The remaining and heaviest elements (beyond iron and bismuth) are formed through explosive events: core-collapse supernovae generate elements between neon and nickel, while the r-process (rapid neutron capture) in supernovae and, predominantly, neutron star mergers creates elements like uranium and thorium, dispersing them into the interstellar medium for planetary formation.”
From https://www.astronomy.com/science/the-universes-guide-to-cre...
https://en.wikipedia.org/wiki/Dredge-up
"By definition, during a dredge-up, a convection zone extends all the way from the star's surface down to the layers of material that have undergone fusion."
In the early universe, stars had so little in the way of "seeds" for the s-process to act on that the few seeds that were there absorbed large numbers of neutrons, eventually producing weird stars highly enriched in lead (the end point of the s-process). These stars have been detected from lead (and bismuth) in their spectra.
Sure dispersion takes a supernova, but production is a different word ;-)
At iron 56 there is a peak in binding energy, both for lighter and heavier nuclei the binding energy is lower.
It is possible for nuclei with lower binding energy to form after a collision, but the probability for this to happen becomes lower and lower with decreasing binding energy.
Thus if one computes the probabilities of the reactions that happen during collisions one can compute the abundances of chemical elements that are reached when there is an equilibrium between the rates at which a certain chemical element is created and destroyed.
At this equilibrium, there is a maximum abundance for iron 56 and the heavier nuclei have abundances that decrease very quickly with the atomic number. For example, zinc may be 600 to 700 times less abundant than iron and germanium may be 7000 to 8000 times less abundant than iron.
Therefore, in an old star, which reaches equilibrium concentrations of elements, there are elements heavier than iron, but in extremely small concentrations, which become negligible for the elements much heavier than germanium.
Significant quantities of heavy elements cannot be produced by collisions between nuclei in a star, because they are destroyed in later collisions faster than they are produced.
So most of the elements heavier than germanium are produced by a different mechanism, i.e. by neutron capture, followed by beta decay. A small number of the heavy nuclei produced by neutron capture also capture protons after their formation, producing thus also some isotopes that are richer in protons.
In normal stars, the number of neutrons is negligible so neutron capture reactions do not happen often. On the other hand, some catastrophic events, like a supernova explosion or the collision between two neutron stars, can produce huge amounts of neutrons. In this case a lot of neutron capture reactions happens, exactly like on Earth during the explosion of a nuclear fission or fusion bomb.
These neutron capture reactions can produce all the chemical elements until fermium (Z=100), i.e. well beyond uranium. Heavier elements than that are not produced, because they fission spontaneously too quickly, before being able to capture other neutrons.
Of the trans-uranium elements, most decay very quickly, but plutonium 244 has a half-life long enough to reach other stellar systems, together with uranium, thorium, bismuth and all elements lighter than bismuth, except technetium and promethium (the latter 2 elements decay quickly, but technetium can survive for a few tens of millions of years, so small quantities of it may reach a nearby star, but they will disappear very soon after that; the elements between bismuth and thorium, and also protactinium, decay quickly and those that exist on Earth are recently created, through the decay of Th and U). The other primordial elements can survive many billions of years, but the amount of primordial plutonium becomes negligible after a few billions of years.
jmyeet•1mo ago
As many of us know, the fusion in stars produces elements as heavy as iron. It then takes explosions of those stars to scatter those elements into space, ultimately bringing them into the protoplanetary disc of a new star, such that it can form a planet in the right zone. That star then needs to live long enough and the system needs to be stable enough to produce complex life.
But it gets worse because we obviously have elements heavier than iron. So stars of a sufficient size need to form such that when the stars die they do so in an even more violent fashion. The core needs to collapse into neutronium and the resultant supernova can produce heavier elements. They also come from neutron star mergers.
So all the uranium we have on Earth came from such an event. Because of the nuclear decay chain we can estimate when this uranium was made and IIRC that's somewhere between 80 and 200 million years before the Earth formed.
So this all had to happen sufficiently close to the Sun and that material had to be captured in the Sun's protoplanetary disc. We needed the right combination of elements to form a protective magnetic field and produce enough but not too much heat.
We're going to keep discovering mechanisms like this and the importance of particular isotopes, events and things like how amino acids seem to form relatively easily (given the right elements are present), which itself is a consequence of CNO fusion.
But also why did the Sun form at all? It has to be in a nebula of largely hydrogen and helium and something had to trigger that like the shock wave from a nearby supernova or neutron star or black hole merger.
It's kind of why I think sentient life is incredibly rare.
moktonar•1mo ago
lanyard-textile•1mo ago
Talk about sublimity.
kldavis4•1mo ago
any idea how close? like 10s of light years or what?
MarkusQ•1mo ago
pfdietz•1mo ago
jcims•1mo ago
Agreed. The universe is big, but combinatorics are bigger.
I'd be disappointed but ultimately unsurprised if an all-knowing oracle said it has only happened once in the history of the universe. My follow up question, of course, would be whether or not it happened on Earth.
buu700•1mo ago
jcims•1mo ago
kakacik•1mo ago
Of course if speed of light is the hard unavoidable limit it doesnt matter now or for next few trillions of years. Eventually though, if it will keep expanding, the only important thing in universe will be energy. Species that will grok that first may decide to not share and take it all for themselves. Although sustainability of some empire over 10^10^10^10 years and further... its something even my otherwide vivid imagination can't concieve.
mr_mitm•1mo ago
dhosek•1mo ago
BurningFrog•1mo ago
I don't understand the question. There must have been a cloud of gas big and dense enough to provide the mass for the solar system.
Once that exists gravity does the rest, right?
> all the uranium we have on Earth came from such an event
That must mean the Sun also has its fair share of that Uranium? Or maybe more of it, since the heavy elements were more drawn to the center of the solar system?
pixl97•1mo ago
That's a good question. I would assume the sun captured a whole pile of uranium around the time the earth was forming. And it likely sunk to the core. The question is what happened then. The core area is dense enough to fuse hydrogen into helium, without any calculation I'd guess a lot of this is now in much smaller elements as there are a lot of neutrons to break it apart.
lawlessone•1mo ago
https://en.wikipedia.org/wiki/Przybylski%27s_Star#Chemically...
marcus_holmes•1mo ago
mr_toad•1mo ago
Very large clouds of gas can exist with gravitation attraction balanced by gas pressure. This delicate balance can be disturbed by passing stars, supernovae, galactic mergers and other events.
01100011•1mo ago
mcswell•1mo ago
jmyeet•1mo ago
The first few include Nickel, Copper and Zinc. Think of all those alloys and direct uses, particularly copper as an electrical conductor. Or all the rare earths for magnets and semiconductors. Or gold or lead even.
Then there's Iodine, which is actually essential for human life. Zinc, selenium and others are used too, possibly others.
The scarier question is what happens when the universe runs out of hydrogen in vast quantities? It will only be around for stars to burn for so long. Most are in the billions of years. A handful are in the trillions. But eventually they will run out too.
zackmorris•1mo ago
They just found the building blocks of life in asteroid Bennu:
https://science.nasa.gov/mission/osiris-rex/
https://physicsworld.com/a/components-of-rna-among-lifes-bui...
“So far we have not seen any evidence for a preferred chirality,” (Dan) Glavin says (important for understanding why amino acids on Earth seem to all be left-handed):
https://physicsworld.com/a/asteroid-bennu-contains-the-stuff...
Life is probably abundant everywhere in the universe. Also, evolution seems to spring up everywhere, in any system of sufficiently advanced complexity, regardless of what substrate it operates on. So I think that we'll start seeing life-like emergent behavior in computing, especially quantum computing, in the next 5-10 years.
So the question becomes: what great filter (in the sense of the Drake equation and Fermi's paradox) causes life as we know it to go dark or wipe itself out just after it achieves sentience?
Well, we're finding out the answer right now. Life probably merges with AI and moves into what could be thought of as another dimension. Where time moves, say, a million times faster than our wall clock time, so that it lives out lifetimes in a matter of seconds. Life everywhere that managed to survive probably ascended when it entered the matrix. So that by now, after billions of years since the first life did this and learned all of the answers, we're considered so primitive that Earth is just a zoo for aliens.
Or to rephrase, omnipotent consciousness probably gets bored and drops out of the matrix periodically to experience mortal life in places like Earth. So simulation theory probably isn't real, but divine intervention might be.
jmyeet•1mo ago
I'm not convinced of that. Yes it seems like the building blocks are abundant but there's so many steps beyond that to get to abundant life.
The first life we had in the Archeaen era was dependant on sulfur, which was concentrated around volcanic vents so this already presumes a lot, namely oceans and a geologically active planet. Oxygen leeched a bunch of minerals into the water.
And then came cyanobacteria who no longer needed volcano but had this annoying habit of producing a new waste product: oxygen. This both absolutely killed all the Archeaen life but also cleansed the oceans as ions like iron precipitated into ferric oxide and we can see the layers of these cycles in the rock.
So the Earth needed all these elements and the Sun and Solar System needed to be sufficiently stable for billions of years just to get to this point and there are so many steps beyond this.
I personally believe it's more likely than not that we are the only potentially spacefaring civilization in our entire galaxy.
estimator7292•1mo ago
We haven't surveyed nearly enough other planetary systems to have any real idea if our system is unique. We barely have the ability to even see systems like ours in the first place. There's so little data available that it's not reasonable to draw a conclusion either way.
Brian_K_White•1mo ago