The Alchemy of Neutron Star Collisions

Carl Sagan's famous words we are star stuff refers to a mind-blowing idea that most atomic nuclei in our bodies were created in the nuclear furnaces and explosive deaths of stars that lived in the ancient universe in recent years it's become clear that the truth is even more mind-blowing many of Earth's heavy elements including most precious metals were produced in an even more spectacular events the collision of neutron stars when I was in astrophysicists school they taught us that all the elements of the periodic table between carbon and iron were produced in onion shells by nuclear fusion in the cores of very massive stars during the last phases of their lives and that elements heavier than iron were synthesized in the following supernova explosion that latter process is well understood the stars dead core collapses and protons are convergence and neutrons the surrounding shells ricochet outwards along with a layer of the iron and nickel core the latter is blasted by a way of neutrons which get rammed into the escaping nuclei some of those captured neutrons convert back to protons and so elements all the way up the periodic table can be made this is the rapid Neutron capture or our process the rapid part is because neutrons are captured faster than nuclei can decay making it possible to build very heavy nuclei it's a cool story it would be cooler if it were true so the our process is real and must happen to some extent in supernovae but there are some details that destroy missiles for one thing the our process only produces the heaviest isotopes remember that element type location on the periodic table is defined by the number of protons in the nucleus neutron number is variable and defines the isotope of the elements the our process produces only Neutron rich isotopes of most heavy elements neutron poor isotopes are likely made by the slow Neutron capture or s process inside low mass star and actually it turns out that even the heavy our protest isotopes are probably not made in supernovae given the rate at which supernovae go off in the Milky Way the interstellar medium should have way more our process elements than it appears to in addition models of supernova explosions have trouble producing the right conditions for substantial release of our process elements the only modern nearby supernova 1987a appeared to have no enhanced enrichment in our process elements our process elements exist but their source doesn't seem to be supernova explosions one of the proposed alternatives shot to prominence last year when the LIGO and Virgo gravitational wave Observatory z' spotted the space-time ripples from the merger of a pair of neutron stars many of the world's great telescopes monitored the subsequent electromagnetic flash as the ejected material from this collision expanded and faded the spectral signatures of many are protists elements were seen in abundance neutral star merger is now the leading candidate for the production of most of these elements including those here on earth the process is suitably awesome let's take a look like I said neutron stars are the dead cause of massive stars they are composed almost entirely of neutrons a density similar to the atomic nucleus they also have a thin crust of iron densities are so high in fact that they are on the verge of complete gravitational collapse into black holes so take a pair of neutron stars in binary orbit perhaps twin remnants of a once binary pair of massive stars they slowly spiral towards each other as a gravitational radiation saps their orbital energy in the last minute before merger that radiation is so strong that it'll be detected by an as yet unborn civilization a hundred million light years away as will the explosion of electromagnetic radiation that immediately follows as the neutron stars plow into each other at the instant of this collision the other layers of the stars splash into a maelstrom of neutrons and iron in orbit around the merging neutron star interiors that combined core has almost certainly pushed beyond the limits of gravity and collapses into a black hole within milliseconds in the meantime the surrounding vortex undergoes some crazy transformations prior to collision the stars neutrons were stabilised by extreme pressure but once released this nuclear group expands and destabilizers it breaks up into droplets of neutrons many neutrons rapidly undergo beta decay transforming into a proton after ejecting an electron and a neutrino the droplets are now essentially nuclei albeit hopelessly unstable once they break apart and beat a decay into semi stable elements meanwhile the inner part of the vortex is still bathed in a sea of neutrons the hour process begins newly formed nuclei and older iron nuclei absorb neutrons and so heavier and heavier elements are created as beta decay transforms some of the absorbed neutrons into protons some of these heavy elements are sprayed into the surrounding space by the energy of the collision itself but most remain trapped in the intense gravitational field of the newly formed black hole presumably doomed to fall into the event horizon but that same beta decay process the converted neutrons back to protons also provides the mechanism for their escape the beta decay releases both electrons and neutrinos in fact a wind of neutrinos so intense that it drives material outwards we tend to think of neutrinos as ghostly particles that barely interact with matter but here both the neutrino and matter densities are so high that our new nucleons can ride this neutrino wind to freedom our best calculations suggest that neutral start collisions should be much better than supernovae at producing heavy elements and getting the mountains of the galaxy now that we've spotted these elements around the site of a neutron star collision the story is looking better and better a recent nature paper by marker and Bartos has clarified the origin of Earth's heavy elements even further in fact they figured out that some of our our process elements were formed in a single nearby neutron star collision around 80 million years before the formation of the solar system now it would be fair to ask how on earth anyone could know that over 4 and 1/2 billion years later well with lots of cleverness let me explain certain nuclei produced in the r-process are unstable these isotopes undergo radioactive decay into lighter elements after being created in a neutron star collision now the average decay time or half-life differs between different radioactive isotopes some decay faster than others that means the relative abundance between any two isotopes should change over time so if you can measure the ratio in their abundance then you know how long ago they'll formed there's actually one more complication if you want to find the event date to within a reasonable degree of accuracy but then you need to look at isotopes with short half-lives you want accuracy within millions of years then the half-life should be measured in millions of years but earth years billions of years old if Earth's our process elements were produced by a neutron star merger that happened before earth formed then any short-lived isotopes from that merger should have completely decayed by now this is where the cleverness comes in it turns out that the abundances of CERN's short-lived isotopes became locked into the very first minerals to form in our solar system we found ancient meteorites that coalesced when the earth was still forming when they formed billions of years ago they contained radioactive r-process isotopes that themselves were formed in a nearby neutron star merger million of years prior and their ratios reflected that those rocks eventually found their ways to earth and into the hands of scientists these short-lived isotopes completely decayed long ago but they decayed into other stable elements that were still locked in those meteorites the abundances of these daughter products exactly reflect the abundances of the parents of the moment the meteorite material was formed now this study focused on curium 247 with a half-life of 15.6 million years and compared it in ratio to plutonium 244 with its longer half-life of 80 point 8 million years based on the relative abundance of their daughter products in meteorites we know the relative abundance of these radioactive isotopes in the nebula that our solar system formed from the researchers then did simulations to figure out how long ago and how far away the neutron star merger that formed these elements must have been an important factor here is that neutron star mergers are rare all short-lived our process isotopes are likely to have formed from the same merger because of this they were able to identify a single neutron star merger that must have happened between 40 and 120 million years before the formation of the solar system and between 650 and 1,300 light years away that one event produced most of the short-lived r-process elements that were present in the early solar system more stable elements were built up over multiple neutron star mergers which the researchers conclude must happen every 20 million years or so galaxy-wide so what does all of this mean for the elements that make up the earth and that makeup.you neutron star mergers are likely the dominant source of most elements with atomic masses 44 and up that includes most of the LEED Silver Gold rare earth elements and the radioactive stuff like uranium and plutonium also a good fraction of the molybdenum and iodine which are essential for your biology in fact including the non-essential heavy elements your body mass is something like two per million colliding neutron star material that only 1/10 of a gram or so but it's a pretty awesome tenth of a gram it was after all synthesized on the rim of a black hole before surfing a wave of neutrinos into the nebula who would eventually collapse into our solar system and those atoms would eventually find themselves part of a life-form that would figure out the very time and distance of their formation a collision of ancient stellar corpses in an earlier and distant space-time okay last time we talked about the cosmic dark ages that mysterious time before the first stars formed in our universe let's see what you had to say bloody albatross reasonably asks why is it called recombination after all weren't electrons combining with nuclei for the very first time so why not just combination great question bloody albatross I think this is just a case of where a less-than-ideal name became common parlance and now we're stuck with it corker and incorrect ly infers that it should be possible to use the lyman-alpha forest to map where along the line of sight to a quasar there are clouds of hydrogen gas and learn about their size and density based on the shape of the absorption dip yep you can actually do that it's pretty crazy and you can learn a lot about the large-scale structure of the universe by looking at these quasars norlan Westridge summarizes my feelings on the matter it's amazing how much scientists can find out with so little in this case from a single point of light and that is that distant quasar Jan Peter Cornett asks something that I hoped one of you would if the universe was transparent before recombination when electrons were free of their atoms and so could block the paths of photons then it was transparent during the dark ages because electrons bound in atoms don't block most of the light then after the universe was reaiiy annive why didn't it become opaque again Jan then goes on to answer that conundrum it's because the universe after realization was much larger than a tree nation like a factor of a hundred at the beginning of realization and so electrons were more spread out the density was a hundred cubed times lower than at recombination and so the mean free path of photons was a million times larger in a related question Lobby seat-warmer asks wouldn't the dark ages actually be blindingly bright given that the cosmic background radiation wasn't yet stretched to invisible microwave wavelengths actually most of the dark ages would have actually been dark at least to us the dark ages started following recombination and then the cosmic background would still have been visible red orange everywhere reflecting the 3000 Kelvin temperature at that time although even by then most of the light was infrared the universe then expanded by a factor of a hundred over the next couple hundred million years roughly linearly with time so ten percent of the way through that period and the CMB would have red shifted to the far infrared and would no longer be visible to us although we would have felt it on our skin the temperature would have been a comfortable 300 Kelvin and spliff five us another related question how dense was the gas just after recombination well let's figure it out the density of the universe is roughly one hydrogen atom per square meter the universe is now around 1,100 times larger than it was then so can I do this in my head there would have been 1,100 hydrogen atoms per square meter that's still incredibly diffuse but it's enough to stop photons moving very far like only about a thousand light years now that sounds far but it's nothing compared to the size of the universe even back then so we still say that the universe was opaque so at the end of the last episode I commented that in Schrodinger's internet grumpy cat both can and can't haz Cheezburger uh and then the next day grumpy cat sadly passed away look despite insinuations in the comments I'm sure we didn't collapse Grumpy's wavefunction - can't Has Cheezburger just by talking about it quantum mechanics forbids that anyway I'm sure grumpy is now in meme heaven looking down on us disapprovingly alongside a certain noble gorilla rest in peace grumpy [Music]

Channel: PBS Space Time

Views: 363,741

Rating: 4.934144 out of 5

Keywords: Space, Outer Space, Physics, Astrophysics, Quantum Mechanics, Space Physics, PBS, Space Time, Time, PBS Space Time, Matt O’Dowd, Astrobiology, Einstein, Einsteinian Physics, General Relativity, Special Relativity, Dark Energy, Dark Matter, Black Holes, The Universe, Math, Science Fiction, Calculus, Maths, Holographic Universe, Holographic Principle, Holography, Holographs, Reality, Consciousness, EHT Black Hole, Event Horizon Telescope, collide, collision

Id: MmgMboWunkI

Length: 15min 40sec (940 seconds)

Published: Thu Jun 06 2019

Reddit Comments

[u/Captain_Rational]

So if neutron star mergers only happen every 20M years or so, and only a fraction of those occur near star forming nebulae, then high metalicity solar systems like ours must be somewhat rare gems in the galaxy?

If that were true, perhaps this is one of the great filters in the Fermi Paradox.