Space

We know quite a lot about stars. After centuries of pointing telescopes at the night sky, astronomers and amateurs alike can figure out key attributes of any star, like its mass or its composition.

To calculate a star’s mass, just look it its orbital period and do a bit of algebra. To determine what it’s made of, look to the spectrum of light the star emits. But the one variable scientists haven’t quite cracked yet is time.

“The sun is the only star we know the age of,” says astronomer David Soderblom of the Space Telescope Science Institute in Baltimore. “Everything else is bootstrapped up from there.”

Even well-studied stars surprise scientists every now and then. In 2019 when the red supergiant star Betelgeuse dimmed, astronomers weren’t sure if it was just going through a phase or if a supernova explosion was imminent. (Turns out it was just a phase.) The sun also shook things up when scientists noticed that it wasn’t behaving like other middle-aged stars. It’s not as magnetically active compared with other stars of the same age and mass. That suggests that astronomers might not fully understand the timeline of middle age.

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Calculations based on physics and indirect measurements of a star’s age can give astronomers ballpark estimates. And some methods work better for different types of stars. Here are three ways astronomers calculate the age of a star.

Hertzsprung-Russell diagrams

Scientists do have a pretty good handle on how stars are born, how they live and how they die. For instance, stars burn through their hydrogen fuel, puff up and eventually expel their gases into space, whether with a bang or a whimper. But when exactly each stage of a star’s life cycle happens is where things get complicated. Depending on their mass, certain stars hit those points after a different number of years. More massive stars die young, while less massive stars can burn for billions of years.

At the turn of the 20th century, two astronomers — Ejnar Hertzsprung and Henry Norris Russell — independently came up with the idea to plot stars’ temperature against their brightness. The patterns on these Hertzsprung-Russell, or H-R, diagrams corresponded to where different stars were in that life cycle. Today, scientists use these patterns to determine the age of star clusters, whose stars are thought to have all formed at the same time.

The caveat is that, unless you do a lot of math and modeling, this method can be used only for stars in clusters, or by comparing a single star’s color and brightness with theoretical H-R diagrams. “It’s not very precise,” says astronomer Travis Metcalfe of the Space Science Institute in Boulder, Colo. “Nevertheless, it’s the best thing we’ve got.”

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Measuring a star’s age isn’t as easy as you’d think. Here’s how scientists get their ballpark estimates.

Rotation rate

By the 1970s, astrophysicists had noticed a trend: Stars in younger clusters spin faster than stars in older clusters. In 1972, astronomer Andrew Skumanich used a star’s rotation rate and surface activity to propose a simple equation to estimate a star’s age: Rotation rate = (Age) -½.

This was the go-to method for individual stars for decades, but new data have poked holes in its utility. It turns out that some stars don’t slow down when they hit a certain age. Instead they keep the same rotation speed for the rest of their lives.

“Rotation is the best thing to use for stars younger than the sun,” Metcalfe says. For stars older than the sun, other methods are better.

Stellar seismology

The new data that confirmed rotation rate wasn’t the best way to estimate an individual star’s age came from an unlikely source: the exoplanet-hunting Kepler space telescope. Not just a boon for exoplanet research, Kepler pushed stellar seismology to the forefront by simply staring at the same stars for a really long time.

Watching a star flicker can give clues to its age. Scientists look at changes in a star’s brightness as an indicator of what’s happening beneath the surface and, through modeling, roughly calculate the star’s age. To do this, one needs a really big dataset on the star’s brightness — which the Kepler telescope could provide.

“Everybody thinks it was all about finding planets, which was true,” Soderblom says. “But I like to say that the Kepler mission was a stealth stellar physics mission.”

This approach helped reveal the sun’s magnetic midlife crisis and recently provided some clues about the evolution of the Milky Way. Around 10 billion years ago, our galaxy collided with a dwarf galaxy. Scientists have found that stars left behind by that dwarf galaxy are younger or about the same age as stars original to the Milky Way. Thus, the Milky Way may have evolved more quickly than previously thought.

As space telescopes like NASA’s TESS and the European Space Agency’s CHEOPS survey new patches of sky, astrophysicists will be able to learn more about the stellar life cycle and come up with new estimates for more stars.

Aside from curiosity about the stars in our own backyard, star ages have implications beyond our solar system, from planet formation to galaxy evolution — and even the search for extraterrestrial life.

“One of these days — it’ll probably be a while — somebody’s going to claim they see signs of life on a planet around another star. The first question people will ask is, ‘How old is that star?’” Soderblom says. “That’s going to be a tough question to answer.” More

Violent explosions of massive, magnetized stars may forge most of the universe’s heavy elements, such as silver and uranium.

These r-process elements, which include half of all elements heavier than iron, are also produced when neutron stars merge (SN: 10/16/17). But collisions of those dead stars alone can’t form all of the r-process elements seen in the universe. Now, scientists have pinpointed a type of energetic supernova called a magnetorotational hypernova as another potential birthplace of these elements.

The results, described July 7 in Nature, stem from the discovery of an elderly red giant star — possibly 13 billion years old — in the outer regions of the Milky Way. By analyzing the star’s elemental makeup, which is like a star’s genetic instruction book, astronomers peered back into the star’s family history. Forty-four different elements seen in the star suggest that it was formed from material left over “by a special explosion of one massive star soon after the Big Bang,” says astronomer David Yong of the Australian National University in Canberra.

The ancient star’s elements aren’t from the remnants of a neutron star merger, Yong and his colleagues say. Its abundances of certain heavy elements such as thorium and uranium were higher than would be expected from a neutron star merger. Additionally, the star also contains lighter elements such as zinc and nitrogen, which can’t be produced by those mergers. And since the star is extremely deficient in iron — an element that builds up over many stellar births and deaths — the scientists think that the red giant is a second-generation star whose heavy elements all came from one predecessor supernova-type event.

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Simulations suggest that the event was a magnetorotational hypernova, created in the death of a rapidly spinning, highly magnetized star at least 25 times the mass of the sun. When these stars explode at the end of their lives as a souped-up type of supernova, they may have the energetic, neutron-rich environments needed to forge heavy elements.

Magnetorotational hypernovas might be similar to collapsars — massive, spinning stars that collapse into black holes instead of exploding. Collapsars have previously been proposed as birthplaces of r-process elements, too (SN: 5/8/19).

The researchers think that magnetorotational hypernovas are rare, composing only 1 in 1,000 supernovas. Even so, such explosions would be 10 times as common as neutron star mergers today, and would produce similar amounts of heavy elements per event. Along with their less energetic counterparts, called magnetorotational supernovas, these hypernovas could be responsible for creating 90 percent of all r-process elements, the researchers calculate. In the early universe, when massive, rapidly rotating stars were more common, such explosions could have been even more influential.

The observations are impressive, says Stan Woosley, an astrophysicist at the University of California, Santa Cruz, who was not involved in the new study. But “there is no proof that the [elemental] abundances in this metal-deficient star were made in a single event. It could have been one. It could have been 10.” One of those events might even have been a neutron star merger, he says.

The scientists hope to find more stars like the elderly red giant, which could reveal how frequent magnetorotational hypernovas are. For now, the newly analyzed star remains “incredibly rare and demonstrates the need for … large surveys to find such objects,” Yong says. More

Moons do it, stars do it, even whole galaxies do it. Now, two teams of scientists say cosmic filaments do it, too. These tendrils stretching hundreds of millions of light-years spin, twirling like giant corkscrews.

Cosmic filaments are the universe’s largest known structures and contain most of the universe’s mass (SN: 1/20/14). These dense, slender strands of dark matter and galaxies connect the cosmic web, channeling matter toward galaxy clusters at each strand’s end (SN: 7/5/12).

At the instant of the Big Bang, matter didn’t rotate; then, as stars and galaxies formed, they began to spin. Until now, galaxy clusters were the largest structures known to rotate. “Conventional thinking on the subject said that’s where spin ends. You can’t really generate torques on larger scales,” says Noam Libeskind, cosmologist at the Leibniz Institute for Astrophysics Potsdam in Germany.

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So the discovery that filaments spin — at a scale that makes galaxies look like specks of dust — presents a puzzle. “We don’t have a full theory of how every galaxy comes to rotate, or every filament comes to rotate,” says Mark Neyrinck, cosmologist at University of the Basque Country in Bilbao, Spain.

To test for rotation, Neyrinck and colleagues used a 3-D cosmological simulation to measure the velocities of dark matter clumps as the clumps moved around a filament. He and his colleagues describe their results in a paper posted in 2020 at arXiv.org and now in press with the Monthly Notices of the Royal Astronomical Society. Meanwhile, Libeskind and colleagues searched for rotation in the real universe, they report June 14 in Nature Astronomy. Using the Sloan Digital Sky Survey, the team mapped galaxies’ motions and measured their velocities perpendicular to filaments’ axes.

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A computer simulation shows how a cosmic filament twists galaxies and dark matter into a strand of the cosmic web. Filaments pull matter into rotation and toward clusters at their ends, visualized here with “test particles” shaped like comets.  

The two teams detected similar rotational velocities for filaments despite differing approaches, Neyrinck says, an “encouraging [indication] that we’re looking at the same thing.”

Next, researchers want to tackle what makes these giant space structures spin, and how they get started. “What is that process?” Libeskind says. “Can we figure it out?” More

Earth is on an orderly path around the sun, orbiting in nearly the same plane as our star’s equator. In 2008, however, astronomers began finding worlds in other solar systems that sail far above and below their star’s equatorial plane.

Now a surprising discovery about these wrong-way worlds may eventually reveal their origin: Most of them follow polar orbits (SN: 6/17/16). If Earth had such an orbit, every year we’d pass over the sun’s north pole, dive through its equatorial plane, then pass below the sun’s south pole before coming back up again.

Astronomers Simon Albrecht and Marcus Marcussen at Aarhus University in Denmark and colleagues analyzed 57 planets in other solar systems for which the researchers could determine the true tilt between a planet’s orbit and its star’s equatorial plane. Two-thirds of the planets have normal orbits, tilted no more than 40 degrees, the team found. The other 19 planets are misaligned.

But the orbits of those misaligned planets don’t make just any old angle with their star’s equator. Instead, they pile up around 90 degrees. In fact, all but one of the misaligned planets are on polar orbits, having tilts from 80 to 125 degrees, the astronomers report online May 20 at arXiv.org.

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“It’s very, very strange,” says Amaury Triaud, an astronomer at the University of Birmingham in England who has found a number of misaligned planets but was not involved with the new study. “It’s a beautifully executed idea, and the result is most intriguing,” he says. “It’s so new and so weird.”

The result may lend insight into the biggest mystery about these planets: how they arose (SN: 10/18/13). Such worlds were a shock to astronomers, because planets form inside pancake-shaped disks of gas and dust orbiting in their stars’ equatorial planes. Thus, planets should lie near the plane of their sun’s equator, too. In our solar system, for example, Earth’s orbit tilts only 7 degrees from the solar equatorial plane, and even Pluto — which many astronomers no longer call a planet — has an orbit tilted only 12 degrees from that plane (and 17 degrees from the Earth’s orbital plane).

“At the moment, we are not sure what is the underlying mechanism” or mechanisms for creating misaligned planets, Albrecht admits. Whatever it is, though, it should account for the newly discovered plethora of perpendicular planets, he says.

A possible clue, Albrecht says, comes from the single exception to the rule: the one misaligned planet in the sample that is not on a polar orbit. This planet also happens to be the most massive in the sample, packing the mass of between five and eight Jupiters. Albrecht says that may be just a coincidence — or it may reveal something about how the other planets became misaligned.

In the future, the astronomers hope to understand how these wayward worlds acquired their odd orbits. All known misaligned planets orbit close to their stars, but are these worlds more likely than normal, close-in planets to have giant planets near them? The scientists don’t yet know, but if they find such a correlation, those companions may have somehow flung these bizarre worlds onto their peculiar planetary paths. More

China’s first Mars rover is taking in the view of its new home. The Zhurong rover touched down on the Red Planet on May 14, and its first images reached Earth on May 19.

Zhurong, named for an ancient Chinese god of fire, has been orbiting the Red Planet since February 10, when China’s Tianwen-1 spacecraft entered Martian orbit. The rover landed in a vast plain called Utopia Planitia — also where NASA’s Viking 2 lander touched down in 1976, although Viking 2’s site was much farther north (SN:  9/11/76).

The orbiter and rover together mark China’s first Mars mission and make China only the second country to successfully land a rover there. China has previously landed two rovers on the moon, named Yutu and Yutu-2, with the Chang’e-3 and Chang’e-4 missions (SN: 1/3/19).

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The Tianwen-1 orbiter captured a video of the lander and rover separating from the orbiter before plunging into the Martian atmosphere.

Unlike NASA’s Perseverance rover, which landed on Mars in February and beamed photos back almost immediately (SN: 2/17/21), Zhurong took a few days to send its first glimpses of the Martian surface back to Earth. That’s because the rover had to wait for the Tianwen-1 orbiter to move into a lower orbit to allow it to relay more data between Mars and Earth.

This image was taken with Zhurong’s rear navigation camera. It shows the rover’s solar panels and antenna.CNSA

The first images are from Zhurong’s hazard avoidance and navigation cameras. For now, the rover is still perched atop its landing platform. After several days looking around and checking out its instruments, Zhurong will roll down the lander’s ramps and onto the Martian soil, possibly on May 21 or 22, according to a report from China’s state-run Xinhua news agency after the landing.

Zhurong will spend at least three months studying the geology at Utopia Planitia and searching for water ice beneath the surface. The rover carries a ground-penetrating radar that can help distinguish between rock and ice beneath the surface, similar to a technique used by the Yutu-2 rover on the moon (SN: 2/26/20).  It also carries an instrument to analyze surface chemistry.

The Tianwen-1 orbiter will remain active for a full Martian year (about 687 Earth days), observing the ground from space with a high-resolution camera. More

The chemistry leading to life may start before stars are even born.

In the planet-forming disk of gas and dust around a young star, astronomers have detected methanol. The disk is too warm for the methanol to have formed there, so this complex organic molecule probably originated in the interstellar cloud that collapsed to form the star and its disk, researchers report online May 10 in Nature Astronomy. This finding offers evidence that at least some organic matter from interstellar space can seed the disks around newborn stars to provide potential ingredients for life on new planets.

“That’s pretty exciting, because it means that, in principle, all planets forming around any kind of star could have this material,” says Viviana Guzmán, an astrochemist at the Pontifical Catholic University of Chile in Santiago not involved in the work.

Complex organic molecules have been observed in interstellar clouds of gas and dust (SN: 3/22/21), as well as in planet-forming disks around young stars (SN: 2/18/08). But astronomers didn’t know whether organic material from interstellar space could survive the formation of a protoplanetary disk, or whether organic chemistry had to start from scratch around new stars.

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“When you form a star and its disk, it’s not a very easy, breezy process,” says Alice Booth, an astronomer at Leiden University in the Netherlands. Radiation from the new star and shock waves in the imploding material, she says, “could destroy a lot of the molecules that were originally in your initial cloud.”

Using the ALMA radio telescope array in Chile, Booth and colleagues observed the disk around a bright, young star named HD 100546, about 360 light-years away. There, the team spotted methanol, which is thought to be a building block for life’s molecules, such as amino acids and proteins.

Methanol could not have originated in the disk, because this molecule forms when hydrogen interacts with carbon monoxide ice, which freezes below temperatures of about –253° Celsius. The disk around HD 100546 is much warmer than that, heated by a star whose surface is roughly 9,700° C — some 4,000 degrees hotter than the sun. So the disk must have inherited its methanol from the interstellar cloud that forged its central star, the researchers conclude.

“This is the first evidence that the really interesting chemistry we see early on [in star formation] actually survives incorporation into the planet-forming disk,” says Karin Öberg, an astrochemist at Harvard University who was not involved in the work. Astronomers should next search the disks around other young stars for methanol or other organic molecules, she says, to “explore whether this is a one-time, get lucky kind of thing, or whether we can safely assume that planet-forming disks always inherit these kinds of molecules.” More

A rare glimpse of a star before it exploded in a fiery supernova looks nothing like astronomers expected, a new study suggests.

Images from the Hubble Space Telescope reveal that a relatively cool, puffy star ended its life in a hydrogen-free supernova. Until now, supernovas without hydrogen were thought to originate only from extremely hot, compact stars.

The discovery “is a very important test case for stellar evolution,” says Sung-Chul Yoon, an astrophysicist at Seoul National University in South Korea, who was not involved in the work. Theorists have some ideas about how massive stars behave right before they blow up, but such hefty stars are scant in the local universe and many are nowhere near ready to go supernova, Yoon says. Retroactively identifying the star responsible for a supernova provides an opportunity to test scenarios of how stars evolve right before exploding.

Finding those stars, however, is difficult, explains Charlie Kilpatrick, an astronomer at Northwestern University in Evanston, Ill. A telescope must have looked at that exact region of the sky in the years leading up to the supernova. And the explosion must have happened close enough for light from its much fainter source star to have reached a telescope.

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Although both conditions are tricky to meet, Kilpatrick is undaunted by the hunt. After scientists discovered a supernova in December 2019, in a galaxy called NGC 4666 about 46 million light-years away, he and colleagues rushed to check old Hubble observations from the same region of the sky. They wanted to find the star behind the explosion, dubbed SN 2019yvr.

After pouring over images and cross-checking observations with those from ground-based telescopes, the team found their quarry: a star at the same spot as the supernova, observed about 2.6 years before the explosion. It appeared to be a yellow star about 6,500° Celsius and about 320 times wider than the sun.

“I was kind of puzzled by all that,” Kilpatrick says. The supernova SN 2019yvr lacked hydrogen, so its progenitor was expected to be hydrogen-deficient, too. But “if a star lacks a hydrogen envelope, then you expect to be seeing deeper inside of the star to the hotter layers,” Kilpatrick says. That is, the star should have looked extremely hot and blue and compact — maybe 10,0000 to 50,000° C, and no more than 50 times wider than the sun. The cool, large, yellow progenitor of SN 2019yvr, on the other hand, appeared to be padded with lots of hydrogen. The researchers report the results May 5 in the Monthly Notices of the Royal Astronomical Society.

For this kind of star to have produced a supernova like SN 2019yvr, it must have shed much of its hydrogen before blowing up, Kilpatrick says. But how?

He and colleagues have come up with a couple scenarios. The star could have expelled much of its hydrogen into space through violent eruptions, possibly caused by some instability in the star’s core or interference from another star nearby. Or perhaps the star’s hydrogen could have been stripped off by another star that was in orbit around it.

To whittle these possibilities down, Jan Eldridge, an astrophysicist at the University of Auckland in New Zealand, suggests turning the Hubble telescope back on that area of the sky. Astronomers should first make sure that the star seen 2.6 years before SN 2019yvr really is gone now, says Eldridge, who was not involved in the work. Researchers could also check whether a star that once orbited SN 2019yvr’s progenitor still remains.

“They’ve found a mystery, and they’ve got some solutions,” Eldridge notes. Trying to figure out how such an unlikely star pulled off this particular supernova, she says, “is going to be fun.” More

Fourteen pinpricks of light on a gamma-ray map of the sky could fit the bill for antistars, stars made of antimatter, a new study suggests.

These antistar candidates seem to give off the kind of gamma rays that are produced when antimatter — matter’s oppositely charged counterpart — meets normal matter and annihilates. This could happen on the surfaces of antistars as their gravity draws in normal matter from interstellar space, researchers report online April 20 in Physical Review D.

“If, by any chance, one can prove the existence of the antistars … that would be a major blow for the standard cosmological model,” says Pierre Salati, a theoretical astrophysicist at the Annecy-le-Vieux Laboratory of Theoretical Physics in France not involved in the work. It “would really imply a significant change in our understanding of what happened in the early universe.”

It’s generally thought that although the universe was born with equal amounts of matter and antimatter, the modern universe contains almost no antimatter (SN: 3/24/20). Physicists typically think that as the universe evolved, some process led to matter particles vastly outnumbering their antimatter alter egos (SN: 11/25/19). But an instrument on the International Space Station recently cast doubt on this assumption by detecting hints of a few antihelium nuclei. If those observations are confirmed, such stray antimatter could have been shed by antistars.

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Intrigued by the possibility that some of the universe’s antimatter may have survived in the form of stars, a team of researchers examined 10 years of observations from the Fermi Gamma-ray Space Telescope. Among nearly 5,800 gamma-ray sources in the catalog, 14 points of light gave off gamma rays with energies expected of matter-antimatter annihilation, but did not look like any other known type of gamma-ray source, such as a pulsar or black hole.

Based on the number of observed candidates and the sensitivity of the Fermi telescope, the team calculated how many antistars could exist in the solar neighborhood. If antistars existed within the plane of the Milky Way, where they could accrete lots of gas and dust made of ordinary matter, they could emit lots of gamma rays and be easy to spot. As a result, the handful of detected candidates would imply that only one antistar exists for every 400,000 normal stars.

If, on the other hand, antistars tended to exist outside the plane of the galaxy, they would have much less opportunity to accrete normal matter and be much harder to find. In that scenario, there could be up to one antistar lurking among every 10 normal stars.

But proving that any celestial object is an antistar would be extremely difficult, because besides the gamma rays that could arise from matter-antimatter annihilation, the light given off by antistars is expected to look just like the light from normal stars. “It would be practically impossible to say that [the candidates] are actually antistars,” says study coauthor Simon Dupourqué, an astrophysicist at the Institute of Research in Astrophysics and Planetology in Toulouse, France. “It would be much easier to disprove.”

Astronomers could watch how gamma rays or radio signals from the candidates change over time to double-check that these objects aren’t really pulsars. Researchers could also look for optical or infrared signals that might indicate the candidates are actually black holes.

“Obviously this is still preliminary … but it’s interesting,” says Julian Heeck, a physicist at the University of Virginia in Charlottesville not involved in the work.

The existence of antistars would imply that substantial amounts of antimatter somehow managed to survive in isolated pockets of space. But Heeck doubts that antistars, if they exist, would be abundant enough to account for all the universe’s missing antimatter. “You would still need an explanation for why matter overall dominates over antimatter.” More