Space & Astronomy

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

The cosmos keeps outdoing itself.

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

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

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

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

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