Wednesday, March 1, 2023

3897 - HEAVIEST ELEMENTS - how did we get here?

 

-  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?

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-    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.

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-   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.

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-   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.

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-    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.

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-    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.

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-    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.

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-    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.

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-    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.

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-   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.

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-   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.

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-      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.

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-    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.

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-   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.

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-     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.

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-    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.

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-    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.

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-    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.

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-    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.

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-     Eventually the gravitational waves traveling at light speed and the light from the merger reached Earth together.  Humanity had just witnessed heavy-element production.

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-  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.

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-    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.

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-    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.

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-    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.

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-    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.

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-  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.

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-  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.

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-    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.

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-    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.

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-    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.

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-    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.

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-    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.

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-   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.

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-    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.

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-    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.

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-    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”.

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-    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.

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-    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.

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-     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.

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-    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.

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-   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.

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-   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.

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-   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.

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-   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.

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-   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.

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-     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.

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-    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.

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-    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.

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-   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!

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            March 1, 2023       HEAVIEST  ELEMENTS  -  how did we get here?                3897                                                                                                                         

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