Space & Astronomy

The nearest black hole to Earth isn’t a black hole at all. Instead, what scientists thought was a stellar triplet — two stars and a black hole — is actually a pair of stars caught in a unique stage of evolution.

In May 2020, a team of astronomers reported that the star system HR 6819 was probably made up of a bright, massive star locked in a tight, 40-day orbit with a nonfeeding, invisible black hole plus a second star orbiting farther away. At about 1,000 light-years from Earth, that would make this black hole the nearest to us (SN: 5/6/20). But over the following months, other teams analyzed the same data and came to a different conclusion: The system hosts only two stars and no black hole.

Now, the original team and one of the follow-up teams have joined forces and looked at HR 6819 with more powerful telescopes that collect a different type of data. The new data can make out finer details on the sky, allowing the astronomers to definitively see how many objects are in the system and what type of objects they are, the teams report in the March Astronomy & Astrophysics.

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“Ultimately, it was the binary system that best explains everything,” says astronomer Abigail Frost of KU Leuven in Belgium.

Previous observations of HR 6819 showed it as a unit, so astronomers couldn’t differentiate the objects in the system nor their masses. To nail down HR 6819’s true nature, Frost and colleagues turned to the Very Large Telescope Array, a network of four interconnected telescopes in Chile that can essentially see the separate stars.

“It allowed us to disentangle that original signal definitively, which is really important to determine how many stars were in it, and whether one of them was a black hole,” Frost says.

The scientists think one of the stars is a massive bright blue star that has been siphoning material from its companion star’s bloated atmosphere. That companion star now has little gaseous atmosphere left. “It’s already gone through its main life, but because the outside has been stripped off, and you only see the exposed core, it has similar temperature and luminosity and radius to a young star,” says Kareem El-Badry, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. El-Badry was not involved in the new study, but he suggested in 2021 that HR 6819 is a binary system.

This siphoned star’s core color and brightness could fool astronomers looking at the older data into thinking it was a young star with 10 times as much mass. It originally appeared as though this star was orbiting something massive but invisible — a black hole.

Once the researchers unraveled the system’s details, they realized this system is a unique one, showing astronomers a phase not seen before among systems with massive stars. “It is a missing link in binary star evolution,” says astrophysicist Maxwell Moe of the University of Arizona in Tucson, who was also not part of the new study.

Astronomers for years have seen binary systems where one star is actively pulling gas off the other, and they’ve seen systems where the donor star is just a naked stellar core. But in HR 6819, the donor star has stopped giving mass to the other. “It still has a little bit of envelope left but is quickly contracting, evolving to become a remnant core,” Moe says.

Frost and her colleagues are using the Very Large Telescope Array to monitor HR 6819 over a year to track precisely how the stars are moving. “We want to really understand how the individual stars in the system are ticking,” she says. The team will then use that information in computer simulations of binary star evolution. “[It’s] exciting to now have a system that we can use as kind of a cornerstone to investigate this in more detail,” Frost says.

Even though HR 6819 doesn’t have the nearest black hole to Earth, it appears to have something more useful to astronomers. More

Huge rings of gas surround a large red star named V Hydrae, new images show, signaling its eventual transformation into a much smaller and bluer star.

“It’s definitely going through its metamorphosis,” says Raghvendra Sahai, an astronomer at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “Such ringlike structures have never been seen in any object like this before.”

Observations of the three concentric rings, all ejected from the star during the last 800 years, could help astronomers understand how giant stars lose mass toward the end of their lives and seed the cosmos with planet- and life-building elements.

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Born roughly twice as massive as the sun and lying about 1,300 light-years from Earth, V Hydrae is what’s known as an asymptotic giant branch star. It once fused hydrogen in its core, as the sun does. But now it is a cool, brilliant, puffed-up star that alternately burns hydrogen and helium in shells around a carbon-oxygen core. Such stars cast lots of material into space.

“The processes by which this happens are not well-understood,” says Sahai, who has studied V Hydrae since the 1980s.

His team used the Atacama Large Millimeter/submillimeter Array of radio telescopes in Chile, also known as ALMA, to detect the three rings of gas. Beyond them lie three additional rings, which are fainter and seen only partially, Sahai and colleagues report in a paper submitted February 18 at arXiv.org.

The outermost complete ring now sits about 260 billion kilometers from the star, or 1,740 times as far as Earth is from the sun — more than 40 times Pluto’s distance from Earth. By measuring the speed at which the three complete rings are moving outward and their current distances from the star, the astronomers calculate that it cast them off about 270, 485 and 780 years ago.

It’s thought that another star orbits the main one every few hundred years on an elliptical orbit. When the companion dives in, it can trigger the giant star to cast more material into space, the team says.

The new image is striking and unusual, and it illustrates how a companion star enhances a giant star’s loss of mass, says Joel Kastner, an astronomer at the Rochester Institute of Technology in New York who was not part of the study. “Mass loss is very important because it’s how the elements of life get distributed from stars into the universe.”

Stars like V Hydrae forged most of the nitrogen in Earth’s air as well as much of our planet’s carbon, the basis for all terrestrial life (SN: 2/12/21; SN: 11/18/21). V Hydrae has so many carbon compounds in its atmosphere that it’s classified as a carbon star. It’s also one of the reddest stars known because those compounds as well as dust particles absorb its blue and violet light.

Sahai expects the star’s ejection of material to continue, but, he says, “it’s anybody’s guess as to how many more rings will be produced.”

When the star loses all of its atmosphere, probably many thousands of years from now, it will expose its hot core, whose ultraviolet light will set the cast-off material aglow, creating a beautiful bubble of gas known as a planetary nebula.

When the nebula dissipates, all that will remain of the magnificent red star will be a tiny blue one — a white dwarf — a little larger than Earth, plus innumerable life-giving elements floating through the Milky Way. More

A new study casts a haze over a hint of the universe’s first glimmers of starlight.

In 2018, researchers claimed that a subtle signature in radio waves from early in the universe’s history had revealed the era when the first stars switched on, known as the cosmic dawn. But the first experiment to test that study’s conclusions found no sign of those early stars, scientists report February 28 in Nature Astronomy.

Just after the Big Bang about 13.8 billion years ago, the universe was a hot stew of matter. Stars probably didn’t flicker on until at least 100 million years later — a poorly understood era of the cosmos. Finding signs of the first beams of starlight would flesh out the cosmic origin story. So the 2018 claim of pinpointing those earliest gleams, from the EDGES experiment in the Australian outback, caused an astronomical hubbub (SN: 2/28/18).

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“It definitely completely excited our whole community with this fascinating result,” says radio astronomer Saurabh Singh of the Raman Research Institute in Bangalore, India.

The researchers reported detecting a dip across particular wavelengths of radio waves, a sign of light from the first stars interacting with surrounding hydrogen gas. But the result quickly raised skepticism, because the dip was deeper than expected. To know whether the hint of the first starlight was real, scientists would need to make more measurements.

Singh and colleagues did just that with the Shaped Antenna Measurement of the Background Radio Spectrum 3, or SARAS 3. Similar to EDGES, the experiment uses an antenna to pick up radio waves. But SARAS 3 has a different design from EDGES, with a differently shaped antenna. And SARAS 3 is designed to float atop a lake. “That gives us a very distinctive advantage,” Singh says.

On Earth, radio waves come from a variety of sources, which must be carefully accounted for to reveal the subtler signal from the cosmic dawn. Misunderstanding those other sources of radio waves could lead to an unaccounted-for experimental error that might give incorrect results.

In particular, experiments on land must contend with radio waves emitted from the ground, which are difficult to estimate due to the complex, layered nature of soil. When the antenna is atop a lake, it’s easier to estimate what kinds of radio waves come from the uniform water below. Data taken from two lakes in India revealed no sign of the dip.

The new study “highlights just how difficult this measurement is,” says physicist H. Cynthia Chiang of McGill University in Montreal. It’s uncomfortable that the two studies disagree, she says, but notes that the disagreement “isn’t quite enough to make any definitive conclusions at this point.”

And some of the same types of experimental issues that may affect EDGES could also affect SARAS 3, says experimental cosmologist Judd Bowman of Arizona State University in Tempe, a member of the EDGES team. “We still have more work ahead to reach the final outcome.”

An improved version of EDGES will be deployed later this year, and the SARAS 3 team has additional deployments planned. Other experiments are also working on similar measurements. Those tests may finally illuminate the universe’s transition from darkness to light. More

In a galaxy not so far away, astronomers have located a surprising source of a mysterious, rapid radio signal.

The signal, a repeating fast radio burst, or FRB, was observed over several months in 2021, allowing astronomers to pinpoint its location to a globular cluster — a tight, spherical cluster of stars — in M81, a massive spiral galaxy 12 million light-years away. The findings, published February 23 in Nature, are challenging astronomers’ assumptions of what objects create FRBs.

“This is a very revolutionary discovery,” says Bing Zhang, an astronomer at the University of Nevada, Las Vegas who was not involved in the study. “It is exciting to see an FRB from a globular cluster. That is not the favorited place people imagined.”

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Astronomers have been puzzling over these mysterious cosmic radio signals, which typically last less than a millisecond, since their discovery in 2007 (SN: 7/25/14). But in 2020, an FRB was seen in our own galaxy, helping scientists determine one source must be magnetars — young, highly magnetized neutron stars with magnetic fields a trillion times as strong as Earth’s (SN: 6/4/20).

The new findings come as a surprise because globular clusters harbor only old stars — some of the oldest in the universe. Magnetars, on the other hand, are young leftover dense cores typically created from the death of short-lived massive stars. The magnetized cores are thought to lose the energy needed to produce FRBs after about 10,000 years. Globular clusters, whose stars average many billions of years old, are much too elderly to have had a sufficiently recent young stellar death to create this type of magnetar. 

To pinpoint the FRB, astronomer Franz Kirsten and colleagues used a web of 11 radio telescopes spread across Europe and Asia to catch five bursts from the same source. Combining the radio observations, the astronomers were able to zero in on the signal’s origins, finding it was almost certainly from within a globular cluster.

“This is a very exciting discovery because it was completely unexpected,” says Kirsten, of ASTRON, the Netherlands Institute for Radio Astronomy, who is based at the Onsala Space Observatory in Sweden.

The new FRB might still be caused by a magnetar, the team proposes, but one that formed in a different way, such as from old stars common in globular clusters. For example, this magnetar could have been created from a remnant stellar core known as a white dwarf that had gathered too much material from a companion star, causing it to collapse.

“This is a [magnetar] formation channel that has been predicted, but it’s hard to see,” Kirsten says. “Nobody has actually seen such an event.”

Alternatively, the magnetar could have been formed from the merger of two stars — such as a pair of white dwarfs, a pair of neutron stars or one of each — in close orbit around one another, but this scenario is less likely, Kirsten says. It’s also possible the FRB source isn’t a magnetar at all but a very energetic millisecond pulsar, which is also a type of neutron star that could be found in a globular cluster, but one that has a weaker magnetic field.

To date, only a few FRB sources have been precisely pinpointed, and their locations are all in or close to star-forming regions in galaxies. Besides adding a new source for FRBs, the findings suggest that magnetars created from something other than the death of young stars might be more common than expected. 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

Strange giant planets known as hot Jupiters, which orbit close to their suns, got kicked onto their peculiar paths by nearby planets and stars, a new study finds.

After analyzing the orbits of dozens of hot Jupiters, a team of astronomers found a way to catch giant planets in the process of getting uncomfortably close to their stars. The new analysis, submitted January 27 to arXiv.org, pins the blame for the weird worlds on gravitational kicks from other massive objects orbiting the same star, many of which destroyed themselves in the process.

“It’s a pretty dramatic way to create your hot Jupiters,” says Malena Rice, an astrophysicist at Yale University.

Hot Jupiters have long been mysterious. They orbit very close to their stars, whirling around in a few days or less, whereas all the giant planets in our solar system lie at vast distances from the sun (SN: 6/5/17). To explain the odd planets, astronomers have proposed three main ideas (SN: 5/11/18). Perhaps the hot Jupiters formed next to their stars and stayed put, or maybe they started off farther out and then slowly spiraled inward. In either case, the planets should have circular orbits aligned with their stars’ equators, because the worlds inherited their paths from material in the protoplanetary disks that gave them birth.

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The new study, though, favors the third idea: Gravitational interactions with another giant planet or a companion star first hurl a Jupiter-sized planet onto a highly elliptical and inclined orbit that brings it close to its star. In some cases, the planet even revolves the wrong way around its star, opposite the way it spins.

In this scenario, every time the tossed planet sweeps past its sun, the star’s gravity robs the planet of orbital energy. This shrinks the orbit, gradually making it more circular and less inclined, until the planet becomes a hot Jupiter on a small, circular orbit, realigned to be in the same plane as the star’s equator.

Stars usually circularize a planet’s orbit before they realign it, and cool stars realign an orbit faster than warm stars do. So Rice and her colleagues looked for relationships between the shapes and tilts of the orbits of several dozen hot Jupiters that go around stars of different temperatures.

Generally speaking, the team found that the hot Jupiters around cool stars tend to be on well-aligned, circular orbits, whereas the hot Jupiters around warm stars are often on orbits that are elongated and off-kilter. Put another way, many of the orbits around warm stars haven’t yet had time to settle down into their final size and orientation. These orbits still bear the marks of having been shaped by gravitational run-ins with neighboring bodies in the system, the team concludes.

It’s a “simple, elegant argument,” says David Martin, an astrophysicist at Ohio State University in Columbus who was not involved with this study. “They’re presenting the evidence in a new way that helps strengthen” the idea that other massive objects in the same solar system produce hot Jupiters. He suspects this theory probably explains the majority of these planets.

But it means that innumerable giant worlds have suffered terrible fates. Some of the planets that hurled their brethren close to their stars ended up plunging into those same stars themselves, Rice says. And many other planets got ejected from their solar systems altogether, so today these wayward worlds wander the deep freeze of interstellar space, far from the light of any sun. More