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      Measuring a black hole’s mass isn’t easy. A new technique could change that

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      An actively feeding black hole surrounds itself with a disk of hot gas and dust that flickers like a campfire. Astronomers have now found that monitoring changes in those flickers can reveal something that is notoriously hard to measure: the behemoth’s heft.

      “It’s a new way to weigh black holes,” says astronomer Colin Burke of the University of Illinois at Urbana-Champaign. What’s more, the method could be used on any astrophysical object with an accretion disk, and may even help find elusive midsize black holes, researchers report in the Aug. 13 Science.

      It’s not easy to measure a black hole’s mass. For one thing, the dark behemoths are notoriously difficult to see. But sometimes black holes reveal themselves when they eat. As gas and dust falls into a black hole, the material organizes into a disk that is heated to white-hot temperatures and can, in some cases, outshine all the stars in the galaxy combined.

      Measuring the black hole’s diameter can reveal its mass using Einstein’s general theory of relativity. But only the globe-spanning Event Horizon Telescope has made this sort of measurement, and for only one black hole so far (SN: 4/22/19). Other black holes have been weighed via observations of their influence on the material around them, but that takes a lot of data and doesn’t work for every supermassive black hole.

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      So, looking for another way, Burke and colleagues turned to accretion disks. Astronomers aren’t sure how black holes’ disks flicker, but it seems like small changes in light combine to brighten or dim the entire disk over a given span of time. Previous research had hinted that the time it takes a disk to fade, brighten and fade again is related to the mass of its central black hole. But those claims were controversial, and didn’t cover the full range of black hole masses, Burke says.

      So he and colleagues assembled observations of 67 actively feeding black holes with known masses. The behemoths spanned sizes from 10,000 to 10 billion solar masses. For the smallest of these black holes, the flickers changed on timescales of hours to weeks. Supermassive black holes with masses between 100 million and 10 billion solar masses flickered more slowly, every few hundred days.

      “That gives us a hint that, okay, if this relation holds for small supermassive black holes and big ones, maybe it’s sort of a universal feature,” Burke says.

      Out of curiosity, the team also looked at white dwarfs, the compact corpses of stars like the sun, which are some of the smallest objects to sport consistent accretion disks. Those white dwarfs followed the same relationship between flicker speed and mass.

      The analyzed black holes didn’t cover the entire possible range of masses. Known black holes that are from about 100 to 100,000 times the mass of the sun are rare. There are several potential candidates, but only one has been confirmed (SN: 9/2/20). In the future, the relationship between disk flickers and black hole mass could tell astronomers exactly what kind of disk flickers to look for to help bring these midsize beasts out of hiding, if they’re there to be found, Burke says.

      Astrophysicist Vivienne Baldassare of Washington State University in Pullman studies black holes in dwarf galaxies, which may preserve some of the properties of ancient black holes that formed in the early universe. One of the biggest challenges in her work is measuring black hole masses. The study’s “super exciting results … will have a large impact for my research, and I expect many others as well,” she says.

      The method offers a simpler way to weigh black holes than any previous technique, Burke says — but not necessarily a faster one. More massive black holes, for example, would need hundreds of days, or possibly years, of observations to reveal their masses.

      Upcoming observatories are already planning to take that kind of data. The Vera C. Rubin Observatory is expected to start observing the entire sky every night beginning in 2022 or 2023 (SN: 1/10/20). Once the telescope has been running long enough, the observations needed to weigh black holes “will fall out for free” from the Rubin Observatory data, Burke says. “We’re already building it. We may as well do this.” More

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      A bounty of potential gravitational wave events hints at exciting possibilities

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      A new crew of potential ripples in spacetime has just debuted — emphasis on the word “potential.”

      By loosening the criteria for what qualifies as evidence for gravitational waves, physicists identified 1,201 possible tremors. Most are probably fakes, spurious jitters in the data that can mimic the cosmic vibrations, the team reports August 2 at arXiv.org. But by allowing in more false alarms, the new tally may also include some weak but genuine signals that would otherwise be missed, potentially revealing exciting new information about the sources of gravitational waves.

      Scientists can now look for signs that may corroborate some of the uncertain detections, such as flashes of light in the sky that flared from the cosmic smashups that set off the ripples. Gravitational waves are typically spawned by collisions of dense, massive objects, such as black holes or neutron stars, the remnants of dead stars (SN: 1/21/21).

      To come up with the new census, physicists reanalyzed six months of data from the Advanced Laser Interferometer Gravitational-Wave Observatory, or LIGO, and Virgo gravitational wave observatories. Scientists had already identified 39 of the events as likely gravitational waves in earlier analyses.

      Eight events that hadn’t been previously identified stand a solid chance of being legitimate — with greater than a 50 percent probability of coming from an actual collision.

      The physicists analyzed the data from those eight events to see how they might have occurred. In one, two black holes may have slammed together, melding into a whopper black hole with about 180 times the mass of the sun, which would make it the biggest black hole merger seen yet (SN: 9/2/20). Another event could be a rare sighting of a black hole swallowing a neutron star (SN: 6/29/21). More

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      Black holes born with magnetic fields quickly shed them

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      Like a shaggy dog in springtime, some black holes have to shed. New computer simulations reveal how black holes might discard their magnetic fields.

      Unlike dogs with their varied fur coats, isolated black holes are mostly identical. They are characterized by only their mass, spin and electric charge. According to a rule known as the no-hair theorem, any other distinguishing characteristics, or “hair,” are quickly cast off. That includes magnetic fields.

      The rule applies to black holes in a vacuum, where magnetic fields can simply slip away. But, says astrophysicist Ashley Bransgrove of Columbia University, “what we were thinking about is what happens in a more realistic scenario.” A magnetized black hole would typically be surrounded by electrically charged matter called plasma, and scientists didn’t know how — or even if — such black holes would undergo hair loss.

      Black holes can be born with magnetic fields or gain them later, for example by swallowing a neutron star, a highly magnetic dead star (SN: 6/29/21). When Bransgrove and colleagues simulated the plasma surrounding a magnetized black hole, they found that a process called magnetic reconnection allows the magnetic field to escape the black hole. The magnetic field lines that map out the field’s direction break apart and reconnect. Loops of magnetic field form around blobs of plasma, some of which blast outward, while others fall into the black hole. That process eliminates the black hole’s magnetic field, the researchers report in the July 30 Physical Review Letters.

      Magnetic reconnection in balding black holes could spew X-rays that astronomers could detect. So scientists may one day glimpse a black hole losing its hair. More

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      A super-short gamma-ray burst defies astronomers’ expectations

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      A surprisingly short gamma-ray burst has astronomers rethinking what triggers these celestial cataclysms.

      The Fermi Gamma-ray Space Telescope detected a single-second-long blast of gamma rays, dubbed GRB 200826A, in August 2020. Such fleeting gamma-ray bursts, or GRBs, are usually thought to originate from neutron star smashups (SN: 10/16/17). But a closer look at the burst revealed that it came from the implosion of a massive star’s core.

      In this scenario, the core of a star collapses into a compact object, such as a black hole, that powers high-speed particle jets. Those jets punch through the rest of the star and radiate powerful gamma rays before the outer layers of the star explode in a supernova (SN: 5/8/19). That process is typically thought to produce longer GRBs, lasting more than two seconds.

      Discovering such a brief gamma-ray burst from a stellar explosion suggests that some bursts previously classified as stellar mergers may actually be from the deaths of massive stars, researchers report online July 26 in two studies in Nature Astronomy.

      The first clues about GRB 200826A’s origin came from the burst itself. The wavelengths of light and amount of energy released in the burst were more similar to collapse-related GRBs than collision-produced bursts, Bing Zhang, an astrophysicist at the University of Nevada, Las Vegas, and colleagues report. Plus, the burst hailed from the middle of a star-forming galaxy, where astronomers expect to find collapsing massive stars, but not neutron star mergers — which are generally found on the fringes of tranquil galaxies.

      Another group, led by astronomer Tomás Ahumada-Mena of the University of Maryland in College Park, searched for the supernova that’s expected to follow a GRB produced by a collapsing star. Using the Gemini North Telescope in Hawaii to observe GRB 200826A’s host galaxy, the team was able to pick out the telltale infrared light of the supernova. The burst may have been so brief because its jets had just barely punched through the surface of the star before they petered out and the star blew up, Ahumada-Mena says. More

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      The tiny dot in this image may be the first look at exomoons in the making

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      New telescope images may provide the first view of moons forming outside the solar system.

      The Atacama Large Millimeter/submillimeter Array in Chile glimpsed a dusty disk of potentially moon-forming material around a baby exoplanet about 370 light-years from Earth. The Jupiter-like world is surrounded by enough material to make up to 2.5 Earth moons, researchers report online July 22 in the Astrophysical Journal Letters. Observations of this system could offer new insight into how planets and moons are born around young stars.

      ALMA observed two planets, dubbed PDS 70b and 70c, circling the star PDS 70 in July 2019. Unlike most other known exoplanets, these two Jupiter-like worlds are still forming — gobbling up material from the disk of gas and dust swirling around their star (SN: 7/2/18). During this formation process, planets are expected to wrap themselves in their own debris disks, which control how planets pack on material and form moons.

      Around PDS 70c, ALMA spotted a disk of dust about as wide as Earth’s orbit around the sun. With previously reported exomoon sightings still controversial, the new observations offer some of the best evidence yet that planets orbiting other stars have moons (SN: 4/30/19).

      Unlike PDS 70c, 70b does not appear to have a moon-forming disk. That may be because it has a narrower orbit than PDS 70c, which is nearly as far from its star as Pluto is from the sun. That puts PDS 70c closer to an outer disk of debris surrounding the star.

      Just inside a ring of debris surrounding a young star is the planet PDS 70c, which is surrounded by its own disk of possible moon-forming material (bright dot at center).ALMA/ESO, NAOJ and NRAO, M. Benisty et al

      “C is getting all the material from the outer disk, and b is getting starved,” says study coauthor Jaehan Bae, an astrophysicist at the Carnegie Institution for Science in Washington, D.C.

      “In the past, b must have gotten some material in its [disk], and it could have already formed moons,” Bae says. But to make the new images, ALMA observed wavelengths of light emitted by sand-sized dust grains, not large objects, so those moons would not be visible. More

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      How do scientists calculate the age of a star?

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      We know quite a lot about stars. After centuries of pointing telescopes at the night sky, astronomers and amateurs alike can figure out key attributes of any star, like its mass or its composition.

      To calculate a star’s mass, just look it its orbital period and do a bit of algebra. To determine what it’s made of, look to the spectrum of light the star emits. But the one variable scientists haven’t quite cracked yet is time.

      “The sun is the only star we know the age of,” says astronomer David Soderblom of the Space Telescope Science Institute in Baltimore. “Everything else is bootstrapped up from there.”

      Even well-studied stars surprise scientists every now and then. In 2019 when the red supergiant star Betelgeuse dimmed, astronomers weren’t sure if it was just going through a phase or if a supernova explosion was imminent. (Turns out it was just a phase.) The sun also shook things up when scientists noticed that it wasn’t behaving like other middle-aged stars. It’s not as magnetically active compared with other stars of the same age and mass. That suggests that astronomers might not fully understand the timeline of middle age.

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      Calculations based on physics and indirect measurements of a star’s age can give astronomers ballpark estimates. And some methods work better for different types of stars. Here are three ways astronomers calculate the age of a star.

      Hertzsprung-Russell diagrams

      Scientists do have a pretty good handle on how stars are born, how they live and how they die. For instance, stars burn through their hydrogen fuel, puff up and eventually expel their gases into space, whether with a bang or a whimper. But when exactly each stage of a star’s life cycle happens is where things get complicated. Depending on their mass, certain stars hit those points after a different number of years. More massive stars die young, while less massive stars can burn for billions of years.

      At the turn of the 20th century, two astronomers — Ejnar Hertzsprung and Henry Norris Russell — independently came up with the idea to plot stars’ temperature against their brightness. The patterns on these Hertzsprung-Russell, or H-R, diagrams corresponded to where different stars were in that life cycle. Today, scientists use these patterns to determine the age of star clusters, whose stars are thought to have all formed at the same time.

      The caveat is that, unless you do a lot of math and modeling, this method can be used only for stars in clusters, or by comparing a single star’s color and brightness with theoretical H-R diagrams. “It’s not very precise,” says astronomer Travis Metcalfe of the Space Science Institute in Boulder, Colo. “Nevertheless, it’s the best thing we’ve got.”

      [embedded content]
      Measuring a star’s age isn’t as easy as you’d think. Here’s how scientists get their ballpark estimates.

      Rotation rate

      By the 1970s, astrophysicists had noticed a trend: Stars in younger clusters spin faster than stars in older clusters. In 1972, astronomer Andrew Skumanich used a star’s rotation rate and surface activity to propose a simple equation to estimate a star’s age: Rotation rate = (Age) -½.

      This was the go-to method for individual stars for decades, but new data have poked holes in its utility. It turns out that some stars don’t slow down when they hit a certain age. Instead they keep the same rotation speed for the rest of their lives.

      “Rotation is the best thing to use for stars younger than the sun,” Metcalfe says. For stars older than the sun, other methods are better.

      Stellar seismology

      The new data that confirmed rotation rate wasn’t the best way to estimate an individual star’s age came from an unlikely source: the exoplanet-hunting Kepler space telescope. Not just a boon for exoplanet research, Kepler pushed stellar seismology to the forefront by simply staring at the same stars for a really long time.

      Watching a star flicker can give clues to its age. Scientists look at changes in a star’s brightness as an indicator of what’s happening beneath the surface and, through modeling, roughly calculate the star’s age. To do this, one needs a really big dataset on the star’s brightness — which the Kepler telescope could provide.

      “Everybody thinks it was all about finding planets, which was true,” Soderblom says. “But I like to say that the Kepler mission was a stealth stellar physics mission.”

      This approach helped reveal the sun’s magnetic midlife crisis and recently provided some clues about the evolution of the Milky Way. Around 10 billion years ago, our galaxy collided with a dwarf galaxy. Scientists have found that stars left behind by that dwarf galaxy are younger or about the same age as stars original to the Milky Way. Thus, the Milky Way may have evolved more quickly than previously thought.

      As space telescopes like NASA’s TESS and the European Space Agency’s CHEOPS survey new patches of sky, astrophysicists will be able to learn more about the stellar life cycle and come up with new estimates for more stars.

      Aside from curiosity about the stars in our own backyard, star ages have implications beyond our solar system, from planet formation to galaxy evolution — and even the search for extraterrestrial life.

      “One of these days — it’ll probably be a while — somebody’s going to claim they see signs of life on a planet around another star. The first question people will ask is, ‘How old is that star?’” Soderblom says. “That’s going to be a tough question to answer.” More

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      Marsquakes reveal the Red Planet boasts a liquid core half its diameter

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      Mars has had its first CT scan, thanks to analyses of seismic waves picked up by NASA’s InSight lander. Diagnosis: The Red Planet’s core is at least partially liquid, as some previous studies had suggested, and is somewhat larger than expected.

      InSight reached Mars in late 2018 and soon afterward detected the first known marsquake (SN: 11/26/18; SN: 4/23/19). Since then, the lander’s instruments have picked up more than a thousand temblors, most of them minor rumbles. Many of those quakes originated at a seismically active region more than 1,000 kilometers away from the lander. A small fraction of the quakes had magnitudes ranging from 3.0 to 4.0, and the resulting vibrations have enabled scientists to probe Mars and reveal new clues about its inner structure.

      Simon Stähler, a seismologist at ETH Zurich, and colleagues analyzed seismic waves from 11 marsquakes, looking for two types of waves: pressure and shear. Unlike pressure waves, shear waves can’t pass through a liquid, and they move more slowly, traveling side to side through solid materials, rather than in a push-and-pull motion in the same direction a wave is traveling like pressure waves do.

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      Of those 11 events, six sets of vibrations included shear waves strong enough to stand out from background noise. The strength of those shear waves suggests that they reflected off of the outer surface of a liquid core, rather than entering a solid core and being partially absorbed, Stähler says. And the difference in arrival times at InSight for the pressure waves and shear waves for each quake suggest that Mars’ core is about 3,660 kilometers in diameter, he and colleagues report in the July 23 Science.

      That’s a little more than half of the diameter of the entire planet, larger than most previous estimates. The Red Planet’s core is so big, in fact, that it blocks InSight from receiving certain types of seismic waves from a large part of the planet. That, in turn, suggests that Mars may be more seismically active than the lander’s sensors can detect. Indeed, one of the regions in the lander’s seismic blind spot is the Tharsis region, home to some of Mars’ largest volcanoes. Volcanic activity there, as well as the motion of molten rock within the crust in that region, could trigger quakes or seismic waves.

      Seismic waves (red lines in this illustration) traveling through Mars from a quake’s source (example, red dot) to the InSight lander (white dot) reveal the Red Planet’s internal structure, including a massive core (yellow-white) more than half the diameter of the planet.Chris Bickel/Science

      While the newly analyzed data confirm the planet’s outer core is liquid, it’s not clear yet whether Mars has a solid inner core like Earth, says study coauthor Amir Khan, a geophysicist also at ETH Zurich. “The signal should be there in the seismic data,” he says. “We just need to locate it.”

      In a separate analysis also published in Science, Khan and colleagues suggest that InSight’s seismic blind spot may also stem, in part, from the way that seismic waves slow down and bend as they travel deep within the planet. Changes in seismic wave speed and direction can result from gradual variations in rock temperature or density, for example.

      Mars’ seismic waves also hint at the thickness of the planet’s crust. As they bounce back and forth within the planet, the waves bounce off interfaces between different layers and types of rocks, says Brigitte Knapmeyer-Endrun, a seismologist at the University of Cologne in Bergisch Gladbach, Germany. In a separate study in Science, she and her team analyzed seismic signals that reflected off several such interfaces near Mars’ surface, making it difficult to determine the depth at which the planet’s crust ends and the underlying mantle begins, she says. The researchers concluded, however, that the average thickness of the crust likely lies between 24 and 72 kilometers. For comparison, Earth’s oceanic crust is about 6 to 7 kilometers thick, while the planet’s continental crust averages from 35 to 40 kilometers thick.

      Together, these seismic analyses are the first to investigate the innards of a rocky planet other than Earth, Stähler says. As such, they provide “ground truth” for measurements made by spacecraft orbiting Mars, and could help scientists better interpret data gathered from orbit around other planets, such as Mercury and Venus.

      The findings could also provide insights that would help planetary scientists better understand how Mars formed and evolved over the life of the solar system, and how the Red Planet ended up so unalike Earth, says Sanne Cottaar, a geophysicist at the University of Cambridge. Cottaar wrote a commentary, also published in Science, on the new research. “Mars was put together with similar building blocks” as Earth, she says, “but had a different result.” More

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      NASA’s Perseverance Mars rover has begun its first science campaign

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      NASA’s Perseverance rover on Mars has seen its future, and it’s full of rocks. Lots and lots of rocks. After spending the summer trundling through Jezero Crater and checking out the sights, it’s now time for Percy to get to work, teasing out the geologic history of its new home and seeking out signs of ancient microbial life.

      “We’ve actually been on a road trip,” project manager Jennifer Trosper, who is based at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., said at a July 21 news conference. “And during it, we will take our very first sample from the surface of Mars.”

      Percy is about 1 kilometer south of where it landed on February 18 (SN: 2/17/21). After driving itself around a region of sand dunes, accompanied by its tagalong helicopter Ingenuity (SN: 4/30/21), the robotic explorer has pulled up to its first sampling spot: a garden of flat, pale stones dubbed paver stones. “This is the area where we are really going to be digging in, both figuratively and literally, to understand the rocks that we have been on for the last several months,” said Kenneth Farley, Perseverance project scientist at Caltech.

      The team has been trying to figure out whether these rocks are volcanic or sedimentary. “We still don’t have the answer,” Farley said. Images taken a few centimeters above the surface show what the team is up against: The rocks are littered with dust and pebbles, probably blown in from elsewhere, and the smoother surfaces have a mysterious purplish coating. “All of these factors conspire to prevent us from peering into the rock and actually seeing what it is made out of,” he said.

      In the coming weeks, Percy will bore a smooth cavity in one of those rocks and get below the surface crud. Instruments on its robotic arm will then move in close to produce detailed chemical and mineralogical maps that will reveal the rocks’ true nature. Then, sometime in mid-August, the team will extract its first sample. That sample will go into a tube that will eventually get dropped off — along with samples from other locales — for some future mission to pick up and bring to Earth (SN: 7/28/20).

      Cameras scouting farther afield have turned up future sampling sites. A small far-off hill shows hints of finely layered rock that may be mud deposits. “This is exactly the kind of rock that we are most interested in investigating for looking for potential biosignatures,” Farley said.

      And the way that rocks are strewn about an ancient river delta in the distance suggests that the lake that once filled Jezero Crater went through multiple episodes of filling in and drying up. If true, Farley said, then the crater may have preserved “multiple time periods when we might be able to look for evidence of ancient life that might have existed on the planet.” More