Nearly a decade ago an object called G2 first enthralled astronomers around the world. G2 was a bizarre hybrid. Containing several Earth masses of material, it looked like a compact mix of gas and dust, yet it moved more like a star, although telescopes could see no signs of starlight seeping out of its interior. The object might have been dismissed as a random astronomical oddity if it did not also happen to be on a path that would bring it dangerously close to the supermassive black hole parked at our galaxy’s center.
Astronomers expected that black hole—called Sagittarius A*, or SgrA*—to shred G2 in the summer of 2014, causing a brief blaze of radiation as the ripped-up object fed the black hole’s insatiable appetite. But that did not happen: G2 slid right on by SgrA*—scarcely worse for wear, save for some slight deformation of its shape—and continued on its galactic journey.
To this day, astronomers do not agree about what, exactly, G2 is. And a similar SgrA*-orbiting body called G1 is also unexplained. At turns suspected of being compact, gassy clouds or dim stars cloaked by an obscuring shroud, the two objects remain deeply enigmatic.
It turns out that they are not alone. Published in January 2020 in the journal Nature, a study reported an additional four so-called G-objects in the Milky Way’s center. And just like G1 and G2, they are cosmic chimeras.
“When you have just two of something, it is all mysterious,” says Elena Murchikova of the Institute for Advanced Study in Princeton, N.J., who was not involved in the new paper. “This study is a game changer. It triples the number of G-objects and demonstrates that they all are on different orbits, so they are not unique and unlikely results of rare conditions coming together. And they clearly are not a part of the same structure.” In other words, the six identified G-objects arose independently, suggesting that whatever cosmic mechanism sculpted them is not rare—and might, in fact, soon be resolved.
The study’s lead author, Anna Ciurlo of the University of California, Los Angeles, worked with her colleagues to spot the new objects in 13 years of galactic-center data gathered by an instrument on one of the twin Keck telescopes situated atop Hawaii’s dormant volcano Mauna Kea. Initially, she says, she was studying how SgrA* influences the gas near the galactic center.
“But then we kept on finding these very compact objects,” she says. “They were not behaving like a gas cloud—they would not get stretched; they would not get eaten by the black hole itself. They were orbiting around the black hole as a star would.” In other words, the newly discerned objects closely resembled the enigmatic G1 and G2.
In the end, Ciurlo flagged four dense, blobby new objects, dubbing them G3 through G6 (although G3 and G6 had been independently detected by other groups). They bring the number of known G-objects to six. Each object is situated within 0.13 light-year of SgrA*. Like G1 and G2, they are all on the order of 100 astronomical units in size (one astronomical unit is roughly the Earth-to-sun distance). But unlike those first two objects, which appear to be on similar orbits, each member of the new quartet traces a wildly divergent path from the others around our galactic nucleus, with orbital periods ranging from 170 to 1,600 years.
“All of this looks very, very believable—there’s no doubt of the data,” says Stefan Gillessen of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, who discovered G2 and was not involved in the new study. “I really find it is a nice paper.”
What are They?
In addition to indicating that G-objects are not rare celestial one-offs, the new observations may help solve the fundamental mystery of their origins. Ciurlo and her colleagues think the mysterious hybrids are merged binary stars shrouded in cast-off star stuff. As the team’s reasoning goes, most of the stars throughout our galaxy form in pairs, even within the galactic center. But in that chaotic environment, binary stars are not just orbiting each other—they are also being jostled and battered by SgrA*. “The likelihood that these systems would merge is much higher than elsewhere in the galaxy,” Ciurlo says.
The resulting merger is likely very messy as each star rips material from the other, eventually expelling about a sun’s mass of material to form a gassy, dusty cloud that obscures the newly formed star. “Of course, this needs to be confirmed in the future. But I think it’s the most fascinating hypothesis, given what we know about this region,” Ciurlo says.
Gillessen, though, is unconvinced that colliding stars are a satisfying explanation for G-objects. Instead he thinks such objects are more likely to be compact gas clouds, perhaps created by a variety of phenomena. “I would be surprised if all of those are of the same category,” he says.
Like Ciurlo and her colleagues, Gillessen and his team have studied the galactic center for years, and he does not believe there are sufficient stars in the region to produce mergers often enough for the aftermath of six collisions to be simultaneously visible. “Out of the 40 stars we are seeing [in the galactic center], we have six mergers,” he says. “That means that unless these mergers are very long-lived, you immediately start overproducing the number of stars that should be there.”
Ciurlo explains that the dusty messiness surrounding a newly merged star could last for a few million years—although she notes that not much is really known about that scenario. And, she observes, the most recent burst of star formation near SgrA* is thought to have occurred between six million and four million years ago. “These types of mergers tend to happen quite early once you form stars,” Ciurlo says. “Therefore, we only see the mergers that happened at the beginning, soon after the stellar population formed.”
Piercing the Veil
Ultimately the G-objects’ identities and origins could be unveiled by more detailed observations, made by Keck, instruments on Chile’s Very Large Telescope, or future facilities such as the James Webb Space Telescope or the ground-based Thirty Meter Telescope. In the interim, Gillessen says, closely studying G2 over the next few years could be revealing.
Such investigations are promising because stars and gas behave quite differently when they approach a supermassive black hole, and G2’s close passage to SgrA* in 2014 could affect the motion of its gassy shroud in a telltale way. Already Gillessen’s observations suggest that SgrA* put the brakes on G2, causing the object—or at least its murky exterior—to decelerate. But a dusty, denser star is tougher to slow down. And over time astronomers should perhaps differentiate between emission from the star and its former shroud.
“The gas actually feels different forces than the stellar object, which might reside inside,” Gillessen says. “If a few years from now, we see that G2 has separated into a different gaseous and dust component, then the case is clear that there is something inside.”