For decades, astronomers have watched a mysterious clump named X7 drift around the supermassive black hole at the heart of the Milky Way, and wondered where it came from.
Analyzing 20 years of observations, a team of researchers led by University of California Los Angeles astrophysicist Anna Ciurlo has revealed a dramatic transformation in X7’s shape, stretching to almost twice its original length.
This change in structure suggests that the strange blob is most likely made of debris ejected during a relatively recent collision between two stars.
“No other object in this region has shown such an extreme evolution,” explains Ciurlo.
“It started out comet-shaped, and people thought maybe it got that shape from stellar wind or jets of particles from the black hole. But when we followed it for 20 years, we saw that it became more elongated. Something must have put this cloud on it. particular road with its particular orientation.”
If a debris cloud is indeed what the object is, the discovery sheds light on some of the fascinating dynamics of the galactic center, such as the frequency of stellar collisions and the effects of extreme gravity. In just a few years, the cloud of dust and gas will become more and more spaghettified, and fall into the black hole Sagittarius A* (SgrA*.).
By studying X7 over a number of years, scientists have been able to calculate its mass, which weighs around 50 times that of Earth. That might be a lot for an Earthling, but in space it’s practically a sneeze, not even one-sixth the mass of a Jupiter.
Changes in the debris cloud’s position and speed also suggest that it is in an elliptical orbit around the galactic center, with a period of about 170 years. Or rather, it would be, if it were a little more together. Simulations suggest it won’t get a chance to complete a single orbit.
Its closest approach to Sgr A*, known as periastron, is expected to occur in 2036. At this point, the gravitational environment will tear the cloud apart, leaving diffuse remnants to continue circling the black hole until they disappear irretrievably beyond its event horizon. When this finally happens, anyone who happens to be watching can see fireworks.
“It is exciting to see significant changes to X7’s shape and dynamics in such great detail over a relatively short time scale as the gravitational forces of the supermassive black hole at the center of the Milky Way affect this object,” says astronomer and co-author Randy Campbell of Keck Observatory.
X7 shares some similarities with other mysterious blocks orbiting the galactic center, known as G-objects. First discovered around 20 years ago, these posed a significant puzzle: they looked like gas clouds but behaved like stars, stretching out at periastron but emerging intact and shrinking back to a more compact form to continue their orbits theirs.
Astronomers assumed that the G objects were stars that had merged, producing a huge cloud of material that stayed within the gravitational field of the newly merged star, hiding it from view. Then a study published in 2021 found that one of those objects, G2, was a molecular cloud hiding three baby stars; but the identities of the others remain unknown.
Although there are similarities, the X7 is significantly different from the G objects as well. Its evolution has been more dramatic than the G objects, both in shape and speed as it stretches out and accelerates towards Sgr A*.
So X7 may not be the same type of object as the G objects, but it is possible that it is related.
“One possibility is that X7’s gas and dust were ejected at the moment two stars merged,” says Ciurlo. “In this process, the merged star is hidden inside a shell of dust and gas, which may fit the description of the G objects. And the ejected gas may have produced X7-like objects.”
Because the X7 is not held together by a mass lurking in the middle, it is expected to have a much shorter lifespan than the G objects; this may be why others of this type have not yet been discovered. Meanwhile, the merged star from which X7 breathed may still be out there in the galactic center, on its own separate orbit. The researchers note that its orbit is very similar to the orbit of the G object G3, and suggest that G3 may be the parent object.
However, it is currently not easy to rule out other possibilities. For example, X7 could have been a piece of debris removed from a larger cloud. Further observations may help narrow it down.
And of course, watching the X7 itself should be interesting and rewarding as it moves closer and closer to its doom.
“Continued monitoring of X7 will allow us to witness these extreme changes,” the researchers write, “culminating in the ultimate tidal dispersal of the remains of this intriguing structure.”
The research is published in The Astrophysical Journal.