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      Betelgeuse went dark, but didn’t go supernova. What happened?

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      Astrophysicist Miguel Montargès has a clear memory of the moment the stars became real places to him. He was 7 or 8 years old, looking up from the garden of his parents’ apartment in the south of France. A huge, red star winked in the night. The young space fan connected the star to a map he had studied in an astronomy magazine and realized he knew its name: Betelgeuse.
      Something shifted for him. That star was no longer an anonymous speck floating in a vast uncharted sea. It was a destination, with a name.
      “I thought, wow, for the first time … I can name a star,” he says. The realization was life-changing.
      Since then, Montargès, now at the Paris Observatory, has written his Ph.D. thesis and about a dozen papers about Betelgeuse. He considers the star an old friend, observing it many times a year, for work and for fun. He says good-bye every May when the star slips behind the sun from the perspective of Earth, and says hello again in August when the star comes back.

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      So in late 2019, when the bright star suddenly dimmed for no apparent reason, Montargès was a little alarmed. Some people speculated that Betelgeuse was about to explode in a brilliant supernova that would outshine the full moon. Astronomers know the star is old and its days are numbered, but Montargès wasn’t ready to see it go.
      “It’s my favorite star,” he says. “I don’t want it to die.”
      Other researchers, though, were eager to watch Betelgeuse explode in real time. Supernovas mark the violent deaths of stars that are at least eight times as massive as the sun (SN: 11/7/20, p. 20). But astronomers still don’t know what would signal that one is about to blow. The outbursts sprinkle interstellar space with elements that ultimately form the bulk of planets and people — carbon, oxygen, iron (SN: 2/18/17, p. 24). So the question of how supernovas occur is a question of our own origins.

      But the explosions are rare — astronomers estimate that one occurs in our galaxy just a few times a century. The last one spotted nearby, SN 1987A, was more than 33 years ago in a neighboring galaxy (SN: 2/18/17, p. 20). Betelgeuse is just one of the many aging, massive stars — called red supergiants — that could go supernova at any moment. But as one of the closest and brightest, Betelgeuse is the one that space enthusiasts know best.
      So when the star started acting strangely at the end of last year, Montargès and a small band of Betelgeuse diehards aimed every telescope they could at the dimming giant. Over the following months, the star returned to its usual brightness, and the excitement over an imminent supernova faded. But the flurry of data collected in the rush to figure out what was happening might help answer a different long-standing question: How do massive, old stars send their planet-building star stuff into the cosmos even before they explode?
      Orion’s shoulder
      If you’ve looked up at the stars during winter in the Northern Hemisphere, you’ve probably seen Betelgeuse, whether you realized it or not. The star is the second brightest in the constellation Orion, marking the hunter’s left shoulder from our perspective.
      And it’s huge. Estimates for Betelgeuse’s vital statistics vary, but if it sat at the center of our solar system, the star would fill much of the space between the sun and Jupiter. At about 15 to 20 times as massive as the sun, somewhere between 750 and 1,000 times its diameter and just about 550 light-years from Earth, Betelgeuse is typically between the sixth- and seventh-brightest star in the sky.
      Betelgeuse’s brightness varies, even under normal circumstances. Its outer layers are a bubbling cauldron of hot gas and plasma. As hot material rises to the surface, the star brightens; as material falls toward the core, the star dims. That convection cycle puts Betelgeuse on a semiregular dimmer switch that fluctuates roughly every 400 days or so. The star’s brightness also varies about every six years, though astronomers don’t know why.

      What they do know is that Betelgeuse is running out of time. It’s less than 10 million years old, a youngster compared with the roughly 4.6-billion–year-old sun. But because Betelgeuse is so massive and burns through its fuel so quickly, it’s already in the final life stage of a red supergiant. Someday in the not too distant future, the star won’t be able to support its own weight — it will collapse in on itself and rebound in a supernova.
      “We know one day it’s going to die and explode,” says Emily Levesque, an astrophysicist at the University of Washington in Seattle. But no one knows when. “In astronomical terms, ‘one day’ means sometime in the next 200,000 years.”
      In October 2019, Betelgeuse started dimming, which wasn’t too strange in and of itself. The change fit within the normal 400ish-day cycle, says astronomer Edward Guinan of Villanova University in Pennsylvania, who has been tracking Betelgeuse’s cycles of brightness since the 1980s.
      But by Christmas, Betelgeuse was the dimmest it had been in the 100-plus years that astronomers have measured it. And the dimming continued all the way through February.
      Guinan was one of the first to sound the alarm. On December 7, and again on December 23, he and colleagues posted a bulletin on The Astronomer’s Telegram website announcing the star’s “fainting” and encouraging fellow astronomers to take a look.
      There was no reason to think that the dimming was a harbinger of a supernova. “I never said it was going to be one,” Guinan says. But because these explosions are so rare, astronomers don’t know what the signals of an imminent supernova are. Dimming could be one of them.
      That report of odd behavior was all astronomers and amateur space enthusiasts needed to hear. Online, the story caught fire.
      “On Twitter, it was hysterical,” says Andrea Dupree, an astrophysicist at the Harvard & Smithsonian’s Center for Astrophysics in Cambridge, Mass. She recalls seeing one tweet suggesting that the explosion was going to happen that night, with the hashtag #HIDE. “Where am I going to hide? Under my desk?” (When Betelgeuse finally explodes, it probably won’t hurt life on Earth — it’s a safe distance away.)

      Most astronomers didn’t really believe that Betelgeuse’s end was nigh, even as they rushed to schedule telescope time. But some got caught up in the excitement.
      “I don’t expect it to blow,” Guinan recalls thinking. “But I don’t want to blink.” He signed up for phone alerts from telescopes that detect invisible particles called neutrinos and ripples in spacetime called gravitational waves. A detection of either one might be an early sign of a supernova. He found himself outside at 1 a.m. in January after a report of gravitational waves from the direction of Orion. “It was cloudy, but I thought I might see a brightening,” he says. “I’ve gotten crazy about it.”
      Others were believers too, until their data cast doubt on the notion.
      “I thought it might,” says astrophysicist Thavisha Dharmawardena of the Max Planck Institute for Astronomy in Heidelberg, Germany. “We knew there were other explanations, and we might have to look into it. But we know Betelgeuse is an old star, close to the end of its life. It was exciting.”
      Two camps
      Once the star started returning to its usual brightness in mid-February, talk of an imminent supernova faded. A paper published in the Oct. 10 Astrophysical Journal boosted confidence in Betelgeuse’s longevity, suggesting that the star is just at the beginning of its old age and has at least 100,000 years to go before it explodes. But what was it up to, if it was not on the verge of exploding?
      As results from telescopes all over the world and in space flooded in, most astronomers have fallen into two camps. One says Betelgeuse’s dimming was caused by a cloud of dust coughed out by the star itself, blocking its glow. The other camp isn’t sure what the explanation is, but says “no” to the dust speculation.
      One explanation for why Betelgeuse went dark in 2019 is that the star sneezed out a burst of gas and dust (illustrated, left), which condensed into a dark cloud. That cloud blocked the star’s face from the perspective of Earth (right).NASA, ESA, E. Wheatley/STScI
      If the dust theory proves true, it could have profound implications for the origins of complex chemistry, planets and even life in the universe. Red supergiants are surrounded by diffuse clouds of gas and dust that are full of elements that are forged only in stars — and those clouds form before the star explodes. Even before they die, supergiants seem to bequeath material to the next generation of stars.
      “The carbon, oxygen in our body, it’s coming from there — from the supernova and from the clouds around dying stars,” Montargès says. But it’s not clear how those elements escape the stars in the first place. “We have no idea,” he says.
      Montargès hoped studying Betelgeuse’s dimming would let scientists see that process in action.
      In December 2019, he and colleagues took an image of Betelgeuse in visible light with the SPHERE instrument on the Very Large Telescope in Chile. That image showed that, yes, Betelgeuse was much dimmer than it had been 11 months earlier — but only the star’s bottom half. Perhaps an asymmetrical dust cloud was to blame.
      Observations from February 15, 2020, seem to support that idea (SN: 4/11/20, p. 6). Levesque and Philip Massey of the Lowell Observatory in Flagstaff, Ariz., compared the February observations with similar ones from 2004. The star’s temperature hadn’t dropped as much as would be expected if the dimming was from something intrinsic to the star, like its convection cycles, the pair reported in the March 10 Astrophysical Journal Letters.
      That left dust as a reasonable explanation. “We know Betelgeuse sheds mass and produces dust around itself,” Levesque says. “Dust could have come toward us, cooled and temporarily blocked the light.”
      Dark cloud
      A strong vote for dust came from Dupree, who was watching Betelgeuse with the Hubble Space Telescope. Like Guinan, she has a decades-long relationship with Betelgeuse. In 1996, she and colleague Ronald Gilliland looked at Betelgeuse with Hubble to make the first real image of any star other than the sun. Most stars are too far and too faint to show up as anything but a point. Betelgeuse is one of the few stars whose surface can be seen as a two-dimensional disk — a real place.
      By the end of 2019, Dupree was observing Betelgeuse with Hubble several times a year. She had assembled an international team of researchers she calls the MOB, for Months of Betelgeuse, to observe the star frequently in a variety of wavelengths of light.

      The goal was the same as Montargès’: to answer fundamental questions about how Betelgeuse, and perhaps other red supergiants, lose material. The MOB had baseline observations from before the dimming and already had Hubble time scheduled to track the star’s brightness cycles.
      Those observations showed that in January and March 2019, Betelgeuse looked “perfectly normal,” Dupree says. But from September through November, just before the dimming event, the star gave out more ultraviolet light — up to four or five times its usual UV brightness — over its southern hemisphere.
      The temperature and electron density in that region went up, too. And material seemed to be moving outward, away from the star and toward Earth.
      Dupree and colleagues’ theory of what happened, reported in the Aug. 10 Astrophysical Journal, is that one of the giant bubbles of hot plasma always churning in the star’s outer layers rose to the edge of the star’s atmosphere and escaped, sending huge amounts of material flowing into interstellar space. That could be one way that red supergiants shed material before exploding.
      Once it had fled the star, that hot stuff cooled, condensed into dust and floated in front of Betelgeuse for several months. As the dust cleared, Betelgeuse appeared brighter again.
      “It seems to us that what we saw with the ultraviolet is kind of the smoking gun,” Dupree says. “This material moved on out, condensed and formed this dark, dark dust cloud.”
      Paul Hertz, director of NASA’s astrophysics division, shared the Hubble results in a NASA online town hall meeting on September 10 as if it were the final answer. “Mystery solved,” he said. “Not gonna supernova anytime soon.”
      Cycles and spots
      Maybe not — but that doesn’t mean dust explains the dimming.
      In the July 1 Astrophysical Journal Letters, Dharmawardena and colleagues published observations of Betelgeuse that ran counter to the dust explanation. Her team used the James Clerk Maxwell Telescope in Hawaii in January, February and March to look at Betelgeuse in submillimeter wavelengths of light. “If we think it’s a dust cloud, the submillimeter is the perfect wavelength to look at,” she says.
      Dust should have made Betelgeuse look brighter in those wavelengths, as floating grains absorbed and reemitted starlight. But it didn’t. If anything, the star dimmed slightly. “Our first thought was that we’d done something wrong — everyone in the community expected it to be dust,” she says. But “the fact that it didn’t increase or stay constant in the submillimeter was pretty much a dead giveaway that it’s not dust.”
      Infrared observations with the airborne SOFIA telescope should have found the glowing signature of dust too, if it existed. “It never showed up,” Guinan says. “I don’t think it’s dust.”
      Instead, Guinan thinks the dimming may have been part of Betelgeuse’s natural convection cycle. The star’s outer atmosphere constantly pulsates and “breathes” in and out as enormous bubbles of hot plasma rise to the surface and sink down again. “It’s driven by the internal core of the star,” he says. “You have hot blobs rising up, they cool, they get more dense, they fall back.”
      Multiple cycles syncing up could explain why the 2019 dimming was so extreme. Guinan and colleagues analyzed about 180 years of observations of Betelgeuse, dating back to astronomer John Herschel’s 1839 discovery that the star’s brightness varies. Guinan’s group found that, in addition to the roughly six-year and 400-day cycles, Betelgeuse might have a third, smaller cycle of about 187 days. It looks like all three cycles might have hit their brightness nadirs at the same time in late 2019, Guinan says.
      Or maybe the darkness in the southern hemisphere that Montargès’ team saw with SPHERE was an enormous star spot, Dharmawardena offers. In the sun’s case, those dark splotches, called sunspots, mark the sites of magnetic activity on the surface. Betelgeuse is one of a handful of stars on which star spots have been directly seen.
      But to cause Betelgeuse’s dimming, a star spot would have to be enormous. Typical star spots cover about 20 to 30 percent of a star’s surface, Dharmawardena says. This one would need to cover at least half, maybe up to 70 percent.
      “That’s rare,” Dharmawardena admits. “But so is this kind of dimming.”
      Pandemic disruptions
      Analyses are still coming in. But just as Betelgeuse was returning to its normal brightness, the COVID-19 pandemic hit.
      “We were hoping to have a lot more data,” Dharmawardena says.
      A few observations came in right under the wire. The SOFIA observations were made on one of the last flights before the pandemic grounded the plane that carries the telescope. And Montargès took another look with SPHERE just days before its observatory shut down in mid-March.
      In mid-July 2020, astronomers announced that STEREO, a sun-watching spacecraft, had seen signs that the star Betelgeuse was beginning to dim yet again. HI/Stereo/NASA
      In mid-July 2020, astronomers announced that STEREO, a sun-watching spacecraft, had seen signs that the star Betelgeuse was beginning to dim yet again. HI/Stereo/NASA
      But one of Montargès’ most hoped-for results may never come. Eager to solve the dust versus not-dust mystery, his plan was to combine two kinds of observations: making a 2-D picture of the whole star’s disk, like Dupree did with Hubble in the ’90s, but in longer wavelengths such as infrared or submillimeter, like Dharmawardena’s images from early 2020. That way, you could differentiate the dust from the star, he reasoned.
      Only one observatory can do both at once: the Atacama Large Millimeter/submillimeter Array, or ALMA, in Chile. Montargès had planned to ask to observe Betelgeuse with ALMA in June and July, when the winter skies in the Southern Hemisphere are most free of turbulence. But ALMA closed in March and was still closed in September.
      “When I realized ALMA will not get the time in June, I thought … we are never going to solve it,” he says. “We may never be completely certain, because of COVID.”
      Any other star
      Montargès and his colleagues have submitted their analysis of the SPHERE pictures from March for publication. Though he’s not yet willing to share the results, he thinks they could pull the two camps together.
      Ultimately, if Betelgeuse did cough out a cloud of dust last year, it could teach us about the origins of life in the universe, Montargès says. If the dust camp is even partially right, Betelgeuse’s dimming may have been the first time humans have watched the seeds of life being launched into the cosmos.
      In the meantime, he’s relieved to see his favorite star shining bright again. “I must admit that since [last] December, since this whole stuff started, every time I see it, I am like, phew, it’s still there,” he says.
      People keep asking him if he would like ​Betelgeuse to go supernova so he can study it. “I would like another star to go supernova,” he says. “Antares, I don’t care about it; it can explode anytime. But not Betelgeuse.” More

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      Arecibo Observatory, an ‘icon of Puerto Rican science,’ will be demolished

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      Arecibo’s days are done. After two support cables failed in recent months, the radio observatory’s 305-meter-wide dish is damaged beyond repair, the National Science Foundation announced on November 19. It will be decommissioned and dismantled.
      “It’s a death in the family,” says astronomer Martha Haynes of Cornell University, who has used the telescope in Puerto Rico to study hydrogen in the universe since she was fresh out of college in 1973. “For those of us who use Arecibo and had hoped to use it in the future, it’s a disaster.”
      The telescope, famous for appearances in movies like GoldenEye and Contact, consists of a wide dish to collect radio waves from space and focus them into detectors housed in a dome suspended above the dish. In August, one of the cables that holds up the dome slipped out of a socket and punched a hole in the dish.
      The NSF and the University of Central Florida, which manages the telescope, had plans to repair the cable, Haynes said. But then a second cable unexpectedly broke on November 6. If a third cable were to break, it could send the platform holding up the dome swinging, or the whole structure could collapse.

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      The NSF determined that there was no safe way to repair the telescope, the agency announced on November 19.
      “Until these assessments came in, our question was not if the observatory should be repaired but how,” said Ralph Gaume, director of NSF’s Division of Astronomical Sciences, in a statement. “But in the end, a preponderance of data showed that we simply could not do this safely. And that is a line we cannot cross.”
      The closure is the last in a series of near disasters for Arecibo. A different cable was damaged in an earthquake in 2014. Repairs on that cable were delayed by Hurricane Maria in 2017, which temporarily shut down the observatory as Puerto Rico weathered widespread power outages and humanitarian crises (SN: 9/29/17). And the observatory has been the victim of threatened or actual budget cuts for years (SN: 11/17/17).
      But its loss is a major blow for astronomy. Built in 1963, Arecibo was one of the best facilities in the world for observations ranging from mysterious blasts of radio waves from deep space (SN: 2/7/20) to tracking near-Earth asteroids that could potentially crash into our planet (SN: 1/20/20). It also was used in the early days of the search for extraterrestrial intelligence, or SETI (SN: 5/29/12).
      The Arecibo Observatory starred in major films, scanned the sky for hazardous asteroids and spotted mysterious radio bursts from space, among other things.University of Central Florida
      “Astronomers don’t have a lot of facilities,” Haynes says. Each new one is designed to have unique advantages over existing telescopes. “So when you lose one, it’s gone.”
      The observatory’s end is also a symbolic and practical loss for Puerto Rico, says radio astronomy researcher Kevin Ortiz Ceballos, a senior at the University of Puerto Rico at Arecibo who used the observatory to study the first known interstellar comet and stars that host exoplanets (SN: 10/14/19).
      “Arecibo is like an icon of Puerto Rican science,” he says. “This is absolutely devastating.”
      Ortiz Ceballos grew up watching Puerto Rican cartoons in which the characters went to Arecibo to use the telescope. His parents drove him an hour and a half to visit the telescope. He credits it with sparking his interest in astronomy, and he had hoped to come back to Puerto Rico to work at Arecibo after completing his Ph.D.
      “Puerto Rico has a huge mass emigration problem,” he says. “It’s a lot of people, and they’re all my age. It’s a huge brain drain. Being able to do what I love without having to leave, it was a huge dream for me.”
      And not just him, he notes: Dozens of students at the university and the observatory, plus more than 200 Puerto Rican students who went through the observatory’s high school program, have a similar story.
      “Losing this, especially after all that we’ve lost over the past half decade, makes me feel like we’re condemned to have our country just be ruins,” he says. “It becomes a signifier of a broader collapse. That’s just really tragic.” More

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      Farming on Mars will be a lot harder than ‘The Martian’ made it seem

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      In the film The Martian, astronaut Mark Watney (played by Matt Damon) survives being stranded on the Red Planet by farming potatoes in Martian dirt fertilized with feces.
      Future Mars astronauts could grow crops in dirt to avoid solely relying on resupply missions, and to grow a greater amount and variety of food than with hydroponics alone (SN: 11/4/11). But new lab experiments suggest that growing food on the Red Planet will be a lot more complicated than simply planting crops with poop (SN: 9/22/15).
      Researchers planted lettuce and the weed Arabidopsis thaliana in three kinds of fake Mars dirt. Two were made from materials mined in Hawaii or the Mojave Desert that look like dirt on Mars. To mimic the makeup of the Martian surface even more closely, the third was made from scratch using volcanic rock, clays, salts and other chemical ingredients that NASA’s Curiosity rover has seen on the Red Planet (SN: 1/31/19). While both lettuce and A. thaliana survived in the Marslike natural soils, neither could grow in the synthetic dirt, researchers report in the upcoming Jan. 15 Icarus.
      “It’s not surprising at all that as you get [dirt] that’s more and more accurate, closer to Mars, that it gets harder and harder for plants to grow in it,” says planetary scientist Kevin Cannon of the Colorado School of Mines in Golden, Colo., who helped make the synthetic Mars dirt but wasn’t involved in the new study.

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      Soil on Earth is full of microbes and other organic matter that helps plants grow, but Mars dirt is basically crushed rock. The new result “tells you that if you want to grow plants on Mars using soil, you’re going to have to put in a lot of work to transform that material into something that plants can grow in,” Cannon says.
      Biochemist Andrew Palmer and colleagues at the Florida Institute of Technology in Melbourne planted lettuce and A. thaliana seeds in imitation Mars dirt under controlled lighting and temperature indoors, just as astronauts would on Mars. The plants were cultivated at 22° Celsius and about 70 percent humidity.
      Seeds of both species germinated and grew in dirt mined from Hawaii or the Mojave Desert, as long as the plants were fertilized with a cocktail of nitrogen, potassium, calcium and other nutrients. No seeds of either species could germinate in the synthetic dirt, so “we would grow up plants under hydroponic-like conditions, and then we would transfer them” to the artificial dirt, Palmer says. But even when given fertilizer, those seedlings died within a week of transplanting.
      In lab experiments, lettuce was able to grow in Marslike soil from the Mojave Desert (pictured) as long as the soil was fertilized with nitrogen, potassium, calcium and other nutrients.Nathan Hadland
      Palmer’s team suspected that the problem with the synthetic Mars dirt was its high pH, which was about 9.5. The two natural soils had pH levels around 7. When the researchers treated the synthetic dirt with sulfuric acid to lower the pH to 7.2, transplanted seedlings survived an extra week but ultimately died.
      The team also ran up against another problem: The original synthetic dirt recipe did not include calcium perchlorate, a toxic salt that recent observations suggest make up to about 2 percent of the Martian surface. When Palmer’s team added it at concentrations similar to those seen on Mars, neither lettuce nor A. thaliana grew at all in the dirt.
      “The perchlorate is a major problem” for Martian farming, says Edward Guinan, an astrobiologist at Villanova University in Pennsylvania who was not involved in the work. But calcium perchlorate may not have to be a showstopper. “There are bacteria on Earth that enjoy perchlorates as a food,” Guinan says. As the microbes eat the salt, they give off oxygen. If these bacteria were taken from Earth to Mars to munch on perchlorates in Martian dirt, Guinan imagines that the organisms could not only get rid of a toxic component of the dirt, but perhaps also help produce breathable oxygen for astronauts.
      What’s more, the exact treatment required to make Martian dirt farmable may vary, depending on where astronauts make their homestead. “It probably depends where you land, what the geology and chemistry of the soil is going to be,” Guinan says.
      To explore how that variety might affect future agricultural practices, geochemist Laura Fackrell of the University of Georgia in Athens and colleagues mixed up five new types of faux Mars dirt. The recipes for these fake Martian materials, also reported in the Jan. 15 Icarus, are based on observations of Mars’ surface from the Curiosity, Spirit and Opportunity rovers, as well as NASA’s Mars Global Surveyor spacecraft and Mars Reconnaissance Orbiter.
      Each new artificial Mars dirt represents a mix of materials that could be found or made on the Red Planet. One is designed to represent the average composition across Mars, similar to the synthetic material created by Cannon’s team. The other four varieties have slightly different makeups, such as dirt that is particularly rich in carbonates or sulfates. This collection “expands the palette of what we have available” as test-beds for agricultural experiments, Fackrell says.
      She’s now using her stock to run preliminary plant growth experiments. So far, a legume called moth bean, which has similar nutritional content to a soybean but is more drought resistant, has grown the best. “But they’re not necessarily super healthy,” Fackrell says. Future experiments could explore what nutrient cocktails help plants survive in the various fake Martian terrains. But this much is clear, Fackrell says: “It’s not quite as easy as it looks in The Martian.” More

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      Planets with many neighbors may be the best places to look for life

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      If you’re looking for life beyond the solar system, there’s strength in numbers.
      A new study suggests that systems with multiple planets tend to have rounder orbits than those with just one, indicating a calmer family history. Only child systems and planets with more erratic paths hint at past planetary sibling clashes violent enough to knock orbits askew, or even lead to banishment. A long-lasting abundance of sibling planets might therefore have protected Earth from destructive chaos, and may be part of what made life on Earth possible, says astronomer Uffe Gråe Jørgensen of the Niels Bohr Institute in Copenhagen.
      “Is there something other than the Earth’s size and position around the star that is necessary in order for life to develop?” Jørgensen says. “Is it required that there are many planets?”
      Most of the 4,000-plus exoplanets discovered to date have elongated, or eccentric, orbits. That marks a striking difference from the neat, circular orbits of the planets in our solar system. Rather than being an oddity, those round orbits are actually perfectly normal — for a system with so many planets packed together, Jørgensen and his Niels Bohr colleague Nanna Bach-Møller report in a paper  published online October 30 in the Monthly Notices of the Royal Astronomical Society.

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      Bach-Møller and Jørgensen analyzed the eccentric paths of 1,171 exoplanets orbiting 895 different stars. The duo found a tight correlation between number of planets and orbit shape. The more planets a system has, the more circular their orbits, no matter where you look or what kind of star they orbit.
      Earlier, smaller studies also saw a correlation between number of planets and orbit shapes, says astrophysicist Diego Turrini of the Italian National Astrophysics Institute in Rome. Those earlier studies used only a few hundred planets.
      “This is a very important confirmation,” Turrini says. “It is providing us an idea of … how likely it is there will be no fight in the family, no destructive events, and your planetary system will remain as it formed … long enough to produce life.”
      Systems with as many planets as ours are exceedingly rare, though. Only one known system comes close: the TRAPPIST-1 system, with seven roughly Earth-sized worlds (SN: 2/22/17). Astronomers have found no solar systems so far, other than ours, with eight or more planets. Extrapolating out to the number of stars expected to have planets in the galaxy, Jørgensen estimates that about 1 percent of planetary systems have as many planets as we do.
      “It’s not unique, but the solar system belongs to a rare type of planetary system,” he says.
      That could help explain why life seems to be rare in the galaxy, Jørgensen suggests. Exoplanet studies indicate that there are billions of worlds the same size as Earth, whose orbits would make them good places for liquid water. But just being in the so-called “habitable zone” is not enough to make a planet habitable (SN: 10/4/19).
      “If there are so many planets where we could in principle live, why are we not teeming with UFOs all the time?” Jørgensen says. “Why do we not get into traffic jams with UFOs?”
      The answer might lie in the different histories of planetary systems with eccentric and circular orbits. Theories of solar system formation predict that most planets are born in a disk of gas and dust that encircles a young star. That means young planets should have circular orbits, and all orbit in the same plane as the disk.
      “You want the planets to not come too close to each other, otherwise their interactions might destabilize the system,” says Torrini. “The more planets you have the more delicate the equilibrium is.”
      Planets that end up on elliptical orbits may have gotten there via violent encounters with neighboring planets, whether direct collisions that break both planets apart or near-misses that toss the planets about (SN: 2/27/15). Some of those encounters may have ejected planets from their solar systems altogether, possibly explaining why planets with eccentric orbits have fewer siblings (SN: 3/20/15).
      Earth’s survival may therefore have depended on its neighbors playing nice for billions of years (SN: 5/25/05). It doesn’t need to have escaped violence altogether, either, Jørgensen says. One popular theory holds that Jupiter and Saturn shifted in their orbits billions of years ago, a reshuffling that knocked the orbits of distant comets askew and send them careening into the inner solar system. Several lines of evidence suggest comets could have brought water to the early Earth (SN: 5/6/15).
      “It’s not the Earth that is important,” Jørgensen says. “It’s the whole configuration of the planetary system that’s important for life to originate on an earthlike planet.” More

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      Chemical reactions high in Mars’ atmosphere rip apart water molecules

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      Mars’ water is being skimmed off the top. NASA’S MAVEN spacecraft found water lofted into Mars’ upper atmosphere, where its hydrogen and oxygen atoms are ripped apart, scientists report in the Nov. 13 Science.
      “This completely changes how we thought hydrogen, in particular, was being lost to space,” says planetary chemist Shane Stone of the University of Arizona in Tucson.
      Mars’ surface was shaped by flowing water, but today the planet is an arid desert (SN: 12/8/14). Previously, scientists thought that Mars’ water was lost in a “slow and steady trickle,” as sunlight split water in the lower atmosphere and hydrogen gradually diffused upward, Stone says.
      But MAVEN, which has been orbiting Mars since 2014, scooped up water molecules in Mars’ ionosphere, at altitudes of about 150 kilometers. That was surprising — previously the highest water had been seen was about 80 kilometers (SN: 1/22/18).
      That high-up water varied in concentration as the seasons changed on Mars, with the peak in the southern summer, when seasonal dust storms are most frequent (SN: 7/14/20). During a global dust storm in 2018, water levels jumped even higher, suggesting dust storms lift water in a “sudden splash,” Stone says.
      The top of Mars’ atmosphere is full of charged molecules that are primed for rapid chemical reactions, especially with water. So water up there is split apart quickly, on average lasting only four hours, leaving hydrogen atoms to float away (SN: 11/27/15). That process is 10 times faster than previously known ways for Mars to lose water, Stone and his colleagues calculated.
      This process could account for Mars losing the equivalent of a 44-centimeter-deep global ocean in the past billion years, plus another 17-centimeter-deep ocean during each global dust storm, the team found. That can’t explain all of Mars’ water loss, but it’s a start. More

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      Giant lasers help re-create supernovas’ explosive, mysterious physics

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      When one of Hye-Sook Park’s experiments goes well, everyone nearby knows. “We can hear Hye-Sook screaming,” she’s heard colleagues say.
      It’s no surprise that she can’t contain her excitement. She’s getting a closeup look at the physics of exploding stars, or supernovas, a phenomenon so immense that its power is difficult to put into words.
      Rather than studying these explosions from a distance through telescopes, Park, a physicist at Lawrence Livermore National Laboratory in California, creates something akin to these paroxysmal blasts using the world’s highest-energy lasers.
      About 10 years ago, Park and colleagues embarked on a quest to understand a fascinating and poorly understood feature of supernovas: Shock waves that form in the wake of the explosions can boost particles, such as protons and electrons, to extreme energies.

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      “Supernova shocks are considered to be some of the most powerful particle accelerators in the universe,” says plasma physicist Frederico Fiuza of SLAC National Accelerator Laboratory in Menlo Park, Calif., one of Park’s collaborators.
      Some of those particles eventually slam into Earth, after a fast-paced marathon across cosmic distances. Scientists have long puzzled over how such waves give energetic particles their massive speed boosts. Now, Park and colleagues have finally created a supernova-style shock wave in the lab and watched it send particles hurtling, revealing possible new hints about how that happens in the cosmos.
      Bringing supernova physics down to Earth could help resolve other mysteries of the universe, such as the origins of cosmic magnetic fields. And there’s a more existential reason physicists are fascinated by supernovas. These blasts provide some of the basic building blocks necessary for our existence. “The iron in our blood comes from supernovae,” says plasma physicist Carolyn Kuranz of the University of Michigan in Ann Arbor, who also studies supernovas in the laboratory. “We’re literally created from stars.”
      Lucky star
      As a graduate student in the 1980s, Park worked on an experiment 600 meters underground in a working salt mine beneath Lake Erie in Ohio. Called IMB for Irvine-Michigan-Brookhaven, the experiment wasn’t designed to study supernovas. But the researchers had a stroke of luck. A star exploded in a satellite galaxy of the Milky Way, and IMB captured particles catapulted from that eruption. Those messengers from the cosmic explosion, lightweight subatomic particles called neutrinos, revealed a wealth of new information about supernovas.
      But supernovas in our cosmic vicinity are rare. So decades later, Park isn’t waiting around for a second lucky event.
      Physicist Hye-Sook Park, shown as a graduate student in the 1980s (left) and in a recent photo (right), uses powerful lasers to study astrophysics.from left: John Van der Velde; Lanie L. Rivera/Lawrence Livermore National Laboratory
      Instead, her team and others are using extremely powerful lasers to re-create the physics seen in the aftermath of supernova blasts. The lasers vaporize a small target, which can be made of various materials, such as plastic. The blow produces an explosion of fast-moving plasma, a mixture of charged particles, that mimics the behavior of plasma erupting from supernovas.
      The stellar explosions are triggered when a massive star exhausts its fuel and its core collapses and rebounds. Outer layers of the star blast outward in an explosion that can unleash more energy than will be released by the sun over its entire 10-billion-year lifetime. The outflow has an unfathomable 100 quintillion yottajoules of kinetic energy (SN: 2/8/17, p. 24).
      Supernovas can also occur when a dead star called a white dwarf is reignited, for example after slurping up gas from a companion star, causing a burst of nuclear reactions that spiral out of control (SN: 4/30/16, p. 20).
      Supernova remnants like W49B (shown in X-ray, radio and infrared light) accelerate electrons and protons to high energies in shock waves.NASA, CXC, MIT L. Lopez et al (X-ray), Palomar (Infrared), VLA/NRAO/NSF (Radio)
      In both cases, things really get cooking when the explosion sends a blast of plasma careening out of the star and into its environs, the interstellar medium — essentially, another ocean of plasma particles. Over time, a turbulent, expanding structure called a supernova remnant forms, begetting a beautiful light show, tens of light-years across, that can persist in the sky for many thousands of years after the initial explosion. It’s that roiling remnant that Park and colleagues are exploring.
      Studying supernova physics in the lab isn’t quite the same thing as the real deal, for obvious reasons. “We cannot really create a supernova in the laboratory, otherwise we would be all exploded,” Park says.
      In lieu of self-annihilation, Park and others focus on versions of supernovas that are scaled down, both in size and in time. And rather than reproducing the entirety of a supernova all at once, physicists try in each experiment to isolate interesting components of the physics taking place. Out of the immense complexity of a supernova, “we are studying just a tiny bit of that, really,” Park says.
      For explosions in space, scientists are at the mercy of nature. But in the laboratory, “you can change parameters and see how shocks react,” says astrophysicist Anatoly Spitkovsky of Princeton University, who collaborates with Park.
      The laboratory explosions happen in an instant and are tiny, just centimeters across. For example, in Kuranz’s experiments, the equivalent of 15 minutes in the life of a real supernova can take just 10 billionths of a second. And a section of a stellar explosion larger than the diameter of Earth can be shrunk down to 100 micrometers. “The processes that occur in both of those are very similar,” Kuranz says. “It blows my mind.”
      [embedded content]
      Powerful, mysterious stellar explosions are difficult to understand from afar, so researchers have figured out how to re-create supernovas’ extreme physics in the lab and study how outbursts seed the cosmos with elements and energetic particles.
      Laser focus
      To replicate the physics of a supernova, laboratory explosions must create an extreme environment. For that, you need a really big laser, which can be found in only a few places in the world, such as NIF, the National Ignition Facility at Lawrence Livermore, and the OMEGA Laser Facility at the University of Rochester in New York.
      At both places, one laser is split into many beams. The biggest laser in the world, at NIF, has 192 beams. Each of those beams is amplified to increase its energy exponentially. Then, some or all of those beams are trained on a small, carefully designed target. NIF’s laser can deliver more than 500 trillion watts of power for a brief instant, momentarily outstripping the total power usage in the United States by a factor of a thousand.
      A single experiment at NIF or OMEGA, called a shot, is one blast from the laser. And each shot is a big production. Opportunities to use such advanced facilities are scarce, and researchers want to have all the details ironed out to be confident the experiment will be a success.
      When a shot is about to happen, there’s a space-launch vibe. Operators monitor the facility from a control room filled with screens. When the time of the laser blast nears, a voice begins counting down: “Ten, nine, eight …”
      “When they count down for your shot, your heart is pounding,” says plasma physicist Jena Meinecke of the University of Oxford, who has worked on experiments at NIF and other laser facilities.
      At the moment of the shot, “you kind of want the Earth to shake,” Kuranz says. But instead, you might just hear a snap — the sound of the discharge from capacitors that store up huge amounts of energy for each shot.
      Then comes a mad dash to review the results and determine if the experiment has been successful. “It’s a lot of adrenaline,” Kuranz says.
      At the National Ignition Facility’s target chamber (shown during maintenance), 192 laser beams converge. The blasts produce plumes of plasma that can mimic some aspects of supernova remnants.Lawrence Livermore National Laboratory
      Lasers aren’t the only way to investigate supernova physics in the lab. Some researchers use intense bursts of electricity, called pulsed power. Others use small amounts of explosives to set off blasts. The various techniques can be used to understand different stages in supernovas’ lives.
      A real shocker
      Park brims with cosmic levels of enthusiasm, ready to erupt in response to a new nugget of data or a new success in her experiments. Re-creating some of the physics of a supernova in the lab really is as remarkable as it sounds, she says. “Other­wise I wouldn’t be working on it.” Along with Spitkovsky and Fiuza, Park is among more than a dozen scientists involved in the Astrophysical Collisionless Shock Experiments with Lasers collaboration, or ACSEL, the quest Park embarked upon a decade ago. Their focus is shock waves.
      The result of a violent input of energy, shock waves are marked by an abrupt increase in temperature, density and pressure. On Earth, shock waves cause the sonic boom of a supersonic jet, the clap of thunder in a storm and the damaging pressure wave that can shatter windows in the aftermath of a massive explosion. These shock waves form as air molecules slam into each other, piling up molecules into a high-density, high-pressure and high-temperature wave.
      In cosmic environments, shock waves occur not in air, but in plasma, a mixture of protons, electrons and ions, electrically charged atoms. There, particles may be diffuse enough that they don’t directly collide as they do in air. In such a plasma, the pileup of particles happens indirectly, the result of electromagnetic forces pushing and pulling the particles. “If a particle changes trajectory, it’s because it feels a magnetic field or an electric field,” says Gianluca Gregori, a physicist at the University of Oxford who is part of ACSEL.
      But exactly how those fields form and grow, and how such a shock wave results, has been hard to decipher. Researchers have no way to see the process in real supernovas; the details are too small to observe with telescopes.
      These shock waves, which are known as collisionless shock waves, fascinate physicists. “Particles in these shocks can reach amazing energies,” Spitkovsky says. In supernova remnants, particles can gain up to 1,000 trillion electron volts, vastly outstripping the several trillion electron volts reached in the biggest human-made particle accelerator, the Large Hadron Collider near Geneva. But how particles might surf supernova shock waves to attain their astounding energies has remained mysterious.

      Magnetic field origins
      To understand how supernova shock waves boost particles, you have to understand how shock waves form in supernova remnants. To get there, you have to understand how strong magnetic fields arise. Without them, the shock wave can’t form.
      Electric and magnetic fields are closely intertwined. When electrically charged particles move, they form tiny electric currents, which generate small magnetic fields. And magnetic fields themselves send charged particles corkscrewing, curving their trajectories. Moving magnetic fields also create electric fields.
      The result is a complex feedback process of jostling particles and fields, eventually producing a shock wave. “This is why it’s so fascinating. It’s a self-modulating, self-controlling, self-reproducing structure,” Spitkovsky says. “It’s like it’s almost alive.”
      All this complexity can develop only after a magnetic field forms. But the haphazard motions of individual particles generate only small, transient magnetic fields. To create a significant field, some process within a supernova remnant must reinforce and amplify the magnetic fields. A theoretical process called the Weibel instability, first thought up in 1959, has long been expected to do just that.
      In a supernova, the plasma streaming outward in the explosion meets the plasma of the interstellar medium. According to the theory behind the Weibel instability, the two sets of plasma break into filaments as they stream by one another, like two hands with fingers interlaced. Those filaments act like current-­carrying wires. And where there’s current, there’s a magnetic field. The filaments’ magnetic fields strengthen the currents, further enhancing the magnetic fields. Scientists suspected that the electromagnetic fields could then become strong enough to reroute and slow down particles, causing them to pile up into a shock wave.
      In 2015 in Nature Physics, the ACSEL team reported a glimpse of the Weibel instability in an experiment at OMEGA. The researchers spotted magnetic fields, but didn’t directly detect the filaments of current. Finally, this year, in the May 29 Physical Review Letters, the team reported that a new experiment had produced the first direct measurements of the currents that form as a result of the Weibel instability, confirming scientists’ ideas about how strong magnetic fields could form in supernova remnants.
      For that new experiment, also at OMEGA, ACSEL researchers blasted seven lasers each at two targets facing each other. That resulted in two streams of plasma flowing toward each other at up to 1,500 kilometers per second — a speed fast enough to circle the Earth twice in less than a minute. When the two streams met, they separated into filaments of current, just as expected, producing magnetic fields of 30 tesla, about 20 times the strength of the magnetic fields in many MRI machines.
      “What we found was basically this textbook picture that has been out there for 60 years, and now we finally were able to see it experimentally,” Fiuza says.
      Surfing a shock wave
      Once the researchers had seen magnetic fields, the next step was to create a shock wave and to observe it accelerating particles. But, Park says, “no matter how much we tried on OMEGA, we couldn’t create the shock.”
      They needed the National Ignition Facility and its bigger laser.
      There, the researchers hit two disk-shaped targets with 84 laser beams each, or nearly half a million joules of energy, about the same as the kinetic energy of a car careening down a highway at 60 miles per hour.
      Two streams of plasma surged toward each other. The density and temperature of the plasma rose where the two collided, the researchers reported in the September Nature Physics. “No doubt about it,” Park says. The group had seen a shock wave, specifically the collisionless type found in supernovas. In fact there were two shock waves, each moving away from the other.

      Learning the results sparked a moment of joyous celebration, Park says: high fives to everyone.
      “This is some of the first experimental evidence of the formation of these collisionless shocks,” says plasma physicist Francisco Suzuki-Vidal of Imperial College London, who was not involved in the study. “This is something that has been really hard to reproduce in the laboratory.”
      The team also discovered that electrons had been accelerated by the shock waves, reaching energies more than 100 times as high as those of particles in the ambient plasma. For the first time, scientists had watched particles surfing shock waves like the ones found in supernova remnants.
      But the group still didn’t understand how that was happening.
      In a supernova remnant and in the experiment, a small number of particles are accelerated when they cross over the shock wave, going back and forth repeatedly to build up energy. But to cross the shock wave, the electrons need some energy to start with. It’s like a big-wave surfer attempting to catch a massive swell, Fiuza says. There’s no way to catch such a big wave by simply paddling. But with the energy provided by a Jet Ski towing surfers into place, they can take advantage of the wave’s energy and ride the swell.
      A computer simulation of a shock wave (structure shown in blue) illustrates how electrons gain energy (red tracks are higher energy, yellow and green are lower).F. Fiuza/SLAC National Accelerator Laboratory
      “What we are trying to understand is: What is our Jet Ski? What happens in this environment that allows these tiny electrons to become energetic enough that they can then ride this wave and be accelerated in the process?” Fiuza says.
      The researchers performed computer simulations that suggested the shock wave has a transition region in which magnetic fields become turbulent and messy. That hints that the turbulent field is the Jet Ski: Some of the particles scatter in it, giving them enough energy to cross the shock wave.
      Wake-up call
      Enormous laser facilities such as NIF and OMEGA are typically built to study nuclear fusion — the same source of energy that powers the sun. Using lasers to compress and heat a target can cause nuclei to fuse with one another, releasing energy in the process. The hope is that such research could lead to fusion power plants, which could provide energy without emitting greenhouse gases or dangerous nuclear waste (SN: 4/20/13, p. 26). But so far, scientists have yet to get more energy out of the fusion than they put in — a necessity for practical power generation.
      So these laser facilities dedicate many of their experiments to chasing fusion power. But sometimes, researchers like Park get the chance to study questions based not on solving the world’s energy crisis, but on curiosity — wondering what happens when a star explodes, for example. Still, in a roundabout way, understanding supernovas could help make fusion power a reality as well, as that celestial plasma exhibits some of the same behaviors as the plasma in fusion reactors.
      At NIF, Park has also worked on fusion experiments. She has studied a wide variety of topics since her grad school days, from working on the U.S. “Star Wars” missile defense program, to designing a camera for a satellite sent to the moon, to looking for the sources of high-energy cosmic light flares called gamma-ray bursts. Although she is passionate about each topic, “out of all those projects,” she says, “this particular collisionless shock project happens to be my love.”
      Early in her career, back on that experiment in the salt mine, Park got a first taste of the thrill of discovery. Even before IMB captured neutrinos from a supernova, a different unexpected neutrino popped up in the detector. The particle had passed through the entire Earth to reach the experiment from the bottom. Park found the neutrino while analyzing data at 4 a.m., and woke up all her collaborators to tell them about it. It was the first time anyone working on the experiment had seen a particle coming up from below. “I still clearly remember the time when I was seeing something nobody’s seen,” Park recalls.
      Now, she says, she still gets the same feeling. Screams of joy erupt when she sees something new that describes the physics of unimaginably vast explosions.

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      in Space & Astronomy

      Jupiter’s icy moon Europa may glow in the dark

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      Jupiter’s icy moon Europa could give the word “moonlight” a whole new meaning. New lab experiments suggest the nightside of this moon glows in the dark.
      Europa’s surface, thought to be mostly water ice laced with various salts, is continually bombarded with energetic electrons by Jupiter’s intense magnetic field (SN: 5/19/15). When researchers simulated that interaction in the lab by shooting electrons at salty ice samples, the ice glowed. The brightness of that glow depended on the kind of salt in the ice, researchers report online November 9 in Nature Astronomy.
      If the same interaction on Europa creates this never-before-seen kind of moonlight, a future mission there, such as NASA’s planned Europa Clipper spacecraft, may be able to use this ice glow map Europa’s surface composition. That, in turn, could give insight into the salinity of the ocean thought to lurk under Europa’s icy crust (SN: 6/14/19).
      “That has implications for the temperature of that liquid water — the freezing point; it has implications for the thickness of the ice shell; it has implications for the habitability of that liquid water,” says Jennifer Hanley, a planetary scientist at Lowell Observatory in Flagstaff, Ariz. not involved in the new work. Europa’s subsurface ocean is considered one of the most promising places to look for extraterrestrial life in the solar system (SN: 4/8/20).

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      The discovery of Europa’s potential ice glow “was serendipity,” says Murthy Gudipati, who studies the physics and chemistry of ices at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. Gudipati and colleagues originally set out to investigate how electron bombardment might change the chemistry of Europa’s surface ice. But in video footage of their initial experiments, the team noticed that ice samples pelted with electrons gave off an unexpected glow.
      Intrigued, the researchers turned their electron beam on samples of pure water ice, as well as water ice mixed with different salts. Each ice core was cooled to the surface temperature of Europa (about –173° Celsius) and showered with electrons that had the same energies as those that strike Europa. Over 20 seconds of irradiation, a spectrometer measured the wavelengths of light, or spectrum, given off by the ice.
      The ice samples all gave off a whitish glow, because they emitted light at many different wavelengths. But the brightness of each ice sample depended on its composition. Ice containing sodium chloride, also known as table salt, or sodium carbonate appeared dimmer than pure water ice. Ice mixed with magnesium sulfate, on the other hand, was brighter.

      “I was doing some back of the envelope calculations [of] what would be the brightness of Europa, if we were to be standing on it in the dark,” Gudipati says. “It’s approximately … as bright as me walking on the beach in full moonlight.”
      Based on the specs proposed for a camera to fly on the Europa Clipper mission, Gudipati and colleagues estimate that the spacecraft could see Europa’s ice glow during a flyby of the dark side of the moon. Dark patches of Europa could reveal sodium-rich regions, while brighter areas may be rich in magnesium.
      But seeing ice glow in the lab does not necessarily mean it happens the same way on Europa, Hanley cautions. Jupiter’s icy moon has been barraged by high-energy electrons for a lot longer than 20 seconds. “Is there ever a point where you might break down the salts, and this glow stops happening?” she wonders.
      Other planetary scientists, meanwhile, are not convinced that Europa’s surface is highly salted. These researchers, including Roger Clark of the Planetary Science Institute in Lakewood, Colo., think the apparent hints of salts on Europa are actually created by acids, such as sulfuric acid. Europa’s surface may be coated in both salts and acids, Clark says. “What [the researchers] need to do next is irradiate acids … to see if they can tell the difference between salt with water ice and acids with water ice.” More

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      in Space & Astronomy

      The Milky Way makes little galaxies bloom, then snuffs them out

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      If you’re a small galaxy and want to mint new stars, come to the Milky Way — but don’t get too close if you want a long-lasting star-making career. New observations with the Gaia space telescope show that our galaxy is both friend and foe to the lesser galaxies that revolve around it.
      Some 60 known galaxies orbit the Milky Way. About a dozen of these satellite galaxies are dim dwarf spheroidals, which each emit just 0.0005 to 0.1 percent as much light as the Milky Way (SN: 12/22/14). Their few stars are spread out from one another, giving the galaxies such a ghostly appearance that the first one found was initially suspected to be only a fingerprint on a photographic plate.
      But these ghostly galaxies once sparkled with young stars. A new study finds that most of these galaxies lit up when they first crossed into our galaxy’s gravitational domain as fresh stars arose. But then, in most cases, the little galaxies stopped making stars soon afterward, because the Milky Way stripped the dwarf galaxies of gas, the raw material for star formation.
      Astronomer Masashi Chiba of Tohoku University in Sendai, Japan, and his then-graduate student Takahiro Miyoshi studied seven of the dwarf spheroidal galaxies orbiting the Milky Way. The researchers used the European Space Agency’s Gaia spacecraft, which had measured the galaxies’ motions, to compute their orbits around the Milky Way’s center. The orbits are elliptical, so the galaxies approach and then recede from our galaxy’s center. The astronomers then compared those paths to the times when the galaxies formed their stars.

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      “We found that there’s a very nice coincidence between the timing of the first infall of the satellite [toward the Milky Way] and the peak in the star formation history,” Chiba says. In work posted online at arXiv.org on October 23, the astronomers attribute the burst of star formation in the small galaxies to the Milky Way. Encountering the giant galaxy squeezes the dwarf galaxies’ gas, causing that gas to collapse and spawn lots of new stars.
      As an example, the Draco dwarf galaxy first crossed into the Milky Way’s domain 11 billion years ago and formed numerous stars then — but never again. More recently, the Leo I dwarf galaxy entered our galaxy’s realm just 2 billion years ago, a time that coincided with its last burst of star birth. But today Leo I creates no new stars and, like Draco, has no gas to do so.
      Dwarf galaxies that kept their distance also kept their gas longer, the researchers found. The galaxies that came closest to the Milky Way’s center, such as Draco and Leo I, ceased all star formation soon after crossing the Milky Way’s frontier. However, the galaxies that entered our galaxy’s domain but remained farther out, such as Fornax and Carina, fared better.
      “Those two galaxies kept their interstellar gas inside them, so that the star formation still continued,” Chiba says. Both galaxies managed to eke out new stars for many billions of years after crossing into the Milky Way’s realm. Today, however, neither galaxy has any gas left.
      “I think it all makes sense,” says Vasily Belokurov, an astronomer at the University of Cambridge, who notes how essential the Gaia spacecraft was to the discovery. “It’s a beautiful demonstration of what we’ve never been able to do before Gaia, and suddenly we can do these magical things.” More