Astronomy

The Big Dipper’s stars make up a conspicuous landmark in the sky of the Northern Hemisphere. Even novice stargazers can easily pick out the shape, part of the Ursa Major constellation. Now, scientists have shown that three factors can explain why certain groups of stars form such recognizable patterns.

To replicate how humans perceive the celestial sphere, a team of researchers considered how the eye might travel randomly across this night sky. Human eyes tend to move in discrete jumps, called saccades (SN: 10/31/11), from one point of interest to another. The team created a simulation that incorporated the distribution of lengths of those saccades, combined that with basic details of the night sky as seen from Earth — namely the apparent distances between neighboring stars and their brightnesses.

The technique could reproduce individual constellations, such as Dorado, the dolphinfish. And when used to map the whole sky, the simulation generated groupings of stars that tended to align with the 88 modern constellations recognized by the International Astronomical Union, Sophia David and colleagues reported March 18 at an online meeting of the American Physical Society.

“Ancient people from various cultures connected similar groupings of stars independently of each other,” said David, a high school student at Friends’ Central School in Wynnewood, Penn., who worked with network scientists at the University of Pennsylvania. “And this indicates that there are some fundamental aspects of human learning … that influence the ways in which we organize information.” More

Something’s fishy in the southern constellation Phoenix.

Strange radio emissions from a distant galaxy cluster take the shape of a gigantic jellyfish, complete with head and tentacles. Moreover, the cosmic jellyfish emits only the lowest radio frequencies and can’t be detected at higher frequencies. The unusual shape and radio spectrum tell a tale of intergalactic gas washing over galaxies and gently revving up electrons spewed out by gargantuan black holes long ago, researchers report in the March 10 Astrophysical Journal.

Spanning 1.2 million light-years, the strange entity lies in Abell 2877, a cluster of galaxies 340 million light-years from Earth. Researchers have dubbed the object the USS Jellyfish, because of its ultra-steep spectrum, or USS, from low to high radio frequencies.

“This is a source which is invisible to most of the radio telescopes that we have been using for the last 40 years,” says Melanie Johnston-Hollitt, an astrophysicist at Curtin University in Perth, Australia. “It holds the record for dropping off the fastest” with increasing radio frequency.

Johnston-Hollitt’s colleague Torrance Hodgson, a graduate student at Curtin, discovered the USS Jellyfish while analyzing data from the Murchison Widefield Array, a complex of radio telescopes in Australia that detect low-frequency radio waves. These radio waves are more than a meter long and correspond to photons, particles of light, with the lowest energies. Remarkably, the USS Jellyfish is about 30 times brighter at 87.5 megahertz — a frequency similar to that of an FM radio station — than at 185.5 MHz.

The Murchison Widefield Array consists of 4,096 radio antennas grouped into 256 “tiles” (one pictured) spanning several kilometers in a remote region of Western Australia.Pete Wheeler, ICRAR

“That is quite spectacular,” says Reinout van Weeren, an astronomer at Leiden University in the Netherlands who was not involved with the work. “It is quite a neat result, because this is really extreme.”

The USS Jellyfish bears no relation to previously discovered jellyfish galaxies. “This is absolutely enormous compared to those other things,” Johnston-Hollitt says. Indeed, jellyfish galaxies are a very different kettle of celestial fish. Although they also inhabit galaxy clusters, they are individual galaxies passing through hot gas in a cluster. The hot gas tears the galaxy’s own gas out of it, creating a wake of tentacles. The much larger USS Jellyfish, on the other hand, appears to have formed when intergalactic gas and electrons interacted.

Hodgson and his colleagues note that two galaxies in the Abell 2877 cluster coincide with the brightest patches of radio waves in the USS Jellyfish’s head. These galaxies, the researchers say, probably have supermassive black holes at their centers. The team ran computer simulations and found that the black holes were probably accreting material some 2 billion years ago. As they did so, disks of hot gas formed around each of them, spewing huge jets of material into the surrounding galaxy cluster.

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This ejected material had electrons that whirled around magnetic fields at nearly the speed of light, so the electrons emitted radio waves. Over time, though, the electrons lost energy, and the most energetic electrons, which had been emitting the highest radio frequencies, faded the most. Then a wave of gas sloshed through the entire cluster, reaccelerating the electrons around the two galaxies.

“It’s a very gentle process,” Johnston-Hollitt says. “The electrons don’t get that much energy, which means they don’t light up at high frequencies.” Instead, the gentle gas wave caused electrons to emit radio waves with the lowest energies and frequencies, giving the USS Jellyfish the extreme spectrum it has today. More

Astronomers have gotten their first glimpse of the magnetic fields tangled around a black hole.

The Event Horizon Telescope has unveiled the magnetism of the hot, glowing gas around the supermassive black hole at the heart of galaxy M87, researchers report in two studies published online March 24 in the Astrophysical Journal Letters. These magnetic fields are thought to play a crucial role in how the black hole scarfs down matter and launches powerful plasma jets thousands of light-years into space (SN: 3/29/19).

“We’ve known for decades that jets are in some sense powered by accretion onto supermassive black holes, and that the in-spiraling gas and the outflowing plasma are highly magnetized — but there was a lot of uncertainty in the exact details,” says Eileen Meyer, an astrophysicist at the University of Maryland, Baltimore County not involved in the work. “The magnetic field structure of the plasma near the event horizon [of a black hole] is a completely new piece of information.”

The supermassive black hole inside M87 was the first black hole to get its picture taken (SN: 4/10/19). That image showed the black hole’s shadow against its accretion disk — the bright eddy of superhot gas spiraling around the black hole’s dark center. It was created using observations taken in April 2017 by a global network of observatories, which collectively form one virtual, Earth-sized radio dish called the Event Horizon Telescope (SN: 4/10/19).

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Using data from 2017, scientists created the first real picture of the supermassive black hole at the center of galaxy M87. How? We explain.

The new analysis uses the same observations. But unlike the black hole’s initial portrait, the new image accounts for the polarization of the light waves emitted by gas around the black hole. Polarization measures a light wave’s orientation — whether it wiggles up and down, left and right or at an angle — and can be affected by the magnetic field where the light originated. So, by mapping the polarization of light around the edge of M87’s black hole, researchers were able to trace the structure of the underlying magnetic fields.

The team found evidence that some magnetic fields loop around the black hole along with the disk of material swirling into it. That’s to be expected because “when gas is rotating, it’s basically able to carry along the magnetic field with it,” says Jason Dexter, an astrophysicist at the University of Colorado Boulder.

But, he says, “there’s some interesting component of this magnetic field which is not just following the motion of the gas.” At least some of the magnetic field lines are sticking up or down perpendicularly from the accretion disk, or pointing directly toward or away from the black hole, Dexter and colleagues found. These magnetic fields must be very strong to resist being dragged around by the whirl of infalling gas, he says.

Such strong magnetic fields may actually push back against some of the material spiraling in toward the black hole, helping it resist gravity’s pull, says study coauthor Monika Mościbrodzka, an astrophysicist at Radboud University in Nijmegen, the Netherlands. Magnetic fields pointed up and down from the accretion disk could also help launch the black hole’s plasma jets, by channeling material toward the black hole’s poles and giving it a boost in speed, she says.

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Complex carbon-bearing molecules that could help explain how life got started have been identified in space for the first time.

These molecules, called polycyclic aromatic hydrocarbons, or PAHs, consist of several linked hexagonal rings of carbon with hydrogen atoms at the edges. Astronomers have suspected for decades that these molecules are abundant in space, but none had been directly spotted before.

Simpler molecules with a single ring of carbon have been seen before. But “we’re now excited to see that we’re able to detect these larger PAHs for the first time in space,” says astrochemist Brett McGuire of MIT, whose team reports the discovery in the March 19 Science.

Studying these molecules and others like them could help scientists understand how the chemical precursors to life might get started in space. “Carbon is such a fundamental part of chemical reactions, especially reactions leading to life’s essential molecules,” McGuire says. “This is our window into a huge reservoir of them.”

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Since the 1980s, astronomers have seen a mysterious infrared glow coming from spots within our galaxy and others. Many suspected that the glow comes from PAHs, but could not identify a specific source. The signals from several different PAHs overlap too much to tease any one of them apart, like a choir blending so well, the ear can’t pick out individual voices.

Instead of searching the infrared signals for a single voice, McGuire and colleagues turned to radio waves, where different PAHs sing different songs. The team trained the powerful Green Bank Telescope in West Virginia on TMC-1, a dark cloud about 430 light-years from Earth near the constellation Taurus.

The interstellar cloud TMC-1 (top, black filaments) appears as a dark streak on the sky next to the bright Pleiades star cluster (right)Brett A. McGuire

Previously, McGuire had discovered that the cloud contains benzonitrile, a molecule made of a single carbon ring (SN: 10/2/19). So he thought it was a good place to look for more complicated molecules.

The team detected 1- and 2-cyanonaphthalene, two-ringed molecules with 10 carbons, eight hydrogens and a nitrogen atom. The concentration is fairly diffuse, McGuire says: “If you filled the inside of your average compact car with [gas from] TMC-1, you’d have less than 10 molecules of each PAH we detected.”

But it was a lot more than the team expected. The cloud contains between 100,000 and one million times more PAHs than theoretical models predict it should. “It’s insane, that’s way too much,” McGuire says.

There are two ways that PAHs are thought to form in space: out of the ashes of dead stars or by direct chemical reactions in interstellar space. Since TMC-1 is just beginning to form stars, McGuire expected that any PAHs it contains ought to have been built by direct chemical reactions in space. But that scenario can’t account for all the PAH molecules the team found. There’s too much to be explained easily by stellar ash, too. That means something is probably missing from astrochemists’ theories of how PAHs can form in space.

“We’re working in uncharted territory here,” he says, “which is exciting.”

Identifying PAHs in space is “a big thing,” says astrochemist Alessandra Ricca of the SETI Institute in Mountain View, Calif., who was not involved in the new study. The work “is the first one that has shown that these PAH molecules actually do exist in space,” she says. “Before, it was just a hypothesis.”

Ricca’s group is working on a database of infrared PAH signals that the James Webb Space Telescope, slated to launch in October, can look for. “All this is going to be very helpful for JWST and the research on carbon in the universe,” she says. More

A cloud of expanding gas in space is the largest supernova remnant ever seen in the sky, a new study confirms.

The Milky Way has some 300 known supernova remnants, each made of debris from an exploded star mixed with interstellar material swept up by the blast. This supersized one, located in the constellation Antlia, isn’t necessarily the biggest of all physically, but thanks to its proximity to us, it looks the biggest. As seen from Earth, it spans a region of sky more than 40 times the size of a full moon, astronomer Robert Fesen of Dartmouth College and his colleagues report February 25 at arXiv.org. The Antlia remnant appears about three times as large as the previous champion, the Vela supernova remnant (SN: 7/8/20).

The star that created the Antlia supernova remnant exploded roughly 100,000 years ago. Estimates of the remnant’s distance vary, so its physical size has yet to be nailed down. But if the cloud is 1,000 light-years away, then it’s about 390 light-years across; if it’s twice as far, then it’s twice as big. Either way, it’s considerably larger than the Vela supernova remnant, which is about 100 light-years wide.

Vela (shown) had been the largest confirmed supernova remnant as seen from Earth, but the one in Antlia looks three times larger.Robert Gendler, Roberto Colombari, Digitized Sky Survey (POSS II)

The Antlia remnant isn’t new to astronomers. In 2002, researchers discovered the cloud and proposed that it is the nearby remains of a supernova, based on the red glow of its hydrogen atoms as well as its X-ray emission. But hardly anyone had observed the object since. “It wasn’t really firmly established as a supernova remnant,” says team member John Raymond, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

So the astronomers studied the cloud at visible and ultraviolet wavelengths, which demonstrate that the Antlia object is indeed a supernova remnant. In particular, the visible light shows spectral signatures of shock waves, which result when high-speed gas from a supernova slams into gas around it.

“The evidence for it being shocks in a supernova remnant seems to be very good,” says Roger Chevalier, an astronomer at the University of Virginia in Charlottesville not involved with the new work. He notes that the team detected red light from sulfur atoms that are missing one electron, a hallmark of shocks in supernova remnants.

The astronomer who discovered the object two decades ago had little doubt it was a genuine supernova remnant. “They’ve done good work,” says Peter McCullough at the Space Telescope Science Institute in Baltimore. “This is a case where it looks like a duck, quacks like a duck, walks like a duck and now someone else 20 years later comes along and says, `Not only that, it has feathers.’” More

The supermassive black holes at the centers of the Milky Way and Andromeda galaxies are doomed to engulf each other in an ill-fated cosmological dance.
Astronomers have long known that Andromeda is on a collision course with our galaxy (SN: 5/31/12). But not much has been known about what will happen to the gargantuan black holes each galaxy harbors at its core. New simulations reveal their ultimate fate.
The galaxies will coalesce into one giant elliptical galaxy — dubbed “Milkomeda” — in about 10 billion years. Then, the central black holes will begin orbiting one another and finally collide less than 17 million years later, researchers propose February 22 at arXiv.org and in an earlier paper published in Astronomy & Astrophysics. Just before the black holes smash into each other, they’ll radiate gravitational waves with the power of 10 quintillion suns (SN: 2/11/16). Any civilization within 3.25 million light-years from us that has gravitational wave–sensing technology on par with our current abilities would be able to detect the collision, the researchers estimate.
The latest data suggest Andromeda is approaching us at about 116 kilometers per second, says Riccardo Schiavi, an astrophysicist at the Sapienza University of Rome. Using computer simulations that include the gravitational pull of the two spiral galaxies on each other as well as the possible presence of sparse gas and other material between them, Schiavi and his colleagues played out how the galactic collision will unfold.
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A computer simulation shows how the Milky Way (left) and Andromeda (right) galaxies will brush past each other about 4 billion years from now before merging into a single galaxy roughly 6 billion years later. The numbers along the sides denote distance in kiloparsecs (1 kiloparsec equals 3,260 light-years).
Previous simulations have suggested that Andromeda and the Milky Way are scheduled for a head-on collision in about 4 billion to 5 billion years. But the new study estimates that the two star groups will swoop closely past each other about 4.3 billion years from now and then fully merge about 6 billion years later.
The team’s estimate for Milkomeda’s merger date “is a bit longer than what other teams have found,” says Roeland van der Marel, an astronomer at the Space Telescope Science Institute in Baltimore who was not involved in the research. However, he notes, that could be due in part to uncertainty in the measurement of Andromeda’s speed across the sky. More

Planet Nine might be a mirage. What once looked like evidence for a massive planet hiding at the solar system’s edge may be an illusion, a new study suggests.
“We can’t rule it out,” says Kevin Napier, a physicist at the University of Michigan in Ann Arbor. “But there’s not necessarily a reason to rule it in.”
Previous work has suggested that a number of far-out objects in the solar system cluster in the sky as if they are being shepherded by an unseen giant planet, at least 10 times the mass of Earth. Astronomers dubbed the invisible world Planet Nine or Planet X.
Now, a new analysis of 14 of those remote bodies shows no evidence for such clustering, knocking down the primary reason to believe in Planet Nine. Napier and colleagues reported the results February 10 at arXiv.org in a paper to appear in the Planetary Science Journal.

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The idea of a distant planet lurking far beyond Neptune received a surge in interest in 2014, when astronomers Chad Trujillo of Northern Arizona University and Scott Sheppard of the Carnegie Institution for Science reported a collection of distant solar system bodies called trans-Neptunian objects with strangely bunched-up orbits (SN: 11/14/14).
In 2016, Caltech planetary scientists Mike Brown and Konstantin Batygin used six trans-Neptunian objects to refine the possible properties of Planet Nine, pinning it to an orbit between 500 and 600 times as far from the sun as Earth’s (SN: 7/5/16).
But those earlier studies all relied on just a handful of objects that may not have represented everything that’s out there, says Gary Bernstein, an astronomer at the University of Pennsylvania. The objects might have seemed to show up in certain parts of the sky only because that’s where astronomers happened to look.
“It’s important to know what you couldn’t see, in addition to what you did see,” he says.
To account for that uncertainty, Napier, Bernstein and colleagues combined observations from three surveys — the Dark Energy Survey, the Outer Solar System Origins Survey and the original survey run by Sheppard and Trujillo — to assess 14 trans-Neptunian objects, more than twice as many as in the 2016 study. These objects all reside between 233 and 1,560 times as far from the sun as Earth.
The team then ran computer simulations of about 10 billion fake trans-Neptunian objects, distributed randomly all around the sky, and checked to see if their positions matched what the surveys should be able to see. They did.
“It really looks like we just find things where we look,” Napier says. It’s sort of like if you lost your keys at night and searched for them under a streetlamp, not because you thought they were there, but because that’s where the light was. The new study basically points out the streetlamps.
“Once you see where the lampposts really are, it becomes more clear that there is some serious selection bias going on with the discovery of these objects,” Napier says. That means the objects are just as likely to be distributed randomly across the sky as they are to be clumped up.
That doesn’t necessarily mean Planet Nine is done for, he says.
“On Twitter, people have been very into saying that this kills Planet Nine,” Napier says. “I want to be very careful to mention that this does not kill Planet Nine. But it’s not good for Planet Nine.”
There are other mysteries of the solar system that Planet Nine would have neatly explained, says astronomer Samantha Lawler of the University of Regina in Canada, who was not involved in the new study. A distant planet could explain why some far-out solar system objects have orbits that are tilted relative to those of the larger planets or where proto-comets called centaurs come from (SN: 8/18/20). That was part of the appeal of the Planet Nine hypothesis.
“But the entire reason for it was the clustering of these orbits,” she says. “If that clustering is not real, then there’s no reason to believe there is a giant planet in the distant solar system that we haven’t discovered yet.”
Batygin, one of the authors of the 2016 paper, isn’t ready to give up. “I’m still quite optimistic about Planet Nine,” he says. He compares Napier’s argument to seeing a group of bears in the forest: If you see a bunch of bears to the east, you might think there was a bear cave there. “But Napier is saying the bears are all around us, because we haven’t checked everywhere,” Batygin says. “That logical jump is not one you can make.”
Evidence for Planet Nine should show up only in the orbits of objects that are stable over billions of years, Batygin adds. But the new study, he says, is “strongly contaminated” by unstable objects — bodies that may have been nudged by Neptune and lost their position in the cluster or could be on their way to leaving the solar system entirely. “If you mix dirt with your ice cream, you’re going to mostly taste dirt,” he says.
Lawler says there’s not a consensus among people who study trans-Neptunian objects about which ones are stable and which ones are not.
Everyone agrees, though, that in order to prove Planet Nine’s existence or nonexistence, astronomers need to discover more trans-Neptunian objects. The Vera Rubin Observatory in Chile should find hundreds more after it begins surveying the sky in 2023 (SN: 1/10/20).
“There always may be some gap in our understanding,” Napier says. “That’s why we keep looking.” More

The first black hole ever discovered still has a few surprises in store.
New observations of the black hole–star pair called Cygnus X-1 indicate that the black hole weighs about 21 times as much as the sun — nearly 1.5 times heavier than past estimates. The updated mass has astronomers rethinking how some black hole–forming stars evolve. For a star-sized, or stellar, black hole that massive to exist in the Milky Way, its parent star must have shed less mass through stellar winds than expected, researchers report online February 18 in Science.
Knowing how much mass stars lose through stellar winds over their lifetimes is important for understanding how these stars enrich their surroundings with heavy elements. It’s also key to understanding the masses and compositions of those stars when they explode and leave behind black holes.
The updated mass measurement of Cygnus X-1 is “a big change to an old favorite,” says Tana Joseph, an astronomer at the University of Amsterdam not involved in the work. Stephen Hawking famously bet physicist Kip Thorne that the Cygnus X-1 system, discovered in 1964, did not include a black hole — and conceded the wager in 1990, when scientists had broadly accepted that Cygnus X-1 contained the first known black hole in the universe (SN: 4/10/19).

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Astronomers got a new look at Cygnus X-1 using the Very Long Baseline Array, or VLBA. This network of 10 radio dishes stretches across the United States, from Hawaii to the Virgin Islands, collectively forming a continent-sized radio dish. In 2016, the VLBA tracked radio-bright jets of material spewing out of Cygnus X-1’s black hole for six days (the time it took for the black hole and its companion star to orbit each other once). Those observations offered a clear view of how the black hole’s position in space shifted over the course of its orbit. That, in turn, helped researchers refine the estimated distance to Cygnus X-1.
The new observations suggest that Cygnus X-1 is about 7,200 light-years from Earth, rather than the previous estimate of about 6,000 light-years. This implies that the star in Cygnus X-1 is even brighter, and therefore bigger, than astronomers thought. The star weighs about 40.6 suns, the researchers estimate. The black hole must also be more massive in order to explain its gravitational tug on such a massive star. The black hole weighs about 21.2 suns — much heftier than its previously estimated 14.8 solar masses, the scientists say. 
The new mass measurement for Cygnus X-1’s black hole is so big that it challenges astronomers’ understanding of the massive stars that collapse to form black holes, says study coauthor Ilya Mandel, an astrophysicist at Monash University in Melbourne, Australia.
“Sometimes stars are born with quite high masses — there are observations of stars being born with masses of well over 100 solar masses,” Mandel says. But such enormous stars are thought to shed much of their weight through stellar winds before turning into black holes. The bigger the star and the more heavy elements it contains, the stronger its stellar winds. So in heavy element–rich galaxies such as the Milky Way, big stars — no matter their starting mass — are supposed to shrink down to about 15 solar masses before collapsing into black holes.
Cygnus X-1’s 21-solar-mass black hole undermines that idea.
The LIGO and Virgo gravitational wave detectors have discovered black holes weighing tens of solar masses in other galaxies (SN: 1/21/21). But that is probably because LIGO peers at distant galaxies that existed earlier in the universe, Joseph says. Back then, fewer heavy elements existed, so stellar winds were weaker. With the new Cygnus X-1 measurement, “now we have to say, hang on, we’re in a [heavy element]–rich environment compared to the early universe … but we still managed to make this really massive black hole,” she says, “so maybe we’re not losing as much mass through stellar winds as we initially thought.” More