Space & Astronomy

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