Astronomy

If you want to be a successful star by making the minimum possible effort, aim for a surface temperature about a quarter of the sun’s. This is the temperature that a new study says separates red dwarf stars, which shine for a long time, from failed stars known as brown dwarfs.

It’s often hard to distinguish between red and brown dwarfs, because when young they both look the same: red and dim. But only red dwarfs are born with enough mass to sustain the same nuclear reactions that power stars like the sun. In contrast, brown dwarfs glow red primarily from the heat of their birth, but then their nuclear activity sputters out, causing them to cool and fade. Now astrophysicists Dino Hsu and Adam Burgasser at the University of California, San Diego and their colleagues have discerned the dividing line between the two types by exploiting how they move through space.

When a star is born, it revolves around the Milky Way’s center on a fairly circular orbit. Over time, though, gravitational tugs from giant gas clouds, spiral arms and other stars toss the stars to and fro. These perturbations make the stars’ orbits around the galactic center more and more elliptical. Thus, the orbital paths of stars can reveal their approximate age.

Most red dwarfs are fairly old; their predicted lifetimes are far longer than the current age of the universe. But because brown dwarfs cool and fade, any that are still warm are young. Thus, on average, red dwarfs should follow more elliptical orbits around the galaxy than young brown dwarfs do.

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In the new study, Hsu’s team analyzed 172 red and brown dwarfs of different spectral types, classifications based on the objects’ spectra that correlate with their surface temperatures. The researchers found that a sharp break in stellar motions separates warmer objects, which on average have more elliptical orbits and are older, from cooler ones, which on average have more circular orbits and are younger. This break appears at a spectral type between L4 and L6, corresponding to a surface temperature of about 1200° to 1400° Celsius (1,500 to 1,700 kelvins) — a fraction of the sun’s surface temperature of about 5500°C (5,800 K) — the team reports July 5 at arXiv.org.

Above this critical temperature, the dim suns are a mix of long-lived red dwarfs and young brown dwarfs. Below this temperature, though, “it’s all brown dwarfs,” Hsu says. These are the failed stars that are fated to fizzle out. The study will appear in a future issue of the Astrophysical Journal Supplement Series.

This new method for detecting the temperature boundary between red and brown dwarfs is intriguing, but the result is tentative, says Trent Dupuy, an astronomer at the University of Edinburgh who was not involved in the work. “It’s right around where you would expect,” he says. Dupuy says additional red and brown dwarfs should be observed to verify the finding.

Hsu agrees: “We need a more complete sample.” Expanding the sample will be both easy and hard. On the positive side, red dwarfs abound, outnumbering all other stellar types put together, and brown dwarfs are also common. On the negative side, though, red and brown dwarfs are faint. That makes measuring their Doppler shifts, which reveal how fast the objects move toward or away from Earth, a challenge. But knowing this motion is essential for calculating a star’s orbital path around the galaxy. More

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

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

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

The Event Horizon Telescope is expanding its portfolio of black hole images.

In 2019, the telescope unveiled the first image of a black hole, revealing the supermassive beast 55 light-years from Earth at the center of galaxy M87 (SN: 4/10/19). That lopsided orange ring showed the shadow of the black hole on its glowing accretion disk of infalling material. Since then, observations from the Event Horizon Telescope, or EHT, have yielded more detailed views of M87’s black hole (SN: 9/23/20). Now, EHT data have revealed new details of the supermassive black hole at the heart of a galaxy near our own, called Centaurus A.

Rather than zooming in close enough to see the black hole’s shadow, the new picture offers the clearest view yet of the powerful plasma jets erupting from the black hole. This perspective gives insight into how supermassive black holes blast such plasma jets into space, researchers report online July 19 in Nature Astronomy.

“It’s a fairly impressive feat,” says radio astronomer Craig Walker of capturing the new high-resolution image. “These [jets] are some of the most powerful things in the universe,” says Walker, of the National Radio Astronomy Observatory in Socorro, N.M., who was not involved in the work. Because such superfast plasma streams are thought to influence how galaxies grow and evolve, astronomers are keen to understand how the jets form (SN: 3/29/19).

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Researchers pointed the global network of radio dishes that make up the EHT at Centaurus A for six hours in April 2017, during the same observing run that delivered the first picture of a black hole (SN: 4/10/19). About 12 million light-years from Earth, Centaurus A is one of the brightest galaxies in the sky and is known for the huge jets expelled by its central black hole.

“They extend to pretty much the entire scale of the galaxy,” says Michael Janssen, a radio astronomer at the Max Planck Institute for Radio Astronomy in Bonn, Germany. “If we were to see radio light [with our eyes], and we were to look at the night sky, then we would see these jets of Centaurus A as a structure that is 16 times bigger than the full moon.”

Using the EHT, Janssen and colleagues homed in on the base of those jets, which gush out from either side of the black hole’s accretion disk. The new image is 16 times as sharp as previous observations of the jets, probing details less than one light-day across — about four times the distance from the sun to Pluto. One of the most striking features that the image reveals is that only the outer edges of the jets seem to glow.

The supermassive black hole in the galaxy Centaurus A launches two jets of plasma in opposite directions (zoomed-out view of the jets at left). In a new close-up view taken by the Event Horizon Telescope (at right; estimated location of the black hole indicated with an arrow), the jet moving toward Earth points toward the image’s top left, with two bright edges and a dark center. The jet moving away from Earth, also bright only at the edges, points toward the bottom right.M. Janssen et al/Nature Astronomy 2021

“That’s still a puzzle,” Janssen says. One possibility is that the jets are rotating, which might cause material in some regions of the jets to emit light toward Earth, while others don’t. Or the jets could be hollow, Janssen says.

Recent observations of a few other galaxies have hinted that the jets of supermassive black holes are brighter around the edges, says Denise Gabuzda, an astrophysicist at University College Cork in Ireland, who wasn’t involved in the work. “But it’s been hard to know whether it was a common feature, or whether it was something quirky about the few that had been observed.”

The new view of Centaurus A’s black hole provides evidence that this edge-brightening is common, Gabuzda says. “It’s fairly rare to be able to detect the jets coming out in both directions, but in the images of Centaurus A … you can clearly see that both of them are brighter at the edges.”

The next step will be to compare the EHT image of Centaurus A with computer simulations based on Einstein’s general theory of relativity, to test how well relativity holds up in this extreme environment, Janssen says. Examining the polarization, or orientation, of the light waves emanating from Centaurus A’s jets could also reveal the structure of their magnetic fields — just as polarization revealed the magnetism around M87’s black hole (SN: 3/24/21). More

Jupiter may have formed in a shadow that kept the planet’s birthplace colder than Pluto. The frigid temperature could explain the giant world’s unusual abundance of certain gases, a new study suggests.

Jupiter consists mostly of hydrogen and helium, which were the most common elements in the planet-spawning disk that spun around the newborn sun. Other elements that were gases near Jupiter’s birthplace became part of the planet, too, but in only the same proportions as they existed in the protoplanetary disk (SN: 6/12/17).

Astronomers think the sun’s composition of elements largely reflects that of the protoplanetary disk, so Jupiter’s should resemble that solar makeup — at least for elements that were gases. But nitrogen, argon, krypton and xenon are about three times as common on Jupiter, relative to hydrogen, as they are on the sun.

“This is the main puzzle of Jupiter’s atmosphere,” says Kazumasa Ohno, a planetary scientist at the University of California, Santa Cruz. Where did those extra elements come from?

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If Jupiter was born at its current distance from the sun, the temperature of the planet’s birthplace would have been around 60 kelvins, or –213˚ Celsius. In the protoplanetary disk, those elements should be gases at that temperature. But they would freeze solid below about 30 kelvins, or –243˚ C. It’s easier for a planet to accrete solids than gases. So if Jupiter somehow arose in a much colder environment than its current home, the planet could have acquired solid objects laden with those extra elements as ice.

For this reason, in 2019 two different research teams independently made the radical suggestion that Jupiter had originated in the deep freeze beyond the current orbits of Neptune and Pluto, then spiraled inward toward the sun.

Now Ohno and astronomer Takahiro Ueda of the National Astronomical Observatory of Japan propose a different idea: Jupiter formed where it is, but a pileup of dust in between the planet’s orbit and the sun blocked sunlight, casting a long shadow that cooled Jupiter’s birthplace. The frosty temperature made nitrogen, argon, krypton and xenon freeze solid and become a greater part of the planet, the scientists suggest in a study in the July Astronomy & Astrophysics.

The dust that cast the shadow came from rocky objects closer to the sun that collided and shattered. Farther from the sun, where the protoplanetary disk was colder, water froze, giving rise to objects that resembled snowballs. When those snowballs collided, they were more likely to stick together than shatter and thus didn’t cast much of a shadow, the researchers say.

“I think it’s a clever fix of something that might have been difficult to rectify otherwise,” says Alex Cridland, an astrophysicist at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany.

Cridland was one of the scientists who had suggested that Jupiter formed beyond Neptune and Pluto. But that theory, he says, means Jupiter had to move much closer to the sun after birth. The new scenario avoids that complication.

Measuring the atmospheric composition of Saturn may pinpoint the birthplace of Jupiter.NASA, ESA, A. Simon/GSFC, M.H. Wong/UCB, the OPAL Team

How to test the new idea? “Saturn might hold the key,” Ohno says. Saturn is nearly twice as far from the sun as Jupiter is, and the scientists calculate that the dust shadow that chilled Jupiter’s birthplace barely reached Saturn’s. If so, that means Saturn arose in a warmer region and so should not have acquired nitrogen, argon, krypton or xenon ice. In contrast, if the two gas giants really formed in the cold beyond the present orbits of Neptune and Pluto, then Saturn should have lots of those elements, like Jupiter.

Thanks to the Galileo probe, which dove into the Jovian atmosphere in 1995, astronomers know these abundances for Jupiter. What’s needed, the researchers say, is a similar mission to Saturn. Unfortunately, while orbiting Saturn, the Cassini spacecraft (SN: 8/23/17) measured only an uncertain level of nitrogen in the Ringed Planet’s atmosphere and detected no argon, krypton or xenon, so Saturn doesn’t yet constrain where the two gas giants arose.     More

A long-predicted type of cosmic explosion has finally burst onto the scene.

Researchers have found convincing evidence for an electron-capture supernova, a stellar explosion ignited when atomic nuclei sop up electrons within a star’s core. The phenomenon was first predicted in 1980, but scientists have never been sure that they have seen one. A flare that appeared in the sky in 2018, called supernova 2018zd, matches several expected hallmarks of the blasts, scientists report June 28 in Nature Astronomy.

“These have been theorized for so long, and it’s really nice that we’ve actually seen one now,” says astrophysicist Carolyn Doherty of Konkoly Observatory in Budapest, who was not involved with the research.

Electron-capture supernovas result from stars that sit right on the precipice of exploding. Stars with more than about 10 times the sun’s mass go supernova after nuclear fusion reactions within the core cease, and the star can no longer support itself against gravity. The core collapses inward and then rebounds, causing the star’s outer layers to explode outward (SN: 2/8/17). Smaller stars, with less than about eight solar masses, are able to resist collapse, instead forming a dense object called a white dwarf (SN: 6/30/21). But between about eight and 10 solar masses, there’s a poorly understood middle ground for stars. For some stars that fall in that range, scientists have long suspected that electron-capture supernovas should occur.

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During this type of explosion, neon and magnesium nuclei within a star’s core capture electrons. In this reaction, an electron vanishes as a proton converts to a neutron, and the nucleus morphs into another element. That electron capture spells bad news for the star in its war against gravity because those electrons are helping the star fight collapse.

According to quantum physics, when electrons are packed closely together, they start moving faster. Those zippy electrons exert a pressure that opposes the inward pull of gravity. But if reactions within a star chip away at the number of electrons, that support weakens. If the star’s core gives way — boom — that sets off an electron-capture supernova.

But without an observation of such a blast, it remained theoretical. “The big question here was, ‘Does this kind of supernova even exist?’” says astrophysicist Daichi Hiramatsu of the University of California, Santa Barbara and Las Cumbres Observatory in Goleta, Calif. Potential electron-capture supernovas have been reported before, but the evidence wasn’t definitive.

So Hiramatsu and colleagues created a list of six criteria that an electron-capture supernova should meet. For example, the explosions should be less energetic, and should forge different varieties of chemical elements, than more typical supernovas. Supernova 2018zd checked all the boxes.

A stroke of luck helped the team clinch the case. Most of the time, when scientists spot a supernova, they have little information about the star that produced it — by time they see the explosion, the star has already been blown to bits. But in this case, the star showed up in previous images taken by NASA’s Hubble Space Telescope and Spitzer Space Telescope. Its properties matched those expected for the type of star that would produce an electron-capture supernova.

“All together, it really is very promising,” says astrophysicist Pilar Gil-Pons of Universitat Politècnica de Catalunya in Barcelona. Reading the researchers’ results, she says, “I got pretty excited, especially about the identification of the progenitor.” 

Finding more of these supernovas could help unveil their progenitors, misfit stars in that odd mass middle ground. It could also help scientists better nail down the divide between stars that will and won’t explode. And the observations could reveal how often these unusual supernovas occur, an important bit of information for better understanding how supernovas seed the cosmos with chemical elements. More

Only a smidge bigger than the moon, a newfound white dwarf is the smallest of its kind known. 

The white dwarf, a type of remnant left behind when certain stars peter out, has a radius of about 2,100 kilometers, researchers report June 30 in Nature. That’s remarkably close to the moon’s approximately 1,700-kilometer radius. Most white dwarfs are closer to the size of Earth, which has a radius of about 6,300 kilometers.

The white dwarf’s small girth means, counterintuitively, that it is also one of the most massive known objects of its kind, at about 1.3 times the sun’s mass. That’s because white dwarfs shrink as they gain mass (SN: 8/12/20).

“That’s not the only very amazing characteristic of this white dwarf,” astrophysicist Ilaria Caiazzo of Caltech said June 28 in an online news conference. “It is also rapidly rotating.”

The white dwarf spins around approximately once every seven minutes. And it has a powerful magnetic field, more than a billion times the strength of Earth’s. Caiazzo and colleagues discovered the unusual stellar remnant, dubbed ZTF J1901+1458 and located about 130 light-years from Earth, using the Zwicky Transient Facility at Palomar Observatory in California, which searches for objects in the sky that change in brightness.

The white dwarf probably formed when two white dwarfs orbited one another and merged to create a single white dwarf with an extra-large mass and extra-small size, the team says. That convergence would also have spun up the white dwarf and given it a strong magnetic field.

This white dwarf is living on the edge: If it were much more massive, it wouldn’t be able to support its own weight, causing it to explode. Studying such objects can help scientists understand the limits of what’s possible for these dead stars. More