A smattering of plutonium atoms embedded in Earth’s crust are helping to resolve the origins of nature’s heaviest elements.

Scientists had long suspected that elements such as gold, silver and plutonium are born during supernovas, when stars explode. But typical supernovas can’t explain the quantity of heavy elements in our cosmic neighborhood, a new study suggests. That means other cataclysmic events must have been major contributors, physicist Anton Wallner and colleagues report in the May 14 Science.

The result bolsters a recent change of heart among astrophysicists. Standard supernovas have fallen out of favor. Instead, researchers think that heavy elements are more likely forged in collisions of two dense, dead stars called neutron stars, or in certain rare types of supernovas, such as those that form from fast-spinning stars (SN: 5/8/19).

Heavy elements can be produced via a series of reactions in which atomic nuclei swell larger and larger as they rapidly gobble up neutrons. This series of reactions is known as the r-process, where “r” stands for rapid. But, says Wallner, of Australian National University in Canberra, “we do not know for sure where the site for the r-process is.” It’s like having the invite list for a gathering, but not its location, so you know who’s there without knowing where the party’s at.

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Scientists thought they had their answer after a neutron star collision was caught producing heavy elements in 2017 (SN: 10/16/17). But heavy elements show up in very old stars, which formed too early for neutron stars to have had time to collide. “We know that there has to be something else,” says theoretical astrophysicist Almudena Arcones of the Technical University of Darmstadt, Germany, who was not involved with the new study.

If an r-process event had recently happened nearby, ­some of the elements created could have landed on Earth, leaving fingerprints in Earth’s crust. Starting with a 410-gram sample of Pacific Ocean crust, Wallner and colleagues used a particle accelerator to separate and count atoms. Within one piece of the sample, the scientists searched for a variety of plutonium called plutonium-244, which is produced by the r-process. Since heavy elements are always produced together in particular proportions in the r-process, plutonium-244 can serve as a proxy for other heavy elements. The team found about 180 plutonium-244 atoms, deposited into the crust within the last 9 million years.

Scientists analyzed a sample of Earth’s deep-sea crust (shown) to search for atoms of plutonium and iron with cosmic origins.Norikazu Kinoshita

Researchers compared the plutonium count to atoms that had a known source. Iron-60 is released by supernovas, but it is formed by fusion reactions in the star, not as part of the r-process. In another, smaller piece of the sample, the team detected about 415 atoms of iron-60.

Plutonium-244 is radioactive, decaying with a half-life of 80.6 million years. And iron-60 has an even shorter half-life of 2.6 million years. So the elements could not have been present when the Earth formed, 4.5 billion years ago. That suggests their source is a relatively recent event. When the iron-60 atoms were counted up according to their depth in the crust, and therefore how long ago they’d been deposited, the scientists saw two peaks at about 2.5 million years ago and at about 6.5 million years ago, suggesting two or more supernovas had occurred in the recent past.

The scientists can’t say if the plutonium they detected also came from those supernovas. But if it did, the amount of plutonium produced in those supernovas would be too small to explain the abundance of heavy elements in our cosmic vicinity, the researchers calculated. That suggests regular supernovas can’t be the main source of heavy elements, at least nearby.  

That means other sources for the r-process are still needed, says astrophysicist Anna Frebel of MIT, who was not involved with the research. “The supernovae are just not cutting it.”

The measurement gives a snapshot of the r-process in our corner of the universe, says astrophysicist Alexander Ji of Carnegie Observatories in Pasadena, Calif. “It’s actually the first detection of something like this, so that’s really, really neat.” More

Two giant, mysterious bubbles spew from the Milky Way’s heart, and now it appears the bubbles may have doubles.
Scientists have known for a decade that two bubbles of charged particles, or plasma, flank the plane of the Milky Way. Those structures, known as the Fermi bubbles after the telescope that detected them, are visible in high-energy light called gamma rays (SN: 11/9/10). But now, the eROSITA X-ray telescope has revealed larger bubbles, seen in X-rays. The X-ray bubbles extend about 45,000 light-years above and below the center of the galaxy, researchers report online December 9 in Nature.
Previously, researchers had seen an X-ray arc above the galactic plane (SN: 7/8/20). But no such feature was evident below the plane of the galaxy. That lack of symmetry led some scientists to discount the possibility of X-ray bubbles. With the new results, “this argument now has fallen,” says study coauthor Andrea Merloni, an astronomer at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany. The eROSITA data reveal a faint and previously unknown bubble below the galactic plane, and a matching bubble above. The gamma-ray bubbles are nested inside the X-ray bubbles, suggesting that the two features are connected, says Merloni.
Studying the bubbles could help reveal violent events that may have taken place in the galaxy’s past. The supermassive black hole at the center of the Milky Way is currently fairly quiet, as far as black holes go. But a past feeding frenzy might have spewed its leftovers outward, forming the structures. Or the bubbles could have been the result of a period when many stars formed and exploded in the galaxy’s heart. Further study of the X-ray and gamma-ray bubbles could help reveal the cause. More

A long-predicted type of cosmic explosion has finally burst onto the scene.

Researchers have found convincing evidence for an electron-capture supernova, a stellar explosion ignited when atomic nuclei sop up electrons within a star’s core. The phenomenon was first predicted in 1980, but scientists have never been sure that they have seen one. A flare that appeared in the sky in 2018, called supernova 2018zd, matches several expected hallmarks of the blasts, scientists report June 28 in Nature Astronomy.

“These have been theorized for so long, and it’s really nice that we’ve actually seen one now,” says astrophysicist Carolyn Doherty of Konkoly Observatory in Budapest, who was not involved with the research.

Electron-capture supernovas result from stars that sit right on the precipice of exploding. Stars with more than about 10 times the sun’s mass go supernova after nuclear fusion reactions within the core cease, and the star can no longer support itself against gravity. The core collapses inward and then rebounds, causing the star’s outer layers to explode outward (SN: 2/8/17). Smaller stars, with less than about eight solar masses, are able to resist collapse, instead forming a dense object called a white dwarf (SN: 6/30/21). But between about eight and 10 solar masses, there’s a poorly understood middle ground for stars. For some stars that fall in that range, scientists have long suspected that electron-capture supernovas should occur.

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During this type of explosion, neon and magnesium nuclei within a star’s core capture electrons. In this reaction, an electron vanishes as a proton converts to a neutron, and the nucleus morphs into another element. That electron capture spells bad news for the star in its war against gravity because those electrons are helping the star fight collapse.

According to quantum physics, when electrons are packed closely together, they start moving faster. Those zippy electrons exert a pressure that opposes the inward pull of gravity. But if reactions within a star chip away at the number of electrons, that support weakens. If the star’s core gives way — boom — that sets off an electron-capture supernova.

But without an observation of such a blast, it remained theoretical. “The big question here was, ‘Does this kind of supernova even exist?’” says astrophysicist Daichi Hiramatsu of the University of California, Santa Barbara and Las Cumbres Observatory in Goleta, Calif. Potential electron-capture supernovas have been reported before, but the evidence wasn’t definitive.

So Hiramatsu and colleagues created a list of six criteria that an electron-capture supernova should meet. For example, the explosions should be less energetic, and should forge different varieties of chemical elements, than more typical supernovas. Supernova 2018zd checked all the boxes.

A stroke of luck helped the team clinch the case. Most of the time, when scientists spot a supernova, they have little information about the star that produced it — by time they see the explosion, the star has already been blown to bits. But in this case, the star showed up in previous images taken by NASA’s Hubble Space Telescope and Spitzer Space Telescope. Its properties matched those expected for the type of star that would produce an electron-capture supernova.

“All together, it really is very promising,” says astrophysicist Pilar Gil-Pons of Universitat Politècnica de Catalunya in Barcelona. Reading the researchers’ results, she says, “I got pretty excited, especially about the identification of the progenitor.” 

Finding more of these supernovas could help unveil their progenitors, misfit stars in that odd mass middle ground. It could also help scientists better nail down the divide between stars that will and won’t explode. And the observations could reveal how often these unusual supernovas occur, an important bit of information for better understanding how supernovas seed the cosmos with chemical elements. More

The Milky Way’s core harbors two giants: the galaxy’s largest black hole and a cluster of tens of millions of stars around the black hole that is denser and more massive than any other star cluster in the galaxy.
Most of the cluster’s many stars shine within just 20 light-years of the galactic center and all together weigh about 25 million times as much as the sun. New observations suggest that this “nuclear star cluster” owes some of its brilliance to another big group of stars, or even a small galaxy, that the main cluster swallowed.
Nuclear star clusters exist in many galaxies and are the densest star clusters in the universe. Astronomers are trying to figure out how these gatherings get so jam-packed and how they feed the giant black holes at the centers of galaxies.
To get a look at the Milky Way’s core, Tuan Do, an astronomer at UCLA, and colleagues observed about 700 red giant stars within five light-years of the galaxy’s heart. Because dust between Earth and the galactic center blocks the stars’ visible light, the astronomers studied infrared wavelengths, which better penetrate the dust.

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“We noticed a very curious thing about our data, which is that the stars with less metals than our sun seem to be moving differently than the stars with more metals,” Do says.
About 7 percent of the stars in the nuclear star cluster revolve around the galactic center faster than their peers and do so around a different axis, the team found. The data on infrared wavelengths indicate that this fast-revolving population is only 30 percent as metal-rich as the sun. In contrast, most of the other stars in the nuclear star cluster have more metals than the sun.
“This discovery shows that at least some of our nuclear star cluster must have been formed from things falling in,” Do says. A metal-poor star cluster thousands of light-years away from the galactic core probably sank into the main star cluster, he and his colleagues report online September 28 in the Astrophysical Journal Letters.
Do says the infalling star cluster was the victim of dynamical friction, a process that can alter a star cluster’s path through space. In this process, the orbiting star cluster’s gravity attracts material that forms a wake behind it. The backward tug of this material’s gravity then causes the cluster to plunge closer and closer to the galactic center.
Scott Tremaine, an astrophysicist at the Institute for Advanced Study in Princeton, N.J., who was not involved in the work, calls the team’s data on the nuclear cluster’s stars unique. “I think by far the most natural explanation is that [the stars] do come from a cluster that’s spiraled in,” he says.
In a companion study, team member Manuel Arca Sedda at Heidelberg University in Germany and colleagues ran computer models to simulate how a star cluster falling into the Milky Way’s nuclear star cluster could explain the new observations. These simulations indicate that such an event occurred less than 3 billion years ago, and that the devoured cluster was roughly a million times as massive as the sun, the researchers report in a second study also published September 28 in the Astrophysical Journal Letters.
That mass is comparable to Omega Centauri, the Milky Way’s most massive globular cluster, a type of star grouping that’s dense but less extreme than nuclear star clusters. “It’s definitely a lot,” Do says. Just a dozen or so massive globular clusters could have populated the entire nuclear star cluster, he says.
Still, many of the nuclear star cluster’s other stars may have been born in place at the galactic center. And the scientists can’t rule out that the gobbled-up victim was a dwarf galaxy. Both dwarf galaxies and globular clusters can possess a similar number of stars. But their stars have different ratios of chemical elements, so future observations of the nuclear star cluster may be able to distinguish between the two scenarios. More

After 35 years, the Hubble Space Telescope is still churning out hits. In just the last year or so, scientists have used the school bus–sized observatory to confirm the first lone black hole, reveal new space rocks created by a NASA asteroid-impact mission and pinpoint the origin of a particularly intense, mysterious burst of radio waves.

These findings are a testament to the fact that there’s still plenty of science for the telescope to do. And there are some observations that simply can’t be done with any other telescope, including Hubble’s younger sibling, the James Webb Space Telescope. More