Fourteen pinpricks of light on a gamma-ray map of the sky could fit the bill for antistars, stars made of antimatter, a new study suggests.

These antistar candidates seem to give off the kind of gamma rays that are produced when antimatter — matter’s oppositely charged counterpart — meets normal matter and annihilates. This could happen on the surfaces of antistars as their gravity draws in normal matter from interstellar space, researchers report online April 20 in Physical Review D.

“If, by any chance, one can prove the existence of the antistars … that would be a major blow for the standard cosmological model,” says Pierre Salati, a theoretical astrophysicist at the Annecy-le-Vieux Laboratory of Theoretical Physics in France not involved in the work. It “would really imply a significant change in our understanding of what happened in the early universe.”

It’s generally thought that although the universe was born with equal amounts of matter and antimatter, the modern universe contains almost no antimatter (SN: 3/24/20). Physicists typically think that as the universe evolved, some process led to matter particles vastly outnumbering their antimatter alter egos (SN: 11/25/19). But an instrument on the International Space Station recently cast doubt on this assumption by detecting hints of a few antihelium nuclei. If those observations are confirmed, such stray antimatter could have been shed by antistars.

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Intrigued by the possibility that some of the universe’s antimatter may have survived in the form of stars, a team of researchers examined 10 years of observations from the Fermi Gamma-ray Space Telescope. Among nearly 5,800 gamma-ray sources in the catalog, 14 points of light gave off gamma rays with energies expected of matter-antimatter annihilation, but did not look like any other known type of gamma-ray source, such as a pulsar or black hole.

Based on the number of observed candidates and the sensitivity of the Fermi telescope, the team calculated how many antistars could exist in the solar neighborhood. If antistars existed within the plane of the Milky Way, where they could accrete lots of gas and dust made of ordinary matter, they could emit lots of gamma rays and be easy to spot. As a result, the handful of detected candidates would imply that only one antistar exists for every 400,000 normal stars.

If, on the other hand, antistars tended to exist outside the plane of the galaxy, they would have much less opportunity to accrete normal matter and be much harder to find. In that scenario, there could be up to one antistar lurking among every 10 normal stars.

But proving that any celestial object is an antistar would be extremely difficult, because besides the gamma rays that could arise from matter-antimatter annihilation, the light given off by antistars is expected to look just like the light from normal stars. “It would be practically impossible to say that [the candidates] are actually antistars,” says study coauthor Simon Dupourqué, an astrophysicist at the Institute of Research in Astrophysics and Planetology in Toulouse, France. “It would be much easier to disprove.”

Astronomers could watch how gamma rays or radio signals from the candidates change over time to double-check that these objects aren’t really pulsars. Researchers could also look for optical or infrared signals that might indicate the candidates are actually black holes.

“Obviously this is still preliminary … but it’s interesting,” says Julian Heeck, a physicist at the University of Virginia in Charlottesville not involved in the work.

The existence of antistars would imply that substantial amounts of antimatter somehow managed to survive in isolated pockets of space. But Heeck doubts that antistars, if they exist, would be abundant enough to account for all the universe’s missing antimatter. “You would still need an explanation for why matter overall dominates over antimatter.” More

Like a phoenix, some stars may burst to life covered in “ash,” rising from the remains of stars that had previously passed on.

Two newfound fireballs that burn hundreds of times as bright as the sun and are covered in carbon and oxygen, ashy byproducts of helium fusion, belong to a new class of stars, researchers report in the March Monthly Notices of the Royal Astronomical Society: Letters. Though these blazing orbs are not the first stellar bodies found covered in carbon and oxygen, an analysis of the light emitted by the stars suggests they are the first discovered to also have helium-burning cores.

“That [combination] has never been seen before,” says study coauthor Nicole Reindl, an astrophysicist from the University of Potsdam in Germany. “That tells you the star must have evolved differently.”

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The stars may have formed from the merging of two white dwarfs, the remnant hearts of stars that exhausted their fuel, another team proposes in a companion study. The story goes that one of the two was rich in helium, while the other contained lots of carbon and oxygen.These two white dwarfs had already been orbiting one another, but gradually drew together over time. Eventually the helium-rich white dwarf gobbled its partner, spewing carbon and oxygen all over its surface, just as a messy child might get food all over their face.

Such a merger would have produced a stellar body covered in carbon and oxygen with enough mass to reignite nuclear fusion in its core, causing it to burn hot and glow brilliantly, say Tiara Battich, an astrophysicist from the Max Planck Institute for Astrophysics in Garching, Germany, and her colleagues.

To test this hypothesis, Battich and her colleagues simulated the evolution, death and eventual merging of two stars. The team found that aggregating a carbon-and-oxygen-rich white dwarf onto a more massive helium one could explain the surface compositions of the two stars observed by Reindl and her colleagues.

“But this should happen very rarely,” Battich says.

In most cases the opposite should occur — the carbon-oxygen white dwarf should cover itself with the helium one. That’s because carbon-oxygen white dwarfs are usually the more massive ones. For the rarer scenario to occur, two stars slightly more massive than the sun must have formed at just the right distance apart from each other. What’s more, they needed to have then exchanged material at just the right time before both running out of nuclear fuel in order to leave behind a helium white dwarf of greater mass than a carbon-and-oxygen counterpart.

The origins story Battich and her colleagues propose demands a very specific and unusual set of circumstances, says Simon Blouin, an astrophysicist from the University of Victoria in Canada, who was not involved with either study. “But in the end, it makes sense.” Stellar mergers are dynamic and complicated events that can unfold in many ways, he says (SN: 12/1/20). “This is just another.” 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

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

At age 12, Tracy Slatyer felt sorry for a book. She read a newspaper article about how lots of people were buying A Brief History of Time by Stephen Hawking. “But then … nobody was actually reading it,” she says. “People were just leaving it on their coffee tables.”

Determined to rectify this wrong, Slatyer obtained a copy and diligently read each page. The famous physicist’s popular text revealed to her “that math was in some sense an expressive language for describing how things really work,” she says. “That, to me, was exciting.” More