Space & Astronomy

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

The James Webb Space Telescope has finally arrived at its new home. After a Christmas launch and a month of unfolding and assembling itself in space, the new space observatory reached its final destination, a spot known as L2, on January 24.

But the telescope can’t start doing science yet. There are still several months’ worth of tasks on Webb’s to-do list before the telescope is ready to peep at the earliest light in the universe or spy on exoplanets’ alien atmospheres (SN: 10/6/21).

“That doesn’t mean there’s anything wrong,” says astronomer Scott Friedman of the Space Telescope Science Institute in Baltimore, who is managing this next phase of Webb’s journey. “Everything could go perfectly, and it would still take six months” from launch for the telescope’s science instruments to be ready for action, he says.

Here’s what to expect next.

Life at L2

L2, technically known as the second Earth-sun Lagrange point, is a spot about 1.5 million kilometers from Earth in the direction of Mars, where the sun and Earth’s gravity are of equal strength. Pairs of massive objects in space have five such Lagrange points, where the gravitational pushes and pulls from these celestial bodies essentially cancel each other out. That lets objects at Lagrange points stay put without much effort.

The telescope, also known as JWST, isn’t just sitting tight, though. It’s orbiting L2, even as L2 orbits the sun. That’s because L2 is not precisely stable, Friedman says. It’s like trying to stay balanced directly on top of a basketball. If you nudged an object sitting exactly at that point, it would be easy to make it wander off. Circling L2 as L2 circles the sun in a “halo orbit” is much more stable — it’s harder to fall off the basketball when in constant motion. But it takes some effort to stay there.

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“JWST and other astronomical satellites, which are said to be at L2 but are really in halo orbits, need propulsion to maintain their positions,” Friedman says. “For JWST, we will execute what we call station keeping maneuvers every 21 days. We fire our thrusters to correct our position, thus maintaining our halo orbit.”

The amount of fuel needed to maintain Webb’s home in space will set the lifetime of the mission. Once the telescope runs out of fuel, the mission is over. Luckily, the spacecraft had a near-perfect launch and didn’t use much fuel in transit to L2. As a result, it might be able to last more than 10 years, team members say, longer than the original five- to 10-year estimate.

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Webb’s final destination is a spot in space called L2, about 1.5 million kilometers away from Earth. The telescope will actually orbit L2 as L2 orbits the sun (as shown in this animation). This special “halo orbit” helps the spacecraft stay in place without burning much fuel.

Webb has one more feature that helps it stay stable. The telescope’s gigantic kitelike sunshield, which protects the delicate instruments from the heat and light of the sun, Earth and the moon, could pick up momentum from the stream of charged particles that constantly flows from the sun, like a solar sail. If so, that could push Webb off course. To prevent this, the telescope has a flap that acts as a rudder, said Webb sunshield manager Jim Flynn of Northrup Grumman in a January 4 news conference.

Cooling down

Webb sees in infrared light, wavelengths longer than what the human eye can see. But humans do experience infrared radiation as heat. “We’re essentially looking at the universe in heat vision,” says astrophysicist Erin Smith of NASA’s Goddard Space Flight Center in Greenbelt, Md., a project scientist on Webb.

That means that the parts of the telescope that observe the sky have to be at about 40 kelvins (–233° Celsius), which nearly matches the cold of space. That way, Webb avoids emitting more heat than the distant sources in the universe that the telescope will be observing, preventing it from obscuring them from view.

Most of Webb has been cooling down ever since the telescope’s sunshield unfurled on January 4. The observatory’s five-layer sunshield blocks and deflects heat and light, letting the telescope’s mirrors and scientific instruments cool off from their temperature at launch. The sunshield layer closest to the sun will warm to about 85° Celsius, but the cold side will be about –233° Celsius, said Webb’s commissioning manager Keith Parrish in a January 4 webcast.

“You could boil water on the front side of us, and on the backside of us, you’re almost down to absolute zero,” Parrish said.

One of the instruments, MIRI, the Mid-Infrared Instrument, has extra coolant to bring it down to 6.7 kelvins (–266° Celsius) to enable it to see even dimmer and cooler objects than the rest of the telescope. For MIRI, “space isn’t cold enough,” Smith says.

Aligning the mirrors

Webb finished unfolding its 6.5-meter-wide golden mirror on January 8, turning the spacecraft into a true telescope. But it’s not done yet. That mirror, which collects and focuses light from the distant universe, is made up of 18 hexagonal segments. And each of those segments has to line up with a precision of about 10 or 20 nanometers so that the whole apparatus mimics a single, wide mirror.

Starting on January 12, 126 tiny motors on the back of the 18 segments started moving and reshaping them to make sure they all match up. Another six motors went to work on the secondary mirror, which is supported on a boom in front of the primary mirror.

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Before the James Webb Space Telescope can start observing the universe, all 18 segments of its primary mirror need to act as one 6.5-meter mirror. This animation shows the mirror segments moving, tilting and bending to bring 18 separate images of a star (light dots) together into a single, focused image.

This alignment process will take until at least April to finish. In part, that’s because the movements are happening while the mirror is cooling. The changing temperature changes the shape of the mirrors, so they can’t be put in their final alignment until after the telescope’s suite of scientific instruments are fully chilled.

Once the initial alignment is done, light from distant space will first bounce off the primary mirror, then the secondary mirror and finally reach the instruments that will analyze the cosmic signals. But the alignment of the mirror segments is “not just right now, it’s a continuous process, just to make sure that they’re always perfectly aligned,” Scarlin Hernandez, a flight systems engineer at the Space Telescope Science Institute in Baltimore said at a NASA Science Live event on January 24. The process will continue for the telescope’s lifetime.

Calibrating the science instruments

While the mirrors are aligning, Webb’s science instruments will turn on. Technically, this is when Webb will take its first pictures, says astronomer Klaus Pontoppidan, also of the Space Telescope Science Institute. “But they’re not going to be pretty,” Pontoppidan says. The telescope will first test its focus on a single bright star, bringing 18 separate bright dots into one by tilting the mirrors.

After a few final adjustments, the telescope will be “performing as we want it to and presenting beautiful images of the sky to all the instruments,” Friedman says. “Then they can start doing their work.”

These instruments include NIRCam, the primary near-infrared camera that will cover the range of wavelengths from 0.6 to 5 micrometers. NIRCam will be able to image the earliest stars and galaxies as they were when they formed at least 12 billion years ago, as well as young stars in the Milky Way. The camera will also be able to see objects in the Kuiper Belt at the edge of the solar system and is equipped with a coronagraph, which can block light from a star to reveal details of dimmer exoplanets orbiting it.

Next up is NIRSpec, the near-infrared spectrograph, which will cover the same range of light wavelengths as NIRCam. But instead of collecting light and turning it into an image, NIRSpec will split the light into a spectrum to figure out an object’s properties, such as temperature, mass and composition. The spectrograph is designed to observe 100 objects at the same time.

MIRI, the mid-infrared instrument, is kept the coldest to observe in the longest wavelengths, from 5 to 28 micrometers. MIRI has both a camera and a spectrograph that, like NIRCam and NIRSpec, will still be sensitive to distant galaxies and newborn stars, but it will also be able to spot planets, comets and asteroids.

And the fourth instrument, called the FGS/NIRISS, is a two-parter. FGS is a camera that will help the telescope point precisely. And NIRISS, which stands for near-infrared imager and slitless spectrograph, will be specifically used to detect and characterize exoplanets.

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The James Webb Space Telescope’s science instruments are stored behind the primary mirror (as shown in this animation). Light from distant objects hits the primary mirror, then the secondary mirror in front of it, which focuses the light onto the instruments.

First science targets

It will take at least another five months after arriving at L2 to finish calibrating all of those science instruments, Pontoppidan says. When that’s all done, the Webb science team has a top secret plan for the first full color images to be released.

“These are images that are meant to demonstrate to the world that the observatory is working and ready for science,” Pontoppidan says. “Exactly what will be in that package, that’s a secret.”

Partly the secrecy is because there’s still some uncertainty in what the telescope will be able to look at when the time comes. If setting up the instruments takes longer than expected, Webb will be in a different part of its orbit and certain parts of the sky will be out of view for a while. The team doesn’t want to promise something specific and then be wrong, Pontoppidan says.

But also, “it’s meant to be a surprise,” he says. “We don’t want to spoil that surprise.”

Webb’s first science projects, however, are not under wraps. In the first five months of observations, Webb will begin a series of Early Release Science projects. These will use every feature of every instrument to look at a broad range of space targets, including everything from Jupiter to distant galaxies and from star formation to black holes and exoplanets.

Still, even the scientists are eager for the pretty pictures.

“I’m just very excited to get to see those first images, just because they will be spectacular,” Smith says. “As much as I love the science, it’s also fun to ooh and ahh.”    More

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