The Stars Go For the Gold
The middle-aged adage that we are made from stardust, made popular by Carl Sagan back in the 1970s, pops up in my thoughts now and then.
I'm looking at my wedding ring right now, feeling the weight of this bit of gold and using it as a mental peephole back in time, to before our solar system even existed. Though most of the chemical elements that make up the Earth and the solar system at large have similar long lineage, coming from the mixture of gases in the interstellar cloud that the sun and planets condensed from, the atoms of gold have a special distinction. These atoms, as well as other atomic nuclei heavier than the element iron, could only have been forged in the core of a super massive star at the end of its life.
That was a long winded way of saying, "Cool! And I get to wear that stuff around my finger!"
Fast-rewinding a bit, about 13.7 billion years to when the universe began with a Big Bang, the first, and for a while only, chemical elements to form out of the seething, expanding ball of energy that was our early universe were hydrogen and helium—and relatively small amounts of lithium and beryllium. Though we're still working out what exactly energy and matter are, we've known for some time now that they are interchangeable: matter can be converted into energy and from energy can come matter — subatomic particles, like protons, neutrons, electrons, and the like.
So the early universe was a vast expanding ball of super-hot, hot hydrogen and helium gas. What next? Still no gold to be found anywhere, even where you find it.
What came next were stars and galaxies —- though we're still working out which came first (a variation on the chicken and egg problem). Did galaxies begin as vast condensing swirls of hydrogen and helium, possibly aided by the gravitational forces of primordial super-massive black holes? Did stars then condense within this concentrated gas environment? Or did stars form first in smaller clusters and groups which over time were assembled into galaxies?
What's relevant here is that the stars did begin to form, condensing from gases into tighter and denser blobs, until finally the pressure at their cores was high enough for the process of nuclear fusion to take place.
Then, hydrogen atoms fused to form heavier hydrogen isotopes like deuterium and tritium, and those isotopes fused to make helium atoms, adding to the universe's inventory of the latter while using up some of the former. Each time nuclei are fused a bit of energy is released in the process, the energy source that powers a star.
Then, further magic at the cores of stars worked to forge nuclei that had never existed before, like carbon, sulfur, magnesium, and iron. Each time the core of a star would run out of a fusionable fuel it would begin to collapse under its own gravity, thus increasing its core pressure and enabling it to fuse heavier nuclei into even heavier ones—in effect burning the "ashes" of the fuel it had burned.
The most massive stars —- more massive than our sun — would eventually build up a core of iron and go no further. A star can only re-burn atomic "ashes" as long as it can release some energy in the process. But fusion of iron into heavier elements does not release energy; instead it requires energy.
At a point of crisis, the massive iron core of the old super-massive star suddenly has no more fuel to burn, and collapses. And this is when the doomed star suddenly finds a source of energy with which to fuse its stubborn iron into heavier elements: its own powerful gravitation, which draws the core together and raises the internal temperature and pressure to enormous levels.
And the star explodes as a supernova, blasting into space its outer layers of lighter chemicals as well as the iron, gold, uranium, and other heavy elements it has newly fused.
And to make a long story short, those materials mixed with the interstellar clouds from which our sun and the planets formed, providing the materials necessary to make planets like Earth possible. And, of course, the gold that now makes up my wedding ring. How cool is that?