Philippa Kaur

A strange, newly measured clump of stars orbiting the nearby Andromeda galaxy has the lowest level of heavy chemical elements ever seen in one of these mysterious star clusters. Named RBC EXT8, this globular cluster is also surprisingly massive, challenging theories for how such clusters and some galaxies form, astronomers report online October 15 in Science.
“It’s a very unusual object,” says astrophysicist Oleg Gnedin of the University of Michigan in Ann Arbor, who was not involved in the new discovery.
Globular clusters are crowded, spherical collections of stars that orbit a galaxy’s center, though most, including RBC EXT8, live in the galactic outskirts. The clusters are typically billions of years old, so their stars tend to be chemically pristine, meaning they formed before the universe had time to create much of any of the elements heavier than hydrogen or helium, which astronomers lump together as “metals.”
Previous observations of these clusters in the Milky Way and other galaxies had suggested that there’s a limit to how low a globular cluster’s metal content can be. The most metal-poor clusters were about 300 times less rich in heavy elements like iron than the sun, but no less.
But spectra of RBC EXT8, some 2.5 million light-years away, show that the cluster’s metal content is about 800 times less than the sun’s. The globular cluster that held the previous record for lowest “metallicity” has three times that amount.
“It was completely unexpected that we would find a globular cluster that is so metal poor,” says astronomer Søren Larsen of Radboud University in Nijmegen, the Netherlands.
The bigger, fuzzy blob in the inset image at left is RBC EXT8, a globular cluster that orbits about 88,000 light-years from the center of the galaxy Andromeda (shown at right). The cluster has surprisingly few heavy elements for its size, a new study finds.© 2020 ESASky, CFHT
What’s more, given its metal-poor status, this cluster is surprisingly massive, weighing about 1.14 million times the mass of the sun. (A mid-weight globular cluster is about 100,000 solar masses, but some clusters reach 3 million solar masses. RBC EXT8 is heavy, but not the heaviest.)
That mass makes the cluster even harder to explain because across the cosmos, the more massive a galaxy or cluster is, the more heavy elements it normally has.
There are several potential explanations for that trend, but one is simply that more massive galaxies or globular clusters have more stars. A star fuses heavy elements in its core and sprinkles them around its host cluster or galaxy as it ages. Sufficiently massive stars can explode in a supernova, spreading those metals to become part of the next generation of stars (SN: 8/9/19). So more stars means more opportunity for metals to accumulate locally.
More massive objects also have the advantage of gravity, which lets them better hold on to the metals that they do have and remain a cohesive group for billions of years. Less massive globular clusters dissolve into their host galaxies over time.
Those trends together could have explained the apparent “metallicity floor” for globular clusters — all of the less massive, more metal-poor clusters have broken apart over the eons.
RBC EXT8 turns that conventional wisdom on its head. “It’s too big to have as low metallicity as it has,” Gnedin says. “That’s the conundrum.”

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Astronomers aren’t sure how globular clusters form in general, but they probably grow within galaxies, rather than forming outside of them and being pulled in later. And so the clusters reflect the characteristics of their galaxies: small, metal-poor galaxies end up with small metal-poor globular clusters, and vice versa. But based on RBC EXT8’s metal content, it’s galactic birthplace would be less than a million solar masses, so smaller than the globular cluster itself – which is a paradox.
As a result, the cluster challenges some simplified models of galaxy formation. But it doesn’t completely break them, Gnedin says. “It’s one object, it’s not going to overturn things,” he says. “It just makes us people working on these issues have to work harder” and be more open-minded about other ways that galaxies could form.
Open-mindedness and willingness to explore is perhaps responsible for the new finding about RBC EXT8’s metals. Larsen and colleagues spotted the globular cluster at the beginning of a night of observing with the Keck telescope in Hawaii in October 2019. “It was really a serendipitous discovery,” he says. He had a spare hour before the globular clusters in galaxy M33 that his team was planning to look at rose above the horizon, so the observers picked another cluster “more or less at random” to fill the time.
“At first, I couldn’t really believe that what was coming out [in the observations] was right,” Larsen says. “But I kept working on it, and it turned out to hold up.” More

When NASA’s OSIRIS-REx arrived at near-Earth asteroid Bennu, scientists were dismayed to find a surface covered with hazardous-looking boulders.
But new research suggests that those boulders are surprisingly brittle. That’s potentially good news for the spacecraft, which is charged with grabbing a piece of Bennu on October 20 and returning it to Earth in 2023 (SN: 1/15/19). If the rocks are crumbly, that could lower the risk of damaging the spacecraft’s equipment.
That kind of rock also may be too fragile to survive the trip through Earth’s atmosphere without burning up. If so, scientists may be close to getting their hands on a never-before-seen kind of space rock, researchers report in a collection of papers published October 8 in Science and Science Advances.
Data taken from Earth before OSIRIS-REx launched suggested that Bennu’s surface would be sandy. So it was a shock to find a rough landscape strewn with boulders when the spacecraft arrived in 2018 (SN: 12/3/18).
“We had really convinced ourselves that Bennu was a smooth object,” says Daniella DellaGiustina, a planetary scientist at the University of Arizona in Tucson and member of the OSIRIS-REx team. “As everyone saw from the first pictures, that was not the case.”
The team found a relatively clear crater, nicknamed Nightingale, from which to retrieve a sample of the space rock (SN: 12/12/19). Still, the worry remains that the boulders might pose a safety hazard for the sampling system, which was designed to handle pebbles only a few centimeters across.
From late April through early June 2019, planetary scientist Ben Rozitis of the Open University in Milton Keynes, England, and colleagues mapped the way Bennu’s boulders retain heat, a clue to the rocks’ structure. Denser materials hold heat better than finer-grained ones, like how a sandy beach cools quickly after sundown, but single large rocks remain warm.
This map shows where carbon-bearing minerals (represented by redder colors) are located on Bennu’s surface. The opportunity to analyze those minerals could help scientists figure out how carbon got to the early Earth.A. Simon et al/Science 2020
Based on those maps — and maps of other surface properties, described in the series of papers released October 8 — Bennu’s boulders seem to come in two flavors: darker-colored rocks that are weaker and more porous and lighter-colored, denser rocks that are stronger and less porous. Even the denser rocks are much more porous and brittle than meteorites from similar asteroids that have been found on Earth. The least dense meteorites are about 15 percent porous; Bennu’s rocks seem to be between 30 and 50 percent porous, Rozitis and colleagues found.
“This is exciting,” says DellaGiustina, a coauthor of the new papers. The spacecraft and its instruments might “encounter some boulders at the sample site that might otherwise be difficult to ingest,” she says, but “if they’re porous and weak, then they might just break down,” making them easier to collect.
The lighter, denser rocks also appear to be shot through with veins of carbonate, which suggests that they were in the presence of flowing water at some point in their past (SN: 12/10/18). NASA chose Bennu as an asteroid to visit partly because it resembles carbonaceous chondrite meteorites, which scientists think are time capsules of the early solar system. Similar space rocks could have delivered water and organic materials to Earth billions of years ago.
But Bennu’s more porous rocks appear to be unlike anything in scientists’ current assortment of meteorites, Rozitis says. “This is one of the cool things about OSIRIS-REx — it’s quite likely it will pick up new material that isn’t in our meteorite collection,” he says.

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That’s believable, says meteor scientist Bill Cooke of NASA’s Marshall Space Flight Center in Huntsville, Ala. Observations of meteors have shown that low-density space rocks and dust burn up higher in Earth’s atmosphere than higher-density rocks.
“The old conventional wisdom was that the low-density stuff was from comets, and the high-density stuff was from asteroids,” he says. But recent observations show that some of the low-density rocks come from the orbits of asteroids. “So it is very plausible that low-density stuff from Bennu … would ablate higher in the atmosphere and not have a chance to create meteorites at all.”
If Bennu represents a missing piece in our understanding of the solar system’s history, studying that material in labs on Earth “will help us fill in an additional piece of the jigsaw,” Rozitis says. More