- 3897 - HEAVIEST ELEMENTS - how did we get here? Some of the heavy elements arrived on Earth by surfing supernova shock waves. When stars die, they spread the elements they’ve created in their cores out to space.
----------- 3897 - HEAVIEST ELEMENTS - how did we get here?
- Eventually,
that “star stuff” scatters across the galaxy in giant debris clouds. Sometimes
millions of years later, it settles onto planets. What’s the missing link
between element creation and deposition on some distant world?
-
- How did
heavy elements like manganese, iron, and plutonium show up on Earth. It turns
out they’re made in different processes, often in different parts of the Milky
Way. Yet, they’ve been found layered together on Earth’s seabed. That implies
they arrived about the same time, despite their different origins.
-
- The elements
from faraway events are carried by supernova shock fronts just like surfers
catching a wave. First, there are the
Type II supernovae. They occur when a supermassive star dies. That’s one at
least eight times the mass of the Sun. These stars fuse heavier and heavier
elements , such as carbon, in their
cores.
-
- When they
get to creating iron, they don’t have enough energy to keep it up. The cores
collapse and then everything expands outward very rapidly in a supernova
explosion sending its heavy elements racing through space.
-
- Next, there
are Type Ia supernovae. These happen in a binary pair of stars. Material from a
main-sequence star accretes onto its partner, a white dwarf. When too much
material accumulates, there’s an explosion. That results in the
“nucleosynthesis” of heavier elements, including manganese.
-
- Another
catastrophic event that likely creates heavy elements is the collision or
merger of two neutron stars. As they spiral in toward each other and eventually
smash up, they release a shower of neutrons. Those, in turn, bombard nearby
atoms. This “r-process” event very quickly produces heavy elements such as
plutonium.
-
- Somehow,
all this material from different sources ended up on Earth at about the same
time. Scientists found puzzling evidence of that in radioactive isotope
deposits on the seabed in 2021. They weren’t formed normally on Earth or during
the birth of the solar system some 4.5 billion years ago. They had to come from
somewhere else.
-
- Radioactive
isotopes abundance can be measured by gamma-ray telescopes in space as well as
by digging the rocks underwater of the Earth. Computer models showing that
nearly continuous supernova shock waves could be a viable transporter mechanism
to deliver these elements to Earth , or other planets.
-
- Rock
samples from the ocean floor were dissolved, put in an accelerator, and
examined for changes in their
composition layer by layer. Using
computer models they were able to interpret their data to find out how exactly
atoms move throughout the Galaxy.
-
- The modeling
effort shows that isotopes can propagate through large areas of a galaxy via
supernova shock waves. These fronts sweep up collections of elements from
various sites. Understanding this
delivery process is particularly crucial as astronomers begin large-scale
studies of exoplanets where life might be possible. Knowing how they got their
elemental composition is a big step toward understanding the possibilities for
life.
-
- It’s a very
important step forward, as it not only shows us how isotopes propagate through
the Galaxy but also how they become abundant on exoplanets, planets beyond our
solar system.
-
-
Scientists also have new evidence about how cosmic cataclysms forge
gold, platinum and other heavy members of the periodic table. Two neutron stars spiral toward an explosive
collision. Many of the periodic table's
heavier elements form through such crashes.
-
- About half
of the abundance of elements heavier than iron originates in some of the most
violent explosions in the cosmos. As the universe churns and new stars and
planets form out of old gas and dust, these elements eventually make their way
to Earth and other worlds.
-
- After 3.7
billion years of evolution on our planet, humans and many other species have
come to rely on them in our bodies and our lives. Iodine, for instance, is a
component of hormones we need to control our brain development and regulate our
metabolism.
-
- Ocean
microplankton called “Acantharea” use the element strontium to create intricate
mineral skeletons. Gallium is critical for the chips in our smartphones and our
laptop screens. And the mirrors of the JWST are gilded with gold, an element
useful for its unreactive nature and ability to reflect infrared light.
-
- Eons ago a
star more than 10 times as massive as our sun died in a spectacular explosion,
giving birth to one of the strangest objects in the universe: a neutron star.
This newborn star was a remnant of the stellar core compressed to extreme
densities where matter can take forms we do not understand.
-
- The neutron
star might have cooled forever in the depths of space, and that would have been
the end of its story. But most massive stars live in binary systems with a
twin, and the same fate that befell our first star eventually came for its
partner, leaving two neutron stars circling each other. -
- In a dance
that went on for millennia, the stars spiraled in, slowly at first and then
rapidly. As they drew closer together, tidal forces began to rip them apart,
flinging neutron-rich matter into space at velocities approaching one-third the
speed of light. At last the stars merged, sending ripples through spacetime and
setting off cosmic fireworks across the entire electromagnetic spectrum.
-
- At the time
of the crash, our own pale blue planet, in a quiet part of the Milky Way about
130 million light-years away, was home to the dinosaurs. The ripples in
spacetime, called gravitational waves, began making their way across the
cosmos, and in the time it took them to cover the vast distance to Earth, life
on the planet changed beyond recognition.
-
- New species
evolved and went extinct, civilizations rose and fell, and curious humans began
looking up at the sky, developing instruments that could do incredible things
such as measure minute distortions in spacetime.
-
- Eventually
the gravitational waves traveling at light speed and the light from the merger
reached Earth together. Humanity had
just witnessed heavy-element production.
-
- Our newfound
ability to detect gravitational waves, as well as light from the same cosmic
source, promises to help us understand astrophysical explosions and the
synthesis of elements in a way that was previously impossible.
-
- The cosmic
history of the elements of the periodic table extends from a few minutes after
the big bang to the present. The synthesis of the first elements, hydrogen,
helium and lithium, occurred roughly three minutes after the birth of the
universe.
-
- From these
ingredients, the first stars formed, shining bright and fusing new elements in
their cores during both their lives and their explosive deaths. The next
generation of stars was born from the debris of these blasts, enriched with the
elements formed by the first stars.
-
- This
process continues today and accounts for all the elements from helium on the light
end, with two protons per atom, all the way up to iron, which has 26 protons in
its atomic nucleus.
-
- The
heaviest elements, such as “tennessine” with 117 protons, aren’t created by
nature at all. But physicists can force them into being inside particle
accelerators, where they typically last for mere thousandths of a second before
decaying.
-
- Several
decades ago scientists theorized that about half of the elements heavier than
iron are produced through a process called rapid neutron capture, or the
“r-process”. The rest are thought to originate through slow neutron capture, or
the “s-process”, a relatively well-understood sequence of reactions that occurs
in long-lived, low-mass stars.
-
- Both the
r-process and the s-process involve adding one or more neutrons to an atomic
nucleus. Adding neutrons, however, does not produce a new element, because
elements are defined by the number of protons in their nucleus.
-
- What we do
get is a heavier isotope of the same element, a nucleus containing the same
number of protons but a different number of neutrons. This heavy isotope is
often unstable and radioactive. Through
'beta-minus decay', a neutron will transform into a proton, spitting out
an electron and another subatomic particle called a neutrino in the process. In
this way, the number of protons in an atom’s nucleus increases, and a new
element is born.
-
- The key
difference between the s-process and the r-process is speed. In the s-process,
atoms capture neutrons slowly, and there is plenty of time for the newly added
neutron to decay into a proton, creating the next stable element in the
periodic table, with just one proton more, before another neutron comes along
to be captured.
-
- This
happens over thousands of years because there are only small numbers of extra
neutrons lying around in the stars that host the s-process, so atoms are able
to capture new neutrons only occasionally.
-
- The
r-process, in contrast, can produce the entire range of heavy elements in one
spectacular flash of creation that barely lasts a second. In this scenario,
neutrons are plentiful and slam into nuclei one after another before they have
time to decay. A nucleus can rapidly balloon into a highly unstable isotope,
going all the way up to the neutron drip line, the absolute limit of the
neutron-to-proton ratio allowed by nature inside a nucleus.
-
- The
extremely heavy nucleus will then convert many of its neutrons to protons via
beta decays or even break into smaller nuclei, ultimately producing a range of
stable heavy elements.
-
- For a long
time scientists thought core-collapse supernovae, explosive deaths of stars
more than eight to 10 times the mass of our sun, might host the r-process. But
simulations of typical core-collapse supernovae couldn’t reproduce the neutron
richness and thermodynamic conditions needed except, perhaps, in the case of
rare explosions driven by strong magnetic fields.
-
- A neutron
star is born when a massive star runs out of nuclear fuel and its gravity causes
the core to collapse inward. The overwhelming force of the star’s mass on the
core compresses it to extremely high densities, causing electrons and protons
to fuse together to become neutrons.
-
- While the
rest of the star gets expelled in the supernova, the neutron star remains
intact, a compact remnant containing the densest matter known in the universe.
Neutron stars more massive than a certain limit further collapse into black
holes, but we don’t know the exact point of this transition, nor do we know how
“squishy” they are.
-
- The inner
structure of neutron stars might contain
mostly neutrons and a small fraction of protons inside a crust of heavier
nuclei at their surfaces. But their interiors could be even weirder than that.
Deep inside the neutron star, matter may take on truly bizarre forms, ranging
from a soup of quarks and gluons, the particles that make up normal matter, to
a sea of “hyperons,” which are made of “strange quarks”.
-
- There is a
connection between neutron star collisions and gamma-ray bursts, highly
energetic explosions in space that emit a flash of gamma rays. And because
r-process nuclei would be unstable and undergo radioactive decay, they should
be able to heat up the material surrounding them and power an electromagnetic
flare that would carry signatures of the elements produced.
-
- In 2010
science introduced the term “kilonova” to refer to such flares after
determining that they would be approximately 1,000 times brighter than a
regular flash of light called a nova.
-
- In 2015
the Laser Interferometer Gravitational-wave Observatory (LIGO) made the first observation of gravitational
waves, which were generated by two black holes spiraling toward each other and
merging.
-
- The
detection was designated “GW150914”.
LIGO and its sibling observatory “Virgoz' detected more binary black
hole collisions. Yet neutron star mergers remained elusive. Then, in the fall
of 2017 the gravitational-wave signal, astronomers observed a short gamma-ray
burst and something that looked a lot like a kilonova.
-
- The very
next day there were almost 70 new papers about GW170817 on arXiv.org. The event forecasted the promise of
multimessenger astronomy, the ability to see cosmic phenomena through different
“messengers” and combine the information to achieve a fuller understanding of
the event. This was the first time astronomers saw gravitational waves and
light, including radio, optical, x-ray and gamma-ray light,coming from the same
celestial source.
-
- The details
of the signal, such as how the waves’ frequency and strength changed with time,
allowed researchers to estimate that each neutron star had weighed about 1.17
to 1.6 times the mass of our sun and had a radius of roughly 11 to 12
kilometers.
-
- Roughly 1.7
seconds after the gravitational waves came in, gamma-ray telescopes Fermi-GBM
and INTEGRAL detected a faint burst of gamma rays lasting only a couple of
seconds that came from the same direction as GW170817. This discovery
definitively linked neutron star mergers with short gamma-ray bursts for the
first time.
-
- Images taken
with the Henrietta Swopes one-meter telescope at the Las Campanas Observatory
in Chile showed a new source of light located in the old but bright galaxy NGC
4993. By breaking up the light into its constituent colors and examining its
spectrum, astronomers concluded that the signal was consistent with the idea
that heavy elements were being forged there. We were looking at a true
kilonova.
-
- Shorter
wavelengths of light, which are bluer, peaked early, and longer, red
wavelengths became predominant later. These peaks can be explained by the
composition and velocity of the material ejected from the merger.
-
- A blue
kilonova can be produced by fast-moving ejecta made mainly of lighter heavy
elements without any “lanthanides”, the metallic periodic elements from
lanthanum to lutetium, which are highly opaque to blue light. A red kilonova,
in contrast, requires slow-moving ejecta containing lots of heavy elements, including
lanthanides.
-
- The
kilonova spectrum carries signatures of heavy elements, confirming that neutron
star mergers are at least one site where r-process elements are produced. The mechanism that produces short gamma-ray
bursts in mergers is still unclear. Properties of matter ejected in a merger
are also changed in important ways by neutrinos.
-
- As existing
gravitational-wave observatories become increasingly sensitive, new telescopes
will come online to collect light from the transient sky. New projects such as
the Facility for Rare Isotope Beams, which opened in May 2022 at Michigan State
University, will measure the nuclear properties of rare nuclei. Proposed
gravitational-wave observatories such as the ground-based Einstein Telescope
are currently being planned in Europe.
-
- Every
isotope of every element in the periodic table has the potential to tell us
something about the nuclear history of the universe. We needto keep looking!
-
March 1, 2023 HEAVIEST
ELEMENTS - how did we get here? 3897
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---------------------
--- Wednesday, March 1, 2023 ---------------------------
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