Space & Astronomy

The most realistic computer simulation of star formation yet offers stunning views of what the inside of a stellar nursery might look like.

In the Star Formation in Gaseous Environments simulation, or STARFORGE, a giant virtual cloud of gas collapses into a nest of new stars. Unlike other simulations, which could render only a small clump of gas within a larger cloud, STARFORGE simulates an entire star-forming cloud. It’s also the first simulation to account for the whole medley of physical phenomena thought to influence star formation, researchers report online May 17 in Monthly Notices of the Royal Astronomical Society.

“We sort of know the basic story of star formation … but the devil is in the details,” says Mike Grudić, a theoretical astrophysicist at Northwestern University in Evanston, Ill. (SN: 4/21/20). Astronomers still don’t fully understand, for instance, why stars have different masses. “If you really want to get the full picture, then you really have to just simulate the whole thing.”

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In the computer simulation STARFORGE, a massive cloud of cosmic gas — roughly 20 parsecs, or 65 light-years, across — collapses to form new stars. White areas indicate denser regions of gas, including baby stars. Orange highlights places where there’s lots of variation in the gas motion, such as in powerful jets launched by new stars. Gas shown in purple is more tranquil. After 4.3 million years (Myr) have passed, the simulation pauses so the virtual camera can swoop around the cloud, revealing its 3-D structure.

STARFORGE starts with a blob of gas that can be tens to hundreds of light-years across and up to millions of times the mass of the sun. Turbulence inside the cloud creates dense pockets that collapse to forge new stars. Those stars then launch powerful jets, give off radiation, shed stellar winds and explode in supernovas. Eventually, these phenomena blow the last vestiges of the cloud away and leave behind a hive of young stars. The whole process takes millions of years — or months of computing time, even running on supercomputers.

Using STARFORGE, Grudić and colleagues have confirmed that jets launched by new stars help regulate how much material a star amasses. In simulations without jets, typical stars were about 10 times the mass of the sun — way bigger than the actual average star. “As soon as you add this jet feedback to your simulation,” Grudić says, “stellar masses start coming out more or less right on the dot for what they’re observed to be.”

The STARFORGE simulation has helped confirm that jets launched by newborn stars (simulated one shown) determine how much mass stars can accrete.Northwestern University, University of Texas at Austin

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The Milky Way as we know it today was shaped by a collision with a dwarf galaxy about 10 billion years ago. But most of the modern galaxy was already in place even at that early date, new research shows.

Ages of stars left behind by the galactic interloper are a bit younger or on par with stars in the Milky Way’s main disk, researchers report May 17 in Nature Astronomy. And that could mean that the Milky Way grew up faster than astronomers expected, says study author Ted Mackereth, an astrophysicist at the University of Toronto.

“The Milky Way had already built up a lot of itself before this big merger happened,” he says.

Our galaxy’s history is one of violent conquest. Like other giant spiral galaxies in the universe, the Milky Way probably built up its bulk by colliding and merging with smaller galaxies over time. Stars from the unfortunate devoured galaxies got mixed into the Milky Way like cream into coffee, making it difficult to figure out what the galaxies were like before they merged.

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In 2018, astronomers realized that they could identify stars from the last major merger using detailed maps of several million stars from the European Space Agency’s Gaia spacecraft (SN: 5/9/18). Streams of stars orbit the galactic center at an angle to the main disk of stars. Those stars’ motions and chemistries suggest they once belonged to a separate galaxy that plunged into the Milky Way about 10 billion years ago (SN: 11/1/2018).

“Those stars are left there like fossil remnants of the galaxy,” Mackereth says.

Two groups discovered evidence of the ancient galaxy at around the same time. One called the galaxy Gaia-Enceladus; the other group called it the Sausage. The name that stuck was Gaia-Enceladus/Sausage.

Mackereth and his colleagues wondered if they could figure out how well developed the Milky Way was when Gaia-Enceladus/Sausage came crashing in. If the oldest stars in the Milky Way’s disk formed after this merger, then they probably formed as a result of this collision, suggesting that Gaia-Enceladus/Sausage met a proto–Milky Way that still had a lot of growing up to do. On the other hand, if the oldest stars are about the same age or older than the stars from the galactic interloper, then our galaxy was probably pretty well developed at the time of the run-in. 

Previous researchers had made estimates. But Mackereth and his colleagues used a precise tool called asteroseismology to figure out the ages of individual stars from both the Milky Way and from Gaia-Enceladus/Sausage (SN: 8/2/19). Just like seismologists on Earth use earthquakes to probe the interior of our planet, asteroseismologists use variations in brightness caused by starquakes and other oscillations to probe the innards of stars.

“Asteroseismology is the only way we have to access the internal part of the stars,” says physicist and study coauthor Josefina Montalbán of the University of Birmingham in England. From intel on the star’s interior structures, researchers can deduce the stars’ ages.

The team selected about 95 stars that had been observed by NASA’s exoplanet-hunting Kepler space telescope, which ended its mission in 2018 (SN: 10/30/18). Six of those stars were from Gaia-Enceladus/Sausage, and the rest were from the Milky Way’s thick disk. By measuring how the brightnesses of those stars fluttered over time, Mackereth and colleagues deduced ages with about 11 percent precision.

The Gaia-Enceladus/Sausage stars are slightly younger than the Milky Way stars, but all were pretty close to 10 billion years old, the team found. That suggests that a large chunk of the Milky Way’s disk was already in place when Gaia-Enceladus/Sausage came crashing through. It’s still possible that the incoming galaxy sparked the formation of some new stars, though, Mackereth says. To tell how much, they’ll need to get ages of a lot more stars.

Measuring ages for individual stars represents a step forward for galactic astronomy, says astrophysicist Tomás Ruiz-Lara of the University of Groningen, the Netherlands, who studies galactic evolution but was not involved in the new work.

“If you cannot tell the difference between a kid and a teenager and an adult, then we cannot say anything” about a population of people, Ruiz-Lara says. “But if I can distinguish between someone in his 40s or her 50s, you have a better graph of society. With the stars, it’s the same. If we are able to distinguish the age properly, then we can distinguish individual events in the history of the galaxy. In the end, that’s the goal.” More

A smattering of plutonium atoms embedded in Earth’s crust are helping to resolve the origins of nature’s heaviest elements.

Scientists had long suspected that elements such as gold, silver and plutonium are born during supernovas, when stars explode. But typical supernovas can’t explain the quantity of heavy elements in our cosmic neighborhood, a new study suggests. That means other cataclysmic events must have been major contributors, physicist Anton Wallner and colleagues report in the May 14 Science.

The result bolsters a recent change of heart among astrophysicists. Standard supernovas have fallen out of favor. Instead, researchers think that heavy elements are more likely forged in collisions of two dense, dead stars called neutron stars, or in certain rare types of supernovas, such as those that form from fast-spinning stars (SN: 5/8/19).

Heavy elements can be produced via a series of reactions in which atomic nuclei swell larger and larger as they rapidly gobble up neutrons. This series of reactions is known as the r-process, where “r” stands for rapid. But, says Wallner, of Australian National University in Canberra, “we do not know for sure where the site for the r-process is.” It’s like having the invite list for a gathering, but not its location, so you know who’s there without knowing where the party’s at.

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Scientists thought they had their answer after a neutron star collision was caught producing heavy elements in 2017 (SN: 10/16/17). But heavy elements show up in very old stars, which formed too early for neutron stars to have had time to collide. “We know that there has to be something else,” says theoretical astrophysicist Almudena Arcones of the Technical University of Darmstadt, Germany, who was not involved with the new study.

If an r-process event had recently happened nearby, ­some of the elements created could have landed on Earth, leaving fingerprints in Earth’s crust. Starting with a 410-gram sample of Pacific Ocean crust, Wallner and colleagues used a particle accelerator to separate and count atoms. Within one piece of the sample, the scientists searched for a variety of plutonium called plutonium-244, which is produced by the r-process. Since heavy elements are always produced together in particular proportions in the r-process, plutonium-244 can serve as a proxy for other heavy elements. The team found about 180 plutonium-244 atoms, deposited into the crust within the last 9 million years.

Scientists analyzed a sample of Earth’s deep-sea crust (shown) to search for atoms of plutonium and iron with cosmic origins.Norikazu Kinoshita

Researchers compared the plutonium count to atoms that had a known source. Iron-60 is released by supernovas, but it is formed by fusion reactions in the star, not as part of the r-process. In another, smaller piece of the sample, the team detected about 415 atoms of iron-60.

Plutonium-244 is radioactive, decaying with a half-life of 80.6 million years. And iron-60 has an even shorter half-life of 2.6 million years. So the elements could not have been present when the Earth formed, 4.5 billion years ago. That suggests their source is a relatively recent event. When the iron-60 atoms were counted up according to their depth in the crust, and therefore how long ago they’d been deposited, the scientists saw two peaks at about 2.5 million years ago and at about 6.5 million years ago, suggesting two or more supernovas had occurred in the recent past.

The scientists can’t say if the plutonium they detected also came from those supernovas. But if it did, the amount of plutonium produced in those supernovas would be too small to explain the abundance of heavy elements in our cosmic vicinity, the researchers calculated. That suggests regular supernovas can’t be the main source of heavy elements, at least nearby.  

That means other sources for the r-process are still needed, says astrophysicist Anna Frebel of MIT, who was not involved with the research. “The supernovae are just not cutting it.”

The measurement gives a snapshot of the r-process in our corner of the universe, says astrophysicist Alexander Ji of Carnegie Observatories in Pasadena, Calif. “It’s actually the first detection of something like this, so that’s really, really neat.” More

One of Saturn’s rings has revealed properties of its core, hidden deep beneath the planet’s golden atmosphere.

That core isn’t the lump of rock and ice that many scientists had envisioned, the new study finds. Instead, the core is diffuse, pervaded by huge amounts of hydrogen and helium and so spread out that it spans 70,000 kilometers, or about 60 percent of the planet’s diameter, researchers report April 28 at arXiv.org.

The new intel should help planetary scientists better understand not only how giant planets formed in our solar system but also the nature of such worlds orbiting other stars.  

To ascertain the structure of Saturn’s core, astronomer Christopher Mankovich and astrophysicist Jim Fuller, both at Caltech, examined the giant planet’s rings. Just as earthquakes help seismologists probe Earth’s interior, oscillations inside Saturn can reveal its internal composition. These oscillations alter Saturn’s gravitational forces, inducing waves in the rings —especially the C ring, which is the nearest of the three main rings to the planet (SN: 1/22/19).

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By analyzing a wave in that ring, along with data on Saturn’s gravity field from the now-defunct Cassini spacecraft (SN: 9/15/17), Mankovich and Fuller found that the core has about 17 Earth masses of rock and ice. But there’s so much hydrogen and helium mixed in, the core encompasses 55 Earth masses altogether — more than half of Saturn’s total, which is equivalent to the mass of 95 Earths. This “ring seismology” work will appear in a future Nature Astronomy.

“It’s a new way to look at gas giant planets in the solar system,” says Ravit Helled, a planetary scientist at the University of Zurich who was not involved with the work. “This knowledge is important because it reflects on our understanding of giant exoplanets,” and indicates that giant planets in other solar systems probably have more complex structures than many researchers had thought.

The discovery also illuminates how Saturn formed, says Nadine Nettelmann, a planetary scientist at the German Aerospace Center in Berlin.

Older theories posited that a gas giant such as Saturn arises when rock and ice orbiting the sun start to conglomerate. Tenuous gaseous envelopes let additional solid materials sink to the center, forming a compact core. Only later, according to this theory, does the core attract lots of hydrogen and helium — the ingredients that make up most of the planet. Although these elements are gases on Earth, Saturn’s great gravity squeezes most of them into a fluid.

But newer theories say instead that plenty of gas got incorporated into the core of rock and ice when it was taking shape 4.6 billion years ago. As the planet accreted additional mass, the proportion of gas rose. The structure Mankovich and Fuller deduce for Saturn’s core preserves this formation history, Nettelmann says, because the planet’s very center, representing the oldest part of Saturn, has the greatest proportion of rock and ice. The fraction of rock and ice decrease gradually rather than abruptly from the core’s center to its edge, reflecting the core’s development over time.

“I find the conclusions very important and very exciting and the line of reasoning very convincing,” Nettelmann says. Still, she cautions that additional waves in the rings should be analyzed for confirmation.

The type of oscillation that Mankovich and Fuller detect inside Saturn also implies that the core is stable rather than bubbling like a pot of water on a hot stove, which is one way a planet can carry heat from its hot interior outward. The core’s stability may help explain a long-standing puzzle: why Saturn emits more energy than it gets from the sun.

After the planet formed, it was warm with the heat of its birth, but then it cooled off. The core’s stability could have put a lid on some of this cooling, however, which helped the planet retain heat that it still radiates to this day. In contrast, if the core had instead transported heat via the upwelling and downwelling of material, the planet would have cooled off faster and no longer give off so much heat. More