Astronomy

After two decades of debate, scientists are getting closer to figuring out exactly what the sun — and thus the whole universe — is made of.

The sun is mostly composed of hydrogen and helium. There are also heavier elements such as oxygen and carbon, but just how much is controversial. New observations of ghostly subatomic particles known as neutrinos suggest that the sun has an ample supply of “metals,” the term astronomers use for all elements heavier than hydrogen and helium, researchers report May 31 at arXiv.org.

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The results “are fully compatible with [a] high metallicity” for the sun, says Livia Ludhova, a physicist at Research Center Jülich in Germany.

Elements heavier than hydrogen and helium are crucial for creating rock-iron planets like Earth and sustaining life-forms like humans. By far the most abundant of these elements in the universe is oxygen, followed by carbon, neon and nitrogen.

But astronomers don’t know exactly how much of these elements exist relative to hydrogen, the most common element in the cosmos. That’s because astronomers typically use the sun as a reference point to gauge elemental abundances in other stars and galaxies, and two techniques imply very different chemical compositions for our star.

One technique exploits vibrations inside the sun to deduce its internal structure and favors a high metal content. The second technique determines the sun’s composition from how atoms on its surface absorb certain wavelengths of light. Two decades ago, a use of this second technique suggested that oxygen, carbon, neon and nitrogen levels in the sun were 26 to 42 percent lower than an earlier determination found, creating the current conflict.

Another technique has now emerged that could decide the long-standing debate: using solar neutrinos.

These particles arise from nuclear reactions in the sun’s core that turn hydrogen into helium. About 1 percent of the sun’s energy comes from reactions involving carbon, nitrogen and oxygen, which convert hydrogen into helium but do not get used up in the process. So the more carbon, nitrogen and oxygen the sun actually has, the more neutrinos this CNO cycle should emit.

In 2020, scientists announced that Borexino, an underground detector in Italy, had spotted these CNO neutrinos (SN: 6/24/20). Now Ludhova and her colleagues have recorded enough neutrinos to calculate that carbon and nitrogen atoms together are about 0.06 percent as abundant as hydrogen atoms in the sun — the first use of neutrinos to determine the sun’s makeup.

And though that number sounds small, it’s even higher than the one favored by astronomers who support a high-metal sun. And it’s 70 percent greater than the number a low-metal sun should have.

“This is a great result,” says Marc Pinsonneault, an astronomer at Ohio State University in Columbus who has long advocated for a high-metal sun. “They’ve been able to demonstrate robustly that the current low-metallicity solution is inconsistent with the data.”

Still, because of uncertainties in both the observed and predicted neutrino numbers, Borexino can’t fully rule out a low-metal sun, Ludhova says.

The new work is “a significant improvement,” says Gaël Buldgen, an astrophysicist at Geneva University in Switzerland who favors a low-metal sun. But the predicted numbers of CNO neutrinos come from models of the sun that he criticizes as too simplified. Those models neglect the sun’s spin, which could induce mixing of chemical elements over its life and change the amount of carbon, nitrogen and oxygen near the sun’s center, thereby changing the predicted number of CNO neutrinos, Buldgen says.

Additional neutrino observations are needed for a final verdict, Ludhova says. Borexino shut down in 2021, but future experiments could fill the void.

The stakes are high. “We’re arguing about what the universe is made of,” Pinsonneault says, because “the sun is the benchmark for all of our studies.”

So if the sun has much more carbon, nitrogen and oxygen than currently thought, so does the whole universe. “That changes our understanding about how the chemical elements are made. It changes our understanding of how stars evolve and how they live and die,” Pinsonneault says. And, he adds, it’s a reminder that even the best-studied star — our sun — still has secrets. More

A solitary celestial object — more massive than the sun, yet far smaller — is wandering the galaxy a few thousand light-years from Earth. It might be the first isolated stellar-mass black hole to be detected in the Milky Way. Or it might be one of the heaviest neutron stars known.

The interstellar wanderer first revealed itself in 2011, when its gravity briefly magnified the light from a more distant star. But at the time, its true nature eluded researchers. Now, two teams of astronomers have analyzed Hubble Space Telescope images to unmask the traveler’s identity — and have come to somewhat different conclusions.

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The mysterious rogue is a black hole roughly seven times as massive as the sun, one team reports in a study in press in the Astrophysical Journal. Or it’s a bit lighter — a mere two to four times the weight of our nearest star — and therefore either an unusually lightweight black hole or a curiously hefty neutron star, another group reports in a study in press in the Astrophysical Journal Letters.  

Neutron stars and stellar-mass black holes form when massive stars — at least several times the heft of the sun — collapse under their own gravity at the end of their lives. Astronomers believe that about a billion neutron stars and roughly 100 million stellar-mass black holes lurk in our galaxy (SN: 8/18/17). But these objects aren’t easy to spot. Neutron stars are so tiny — about the size of a city — that they don’t produce much light. And black holes emit no light at all.

To detect these kinds of objects, scientists typically observe how they affect their surroundings. “The only way that we can find them is if they influence something else,” says Kailash Sahu, an astronomer at the Space Telescope Science Institute in Baltimore.

To date, scientists have detected nearly two dozen stellar-mass black holes. (These relatively lightweight black holes are puny compared to the supermassive behemoths that sit at the center of most galaxies, including our own (SN: 1/18/21).) To do so, researchers have watched how these objects interact with their nearby celestial neighbors. When a black hole is locked in a gravitational dance with another star, it rips away matter from its partner. As that material falls onto the black hole, it emits X-rays, which telescopes orbiting the Earth can detect.

But finding black holes in binary systems doesn’t paint a whole picture of the black hole kingdom. Because these objects are continually accreting matter, it’s challenging to determine the mass at which they formed. And since birthweight is a key characteristic of a black hole, that’s a significant drawback to looking at binary systems, Sahu says. “If we want to understand the properties of black holes, it’s best to find isolated ones.”

For more than a decade, researchers have been scanning the heavens for solitary black holes. The searches have hinged on Einstein’s theory of general relativity, which states that any massive object, even an unseen one, bends space in its vicinity (SN: 2/3/21). That bending causes light from background stars to be magnified and distorted, a phenomenon known as gravitational microlensing. By measuring changes in the brightness and apparent position of stars, scientists can calculate the mass of the intervening object that’s acting like a lens — a technique that’s rounded up a few extrasolar planets as well (SN: 7/24/17).

In 2011, researchers announced that they had spotted a star that suddenly had gotten more than 200 times brighter. But those initial observations, made using telescopes in Chile and New Zealand, were unable to reveal whether the star’s apparent position was also changing. And that information is key to pinning down the mass of the intervening object. If it’s a heavyweight, its gravity would distort space so much that the star would appear to move. But even a “big” shift in the star’s position would have been extremely small and hard to detect. And unfortunately fine details in astronomical images captured by ground-based telescopes tend to be blurred out because of our planet’s turbulent atmosphere (SN: 7/29/20).

To circumvent this Earthly limitation, two independent teams of astronomers turned to the Hubble Space Telescope. This observatory can capture extremely detailed images since it orbits above most of Earth’s atmosphere.  

Both groups found that the star’s location shifted over the course of several years. One of the teams, led by Sahu, concluded that the star’s apparent dance was caused by an object roughly seven times as hefty as the sun. A star of that mass would have been blazingly bright in the Hubble images, but the researchers saw nothing. Something that heavy and dark must be a black hole, the team reports.

But another group of researchers, led by astronomer Casey Lam at the University of California, Berkeley, found different results. Lam and her colleagues calculated that the mass of the lensing object was lower, only about two to four times the mass of the sun. It could therefore be either a neutron star or a black hole, the group concluded.

Whatever it is, it’s an intriguing object, says astronomer Jessica Lu, a member of Lam’s team also at UC Berkeley. That’s because it’s a bit of an oddball in terms of mass. It’s either one of the most massive neutron stars discovered to date, or it’s one of the least massive black holes known, Lu says. “It falls within this strange region we call the mass gap.”

Despite the disagreement, these are thrilling results, says Will M. Farr, an astrophysicist at Stony Brook University in New York not involved in either study. “To be working at the instrumental limit at the real forefront of what’s measurable is very exciting.” More

1.6 billion stars. 11.4 million galaxies. 158,000 asteroids.

One spacecraft.

The European Space Agency’s Gaia space observatory, which launched in 2013, has long surpassed its goal of charting more than a billion stars in the Milky Way (SN: 10/15/16). On June 13, the mission extended that map into new dimensions, releasing more detailed measurements of hundreds of millions of stars, plus — for the first time — asteroids, galaxies and the dusty medium between stars.

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“Suddenly you have a flood of data,” says Laurent Eyer, an astrophysicist at the University of Geneva who has worked on Gaia for years. For some topics in astronomy, the new results effectively replace all the observations that were taken before, Eyer says. “The data is better. It’s amazing.”

Data in the new survey, which were collected from 2014 to 2017, are already leading to some discoveries — including the presence of surprisingly massive  “starquakes” on the surfaces of thousands of stars (SN: 8/2/19). But more than anything, the release is a new tool for astronomers, one that will aid their efforts to understand how stars, planets and entire galaxies form and evolve.

Here are a few of the long-standing puzzles the data could help solve. 

Asteroid mishmash

The asteroid belt between Mars and Jupiter is a mess of history. After the Earth and other planets formed, the rocky building blocks that were left over smashed into each other, leaving behind jumbled fragments. But if scientists know enough about individual asteroids, they can reconstruct when and where they came from (SN: 4/13/19). And that can provide a peek into the solar system’s earliest days.

Using new Gaia data, astronomers plotted the June 13, 2022, positions of 156,000 asteroids. The trails show their orbits for the last 10 days, and the colors mark different groups of asteroids based on their location (blue, inner solar system; green, the main asteroid belt between Mars and Jupiter; orange, the Trojan asteroids near Jupiter).DPAC/Gaia/ESA, CC BY-SA 3.0 IGO

Gaia’s massive new dataset may help solve this puzzle, says Federica Spoto, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. It includes data on the chemical makeup of over 60,000 asteroids — six times more than researchers had such details on before using other tools. That information can be essential for tracing asteroids back to their shattering origins.

“You can go back in time and try to understand all the formation and evolution of the solar system,” says Spoto, a Gaia collaborator. “That’s something huge that before Gaia we couldn’t even think about.” 

Asteroids aren’t just pieces of the past, though; they’re also dangerous. The new data could reveal asteroids that are next to impossible to spot from Earth because they orbit too close to the sun, says Thomas Burbine, a planetary scientist at Mount Holyoke College in South Hadley, Mass., who is not involved with the mission (SN: 2/15/20). Since these asteroids would have originally come from farther out (say, the asteroid belt), they can tell us about the rocks going past Earth that can potentially hit us. “We’ll know our neighborhood better,” Burbine says.

Dating a star

It is notoriously difficult to measure the age of stars (SN: 7/23/21). “It’s not uncommon to have uncertainty of more than a billion years,” says Alessandro Savino, an astrophysicist at the University of California, Berkeley who is not involved with Gaia. Unlike brightness or location, age is not directly visible. Astronomers have to rely on theories of how stars evolve to predict ages from what they can measure.

If past versions of the Gaia survey were like a photograph of stars, the new release is like shifting the photograph from black and white to color. It provides a deeper look at hundreds of millions of stars by measuring their temperature, gravity and chemistry. “You imagine the star as this point in space, but then they have so many properties,” Spoto says. “That’s what Gaia is giving you.”

Although these kinds of measurements are far from new, they have never been collected in the Milky Way on such a scale before. Those data could provide more insight into how stars evolve. “We can improve the resolution of our clocks,” Savino says. 

Milky Way snacks

Though it may seem unchanging, the Milky Way is actually gorging on a steady diet of smaller galaxies —it’s even in the process of eating one right now. But for decades, predictions of when and how these cosmic mergers happen have been at odds with evidence from our galaxy, says Bertrand Goldman, an astrophysicist at the International Space University in Strasbourg, France, who is not involved in the Gaia data release.  “That has been controversial for a long time,” Goldman says, “but I think that Gaia will certainly shed light.”

The key is to be able to pick apart different structures in the Milky Way and see how old they are (SN: 1/10/20). Gaia’s latest release helps in two ways: By mapping the chemistry of stars and by measuring their motion. Previous versions of the survey described how millions of stars were moving, but mostly in two dimensions. The new catalog quadruples the number of stars with full 3-D trajectories from 7 million to 33 million. 

This has implications beyond our neighborhood. Most of the mass in the universe is contained in galaxies like the Milky Way, so knowing how our own galaxy works goes a long way to understanding space on the largest scales. And the more scientists understand the parts of galaxies they can see, the more they can learn about dark matter, the mysterious substance that exerts gravity but doesn’t interact with light (SN: 6/25/21).

Even as astronomers mine this latest dataset, they are already looking ahead to future treasure hunts. The next round is years off, but it is expected to enable the discovery of thousands of exoplanets, produce rare measurements of black holes and help astronomers clock how fast the universe is expanding. In part, this is because Gaia is designed to track the motion of objects in space, and that gets easier as more time passes. So Gaia’s observations can only get more powerful. “Like good wine, they age very, very well,” Savino says. More

Astronomers have added a new species to the neutron star zoo, showcasing the wide diversity among the compact magnetic remains of dead, once-massive stars.

The newfound highly magnetic pulsar has a surprisingly long rotation period, which is challenging the theoretical understanding of these objects, researchers report May 30 in Nature Astronomy. Dubbed PSR J0901-4046, this pulsar sweeps its lighthouse-like radio beam past Earth about every 76 seconds — three times slower than the previous record holder.

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While it’s an oddball, some of this newfound pulsar’s characteristics are common among its relatives. That means this object may help astronomers better connect the evolutionary phases among mysterious species in the neutron star menagerie.

Astronomers know of many types of neutron stars. Each one is the compact object left over after a massive star’s explosive death, but their characteristics can vary. A pulsar is a neutron star that astronomers detect at a regular interval thanks to its cosmic alignment: The star’s strong magnetic field produces beams of radio waves emanating from near the star’s poles, and every time one of those beams sweeps across Earth, astronomers can see a radio pulse.

The newfound, slowpoke pulsar sits in our galaxy, roughly 1,300 light-years away. Astrophysicist Manisha Caleb of the University of Sydney in Australia and her colleagues found it in data from the MeerKAT radio telescope outside Carnarvon, South Africa.

Further observations with MeerKAT revealed not only the pulsar’s slow, steady radio beat — a measure of how fast it spins — but also another important detail: The rate at which the spin slows as the pulsar ages. And those two bits of info revealed something odd about this pulsar. According to theory, it should not be emitting radio waves. And yet, it is.

As neutron stars age, they lose energy and spin more slowly. According to calculations, “at some point, they’ve exhausted all their energy, and they cease to emit any sort of emission,” Caleb says. They’ve become dead to detectors.

A pulsar’s rotation period and the slowdown of its spin relates to the strength of its magnetic field, which accelerates subatomic particles streaming from the star and, in turn, generates radio waves. Any neutron stars spinning as slowly as PSR J0901-4046 are in this stellar “graveyard” and shouldn’t produce radio signals.

But “we just keep finding weirder and weirder pulsars that chip away at that understanding,” says astrophysicist Maura McLaughlin of West Virginia University in Morgantown, who wasn’t involved with this work.

The newfound pulsar could be its own unique species of neutron star. But in some ways, it also looks a bit familiar, Caleb says. She and her colleagues calculated the pulsar’s magnetic field from the rate its spin is slowing, and it’s incredibly strong, similar to magnetars (SN: 9/17/02). This hints that PSR J0901-4046 could be what’s known as a “quiescent magnetar,” which is a pulsar with very strong magnetic fields that occasionally emits brilliantly energetic bursts of X-rays or other radiation. “We’re going to need either X-ray emission or [ultraviolet] observations to confirm whether it is indeed a magnetar or a pulsar,” she says.

The discovery team still has additional observations to analyze. “We do have a truckload more data on it,” says astrophysicist Ian Heywood of the University of Oxford. The researchers are looking at how the object’s brightness is changing over time and whether its spin abruptly changes, or “glitches.”

The astronomers also are altering their automated computer programs, which scan the radio data and flag intriguing signals, to look for these longer-duration spin periods — or even weirder and more mysterious neutron star phenomena. “The sweet thing about astronomy, for me, is what’s out there waiting for us to find,” Heywood says. More

The windy and chaotic remains surrounding recently exploded stars may be launching the fastest particles in the universe.

Highly magnetic neutron stars known as pulsars whip up a fast and strong magnetic wind. When charged particles, specifically electrons, get caught in those turbulent conditions, they can be boosted to extreme energies, astrophysicists report April 28 in the Astrophysical Journal Letters. What’s more, those zippy electrons can then go on to boost some ambient light to equally extreme energies, possibly creating the very high-energy gamma-ray photons that led astronomers to detect these particle launchers in the first place.

“This is the first step in exploring the connection between the pulsars and the ultrahigh-energy emissions,” says astrophysicist Ke Fang of the University of Wisconsin, Madison, who was not involved in this new work.

Last year, researchers with the Large High Altitude Air Shower Observatory, or LHAASO, in China announced the discovery of the highest-energy gamma rays ever detected, up to 1.4 quadrillion electron volts (SN: 2/2/21). That’s roughly 100 times as energetic as the highest energies achievable with the world’s premier particle accelerator, the Large Hadron Collider near Geneva. Identifying what’s causing these and other extremely high-energy gamma rays could point, literally, to the locations of cosmic rays — the zippy protons, heavier atomic nuclei and electrons that bombard Earth from locales beyond our solar system.

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Some gamma rays are thought to originate in the same environs as cosmic rays. One way they’re produced is that cosmic rays, shortly after being launched, can slam into relatively low-energy ambient photons, boosting them to high-energy gamma rays. But the electrically charged cosmic rays are buffeted by galactic magnetic fields, which means they don’t travel in a straight line, thus complicating efforts to trace the zippy particles back to their source. Gamma rays, however, are impervious to magnetic fields, so astrophysicists can trace their unwavering paths back to their origins — and figure out where cosmic rays are created.

To that end, the LHAASO team traced the hundreds of gamma-ray photons that it detected to 12 spots on the sky. While the team identified one spot as the Crab Nebula, the remnant of a supernova about 6,500 light-years from Earth, the researchers suggested that the rest could be associated with other sites of stellar explosions or even young massive star clusters (SN: 6/24/19).

In the new study, astrophysicist Emma de Oña Wilhelmi and colleagues zeroed in one of those possible points of origin: pulsar wind nebulas, the clouds of turbulence and charged particles surrounding a pulsar. The researchers weren’t convinced such locales could create such high-energy particles and light, so they set out to show through calculations that pulsar wind nebulas weren’t the sources of extreme gamma rays. “But to our surprise, we saw at the very extreme conditions, you can explain all the sources [that LHAASO saw],” says de Oña Wilhelmi, of the German Electron Synchrotron in Hamburg.

The young pulsars at the heart of these nebulas — no more than 200,000 years old — can provide all that oomph because of their ultrastrong magnetic fields, which create a turbulent magnetic bubble called a magnetosphere.

Any charged particles moving in an intense magnetic field get accelerated, says de Oña Wilhelmi. That’s how the Large Hadron Collider boosts particles to extreme energies (SN: 4/22/22). A pulsar-powered accelerator, though, can boost particles to even higher energies, the team calculates. That’s because the electrons escape the pulsar’s magnetosphere and meet up with the material and magnetic fields from the stellar explosion that created the pulsar. These magnetic fields can further accelerate the electrons to even higher energies, the team finds, and if those electrons slam into ambient photons, they can boost those particles of light to ultrahigh energies, turning them into gamma rays.

“Pulsars are definitely very powerful accelerators,” Fang says, with “several places where particle acceleration can happen.”

And that could lead to a bit of confusion. Gamma-ray telescopes have pretty fuzzy vision. For example, LHASSO can make out details only as small as about half the size of the full moon. So the gamma-ray sources that the telescope detected look like blobs or bubbles, says de Oña Wilhelmi. There could be multiple energetic sources within those blobs, unresolved to current observatories.

“With better angular resolution and better sensitivity, we should be able to identify what [and] where the accelerator is,” she says. A few future observatories — such as the Cherenkov Telescope Array and the Southern Wide-field Gamma-ray Observatory — could help, but they’re several years out. More

There’s a new addition to astronomers’ portrait gallery of black holes. 

Astronomers announced May 12 that they have finally assembled an image of the supermassive black hole at the center of our galaxy. 

“This image shows a bright ring surrounding the darkness, the telltale sign of the shadow of the black hole,” astrophysicist Feryal Özel of the University of Arizona in Tucson said at a news conference announcing the result.

The black hole, known as Sagittarius A*, appears as a faint silhouette amidst the glowing material that surrounds it. The image reveals the turbulent, twisting region immediately surrounding the black hole in new detail. The findings also were published May 12 in 6 studies in the Astrophysical Journal Letters.

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A planet-spanning network of radio telescopes, known as the Event Horizon Telescope, worked together to create this much-anticipated look at the Milky Way’s giant. Three years ago, the same team released the first-ever image of a supermassive black hole (SN: 4/10/19). That object sits at the center of the galaxy M87, about 55 million light-years from Earth. 

But Sagittarius A*, or Sgr A* for short, is “humanity’s black hole,” says astrophysicist Sera Markoff of the University of Amsterdam, and a member of the EHT collaboration. 

At 27,000 light-years away, the behemoth is the closest giant black hole to Earth. That proximity means that Sgr A* is the most-studied supermassive black hole in the universe. Yet Sgr A* and others like it remain some of the most mysterious objects ever found. 

That’s because, like all black holes, Sgr A* is an object so dense that its gravitational pull won’t let light escape. Black holes are “natural keepers of their own secrets,” says physicist Lena Murchikova of the Institute for Advanced Study in Princeton, N.J., who is not part of the EHT team. Their gravity traps light that falls within a border called the event horizon. EHT’s images of Sgr A* and the M87 black hole skirt up to that inescapable edge.

[embedded content]
This sonification is a translation into sound of the Event Horizon Telescope’s image of the supermassive black hole Sagittarius A*. The sonification sweeps clockwise around the black hole image. Material closer to the black hole orbits faster than material farther away. Here, the faster-moving material is heard at higher frequencies. Very low tones represent material outside the black hole’s main ring. Louder volume indicates brighter spots in the image.

Sgr A* feeds on hot material pushed off of massive stars at the galactic center. That gas, drawn toward Sgr A* by its gravitational pull, flows into a surrounding disk of glowing material, called an accretion disk. The disk, the stars and an outer bubble of X-ray light “are like an ecosystem,” says astrophysicist Daryl Haggard of McGill University in Montreal and a member of the EHT collaboration. “They’re completely tied together.”

That accretion disk is where the action is — as the gas moves within immensely strong magnetic fields — so astronomers want to know more about how the disk works.

Like the majority of supermassive black holes,  Sgr A* is quiet and faint (SN: 6/5/19 ). The black hole eats only a few morsels fed to it by its accretion disk. Still, “it’s always been a little bit of a puzzle why it’s so, so faint,” says astrophysicist Meg Urry of Yale University, who is not part of the EHT collaboration. M87’s black hole, in comparison, is a monster gorging on nearby material and shooting out enormous, powerful jets (SN: 11/10/21). But that doesn’t mean Sgr A* isn’t producing light. Astrophysicists have seen its region feebly glowing in radio waves, jittering in infrared and burping in X-rays.

In fact, the accretion disk around Sgr A* seems to constantly flicker and simmer. This variability, the constant flickering, is like a froth on top of ocean waves, Markoff says. “​​And so we’re seeing this froth that is coming up from all this activity, and we’re trying to understand the waves underneath the froth.” 

The big question, she adds, has been if astronomers would be able to see something changing in those waves with EHT. In the new work, they’ve seen hints of those changes below the froth, but the full analysis is still ongoing.

By combining about 3.5 petabytes of data, or the equivalent of about 100 million TikTok videos, captured in April 2017, researchers could begin to piece together the picture. To tease out an image from the initial massive jumble of data, the EHT team needed years of work, complicated computer simulations and observations in various types of light from other telescopes. 

[embedded content]
Scientists created a vast library of computer simulations of Sagittarius A* (one shown) to explore the turbulent flow of hot gas that rings the black hole. That rapid flow causes the ring’s appearance to vary in brightness on timescales of minutes. Scientists compared these simulations with the newly released observations of the black hole to better understand its true properties.

Those “multiwavelength” data from the other telescopes were crucial to assembling the image. “By looking at these things simultaneously and all together, we’re able to come up with a complete picture,” says theorist Gibwa Musoke of the University of Amsterdam. 

Sgr A*’s variability, the constant simmering, complicated the analysis because the black hole changes on timescales of just a few minutes, changing as the researchers were imaging it. “It was like trying to take a clear picture of a running child at night,” astronomer José L. Gómez of Instituto de Astrofísica de Andalucía in Granada, Spain, said at a news conference announcing the result. M87 was easier to analyze because it changed over the course of weeks.

Ultimately, a better understanding of what is happening in the disk so close to Sgr A* could help scientists learn how many other similar supermassive black holes work. 

The new EHT observations also confirm the mass of Sgr A* at 4 million times that of the sun. If the black hole replaced our sun, the shadow EHT imaged would sit within Mercury’s orbit. 

The researchers also used the image of Sgr A* to put general relativity to the test (SN: 2/3/21). Einstein’s steadfast theory of gravity passed: The size of the shadow matched the predictions of general relativity. By testing the theory in extreme conditions — like those around black holes — scientists hope to pinpoint any hidden weaknesses.

Scientists have previously tested general relativity by following the motions of stars that orbit very close to Sgr A* — work that also helped confirm that the object truly is a black hole (SN: 7/26/18). For that discovery, researchers Andrea Ghez and Reinhard Genzel won a share of the Nobel Prize in physics in 2020 (SN: 10/6/20).

The two types of tests of general relativity are complementary,  says astrophysicist Tuan Do of UCLA. “With these big physics tests, you don’t want to use just one method.” If one test appears to contradict general relativity, scientists can check for a corresponding discrepancy in the other.

The Event Horizon Telescope, however, tests general relativity much nearer to the black hole’s edge, which could highlight subtle effects of physics beyond general relativity. “The closer you get, the better you are in terms of being able to look for these effects,” says physicist Clifford Will of the University of Florida in Gainesville.

However, some researchers have criticized a similar test of general relativity made using the EHT image of M87’s black hole (SN: 10/1/20). That’s because the test relies on relatively shaky assumptions about the physics of how material swirls around a black hole, says physicist Sam Gralla of the University of Arizona in Tucson. Testing general relativity in this way “would only make sense if general relativity were the weakest link,” but scientists’ confidence in general relativity is stronger than the assumptions that went into the test, he says.

The observations of Sgr A* provide more evidence that the object is in fact a black hole, says physicist Nicolas Yunes of the University of Illinois Urbana-Champaign. “It’s really exciting to have the first image of a black hole that is in our own Milky Way. It’s fantastic.” It sparks the imagination, like early pictures astronauts took of Earth from the moon, he says.

This won’t be the last eye-catching image of Sgr A* from EHT. Additional observations, made in 2018, 2021 and 2022, are still waiting to be analyzed. 

“This is our closest supermassive black hole,” Haggard says. “It is like our closest friend and neighbor. And we’ve been studying it for years as a community. [This image is a] really profound addition to this exciting black hole we’ve all kind of fallen in love with in our careers.” More

In the solar system’s early years, the still-forming giant planets sidestepped, did a do-si-do and then swung one of their partners away from the sun’s gravitational grasp. Things settled, and our planetary system was in its final configuration.

What triggered that planetary shuffle has been unknown. Now, computer simulations suggest that the hot radiation of the young sun evaporating its planet-forming disk of gas and dust led to the scrambling of the giant planets’ orbits, researchers report in the April 28 Nature.

As a result, the four largest planets may have been in their final configuration within 10 million years of the solar system’s birth about 4.6 billion years ago. That’s much quicker than the 500 million years that previous work had suggested.

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The planetary-shuffling mechanism that the team uncovered in the computer simulations is very innovative, says Nelson Ndugu, an astrophysicist who studies forming planetary systems at North-West University in Potchefstroom, South Africa, and Muni University in Arua, Uganda. “It has huge potential.”

Heaps of evidence, including observations of extrasolar planetary systems forming (SN: 7/2/18), had already indicated that something in our solar system’s early history jumbled the giant planets’ orbits, which scientists call the giant-planet instability (SN: 5/25/05).

“The evidence for the giant-planet instability is really robust,” says Seth Jacobson, a planetary scientist at Michigan State University in East Lansing. “It explains many features of the outer solar system,” he says, like the large number of rocky objects beyond Neptune that make up the Kuiper Belt (SN: 12/31/09).

To figure out what triggered that instability, Jacobson and colleagues ran computer simulations of the thousands of ways that the early solar system could have developed. All started with a young star and a planet-forming disk of gas and dust surrounding the star. The team then altered the disk parameters, such as its mass, density and how fast it evolved.

The simulations also included the still-forming giant planets — five of them, in fact. Astronomers think a third ice giant, in addition to Uranus and Neptune, was originally a solar system member (SN: 4/20/12). Jupiter and Saturn round out the final tally of these massive planets.

When the sun officially became a star, that is, the moment it began burning hydrogen at its core — roughly 4.6 billion years ago — its ultraviolet emission would have hit the disk’s gas, ionizing it and heating it to tens of thousands of degrees. “This is a very well-documented process,” Jacobson says. As the gas heats, it expands and flows away from the star, beginning with the inner portion of the disk.

“The disk disperses its gas from inside out,” says Beibei Liu, an astrophysicist at Zhejiang University in Hangzhou, China. He and Jacobson collaborated with astronomer Sean Raymond of Laboratoire d’Astrophysique de Bordeaux in France in the new research.

In the team’s simulations, as the inner part of the disk dissolves, that area loses mass, so the embedded, still-forming planets feel less gravity from that region, Jacobson says. But the planets still feel the same amount of pull from the disk’s outer region. This gravitational torquing, as the team calls it, can trigger a rebound effect: “Originally, the planets migrate in, and they reach the [inner] edge of this disk, and they reverse their migration,” Liu says.

Because of Jupiter’s large mass, it’s mostly unaffected. Saturn, though, moves outward and into the region, which, in the simulations, holds the three ice giant planets. That area becomes crowded, Liu says, and close planetary interactions follow. One ice giant gets kicked out of the solar system entirely, Uranus and Neptune shift a bit farther from the sun, and “they gradually form the orbits close to our solar system’s configuration,” Liu says.

In their computer simulations, the researchers found that as the sun’s radiation evaporates the disk, a planetary reshuffle nearly always ensues. “We can’t avoid this instability,” Jacobson says.

Now that the researchers have an idea of what may have caused this solar system shuffle, the next step is to simulate how the evaporation of the disk could affect other objects.

“We’ve focused really heavily on the giant planets, because their orbits were the original motivation,” Jacobson says. “But now, we have to do the follow-up work to show how this trigger mechanism relates to the small bodies.” More

If you’re an aspiring life-form, you might want to steer clear of planets around orange dwarf stars.

Some astronomers have called these orange suns “Goldilocks stars” (SN: 11/18/09). They are dimmer and age more slowly than yellow sunlike stars, thus offering an orbiting planet a more stable climate. But they are brighter and age faster than red dwarfs, which often spew large flares. However, new observations show that orange dwarfs emit lots of ultraviolet light long after birth, potentially endangering planetary atmospheres, researchers report in a paper submitted March 29 at arXiv.org.

Using data from the Hubble Space Telescope, astronomer Tyler Richey-Yowell and her colleagues examined 39 orange dwarfs. Most are moving together through the Milky Way in two separate groups, either 40 million or 650 million years old.

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To Richey-Yowell’s surprise, she and her team found that the ultraviolet flux didn’t drop off from the younger orange stars to the older ones — unlike the case for yellow and red stars. “I was like, `What the heck is going on?’” says Richey-Yowell, of Arizona State University in Tempe.

In a stroke of luck, another team of researchers supplied part of the answer. As yellow sunlike stars age, they spin more slowly, causing them to be less active and emit less UV radiation. But for orange dwarfs, this steady spin-down stalls when the stars are roughly a billion years old, astronomer Jason Lee Curtis at Columbia University and colleagues reported in 2019.

“[Orange] stars are just much more active for a longer time than we thought they were,” Richey-Yowell says. That means these possibly not-so-Goldilocks stars probably maintain high levels of UV light for more than a billion years.

And that puts any potential life-forms inhabiting orbiting planets on notice. Far-ultraviolet light — whose photons, or particles of light, have much more energy than the UV photons that give you vitamin D — tears molecules in a planet’s atmosphere apart. That leaves behind individual atoms and electrically charged atoms and groups of atoms known as ions. Then the star’s wind — its outflow of particles — can carry the ions away, stripping the planet of its air.

But not all hope is lost for aspiring life-forms that have an orange dwarf sun. Prolonged exposure to far-ultraviolet light can stress planets but doesn’t necessarily doom them to be barren, says Ed Guinan, an astronomer at Villanova University in Pennsylvania who was not involved in the new work. “As long as the planet has a strong magnetic field, you’re more or less OK,” he says.

Though far-ultraviolet light splits water and other molecules in a planet’s atmosphere, the star’s wind can’t remove the resulting ions if a magnetic field as strong as Earth’s protects them. “That’s why the Earth survived” as a life-bearing world, Guinan says. In contrast, Venus might never have had a magnetic field, and Mars lost its magnetic field early on and most of its air soon after.

“If the planet doesn’t have a magnetic field or has a weak one,” Guinan says, “the game is over.”

What’s needed, Richey-Yowell says, is a study of older orange dwarfs to see exactly when their UV output declines. That will be a challenge, though. The easiest way to find stars of known age is to study a cluster of stars, but most star clusters get ripped apart well before their billionth birthday (SN: 7/24/20). As a result, star clusters somewhat older than this age are rare, which means the nearest examples are distant and harder to observe. More