Astronomy

The Milky Way’s core harbors two giants: the galaxy’s largest black hole and a cluster of tens of millions of stars around the black hole that is denser and more massive than any other star cluster in the galaxy.
Most of the cluster’s many stars shine within just 20 light-years of the galactic center and all together weigh about 25 million times as much as the sun. New observations suggest that this “nuclear star cluster” owes some of its brilliance to another big group of stars, or even a small galaxy, that the main cluster swallowed.
Nuclear star clusters exist in many galaxies and are the densest star clusters in the universe. Astronomers are trying to figure out how these gatherings get so jam-packed and how they feed the giant black holes at the centers of galaxies.
To get a look at the Milky Way’s core, Tuan Do, an astronomer at UCLA, and colleagues observed about 700 red giant stars within five light-years of the galaxy’s heart. Because dust between Earth and the galactic center blocks the stars’ visible light, the astronomers studied infrared wavelengths, which better penetrate the dust.

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“We noticed a very curious thing about our data, which is that the stars with less metals than our sun seem to be moving differently than the stars with more metals,” Do says.
About 7 percent of the stars in the nuclear star cluster revolve around the galactic center faster than their peers and do so around a different axis, the team found. The data on infrared wavelengths indicate that this fast-revolving population is only 30 percent as metal-rich as the sun. In contrast, most of the other stars in the nuclear star cluster have more metals than the sun.
“This discovery shows that at least some of our nuclear star cluster must have been formed from things falling in,” Do says. A metal-poor star cluster thousands of light-years away from the galactic core probably sank into the main star cluster, he and his colleagues report online September 28 in the Astrophysical Journal Letters.
Do says the infalling star cluster was the victim of dynamical friction, a process that can alter a star cluster’s path through space. In this process, the orbiting star cluster’s gravity attracts material that forms a wake behind it. The backward tug of this material’s gravity then causes the cluster to plunge closer and closer to the galactic center.
Scott Tremaine, an astrophysicist at the Institute for Advanced Study in Princeton, N.J., who was not involved in the work, calls the team’s data on the nuclear cluster’s stars unique. “I think by far the most natural explanation is that [the stars] do come from a cluster that’s spiraled in,” he says.
In a companion study, team member Manuel Arca Sedda at Heidelberg University in Germany and colleagues ran computer models to simulate how a star cluster falling into the Milky Way’s nuclear star cluster could explain the new observations. These simulations indicate that such an event occurred less than 3 billion years ago, and that the devoured cluster was roughly a million times as massive as the sun, the researchers report in a second study also published September 28 in the Astrophysical Journal Letters.
That mass is comparable to Omega Centauri, the Milky Way’s most massive globular cluster, a type of star grouping that’s dense but less extreme than nuclear star clusters. “It’s definitely a lot,” Do says. Just a dozen or so massive globular clusters could have populated the entire nuclear star cluster, he says.
Still, many of the nuclear star cluster’s other stars may have been born in place at the galactic center. And the scientists can’t rule out that the gobbled-up victim was a dwarf galaxy. Both dwarf galaxies and globular clusters can possess a similar number of stars. But their stars have different ratios of chemical elements, so future observations of the nuclear star cluster may be able to distinguish between the two scenarios. More

The sun’s wispy upper atmosphere, called the corona, is an ever-changing jungle of sizzling plasma. But mapping the strength of the magnetic fields that largely control that behavior has proved elusive. The fields are weak and the brightness of the sun outshines its corona.
Now though, observations taken using a specialized instrument called a coronagraph to block out the sun’s bright disk have allowed solar physicists to measure the speed and intensity of waves rippling through coronal plasma (SN: 3/19/09). “This is the first time we’ve mapped the coronal magnetic field on a large scale,” says Steven Tomczyk, a solar physicist at the High Altitude Observatory in Boulder, Colo., who designed the coronagraph.
In 2017, Tomczyk had been part of a team that took advantage of a total solar eclipse crisscrossing North America to take measurements of the corona’s magnetic field (SN: 8/16/17). He trekked to a mountaintop in Wyoming with a special camera to snap polarized pictures of the corona just as the moon blocked the sun.  (I was there with them, reporting on the team’s efforts to help explain why the corona is so much hotter than the sun’s surface (SN: 8/21/17).) The team observed a tiny slice of the corona to test whether a particular wavelength of light could carry signatures of the corona’s magnetic field. It can (SN: 8/21/18).
But it’s the observations from the coronagraph, made in 2016, that allowed researchers to look at the whole corona all at once. Theorists had shown decades ago that coronal waves’ velocities can be used to infer the strength of the magnetic field. Such waves might also help carry heat from the sun’s surface into the corona (SN: 11/14/19). But no one had measured them across the whole corona before.

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The corona’s magnetic field strength is mostly between 1 and 4 gauss, a few times the strength of the Earth’s magnetic field at the planet’s surface, the researchers report in the Aug. 7 Science.
Making a map is a big step, the team says. But what solar physicists would really like to do is track the corona’s magnetic field continuously, at least once a day.
“The solar magnetic field is evolving all the time,” says solar physicist Zihao Yang of Peking University in Beijing. Sometimes the sun releases magnetic energy explosively, sending bursts of plasma can shooting out into space (SN: 3/7/19). Those ejections can wreak havoc on satellites or power grids when they strike Earth. Continuously monitoring coronal magnetism can help predict those outbursts. “Our work demonstrated that we can use this technique to map the global distribution of coronal magnetic field, but we only showed one map from a single dataset,” Yang says.
Measuring the strength of the corona’s magnetic field is “a really big deal,” says solar physicist Jenna Samra of the Smithsonian Astrophysical Observatory in Cambridge, Mass. “Making global maps of the coronal magnetic field strength … is what’s going to allow us to eventually get better predictions of space weather events,” she says. “This is a really nice step in that direction.”
Tomczyk and colleagues are working on an upgraded version of the coronagraph, called COSMO, for Coronal Solar Magnetism Observatory, that would use the same technique repeatedly with the ultimate goal of predicting the sun’s behavior.
“It’s a milestone to do it,” Tomczyk says. “The goal is to do it regularly, do it all the time.” More

Like the mythical half-human, half-horse creatures, centaurs in the solar system are hybrids between asteroids and comets. Now, astronomers have caught one morphing from one type of space rock to the other, potentially giving scientists an unprecedented chance to watch a comet form in real time in the decades to come.
“We have an opportunity here to see the birth of a comet as it starts to become active,” says planetary scientist Kat Volk of the University of Arizona in Tucson.
The object, called P/2019 LD2, was discovered by the ATLAS telescope in Hawaii in May. Its orbit suggests that it’s a centaur, a class of rocky and icy objects with unstable orbits. Because of that mixed composition and potential to move around the solar system, astronomers have long suspected that centaurs are a missing link between small icy bodies in the Kuiper Belt beyond Neptune and comets that regularly visit the inner solar system (SN: 11/19/94).
These “short-period” comets, which are thought to originate from icy objects in the Kuiper Belt, orbit the sun once a decade or so, and make repeat appearances in Earth’s skies. (Long-period comets, like Halley’s Comet, which visits the inner solar system once a century, probably originate even farther from the sun, in the Oort cloud (SN: 10/25/13).)

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All previously found short-period comets were spotted only after they had transitioned into comets (SN: 8/6/14). But LD2 just came in from the Kuiper Belt recently and will become a comet in as little as 43 years, Volk and colleagues report August 10 at arXiv.org.
“It’s weird to think that this object should be becoming a comet when I’m retiring,” Volk says.
In 2019, she and colleagues showed that there’s a region of space just beyond Jupiter that they call the “Gateway”.  In this area, small planetary objects hang out while warming up and transitioning from outer solar system ice balls to inner solar system comets with their long tails. It’s like a comet incubator, says planetary scientist Gal Sarid of the SETI Institute, who is based in Rockville, Md.
After hearing about LD2, Volk, Sarid and their colleagues simulated thousands of possible trajectories to see where the object had been and where it is going. LD2’s orbit probably took it near Saturn around 1850, and it entered its current orbit past Jupiter after a close encounter with the gas giant in 2017, the team found. The object will leave its present orbit and move in toward the sun in 2063, where heat from the sun will probably sublimate LD2’s volatile elements, giving it a bright cometary tail, the researchers say.
“This will be the first ever comet that we know its history, because we’ve seen it before being a comet,” Sarid says.
The fact that LD2 is fairly new to the inner reaches of the solar system suggests that it’s made of relatively pristine material that has been in the back of the solar system’s freezer for billions of years, unaltered by heat from the sun. That would make it a time capsule of the early solar system. Studying its composition could help planetary scientists learn what the first planets were made of.
The orbital analysis looks “very reasonable,” says Henry Hsieh, a planetary astronomer with the Planetary Science Institute who is based in Honolulu and was not involved in the study. But studying just one transition object is not enough to open the solar system time capsule.
“What we really need to do is study many of these,” he says. “Study this one first, and then study more of them, and figure out whether this object is an outlier or whether we see a consistent picture.” Future sky surveys, like the ones planned using the future Vera Rubin Observatory (SN: 1/10/20), should discover more balls of ice shifting into comets.
Sarid and colleagues think LD2 could be a good target for a spacecraft to visit. NASA has considered sending spacecraft to centaurs, although no missions have been selected for development yet. But considering that LD2 will become a comet in just a few decades, scientists don’t have much time to plan, build and launch a mission to visit it. “The windows are closing,” Sarid says. “We really need to be doing this now.” More

To practice searching for extraterrestrial life, researchers have run a dress rehearsal with the one world they know to be habitable: Earth.
While Earth was between the sun and moon for a lunar eclipse in January 2019, the Hubble Space Telescope observed how chemicals in Earth’s atmosphere blocked certain wavelengths of sunlight from reaching the moon. That observing setup mimicked the way astronomers plan to probe the atmospheres of Earthlike exoplanets as they pass in front of their stars, filtering out some starlight.
“We basically pretend we’re alien observers looking at our planet,” says Giada Arney, a planetary scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md.
Using Hubble, the researchers focused on spotting the effects of atmospheric ozone. Because ozone is both a chemical by-product of oxygen produced in photosynthesis and a shield that protects life from the sun’s harmful ultraviolet rays, astronomers think atmospheric ozone could be a key indicator that a distant world is habitable. During the lunar eclipse, Hubble examined sunlight that had passed through Earth’s atmosphere and reflected off of the moon for signatures of ozone.

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“It’s safer for Hubble to observe sunlight reflected off the moon” than to look directly at the backlit Earth, explains Allison Youngblood, an astronomer at the University of Colorado Boulder. The telescope’s instruments are so sensitive and Earth is so bright that “even the nightside would fry Hubble’s detectors.” 
Those observations revealed prominent dips in particular wavelengths of ultraviolet sunlight that had been absorbed by the ozone, Youngblood, Arney and colleagues report online August 6 in the Astronomical Journal.
The data help confirm that chemicals in the Earth’s atmosphere filter light as expected, based on researchers’ understanding of atmospheric chemistry. That finding gives astronomers more confidence that they will be able to recognize potentially habitable exoplanets. More

Mauna Kea opened, Science News, August 1, 1970 —
The new Mauna Kea Observatory of the University of Hawaii has been completed and dedication ceremonies have been held. Standing at an altitude of 13,780 feet on the island of Hawaii, the new observatory is the highest in the world. Its major instrument is an 88-inch reflecting telescope that cost $3 million to build.
Update
More than a dozen large telescopes now dot Mauna Kea, operated by a variety of organizations. Those telescopes have revolutionized astronomy, helping to reveal the accelerating expansion of the universe and evidence for the black hole at the center of the Milky Way. But the telescopes have long sparked controversy, as the dormant volcano is sacred to Native Hawaiians. Since 2014, protests have flared in response to the attempted construction of the Thirty Meter Telescope. Opponents have kept progress stalled by blocking the only access road to the site. Some scientists have spoken out against the telescope’s location. The Thirty Meter Telescope collaboration is considering the Canary Islands as a backup site. More

The closest cluster of stars to Earth is falling apart and will soon die, astronomers say.
Using the Gaia spacecraft to measure velocities of stars in the Hyades cluster and those escaping from it, researchers have predicted the cluster’s demise. “We find that there’s only something like 30 million years left for the cluster to lose its mass completely,” says Semyeong Oh, an astronomer at the University of Cambridge.
“Compared to the Hyades’ age, that’s very short,” she says. The star cluster, just 150 light-years away and visible to the naked eye in the constellation Taurus, formed about 680 million years ago from a large cloud of gas and dust in the Milky Way.
Stellar gatherings such as the Hyades, known as open star clusters, are born with hundreds or thousands of stars that are held close to one another by their mutual gravitational pull. But numerous forces try to tear them apart: Supernova explosions from the most massive stars eject material that had been binding the cluster together; large clouds of gas pass near the cluster and yank stars out of it; the stars themselves interact with one another and jettison the least massive ones; and the gravitational pull of the whole Milky Way galaxy lures stars away too. As a result, open star clusters rarely reach their billionth birthday.

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The Hyades has survived longer than many of its peers. But astronomers first saw signs of trouble there in 2018, when teams in Germany and Austria independently used the European Space Agency’s Gaia space observatory to find numerous stars that had escaped the cluster. These departing stars form two long tails on opposite sides of the Hyades — the first ever seen near an open star cluster. Each stellar tail stretches hundreds of light-years and dwarfs the cluster itself, which is about 65 light-years across.
In the new work, posted July 6 at arXiv.org, Oh and Cambridge colleague N. Wyn Evans analyzed how the cluster has lost stars over its life. It was born with about 1,200 solar masses but now has only 300 solar masses left. In fact, the two tails of escapees possess more stars than does the cluster. And the more stars the cluster loses, the less gravity it has to hold on to its remaining members, which leads to the escape of additional stars, exacerbating the cluster’s predicament.

Siegfried Röser, an astronomer at Heidelberg University in Germany who led one of the two teams that discovered the cluster’s tails, agrees that the Hyades is in its sunset years. But he worries that it’s too early to pin a precise date on the funeral. “That seems to be a little bit risky to say,” Röser says. Running a computer simulation with the stars’ masses, positions and velocities should better show what will happen in the future, he says.
The main culprit behind the cluster’s coming demise, Oh says, is the Milky Way. Just as the moon causes tides on Earth, lifting the seas on both the side facing the moon and the side facing away, so the galaxy exerts tides on the Hyades: The Milky Way pulls stars out of the side of the cluster that faces the galactic center as well as the cluster’s far side.
Even millions of years after the cluster disintegrates, its stars will continue to drift through space with similar positions and velocities, like parachutists jumping out of the same airplane. “It’s still probably going to be detectable as a coherent structure in position-velocity space,” Oh says, but the stars will be so spread out from one another that they will no longer constitute a star cluster. More

For the first time, an exoplanet family around a sunlike star has had its portrait taken. Astronomers used the Very Large Telescope in Chile to snap a photo of two giant planets orbiting a young star with about the same mass as the sun, researchers report July 22 in The Astrophysical Journal Letters.
The star, called TYC 8998-760-1, is about 300 light-years away in the constellation Musca. At just 17 million years old, the planetary family is a youngster compared with the 4-billion-year-old solar system.
Although astronomers have found thousands of exoplanets, most aren’t observed directly. Instead they are spotted as shadows crossing in front of their stars, or inferred as unseen forces tugging at their stars.
Only a few tens of planets have been photographed around other stars, and just two of those stars have more than one planet. Neither is sunlike, says astronomer Alexander Bohn of Leiden University in the Netherlands — one is more massive than the sun, the other less massive.
Both of this star’s planets are unlike anything seen in the solar system. The inner planet, a giant weighing 14 times the mass of Jupiter, is 160 times farther from its star than Earth is from the sun. The outer one weighs six times Jupiter’s mass and orbits at twice its sibling’s distance. In comparison, the Voyager 1 spacecraft, which flew past the boundary marking the sun’s magnetic influence and into interstellar space in 2012, is still closer to the sun than either planet is to its star (SN: 9/12/13).
This exoplanet family could provide new insight into how solar systems can form. “As with many other exoplanet discoveries, this discovery makes us aware of other scenarios that we did not think of,” Bohn says. More

The sun could come from a large, loose-knit clan or a small family that’s always fighting.
New computer simulations of young stars suggest two pathways to forming the solar system. The sun could have formed in a calm, large association of 10,000 stars or more, like NGC 2244 in the present-day Rosette Nebula, an idea that’s consistent with previous research. Or the sun could be from a violent, compact cluster with about 1,000 stars, like the Pleiades, researchers report July 2 in the Astrophysical Journal.Whether a star forms in a tight, rowdy cluster or a loose association can influence its future prospects. If a star is born surrounded by lots of massive siblings that explode as supernovas before a cluster spreads out, for example, that star will have more heavy elements to build planets with (SN: 8/9/19).
To nail down a stellar birthplace, astronomers have considered the solar system’s chemistry, its shape and many other factors. Most astronomers who study the sun’s birthplace think the gentle, large association scenario is most likely, says astrophysicist Fred Adams of the University of Michigan in Ann Arbor, who was not involved in the new work.
But most previous studies didn’t include stars’ motions over time. So astrophysicists Susanne Pfalzner and Kirsten Vincke, both of the Max Planck Institute for Radio Astronomy in Bonn, Germany, ran thousands of computer simulations to see how often different kinds of young stellar families produce solar systems like ours.

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The main solar system feature that the pair looked for was the distance to the farthest planet from the star. Planet-forming disks can extend to hundreds of astronomical units, or AU, the distance between the Earth and the sun (SN: 7/16/19). Theoretically, planets should be able to form all the way to the edge. But the sun’s planetary material is mostly packed within the orbit of Neptune.
“You have a steep drop at 30 AU, where Neptune is,” Pfalzner says. “And this is not what you expect from a disk.”
In 2018, Pfalzner and her colleagues showed that a passing star could have truncated and warped the solar system’s outer edge long ago. If that’s what happened, it could help point to the sun’s birth environment, Pfalzner reasoned. The key was to simulate groupings dense enough that stellar flybys happen regularly, but not so dense that the encounters happen too often and destroy disks before planets can grow up.
“We were hoping we’d get one answer,” Pfalzner says. “It turned out there are two possibilities.” And they are wildly different from each other.
Large associations have more stars, but the stars are more spread out and generally leave each other alone. Those associations can stay together for up to 100 million years. Compact clusters, on the other hand, see more violent encounters between young stars and don’t last as long. The stars shove each other away within a few million years.
“This paper opens up another channel for what the sun’s birth environment looked like,” Adams says, referring to the violent cluster notion.
The new study doesn’t cover every aspect of how a tight cluster could have affected the nascent solar system. The findings don’t account for how radiation from other stars in the cluster could erode planet-forming disks, for example, which could have shrunk the sun’s disk or even prevented the solar system from forming. The study also doesn’t explain certain heavy elements found in meteorites, which are thought to come from a nearby supernova and so could require the sun come from a long-lived stellar family.
“I think [the research] is an interesting addition to the debate,” Adams says. “It remains to be seen how the pieces of the puzzle fit together.”
Pfalzner thinks that the star cluster would break apart before radiation made a big difference, and there are other explanations for the heavy elements apart from a single supernova. She hopes future studies will be able to use that sort of cosmic chemistry to narrow the sun’s birthplace down even further.
“For us humans, this is an important question,” Pfalzner says. “It’s part of our history.” More