Space & Astronomy

PASADENA, Calif. — The faint dwarf galaxies in a nearby galaxy group seem to have missed the memo. Instead of being dispersed evenly around the group’s most massive galaxy, which is what happens in our own galaxy group, these newly found dwarfs cluster in one region. And astronomers don’t know why.

“This satellite distribution is just weird,” astronomer Eric Bell said June 13 at the American Astronomical Society meeting.

Bell, of the University of Michigan in Ann Arbor, and colleagues used the Subaru telescope in Hawaii to hunt for faint clumps of stars, indicating dwarf galaxies, around the galaxy M81. This Milky Way–like galaxy is the most prominent member in a relatively nearby group of galaxies, all about 12 million light-years from Earth. The team found one definite dwarf galaxy and six possible fainter ones.

Most of the known satellite galaxies (circled in red) in the M81 galaxy group, along with seven newfound candidates (yellow), seem to cluster toward one side of the galaxy M81 (center).Sloan Digital Sky Survey

“The part that’s just bananas,” Bell said, is that the newfound satellite galaxies all sit on one side of M81.

Computer simulations of galaxy evolution suggest that the largest galaxies have many faint, small galaxies sprinkled uniformly throughout the outer part of the dominant galaxy’s diffuse cloudlike halo. Observations in our galaxy group back this up: The dozens of dwarf galaxies known to orbit in the Milky Way’s outskirts are distributed evenly around the galaxy, as are most of the dwarf galaxies seen around our nearest large neighbor, the Andromeda Galaxy (SN: 3/11/15; SN: 8/19/15).

But in the M81 group, the seven newly identified star clumps appear to surround a smaller member of that group, NGC 3077, which is about one-tenth the mass of M81. “The fact that the bigger thing doesn’t have more satellites,” Bell says, “nobody expects that.” 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

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