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Imagine a star 30 times thr mass of the sun made of iron. Explode that star into an expanding sphere of iron atoms and by the time it reaches a radius of 1 light year, I bet you would not have any two iron atoms within an AU of each other.

Let's do the math. The amount of iron ejecta from a Type II supernova isn't known for certain, but one rough estimate claims it to be about half the mass of our own sun, or about 10³⁰ kg. One light year is about 10¹³ km. The volume of a sphere of that size (4/3 pi r²) is about 10²⁶ cubic kilometers. Divide that by the mass of the ejecta (10³⁰ kg) and you get 10,000 kg of iron per cubic kilometer.

This of course assumes a uniform distribution. Actual supernovas produce complex wavefronts (see the Crab Nebula) where the local density might be might higher.

Anyway, at 10,000 kg per cubic km you get about 10 micrograms per cubic meter. The atomic weight of iron is 55g/mol. Multiply by Avogadro's number (6.02 x 10²³ atoms/mol) and you get 100,000,000,000,000,000 atoms per cubic meter.

That's a teeny weeny bit larger than 1 atom per AU.

Well that's big iron. I knew that the sun is about 8 light min from earth so I could have easily calculated 1ly=65700AU a number much smaller than I would have guessed. And a half solar mass of 10³⁰ kg is mind bogglingly large to me, much larger than I would have suspected. In my imagination I was applying the inverse square principle to an under esteemed mass over an over esteemed light year.

Notwithstanding that your formula for the volume of a sphere is off by a order of magnitude of the radius, my imagination stands metrically and mathematically corrected. Thank you sir!

I'm still curios about how all that fast moving matter could coalesce to form asteroids and planets. I'll read the article. I'll also read about the history of the solar mass estimation. That's really facinating.