Astronomy

A brilliant blast from a galaxy 2 billion light-years away is the brightest cosmic “Cow” found yet. It’s the fifth known object in this new class of exploding stars and their long-glowing remnants, and it’s giving astronomers even more hints of what powers these mysterious blasts.

These Cow-like events, named for the first such object discovered in 2018 — which had the unique identifier name of AT2018cow — are a subclass of supernova explosions, making up only 0.1 percent of such cosmic blasts (SN: 6/21/19). They brighten quickly, glow brilliantly in ultraviolet and blue light and continue to show up for months in higher-energy X-rays and lower-energy radio waves.

X-rays from the newest discovery, dubbed AT2020mrf, glowed 20 times as bright as the original Cow a month after the blast, Caltech astronomer Yuhan Yao reported January 10 at a virtual news conference held by the American Astronomical Society. And even one year after this new object’s discovery, its X-rays were 200 times as bright as those from the original Cow. Yao and colleagues also reported the results in a paper submitted December 1 at arXiv.org.

Unraveling all that took a bit of time. The Zwicky Transient Facility at Caltech’s Palomar Observatory near San Diego, Calif., initially noted a bright new burst of light June 12, 2020, but astronomers didn’t realize what it was at the time.

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Then in April 2021, researchers with the Spektrum-Roentgen-Gamma (SRG) space telescope, which studies X-ray light, alerted Yao and her colleagues to an interesting signal in SRG data from July 21–24, 2020, at the same spot in the sky. “I almost immediately realized that this might be another Cow-like event,” says Yao. The astronomers sprang to action and looked at that location with multiple other observatories in different kinds of light.

One of those observatories was the space-based Chandra X-ray Observatory, the world’s most powerful X-ray telescope. In June 2021, a year after the original supernova blast, it captured X-rays from the same location. The source’s signal “was 10 times brighter than what I expected,” says Yao, and 200 times as bright as the original Cow was a year post-explosion.

Even more exciting was that the strengths of both the Chandra X-ray detection and the original SRG X-ray observations also changed within hours to days. That flaring characteristic, it turns out, can tell astronomers a lot.

“X-rays give us information of what’s happening at the heart of these events,” says MIT astrophysicist DJ Pasham, who has studied the original Cow but was not part of this new study. “The duration of the flare gives you a sense of how compact or how big the object is.”  

A compact object like an actively eating black hole or a rapidly spinning and highly magnetic neutron star would create the strong and variable X-ray signals that were seen, Yao says. These were the two most probable leftover remnants of the original cosmic Cow as well, but the AT2020mrf observations provide even greater certainty (SN: 12/13/21).

Further observations and catching these objects earlier in the act with multiple types of light will help researchers learn more about this new class of supernovas and what type of star eventually explodes as a Cow. More

A star’s death usually comes without warning. But an early sign of one star’s imminent demise hints at what happens before some stellar explosions.

In a last hurrah before exploding, a star brightened, suggesting that it blasted some of its outer layers into space. It’s the first time scientists have spotted a pre-explosion outburst from a run-of-the-mill type of exploding star, or supernova, researchers report in the Jan. 1 Astrophysical Journal.

Scientists have previously seen harbingers of unusual types of supernovas. But “what’s nice about this one is it’s a much more normal, vanilla … supernova that’s showing this eruption before explosion,” says astronomer Mansi Kasliwal of Caltech, who was not involved with the research.

On September 16, 2020, scientists discovered the explosion of a star roughly 10 times as massive as the sun, located about 120 million light-years away. Thankfully, telescopes that regularly survey a swath of the sky, as part of an effort called the Young Supernova Experiment, had been observing the star well before it detonated. About 130 days before the explosion, the star brightened, the researchers found, the start of a pre-explosion eruption.

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The final explosion was a commonplace type of stellar detonation called a type 2 supernova, which occurs when the core of an aging star collapses. Precursors to such explosions probably hadn’t been seen before because the early eruptions are faint. For this supernova, scientists had observations of the star sensitive enough to pick up the relatively weak eruption.

Previous post-explosion observations of such supernovas have hinted that the stars slough off layers before death. In 2021, astronomers reported signs of a supernova’s shock wave plowing into material that the star had expelled (SN: 11/2/21). A similar sign of cast-off stellar material was also found in the new study.

Scientists aren’t sure exactly what causes such early outbursts. They could be the result of events happening deep within a star, for example, as the star burns different types of fuel as it nears death. If more such events are found, scientists may eventually be able to predict which stars will go boom, and when.

Precursor outbursts are a sign that stars experience inner turmoil before exploding, says study coauthor Raffaella Margutti, an astrophysicist at the University of California, Berkeley. “The main message that we are getting from the universe is that these stars are really knowing that the end is coming.” More

Some of the same researchers who found the first purported exomoon now say that they’ve found another.

Dubbed Kepler 1708 b i, the satellite has a radius about 2.6 times that of Earth, and circles a Jupiter-sized exoplanet that orbits its parent star about once every two Earth years, the team reports January 13 in Nature Astronomy. That sunlike star lies about 5,700 light-years from Earth.

To find this nugget, the team sorted through a database of more than 4,000 exoplanets detected by NASA’s now-retired Kepler space telescope. Because large planets orbiting far from their parent star are more likely to have moons large enough to be detected, the team focused on a subset of 70 exoplanets.

Each of these planets is between half and twice the size of Jupiter. They all either take more than 400 Earth days to orbit their star or have an estimated average surface temperature less than 300 kelvins (around 27° Celsius), slightly higher than that of Earth.

After further screening, including tossing out exoplanets that don’t have near-circular orbits (which are statistically less likely to host moons), the team identified a strong candidate for an exomoon. It, like its host planet, caused detectable dimming of the parent star’s light when moving across the face of the star.

Discovery of the first possible exomoon, dubbed Kepler 1625 b, has faced a lot of skepticism (SN: 4/30/19). Both proposed exomoons need to be confirmed by further observations by other instruments, such as the recently launched James Webb Space Telescope, the team notes (SN: 10/6/21).

But fresh observations will need to wait: The newfound exomoon candidate and its planet won’t pass in front of the parent star again until March 24, 2023, the researchers calculate. More

A newborn star whizzing past another stellar youngster triggered a cosmic flare-up that began nearly a century ago and is still going strong today, researchers say.

In late 1936, a dim star in the constellation Orion started to erupt in our sky and soon shone over 100 times as brightly as it had before. Only telescopes could detect the star prior to the outburst, but afterward, the star was so bright it was visible through binoculars. The star even lit up part of the previously dark interstellar cloud called Barnard 35 that presumably gave the star birth (SN: 1/10/76).

Amazingly, the star, now named FU Orionis, is still shining almost as brightly today, 85 years later. That means the star wasn’t a nova, a stellar explosion that quickly fades from view (SN: 2/12/21). But the exact cause of the long-lasting flare-up has been a mystery.

Now, computer simulations may offer a clue to what’s kept the celestial beacon shining so brightly. Located about 1,330 light-years from Earth, FU Orionis is actually a double star, consisting of two separate stars that probably orbit each other. One is about as massive as the sun, while the other is only 30 percent to 60 percent as massive. Because the stars are so young, each has a disk of gas and dust revolving around it. It’s the lesser star’s passage through the other star’s disk that triggered and sustains the great flare-up, the simulations suggest.

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“The low-mass star is the one that is in outburst,” says Elisabeth Borchert, an astrophysicist at Monash University in Clayton, Australia.

According to Borchert’s team, the outburst arose as the low-mass star passed 10 to 20 times as far from its mate as the Earth is from the sun — comparable to the distance between the sun and Saturn or Uranus. As the lesser star plowed through the other star’s disk, gas and dust from that disk rained down onto the intruder. In the simulations, this material got hot and glowed profusely, making the low-mass star hundreds of times brighter, behavior that mimicked FU Orionis’ outburst.

The flare-up has endured so long because the gravitational pull of the lesser star captured material that began to orbit the star and is still falling onto it, the researchers explain in a paper submitted online November 24 at arXiv.org. The study will be published in Monthly Notices of the Royal Astronomical Society.

“It is a plausible explanation,” says Scott Kenyon, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., who was not involved with the study. The researchers “get a rise in luminosity about what the observations show,” he says, and “it lasts a long time.”

Kenyon says one way to test the team’s theory is to track how the two stars move relative to each other in the future. That may reveal whether the stars were as close together in 1936 as the simulations suggest. Astronomers discovered the binary nature of FU Orionis only two decades ago, by which time the stars were much farther apart in their elliptical orbit around each other.

Since the discovery of FU Orionis, several other newborn stars have flared up in a similar fashion. The binary model “could be a good explanation for all of them,” Borchert says, if those stars also have stellar companions that recently skirted past. More

The only known duo of pulsars has just revealed a one-of-a-kind heap of cosmic insights.

For over 16 years, scientists have been observing the pair of pulsars, neutron stars that appear to pulsate. The measurements confirm Einstein’s theory of gravity, general relativity, to new levels of precision, and hint at subtle effects of the theory, physicists report in a paper published December 13 in Physical Review X.

Pulsars, spinning dead stars made of densely packed neutrons, appear to blink on and off due to their lighthouse-like beams of radiation that sweep past Earth at regular intervals. Variations in the timing of those pulses can expose pulsars’ movements and effects of general relativity. While physicists have found plenty of individual pulsars, there’s only one known pair orbiting one another. The 2003 discovery of the double-pulsar system, dubbed J0737-3039, opened up a new world of possible ways to test general relativity.

One of the pulsars whirls around roughly 44 times per second while the other spins about once every 2.8 seconds. The slower pulsar went dark in 2008, due to a quirk of general relativity that rotated its beams out of view. But researchers kept monitoring the remaining visible pulsar, combining that new data with older observations to improve the precision of their measurements.

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Now, astrophysicist Michael Kramer of the Max Planck Institute for Radio Astronomy in Bonn, Germany, and colleagues have dropped an exhaustive paper that “just lays it all out,” says physicist Clifford Will of the University of Florida in Gainesville. “To me, it’s just magnificent.”

Here are five insights from the new study:

1. Einstein was right, in so many ways.

The pulsar duo allows for five independent tests of general relativity in one, checking whether various properties of the orbit match predictions of Einstein’s theory. For example, the researchers measure the rate at which the orbit’s ellipse rotates, or precesses, to see if it agrees with expectations. All of the parameters fell in line with Einstein.

What’s more, says astrophysicist Scott Ransom of the National Radio Astronomy Observatory in Charlottesville, Va., “each of the individual tests of general relativity have gotten so precise that …  higher-order effects of general relativity have to be included to match the data.” That means that the measurements are so exacting that they hint at subtle peculiarities of gravity. “All of those things are really amazing,” says Ransom, who was not involved with the research.

2. Gravitational waves are sapping energy.

The observations reveal that the pulsars’ orbit is shrinking. By measuring how long the pulsars take to complete each orbit, the researchers determined that the pair get about seven millimeters closer every day.

That’s because, as they orbit, the pulsars stir up gravitational waves, ripples in spacetime that vibrate outward, carrying away energy (SN: 12/18/15). This telltale shrinkage was seen for the first time in the 1970s in a system with one pulsar and one neutron star, providing early evidence for gravitational waves (SN: 12/16/78). But the new result is 25 times as precise as the earlier measurement.

3. The pulsar is losing mass and that matters.

There’s a subtler effect that tweaks that orbit, too. Pulsars gradually slow down over time, losing rotational energy. And because energy and mass are two sides of the same coin, that means the faster pulsar is losing about 8 million metric tons per second.

“When I realized that for the first time, it really blew me away,” says Kramer. Although it sounds like a lot, that mass loss equates to only a tiny adjustment of the orbit. Previously, scientists could neglect this effect in calculations because the tweak was so small. But the measurement of the orbit is now precise enough that it makes sense to include.

4. We can tell which way the pulsar spins and that hints at its origins.

By studying the timing of the pulses as the light from one pulsar passes by its companion, scientists can tell in what direction the faster pulsar is spinning. The results indicate that the pulsar rotates in the same direction as it orbits, and that provides clues to how the pulsar duo formed.

The two pulsars began as neighboring stars that exploded, one after the other. Often when a star explodes, the remnant it leaves behind gets kicked away, splitting apart such pairs. The fact that the faster pulsar spins in the same direction it orbits means the explosion that formed it didn’t give it much of a jolt, helping to explain how the union stayed intact.

5. We have a clue to the pulsar’s radius.

Gravitational effects are known to cause the orbit’s ellipse to precess, or rotate, by about 17 degrees per year. But there’s a subtle tweak that becomes relevant in the new study. The pulsar drags the fabric of spacetime behind it as it spins, like a twirling dancer’s twisting skirt, altering that precession.

This dragging effect implies that the faster pulsar’s radius must be less than 22 kilometers, an estimate that, if made more precise with future work, could help unveil the physics of the extremely dense neutron star matter that makes up pulsars (SN: 4/20/21).

“The authors have clearly been very meticulous in their study of this amazing system,” says astrophysicist Victoria Kaspi of McGill University in Montreal. “It is wonderful to see that the double pulsar … indeed is living up to its unique promise.” More

A cosmic flare-up called the Cow seems to have left behind a black hole or neutron star.

When the flash was spotted in June 2018, astronomers debated its origins. Now, astrophysicist DJ Pasham of MIT and colleagues have seen the first direct evidence of what the Cow left behind. “We may be seeing the birth of a black hole or neutron star,” Pasham says.  

The burst’s official, random designation is AT2018cow, but astronomers affectionately dubbed it the Cow. The light originated about 200 million light-years away and was 10 times as bright as an ordinary supernova, the explosion that marks the death of a massive star.

Astronomers thought the flare-up could have been from an unusual star being eaten by a black hole or from a weird sort of supernova that left behind a black hole or neutron star (SN: 6/21/19).

So Pasham and colleagues checked the Cow for flickering X-rays, which are typically produced close to a compact object, possibly in a disk of hot material around a black hole or on the surface of a neutron star.

Flickers in these X-rays can reveal the size of their source. The Cow’s X-rays flicker roughly every 4 milliseconds, meaning the object that produces them must be no more than 1,000 kilometers wide. Only a neutron star or a black hole fits the bill, Pasham and colleagues report December 13 in Nature Astronomy.

Because the Cow’s flash was from the explosion that produced either of these objects, a preexisting black hole was probably not responsible for the burst. Pasham admits he was hoping for a black hole eating an exotic star. “I was a little bit disappointed,” he says. “But I’m more blown away that this could be direct evidence of the birth of a black hole. This is an even cooler result.” More

The Milky Way has a “feather” in its cap.A long, thin filament of cold, dense gas extends jauntily from the galactic center, connecting two of the galaxy’s spiral arms, astronomers report November 11 in the Astrophysical Journal Letters. This is the first time that such a structure, which looks like the barb of a feather fanning off the central quill, has been spotted in the Milky Way.

The team that discovered our galaxy’s feather named it the Gangotri wave, after the glacier that is the source of India’s longest river, the Ganges. In Hindi and other Indian languages, the Milky Way is called Akasha Ganga, “the river Ganga in the sky,” says astrophysicist Veena V.S. of the University of Cologne in Germany.She and colleagues found the Gangotri wave by looking for clouds of cold carbon monoxide gas, which is dense and easy to trace, in data from the APEX telescope in San Pedro de Atacama, Chile. The structure stretches 6,000 to 13,000 light-years from the Norma arm of the Milky Way to a minor arm near the galactic center called the 3-kiloparsec arm. So far, all other known gas tendrils in the Milky Way align with the spiral arms (SN: 12/30/15).

The Gangotri wave has another unusual feature: waviness. The filament appears to wobble up and down like a sine wave over the course of thousands of light-years. Astronomers aren’t sure what could cause that, Veena says.

Other galaxies have gaseous plumage, but when it comes to the Milky Way, “it’s very, very difficult” to map the galaxy’s structure from the inside out, she says. She hopes to find more galactic feathers and other bits of our galaxy’s structure. “One by one, we’ll be able to map the Milky Way.” More

The next time you thank your lucky stars, you might want to bless the binaries. New calculations indicate that a massive star whose outer layer gets torn off by a companion star ends up shedding a lot more carbon than if the star had been born a loner.

“That star is making about twice as much carbon as a single star would make,” says Rob Farmer, an astrophysicist at the Max Planck Institute for Astrophysics in Garching, Germany.

All life on Earth is based on carbon, the fourth most abundant element in the cosmos, after hydrogen, helium and oxygen. Like nearly every chemical element heavier than helium, carbon is formed in stars (SN: 2/12/21). For many elements, astronomers have been able to pin down the main source. For example, oxygen comes almost entirely from massive stars, most of which explode, while nitrogen is made mostly in lower-mass stars, which don’t explode. In contrast, carbon arises both in massive and lower-mass stars. Astronomers would like to know exactly which types of stars forged the lion’s share of this vital element.

Farmer and his colleagues looked specifically at massive stars, which are at least eight times heavier than the sun, and calculated how they behave with and without partners. Nuclear reactions at the core of a massive star first turn hydrogen into helium. When the core runs out of hydrogen, the star expands, and soon the core starts converting helium into carbon.

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But massive stars usually have companion stars, adding a twist to the storyline: When the star expands, the companion’s gravity can tear off the larger star’s outer envelope, exposing the helium core. That allows freshly minted carbon to stream into space via a flow of particles.

“In these very massive stars, these winds are quite strong,” Farmer says. For instance, his team’s calculations indicate that the wind of a star born 40 times as massive as the sun with a close companion ejects 1.1 solar masses of carbon before dying. In comparison, a single star born with the same mass ejects just 0.2 solar masses worth of carbon, the researchers report in a paper submitted to arXiv.org October 8 and in press at the Astrophysical Journal.

If the massive star then explodes, it also can outperform a supernova from a solo massive star. That’s because, when the companion star removes the massive star’s envelope, the helium core shrinks. This contraction leaves some carbon behind, outside the core. As a result, nuclear reactions can’t convert that carbon into heavier elements such as oxygen, leaving more carbon to be cast  into space by the explosion. Had the star been single, the core would have destroyed much of that carbon.

By analyzing the output from massive stars of different masses, Farmer’s team concludes that the average massive star in a binary ejects 1.4 to 2.6 times as much carbon through winds and supernova explosions as the average massive star that’s single.

Given how many massive stars are in binaries, astronomer Stan Woosley says emphasizing binary-star evolution, as the researchers have done, is helpful in pinning down the origin of a crucial element. But “I think they are making too strong a claim based on models that may be sensitive to uncertain physics,” says Woosley, of the University of California, Santa Cruz. In particular, he says, mass-loss rates for massive stars are not known well enough to assert a specific difference in carbon production between single and binary stars.

Farmer acknowledges the uncertainty, but “the overall picture is sound,” he says. “The binaries are making more [carbon].” More