As I understand it most of the earth (and hence everything on it…including people) came from the inside of a star that exploded in the distant past.
This is necessary as ‘in the beginning’ there wasn’t much more than hydrogen floating about the universe. This hydrogen eventually coalesced into stars which fused the hydrogen into helium.
As a star goes through its life it eventually runs out of hydrogen and begins fusing helium and so on till you reacg iron. Once all that is left is iron normal stellar processes can no longer continue its fusion engine and the star fizzles out or explodes.
Down the line the expelled material starts to reform and you get another star and maybe planets this time. And maybe, if you’re lucky, those planets evolve life where questions like this can be asked.
I don’t know why but I’m under the impression that we (our atoms that is) have been inside a star 2 or 3 times in the past before ending up here.
While all that is well and good where did heavier elements come from (i.e Uranium)? If they can’t be produced inside a star where else in this universe did the stuff come from?
All the processes up to creating iron release energy. However, heavier elements can be formed at almost any point in a star’s life (although creating them absorbs a bit of energy, there is energy available in the star’s interior). When a star explodes, then there’s lots of extra enrgy available, and heavier elements are formed in noticable quantities.
JonF nailed it but the short answer is that they come from supernovae. When a star goes supernova a shock wave is created that both blows the outer layers off the star and compresses the core. During this core compression all sorts of nucleosynthesis can, and does, occur. There is so much excess energy that the creation of heavy elements is commonplace.
“When you wake up in the morning, Pooh,” said Piglet at last, “what’s the first thing you say to yourself?”
“What’s for breakfast?” said Pooh. “What do you say, Piglet?”
“I say, I wonder what’s going to happen exciting today?” said Piglet.
Pooh nodded thoughtfully.
“It’s the same thing,” he said.
This is correct except that the star doesn’t burn all the hydrogen to helium and only then start burning helium. As the star converts hydrogen to helium it forms a core of helium which can start to fuse into carbon and oxygen as soon as the pressure and temperature are high enough. So what you get are layers where different fusion processes are occuring all at once in a single star.
The sequence, according to a nearby textbook, is as follows:
H -> He
He -> C, O
C -> Ne, Mg
Ne -> O, Mg
O -> Si
Si -> Fe, Ni
However, few stars are massive enough to continue burning all the way to iron. And stars that are massive enough for this invariably go supernova. So even the iron in the earth is supernova detritus.
A related question that I’ve always wondered about is why do we find heavy elements in “clumps” on earth. For example did a particular uranium deposit arrive here as a clump from a supernova (seems unlikely), or is there some process by which it was sorted and coalesced during the formation of the solar system?
The existence of “clumps” is questionable. There are “areas of enrichment”, maybe, where the concentration of a particular element is higher than usual, maybe even much higher, but unless you’re talking about one of the few really common elements (Si, O, Mg, Fe, Al, etc.) the local concentration never gets high enough that a handful of ore is mostly that element. Usually it’s tons of ore yielding ounces of the desired element.
The earth and all its constituent minerals were once molten, and were in that state for a long (hundreds of millions of years) time. Long enough for the iron core to form and for the general differentiation into core, mantle and crust to develop. No remnant of original supernova ejecta remains in it’s created form. The crust is negligibly thin compared to the rest of the earth and is more or less just a “froth” left over from the formation of the “real” planet. Everything has been well-stirred, thoroughly mixed.
Within that froth, typical chemical processes did and do occur. Local differences in temperature, water content, pressure favor solutions of one mineral or another. Convective processes (mostly) move things around. For example, veins of native metals (copper, silver, gold) occur in what once were hot, molten channels of magma moving along cracks or other weaknesses in the surrounding rocks. As the magma cooled, the concentrated elements could no longer stay in solution and were precipitated out. If you can find these old channels – eureka! You’ve hit the mother lode.
“When you wake up in the morning, Pooh,” said Piglet at last, “what’s the first thing you say to yourself?”
“What’s for breakfast?” said Pooh. “What do you say, Piglet?”
“I say, I wonder what’s going to happen exciting today?” said Piglet.
Pooh nodded thoughtfully.
“It’s the same thing,” he said.
Sometime in the astronomically distant past, a large star supernovaed, heavy elements were made, and lo, it was good.
From those gasses, our solar system condensed, and the first differentiation processes (i.e., turning a homogenous substance into “clumps”) began as the heavier elements stayed closer to the center of the solar disk, and lo, it was good.
One such homogenous ball of substance that got more than its fair share of heavy elements became the EARTH, and lo, it was good.
And on the fourth day, the EARTH began its own differentiation. Abundant heavy iron sank to form the core; elements that chemically follow Fe (e.g., Mn, Ni, Co) tended to concentrate there, too. Lighter Si and Al rose to form the crust; elements that prefer to be incorporated into alumino-silicate phases (e.g., Na, K, Ca) tended to concentrate there, too. (Really light stuff and elements that don’t like to form mineral phases differentiated out to form the proto-atmosphere: Noble gasses, N, H). So the first terrestrial “clumps” of elements formed as the once-homogenous EARTH blob differentiated into a crust, mantle, and core.
Within the crust, many other processes serve to differentiate the elements into various ore deposits. As a magma crystallizes, some elements (such as Au, Ag, Hg, U, Th, and many others) don’t join into mineral structures. Result: they remain concentrated in the residual liquid, which may contiue to rise as a magma of more silicic composition. Ultimately, this residual magma may be very silicic and very full of Au, Ag, U, etc., but still not economic.
Secondary near-surface processes of differentiation, usually involving hot fluids full of halogens (which complex with Au, Ag and allow them to be mobile in hydrous systems) leach these relatively-enriched differentiated magmas of their precious cargo. These fluids follow cracks in the country rock; when the cool and/or mix with halogenless fluids, they lose the ability to carry these metals, and–bingo!–ore deposits! And lo, it was good.
And on the seventh day, HE rested.
Different differentiation processes give rise to the variety of rocks and ore deposits… all from an originally homogenous “clump” of proto-planet.
