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 the DES story misses 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
long
starts 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 provenance last
year when the LIGO and Virgo
gravitational wave Observatory x'
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 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 100 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 goop 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 stock collisions should be much
better than supernovae at producing
heavy elements and getting them out into
the galaxy
now that we've spotted these elements
around the sight 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 that formed there's
actually one more complication if you
want to find the event date to within a
reasonable degree of accuracy 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 Cerne
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
as 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 at 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 sources to form trough 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 or 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 parts
million colliding neutron star material
that's only a tenth 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 ok 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 random correctly
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 that is
that distant quasar jean peter cornet
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 realized why didn't it
become opaque again Jen then goes on to
answer that conundrum it's because the
universe after reorganization 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
invisible 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 asks 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
Canon can't Has 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 haz
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
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