Astronomy

Astronomers look for distant planets by watching for the shadow the worlds cast when passing between their star and Earth. If any aliens are searching for other intelligent life, they could spot us using the same trick.

Now, scientists have identified 1,715 star systems whose hypothetical inhabitants could have seen Earth cross in front of the sun sometime in the last 5,000 years. Another 319 stars will come into the right positions for spotting Earth in the next 5,000 years, astrophysicist Jackie Faherty and astronomer Lisa Kaltenegger report in the June 23 Nature.

Those 2,034 stars had or will have “the front row seat to finding Earth as a transiting planet,” says Faherty, of the American Museum of Natural History in New York City.

Seventy-five of the stars are close enough that human-made radio waves have already reached them, and seven of those stars have potentially habitable planets.

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Faherty and Kaltenegger, of Cornell University, used maps of more than 1 billion stars from the European Space Agency’s Gaia spacecraft, which measures stars’ movements and distances from Earth. The researchers identified the Earth transit zone, the region of space from which stars can see Earth cross the sun, and ran the clock backward and forward to see stars move in and out of that zone. “The way I think about this is, ‘Where are we the aliens?’” Kaltenegger says.

Previous research identified stars that can currently see Earth silhouetted against the sun (SN: 4/20/16). But those studies did not take into account stellar movements through space and time. The new work shows that most star systems with a good look at Earth will keep that view for thousands of years.

The newly described stellar collection includes some of the nearest and most well-known stars with planets, including Ross 128 and TRAPPIST-1, with its septet of rocky worlds (SN: 2/22/17). More

Astronomers around the world were startled in late 2019 when Betelgeuse, one of the brightest stars in the sky, grew dark for several months. Rumors swirled that the star was about to go supernova. It didn’t. But debate over what was going on exploded. Now, newly released images taken before and during the “Great Dimming” suggest what happened: The star’s surface cooled and triggered a cloud of dust that temporarily blocked its light.

“This is the best interpretation we can get with the data that we have … without flying our spaceship to Betelgeuse and seeing what’s going on there,” says astrophysicist Emily Cannon of KU Leuven in Belgium.

Cannon and colleagues used the SPHERE instrument on the European Southern Observatory’s Very Large Telescope in Chile to take snapshots of Betelgeuse for more than a year. Serendipitously, the team had captured an image of the star in January 2019, months before the dimming began, and could compare that image with others taken in December 2019 and January and March 2020.

The dimming wasn’t spread uniformly across Betelgeuse’s surface, the team reports June 16 in Nature. A dark splotch was concentrated over the star’s southern hemisphere. The researchers then ran computer simulations of the star, which included incorporating how dynamic gas bubbles constantly churn beneath its surface, to figure out the likeliest explanation for the way that the dimming played out.

Earlier observations of the star had split astronomers into two camps (SN: 11/29/20). One group thought that a cloud of dust had blocked Betelgeuse’s light (SN: 3/12/20). Another thought that there wasn’t enough evidence of dust, and the dimming was due to temporary cooling at Betelgeuse’s surface.

Betelgeuse, one of the brightest stars in the sky, marks the shoulder (circled in red) of the constellation Orion.Nick Risinger/skysurvey.org, ESO

Astrophysicist Miguel Montargès says that now that he’s seen his team’s data, he’s in both camps. “The most natural conclusion is that both events happened,” says Montargès, of the Paris Observatory.

The team’s hypothesis is that in late 2019, a temporary cold patch formed in Betelgeuse’s southern hemisphere due to the normal churning of surface plasma, and that cooling caused the star’s light to dim. The cold patch then allowed gas that had been released from the star’s surface to cool enough to form dust particles, which further blocked the star’s light.

“You start getting a runaway effect,” which makes it easier for more dust to form, says astrophysicist Emily Levesque of the University of Washington in Seattle, who was not involved in the research but wrote a commentary in the same issue of Nature. As the dust spread out, the starlight shone through again.

Some astronomers are still unconvinced that dust is part of the answer. The images plus simulations don’t prove dust was there, says astrophysicist Thavisha Dharmawardena of the Max Planck Institute for Astronomy in Heidelberg, Germany. “This discussion will continue till we obtain direct evidence for dust,” says Dharmawardena, who has looked for — and failed to find — signs of dust during the Great Dimming.

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Montargès thinks the dust was just hard to see using other techniques. “When people say they are not seeing new dust, I think they are wrong,” he says. “It’s that their data does not allow them to see it.”

Both researchers agree that the Atacama Large Millimeter Array in Chile could break the stalemate. That telescope was out of commission last summer due to the COVID-19 pandemic, when its observations would have been most useful. More observations are scheduled for this summer, and if dust is still there, ALMA should see it.

Still, “if we cannot identify it, it’s not because it’s not there,” Montargès says. “It’s because we are too late.”

The Betelgeuse observations may help astronomers recognize similar dimming events in other stars, Levesque says. Betelgeuse is Earth’s closest red supergiant star, a late phase of the stellar life cycle that comes before a supernova explosion. While dust does not predict an explosion, it can be part of how these stars lose mass before they die.

So when will Betelgeuse go out with a bang? “Not today,” Montargès says. “Every day, we are closer to the explosion, that’s for sure. I think it’s not tomorrow, or even in our lifetime, for Betelgeuse.” More

A giant arc of galaxies appears to stretch across more than 3 billion light-years in the distant universe. If the arc turns out to be real, it would challenge a bedrock assumption of cosmology: that on large scales, matter in the universe is evenly distributed no matter where you look.

“It would overturn cosmology as we know it,” said cosmologist Alexia Lopez at a June 7 news conference at the virtual American Astronomical Society meeting. “Our standard model, not to put it too heavily, kind of falls through.”

Lopez, of the University of Central Lancashire in Preston, England, and colleagues discovered the purported structure, which they call simply the Giant Arc, by studying the light of about 40,000 quasars captured by the Sloan Digital Sky Survey. Quasars are the luminous cores of giant galaxies so distant that they appear as points of light. While en route to Earth, some of that light gets absorbed by atoms in and around foreground galaxies, leaving specific signatures in the light that eventually reaches astronomers’ telescopes (SN: 7/12/18).

The Giant Arc’s signature is in magnesium atoms that have lost one electron, in the halos of galaxies about 9.2 billion light-years away. The quasar light absorbed by those atoms traces out a nearly symmetrical curve of dozens of galaxies spanning about one-fifteenth the radius of the observable universe, Lopez reported. The structure itself is invisible on the sky to human eyes, but if you could see it, the arc would span about 20 times the width of the full moon.

Astronomers discovered what they say is a giant arc of galaxies (smile-shaped curve in the middle of this image) by using the light from distant quasars (blue dots) to map out where in the sky that light got absorbed by magnesium atoms in the halos (dark spots) that surround foreground galaxies.A. Lopez

“This is a very fundamental test of the hypothesis that the universe is homogeneous on large scales,” says astrophysicist Subir Sarkar of the University of Oxford, who studies large-scale structures in the universe but was not involved in the new work. If the Giant Arc is real, “this is a very big deal.”

But Sarkar isn’t convinced it is real yet. “Our eye has a tendency to pick up patterns,” Sarkar says, noting that some people have claimed to see cosmologist Stephen Hawking’s initials written in fluctuations in the cosmic microwave background, the oldest light in the universe.

Lopez ran three statistical tests to figure out the odds that galaxies would line up in a giant arc by chance. All three suggest that the structure is real, with one test surpassing physicists’ gold standard that the odds of it being a statistical fluke are less than 0.00003 percent.

That sounds pretty good, but it’s not enough, Sarkar says. “Right now, I would say they still don’t have compelling evidence,” he says. More observations, from Lopez’s group and others, could confirm or refute the Giant Arc.

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If it is real, the Giant Arc would join a growing group of large-scale structures in the universe that, taken together, would break the standard model of cosmology. This model assumes that when you look at large enough volumes of space — above about 1 billion light-years — matter is distributed evenly. The Giant Arc appears about three times as long as that theoretical threshold. It joins other structures with similarly superlative names, like the Sloan Great Wall, the Giant Gamma-Ray Burst Ring and the Huge Large Quasar Group.

“We can have one large-scale structure that could just be a statistical fluke,” Lopez said. “That’s not the problem. All of them combined is what makes the problem even bigger.” More

Five brief, bright blasts of radio waves from deep space now have precise addresses.

The fast radio bursts, or FRBs, come from the spiral arms of their host galaxies, researchers report in a study to appear in the Astrophysical Journal. The proximity of the FRBs to sites of star formation bolsters the case for run-of-the-mill young stars as the origin of these elusive, energetic eruptions.

“This is the first such population study of its kind and provides a unique piece to the puzzle of FRB origins,” says Wen-fai Fong, an astronomer at Northwestern University in Evanston, Ill.

FRBs typically last a few milliseconds and are never seen again. Because the bursts are so brief, it’s difficult to nail down their precise origins on the sky. Although astronomers have detected about 1,000 FRBs since the first was reported in 2007, only 15 or so have been traced to a specific galaxy.

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The first burst to be traced to its source came from a small, blobby dwarf galaxy with a lot of active star formation (SN: 1/4/17). That FRB sends off repeated blasts from a single source, which is an unusual feature, and helped astronomers localize its host galaxy.

“After that, a lot of people thought, well, maybe all FRB hosts are like this,” says astronomer Alexandra Mannings of the University of California, Santa Cruz. But then a second repeating burst was tracked back to a spiral galaxy like the Milky Way (SN: 1/6/20). And a one-off burst was localized to a massive disk-shaped galaxy, also the size of the Milky Way. Others followed.

Mannings, Fong and colleagues thought they could learn more about the FRBs’ sources by localizing their origins even more precisely. Different parts of spiral galaxies tend to host different types of stars. The bright spiral arms tend to mark sites where new stars are being born, while the older and dimmer stars have had time to drift away from the arms into the rest of the galaxy. So figuring out which galactic neighborhoods FRBs call home can reveal a lot about what kind of objects they come from.

Using the Hubble Space Telescope, the researchers took high-resolution images of eight galaxies that were already known to host FRBs, then overlaid the FRBs’ positions onto the images. The five FRBs that came from clearly defined spiral galaxies all lay on or close to the galaxies’ spiral arms, which had not been visible in images from ground-based telescopes. The other three host galaxies had inconclusive shapes, Fong says.

The FRB locales have a fair amount of star formation, but they’re not the brightest and most active parts of their galaxies, Fong says. That suggests FRBs originate with ordinary young stars — not the youngest, most massive stars that occupy the brightest knots in the spiral arms, but not the oldest and dimmest stars that have drifted away from their homes, either.

That finding is consistent with the idea that FRBs come from highly magnetized stellar corpses called magnetars, Mannings says (SN: 6/4/20). There are a couple of ways to produce magnetars from ordinary stars. There’s the slow way, which involves waiting billions of years for a pair of neutron stars to collide (SN: 12/1/20). Or there’s the fast way, which follows the death of a single massive star. It seems like FRBs might come from an in-between process, like the death of a not-so-massive star, Mannings says.

“The fact that FRBs are found to be pretty close to, if not on, the spiral arm, near to these star forming regions, that can give us a better idea of what the timeline is like for the progenitor,” whatever created the FRB, Mannings says. “And if it is a magnetar, it lets us know that it’s not through the delayed channel, like a neutron star merger.”

The finding doesn’t entirely solve the mystery of where FRBs come from, says astrophysicist Emily Petroff of the University of Amsterdam, who was not involved in the new work. But it does help to get a broader picture of their host galaxies.

“FRBs keep throwing a lot of surprises at us, in terms of what they look like, where they’re found, how they repeat,” Petroff says. “This is maybe providing more evidence that FRBs are more related to just sort of general neutron stars.” The next step, of course, is to find more FRBs. More

The cosmos keeps outdoing itself.

Extremely energetic light from space is an unexplained wonder of astrophysics, and now scientists have spotted this light, called gamma rays, at higher energies than ever before.

The Large High Altitude Air Shower Observatory, LHAASO, spotted more than 530 gamma rays with energies above 0.1 quadrillion electron volts, researchers report online May 17 in Nature. The highest-energy gamma ray detected was about 1.4 quadrillion electron volts. For comparison, protons in the largest accelerator on Earth, the Large Hadron Collider, reach mere trillions of electron volts. Previously, the most energetic gamma ray known had just under a quadrillion electron volts (SN: 2/2/21).

In all, the scientists spotted 12 gamma ray hot spots, hinting that the Milky Way harbors powerful cosmic particle accelerators. In order for gamma rays to reach such energies, electromagnetic fields must first rev up charged particles, namely protons or electrons, to immense speeds. Those particles can then produce energetic gamma rays, for example, when protons interact with other matter in space.

Scientists aren’t yet sure what environments are powerful enough to produce light with energies reaching more than a quadrillion electron volts. But the new observations point to two possibilities. One hot spot was associated with the Crab Nebula, the turbulent remains of an exploded star (SN: 6/24/19). Another potential source was the Cygnus Cocoon, a region where massive stars are forming, blasting out intense winds in the process.

LHAASO, located on Haizi Mountain in China’s Sichuan province, is not yet fully operational. When it is completed later this year, it is expected to find even more energetic gamma rays. More

The most realistic computer simulation of star formation yet offers stunning views of what the inside of a stellar nursery might look like.

In the Star Formation in Gaseous Environments simulation, or STARFORGE, a giant virtual cloud of gas collapses into a nest of new stars. Unlike other simulations, which could render only a small clump of gas within a larger cloud, STARFORGE simulates an entire star-forming cloud. It’s also the first simulation to account for the whole medley of physical phenomena thought to influence star formation, researchers report online May 17 in Monthly Notices of the Royal Astronomical Society.

“We sort of know the basic story of star formation … but the devil is in the details,” says Mike Grudić, a theoretical astrophysicist at Northwestern University in Evanston, Ill. (SN: 4/21/20). Astronomers still don’t fully understand, for instance, why stars have different masses. “If you really want to get the full picture, then you really have to just simulate the whole thing.”

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In the computer simulation STARFORGE, a massive cloud of cosmic gas — roughly 20 parsecs, or 65 light-years, across — collapses to form new stars. White areas indicate denser regions of gas, including baby stars. Orange highlights places where there’s lots of variation in the gas motion, such as in powerful jets launched by new stars. Gas shown in purple is more tranquil. After 4.3 million years (Myr) have passed, the simulation pauses so the virtual camera can swoop around the cloud, revealing its 3-D structure.

STARFORGE starts with a blob of gas that can be tens to hundreds of light-years across and up to millions of times the mass of the sun. Turbulence inside the cloud creates dense pockets that collapse to forge new stars. Those stars then launch powerful jets, give off radiation, shed stellar winds and explode in supernovas. Eventually, these phenomena blow the last vestiges of the cloud away and leave behind a hive of young stars. The whole process takes millions of years — or months of computing time, even running on supercomputers.

Using STARFORGE, Grudić and colleagues have confirmed that jets launched by new stars help regulate how much material a star amasses. In simulations without jets, typical stars were about 10 times the mass of the sun — way bigger than the actual average star. “As soon as you add this jet feedback to your simulation,” Grudić says, “stellar masses start coming out more or less right on the dot for what they’re observed to be.”

The STARFORGE simulation has helped confirm that jets launched by newborn stars (simulated one shown) determine how much mass stars can accrete.Northwestern University, University of Texas at Austin

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The Milky Way as we know it today was shaped by a collision with a dwarf galaxy about 10 billion years ago. But most of the modern galaxy was already in place even at that early date, new research shows.

Ages of stars left behind by the galactic interloper are a bit younger or on par with stars in the Milky Way’s main disk, researchers report May 17 in Nature Astronomy. And that could mean that the Milky Way grew up faster than astronomers expected, says study author Ted Mackereth, an astrophysicist at the University of Toronto.

“The Milky Way had already built up a lot of itself before this big merger happened,” he says.

Our galaxy’s history is one of violent conquest. Like other giant spiral galaxies in the universe, the Milky Way probably built up its bulk by colliding and merging with smaller galaxies over time. Stars from the unfortunate devoured galaxies got mixed into the Milky Way like cream into coffee, making it difficult to figure out what the galaxies were like before they merged.

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In 2018, astronomers realized that they could identify stars from the last major merger using detailed maps of several million stars from the European Space Agency’s Gaia spacecraft (SN: 5/9/18). Streams of stars orbit the galactic center at an angle to the main disk of stars. Those stars’ motions and chemistries suggest they once belonged to a separate galaxy that plunged into the Milky Way about 10 billion years ago (SN: 11/1/2018).

“Those stars are left there like fossil remnants of the galaxy,” Mackereth says.

Two groups discovered evidence of the ancient galaxy at around the same time. One called the galaxy Gaia-Enceladus; the other group called it the Sausage. The name that stuck was Gaia-Enceladus/Sausage.

Mackereth and his colleagues wondered if they could figure out how well developed the Milky Way was when Gaia-Enceladus/Sausage came crashing in. If the oldest stars in the Milky Way’s disk formed after this merger, then they probably formed as a result of this collision, suggesting that Gaia-Enceladus/Sausage met a proto–Milky Way that still had a lot of growing up to do. On the other hand, if the oldest stars are about the same age or older than the stars from the galactic interloper, then our galaxy was probably pretty well developed at the time of the run-in. 

Previous researchers had made estimates. But Mackereth and his colleagues used a precise tool called asteroseismology to figure out the ages of individual stars from both the Milky Way and from Gaia-Enceladus/Sausage (SN: 8/2/19). Just like seismologists on Earth use earthquakes to probe the interior of our planet, asteroseismologists use variations in brightness caused by starquakes and other oscillations to probe the innards of stars.

“Asteroseismology is the only way we have to access the internal part of the stars,” says physicist and study coauthor Josefina Montalbán of the University of Birmingham in England. From intel on the star’s interior structures, researchers can deduce the stars’ ages.

The team selected about 95 stars that had been observed by NASA’s exoplanet-hunting Kepler space telescope, which ended its mission in 2018 (SN: 10/30/18). Six of those stars were from Gaia-Enceladus/Sausage, and the rest were from the Milky Way’s thick disk. By measuring how the brightnesses of those stars fluttered over time, Mackereth and colleagues deduced ages with about 11 percent precision.

The Gaia-Enceladus/Sausage stars are slightly younger than the Milky Way stars, but all were pretty close to 10 billion years old, the team found. That suggests that a large chunk of the Milky Way’s disk was already in place when Gaia-Enceladus/Sausage came crashing through. It’s still possible that the incoming galaxy sparked the formation of some new stars, though, Mackereth says. To tell how much, they’ll need to get ages of a lot more stars.

Measuring ages for individual stars represents a step forward for galactic astronomy, says astrophysicist Tomás Ruiz-Lara of the University of Groningen, the Netherlands, who studies galactic evolution but was not involved in the new work.

“If you cannot tell the difference between a kid and a teenager and an adult, then we cannot say anything” about a population of people, Ruiz-Lara says. “But if I can distinguish between someone in his 40s or her 50s, you have a better graph of society. With the stars, it’s the same. If we are able to distinguish the age properly, then we can distinguish individual events in the history of the galaxy. In the end, that’s the goal.” More

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