The Alchemy of Neutron Star Collisions

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This episode is brought to you by Facebook.

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Matt O'Dowd (1:00)

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 event. The collision of neutron stars.

When I was in astrophysicist 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 star's dead core collapses and protons are converted to neutrons. The surrounding shells ricochet outwards along with a layer of the iron and nickel core. The latter is blasted by a wave 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 R, 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 R process is real and must happen to some extent in supernovae, but there are some details that this story misses.

For one thing, the R 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 element. The R 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 lower-mass stars. And actually, it turns out that even the heavy R process 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 R process elements than it appears to. In addition, models of supernova explosions have trouble producing the right conditions for substantial release of R process elements. The only modern nearby supernova, 1987A, appeared to have no enhanced enrichment in R process elements. R 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 observed…

SPEAKER_1 (4:00)

This episode is brought to you by Facebook. So you were scrolling on Marketplace, and there it was, the bike you'd been searching for.

You sent a message, and it turned out the seller was super chatty, kind of funny, and an avid cyclist. The next thing you know, you're in a cycling crew. Well, a community cycling group. The thing about Facebook, you might find more than what you're looking for. From a browse to a bike ride, this summer find more on Facebook.

SPEAKER_2 (4:30)

You know those tiny back to school emergencies that somehow become your problem? That's why I love Uber Eats. You can order school supplies, snacks and lunchbox essentials for $5 or less.

So when your kid casually drops, I don't like peanut butter anymore or I need five green highlighters for a project due tomorrow. Uber Eats has you covered. Get everything you need for back to school today from your favorite brands like Albion Staples on Uber Eats.