Astronomy

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

One of Saturn’s rings has revealed properties of its core, hidden deep beneath the planet’s golden atmosphere.

That core isn’t the lump of rock and ice that many scientists had envisioned, the new study finds. Instead, the core is diffuse, pervaded by huge amounts of hydrogen and helium and so spread out that it spans 70,000 kilometers, or about 60 percent of the planet’s diameter, researchers report April 28 at arXiv.org.

The new intel should help planetary scientists better understand not only how giant planets formed in our solar system but also the nature of such worlds orbiting other stars.  

To ascertain the structure of Saturn’s core, astronomer Christopher Mankovich and astrophysicist Jim Fuller, both at Caltech, examined the giant planet’s rings. Just as earthquakes help seismologists probe Earth’s interior, oscillations inside Saturn can reveal its internal composition. These oscillations alter Saturn’s gravitational forces, inducing waves in the rings —especially the C ring, which is the nearest of the three main rings to the planet (SN: 1/22/19).

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By analyzing a wave in that ring, along with data on Saturn’s gravity field from the now-defunct Cassini spacecraft (SN: 9/15/17), Mankovich and Fuller found that the core has about 17 Earth masses of rock and ice. But there’s so much hydrogen and helium mixed in, the core encompasses 55 Earth masses altogether — more than half of Saturn’s total, which is equivalent to the mass of 95 Earths. This “ring seismology” work will appear in a future Nature Astronomy.

“It’s a new way to look at gas giant planets in the solar system,” says Ravit Helled, a planetary scientist at the University of Zurich who was not involved with the work. “This knowledge is important because it reflects on our understanding of giant exoplanets,” and indicates that giant planets in other solar systems probably have more complex structures than many researchers had thought.

The discovery also illuminates how Saturn formed, says Nadine Nettelmann, a planetary scientist at the German Aerospace Center in Berlin.

Older theories posited that a gas giant such as Saturn arises when rock and ice orbiting the sun start to conglomerate. Tenuous gaseous envelopes let additional solid materials sink to the center, forming a compact core. Only later, according to this theory, does the core attract lots of hydrogen and helium — the ingredients that make up most of the planet. Although these elements are gases on Earth, Saturn’s great gravity squeezes most of them into a fluid.

But newer theories say instead that plenty of gas got incorporated into the core of rock and ice when it was taking shape 4.6 billion years ago. As the planet accreted additional mass, the proportion of gas rose. The structure Mankovich and Fuller deduce for Saturn’s core preserves this formation history, Nettelmann says, because the planet’s very center, representing the oldest part of Saturn, has the greatest proportion of rock and ice. The fraction of rock and ice decrease gradually rather than abruptly from the core’s center to its edge, reflecting the core’s development over time.

“I find the conclusions very important and very exciting and the line of reasoning very convincing,” Nettelmann says. Still, she cautions that additional waves in the rings should be analyzed for confirmation.

The type of oscillation that Mankovich and Fuller detect inside Saturn also implies that the core is stable rather than bubbling like a pot of water on a hot stove, which is one way a planet can carry heat from its hot interior outward. The core’s stability may help explain a long-standing puzzle: why Saturn emits more energy than it gets from the sun.

After the planet formed, it was warm with the heat of its birth, but then it cooled off. The core’s stability could have put a lid on some of this cooling, however, which helped the planet retain heat that it still radiates to this day. In contrast, if the core had instead transported heat via the upwelling and downwelling of material, the planet would have cooled off faster and no longer give off so much heat. More

Scientists have cracked the case of mysterious cosmic objects dubbed “yellowballs.” The celestial specks mark the birthplaces of many kinds of stars with a wide range of masses, rather than single supermassive stars, researchers report April 13 in the Astrophysical Journal.

The stars in the clusters are relatively young, only about 100,000 years old. “I think of these as stars in utero,” says Grace Wolf-Chase, an astronomer at the Planetary Science Institute who is based in Naperville, Ill. For comparison, the massive stars forming in the Orion nebula are about 3 million years old, and the middle-aged sun is 4.6 billion years old.

Volunteers with the Milky Way Project first identified the objects while scouring pictures of the galaxy taken by the Spitzer Space Telescope. The now-defunct observatory saw the cosmos in infrared light, which let astronomers take a sort of stellar ultrasound “to probe what’s going on in these cold environments before the stars are actually born,” says Wolf-Chase.

Citizen scientists had been looking through these images for baby stars thought to be at least 10 times the mass of the sun that were blowing giant bubbles of ionized gas. A year or two into the project, some users began labeling certain objects with the tag #yellowballs¸ because that’s what they looked like in the false-color images. Between 2010 and 2015, the volunteers found 928 yellowballs.

Wolf-Chase’s team initially thought the balls represented early stage gas bubbles. But because yellowballs were a serendipitous discovery, the researchers knew they probably hadn’t caught enough of them to definitively ID the objects. In 2016, the team asked Milky Way Project volunteers to find more. By the following year, the group had spotted more than 6,000 yellowballs.

Astronomers first thought ‘yellowballs’ (circled left) were precursors to gas bubbles blown around massive, young stars (right). But a new study suggests yellowballs are actually clusters of less massive stars.JPL-Caltech/NASA

Wolf-Chase and colleagues compared about 500 of those balls to existing catalogs of star clusters and other structures to try to figure out what they were. “Now we have a good answer: They’re infant star clusters,” Wolf-Chase says. The clusters blow ionized bubbles of their own, similar to the stellar bubbles blown by single young, big stars.

Wolf-Chase hopes researchers will be able to use the work to pick out yellowballs with telescopes like the James Webb Space Telescope, which is due to launch in October, and figure out more about the balls’ physical properties. More

Like a dried-up lemon from the back of the fridge, neutron stars are less squeezable than expected, physicists report.

New measurements of the most massive known neutron star find that it has a surprisingly large diameter, suggesting that the matter within isn’t as squishy as some theories predicted, physicists with the Neutron star Interior Composition Explorer, or NICER, reported April 17 at a virtual meeting of the American Physical Society.

When a dying star explodes, it can leave behind a memento: a remnant crammed with neutrons. These neutron stars are extraordinarily dense — like compressing Mount Everest into a teaspoon, said NICER astrophysicist Zaven Arzoumanian of NASA’s Goddard Space Flight Center in Greenbelt, Md. “We don’t know what happens to matter when it’s crushed to this extreme point.”

The more massive the neutron star, the more extreme the conditions in its core. Jammed together at tremendous densities, particles may form unusual states of matter. For example, particles known as quarks — usually contained within protons and neutrons — may roam freely in a neutron star’s center.

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The core’s composition determines its squeezability. For example, if quarks are free agents within the most massive neutron stars, the immense pressure will compress the neutron star’s core more than if quarks remain within neutrons. Because of that compressibility, for neutron stars, more mass doesn’t necessarily translate to a larger diameter. If neutron star matter is squishy, the objects could counterintuitively shrink as they become more massive (SN: 8/12/20).

To understand how neutron star innards respond to being put through the cosmic wringer, scientists used the X-ray telescope NICER aboard the International Space Station to estimate the diameters of rapidly spinning neutron stars called pulsars. In 2020, NICER sized up a pulsar with a mass about 1.4 times the sun’s: It was about 26 kilometers wide (SN: 1/3/20).

Researchers have now gauged the girth of the heftiest confirmed neutron star, with about 2.1 times the mass of the sun. But the beefy neutron star’s radius is about the same as its more lightweight compatriot’s, according to two independent teams within the NICER collaboration. Combining NICER data with measurements from the European Space Agency’s XMM-Newton satellite, one team found a diameter of around 25 kilometers while the other estimated 27 kilometers, physicists reported in a news conference and in two talks at the meeting.

Many theories predict that the more massive neutron star should have a radius that is smaller. “That it is not tells us that, in some sense, the matter inside neutron stars is not as squeezable as many people had predicted,” said astrophysicist Cole Miller of the University of Maryland in College Park, who presented the second result.

“This is a bit puzzling,” said astrophysicist Sanjay Reddy of the University of Washington in Seattle, who was not involved in the research. The finding suggests that inside a neutron star, quarks are not confined within neutrons, but they still interact with one another strongly, rather than being free to roam about unencumbered, Reddy said.

The measurements reveal another neutron star enigma. Pulsars emit beams of X-rays from two hot spots associated with the magnetic poles of the pulsar. According to the textbook picture, those beams should be emitted from opposite sides. But for both of the neutron stars measured by NICER, the hot spots were in the same hemisphere.

“It implies that we have a somewhat complex magnetic field,” said NICER astrophysicist Anna Watts of the University of Amsterdam, who presented the first team’s result. “Your beautiful cartoon of a pulsar … is for these two stars completely wrong. And that’s brilliant.”

Beams of radiation are emitted from the magnetic poles of spinning neutron stars called pulsars. Scientists typically envision pulsars with two beams on opposite sides, like a lighthouse. But the beams of a newly measured pulsar (illustrated) come from the same hemisphere.NASA’s Goddard Space Flight Center More

The most oxygen-poor star-forming galaxy ever found hints that the first galaxies to arise after the universe’s birth glittered with supermassive stars that left behind big black holes.

Such galaxies are rare now because almost as soon as a galaxy initiates star formation, massive stars produce huge amounts of oxygen, which is the most abundant element in the cosmos after hydrogen and helium. Astronomers prize the few such galaxies found close to home because they offer a glimpse of what conditions were like in the very early universe, before stars had made much oxygen (SN: 8/7/19).

The new galaxy’s oxygen-to-hydrogen ratio — a standard measure of relative oxygen abundance in the cosmos — is well under 2 percent of the sun’s, researchers report in a paper to appear in the Astrophysical Journal and posted online March 22 at arXiv.org.

“It is quite difficult to pick up such a rare object,” says astrophysicist Takashi Kojima, who, along with colleagues, made the discovery while he was at the University of Tokyo.

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Named HSC J1631+4426, the record-breaking galaxy, found by using the Subaru Telescope in Hawaii, is 430 million light-years from Earth in the constellation Hercules. The galaxy is a dwarf, with far fewer stars to create oxygen than the Milky Way has. Those relatively few stars have given the runt just a pinch of oxygen: one oxygen atom for every 126,000 hydrogen atoms. That’s only 1.2 to 1.6 percent of the oxygen level in the sun.

“Any new galaxy is good,” says Trinh Thuan, an astronomer at the University of Virginia in Charlottesville who helped find the previous champion four years ago. “We’re counting the number of [very oxygen-poor galaxies] in the palm of our hand.” The new galaxy’s oxygen-to-hydrogen ratio is 83 percent that of the previous record holder, J0811+4730, which is 620 million light-years away in the constellation Lynx.

A newly discovered galaxy has only about half the oxygen-to-hydrogen ratio of I Zwicky 18 (pictured), which once held the record for the most oxygen-poor star-forming galaxy known.NASA, ESA, A. Aloisi/Space Telescope Science Institute and European Space Agency.

In HSC J1631+4426, Kojima and his colleagues also find odd abundances of another chemical element: iron. While the overall amount of iron in the galaxy is low, “we discovered that the iron-to-oxygen abundance ratio is surprisingly high,” he says.

The same pattern also appears in the oxygen-poor galaxy in Lynx. In contrast, ancient stars in the Milky Way usually have little iron relative to oxygen. That’s because newborn stars get most of their iron from the explosions of long-lived stars. Those explosions had not occurred by the time the Milky Way’s oldest stars formed. But in the two nearly pristine galaxies, the amount of iron relative to oxygen is as high as that of the sun, which acquired large amounts of both elements from previous generations of stars.

“This is a very unusual pattern, and it’s not obvious how to explain that,” says Volker Bromm, an astrophysicist at the University of Texas at Austin who was not involved with the discovery.

Just before Kojima earned his Ph.D. in 2020, he hit upon a possible explanation: High-mass stars in dense star clusters merged together to make stellar goliaths more than 300 times as massive as the sun. These superstars then exploded and showered their galactic homes with both iron and oxygen, leading to high iron-to-oxygen ratios in the two primitive galaxies as well as a source of what little oxygen exists there.

No stars this massive are known to exist in the modern Milky Way. But Kojima says their presence in the two most oxygen-poor star-making galaxies suggests that primordial galaxies had them too.

When the superstars died, they should have left behind intermediate-mass black holes, which are more than 100 times as massive as the sun (SN: 9/2/20). That’s about 10 times as massive as typical black holes, which can form when bright stars die.

Kojima’s team sees evidence for these big black holes in the newly discovered galaxy. Gas swirling around such large black holes should get so hot it emits high-energy photons, or particles of light. Because of their high energy, these photons would tear electrons even from helium atoms, which cling tightly to their electrons, and turn the atoms into positively charged ions. Sure enough, the galaxy in Hercules emits a wavelength of blue light that comes from just such helium ions.

The record-breaking galaxy is “an exciting preview of things to come,” Bromm says. In coming years, he says, enormous telescopes will open that will find even more extreme galaxies (SN: 1/10/20). “Then we will have a wonderfully complementary way to learn about the early universe.” More

The Big Dipper’s stars make up a conspicuous landmark in the sky of the Northern Hemisphere. Even novice stargazers can easily pick out the shape, part of the Ursa Major constellation. Now, scientists have shown that three factors can explain why certain groups of stars form such recognizable patterns.

To replicate how humans perceive the celestial sphere, a team of researchers considered how the eye might travel randomly across this night sky. Human eyes tend to move in discrete jumps, called saccades (SN: 10/31/11), from one point of interest to another. The team created a simulation that incorporated the distribution of lengths of those saccades, combined that with basic details of the night sky as seen from Earth — namely the apparent distances between neighboring stars and their brightnesses.

The technique could reproduce individual constellations, such as Dorado, the dolphinfish. And when used to map the whole sky, the simulation generated groupings of stars that tended to align with the 88 modern constellations recognized by the International Astronomical Union, Sophia David and colleagues reported March 18 at an online meeting of the American Physical Society.

“Ancient people from various cultures connected similar groupings of stars independently of each other,” said David, a high school student at Friends’ Central School in Wynnewood, Penn., who worked with network scientists at the University of Pennsylvania. “And this indicates that there are some fundamental aspects of human learning … that influence the ways in which we organize information.” More

Something’s fishy in the southern constellation Phoenix.

Strange radio emissions from a distant galaxy cluster take the shape of a gigantic jellyfish, complete with head and tentacles. Moreover, the cosmic jellyfish emits only the lowest radio frequencies and can’t be detected at higher frequencies. The unusual shape and radio spectrum tell a tale of intergalactic gas washing over galaxies and gently revving up electrons spewed out by gargantuan black holes long ago, researchers report in the March 10 Astrophysical Journal.

Spanning 1.2 million light-years, the strange entity lies in Abell 2877, a cluster of galaxies 340 million light-years from Earth. Researchers have dubbed the object the USS Jellyfish, because of its ultra-steep spectrum, or USS, from low to high radio frequencies.

“This is a source which is invisible to most of the radio telescopes that we have been using for the last 40 years,” says Melanie Johnston-Hollitt, an astrophysicist at Curtin University in Perth, Australia. “It holds the record for dropping off the fastest” with increasing radio frequency.

Johnston-Hollitt’s colleague Torrance Hodgson, a graduate student at Curtin, discovered the USS Jellyfish while analyzing data from the Murchison Widefield Array, a complex of radio telescopes in Australia that detect low-frequency radio waves. These radio waves are more than a meter long and correspond to photons, particles of light, with the lowest energies. Remarkably, the USS Jellyfish is about 30 times brighter at 87.5 megahertz — a frequency similar to that of an FM radio station — than at 185.5 MHz.

The Murchison Widefield Array consists of 4,096 radio antennas grouped into 256 “tiles” (one pictured) spanning several kilometers in a remote region of Western Australia.Pete Wheeler, ICRAR

“That is quite spectacular,” says Reinout van Weeren, an astronomer at Leiden University in the Netherlands who was not involved with the work. “It is quite a neat result, because this is really extreme.”

The USS Jellyfish bears no relation to previously discovered jellyfish galaxies. “This is absolutely enormous compared to those other things,” Johnston-Hollitt says. Indeed, jellyfish galaxies are a very different kettle of celestial fish. Although they also inhabit galaxy clusters, they are individual galaxies passing through hot gas in a cluster. The hot gas tears the galaxy’s own gas out of it, creating a wake of tentacles. The much larger USS Jellyfish, on the other hand, appears to have formed when intergalactic gas and electrons interacted.

Hodgson and his colleagues note that two galaxies in the Abell 2877 cluster coincide with the brightest patches of radio waves in the USS Jellyfish’s head. These galaxies, the researchers say, probably have supermassive black holes at their centers. The team ran computer simulations and found that the black holes were probably accreting material some 2 billion years ago. As they did so, disks of hot gas formed around each of them, spewing huge jets of material into the surrounding galaxy cluster.

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This ejected material had electrons that whirled around magnetic fields at nearly the speed of light, so the electrons emitted radio waves. Over time, though, the electrons lost energy, and the most energetic electrons, which had been emitting the highest radio frequencies, faded the most. Then a wave of gas sloshed through the entire cluster, reaccelerating the electrons around the two galaxies.

“It’s a very gentle process,” Johnston-Hollitt says. “The electrons don’t get that much energy, which means they don’t light up at high frequencies.” Instead, the gentle gas wave caused electrons to emit radio waves with the lowest energies and frequencies, giving the USS Jellyfish the extreme spectrum it has today. More

Astronomers have gotten their first glimpse of the magnetic fields tangled around a black hole.

The Event Horizon Telescope has unveiled the magnetism of the hot, glowing gas around the supermassive black hole at the heart of galaxy M87, researchers report in two studies published online March 24 in the Astrophysical Journal Letters. These magnetic fields are thought to play a crucial role in how the black hole scarfs down matter and launches powerful plasma jets thousands of light-years into space (SN: 3/29/19).

“We’ve known for decades that jets are in some sense powered by accretion onto supermassive black holes, and that the in-spiraling gas and the outflowing plasma are highly magnetized — but there was a lot of uncertainty in the exact details,” says Eileen Meyer, an astrophysicist at the University of Maryland, Baltimore County not involved in the work. “The magnetic field structure of the plasma near the event horizon [of a black hole] is a completely new piece of information.”

The supermassive black hole inside M87 was the first black hole to get its picture taken (SN: 4/10/19). That image showed the black hole’s shadow against its accretion disk — the bright eddy of superhot gas spiraling around the black hole’s dark center. It was created using observations taken in April 2017 by a global network of observatories, which collectively form one virtual, Earth-sized radio dish called the Event Horizon Telescope (SN: 4/10/19).

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Using data from 2017, scientists created the first real picture of the supermassive black hole at the center of galaxy M87. How? We explain.

The new analysis uses the same observations. But unlike the black hole’s initial portrait, the new image accounts for the polarization of the light waves emitted by gas around the black hole. Polarization measures a light wave’s orientation — whether it wiggles up and down, left and right or at an angle — and can be affected by the magnetic field where the light originated. So, by mapping the polarization of light around the edge of M87’s black hole, researchers were able to trace the structure of the underlying magnetic fields.

The team found evidence that some magnetic fields loop around the black hole along with the disk of material swirling into it. That’s to be expected because “when gas is rotating, it’s basically able to carry along the magnetic field with it,” says Jason Dexter, an astrophysicist at the University of Colorado Boulder.

But, he says, “there’s some interesting component of this magnetic field which is not just following the motion of the gas.” At least some of the magnetic field lines are sticking up or down perpendicularly from the accretion disk, or pointing directly toward or away from the black hole, Dexter and colleagues found. These magnetic fields must be very strong to resist being dragged around by the whirl of infalling gas, he says.

Such strong magnetic fields may actually push back against some of the material spiraling in toward the black hole, helping it resist gravity’s pull, says study coauthor Monika Mościbrodzka, an astrophysicist at Radboud University in Nijmegen, the Netherlands. Magnetic fields pointed up and down from the accretion disk could also help launch the black hole’s plasma jets, by channeling material toward the black hole’s poles and giving it a boost in speed, she says.

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