin addition of the hydrogen protons can allow them to get closer due to magnetic addition. The result will add grains to the hydrogen metal.