The Sagittarius A* (pronounced Sagittarius A Star) is a supermassive black hole in the Milky Way galaxy. For perspective, it has a mass 4 million times bigger than that of the sun. A mysterious object has been seen orbiting the black hole since 2011, and scientists have not come to an agreement as to what it is other than calling it G2.
For one group of scientists, particularly those from the Max Planck Institute for Extraterrestrial Physics, G2 is a gas cloud. In fact, most scientists followed this train of thought until G2 didn't behave like one when it got close to the black hole. As it's not solid, a gas cloud is expected to be absorbed by the black hole. Because the G2 was not devoured by the Sagittarius A*, it must not be a gas cloud then.
This is what researchers from the University of California, Los Angeles used as a counter. Led by Andrea Ghez, the team from UCLA argued that G2 was likely to be a binary star shrouded in a layer of gas and dust. It's just one object now because the powerful gravity from the black hole drove a pair of stars to merge into one. As stars have solid mass, it wasn't as easily taken in by the Sagittarius A* as a gas cloud.
Proponents of the gas cloud theory, however, aren't giving up just yet. Max Planck scientists said that it was possible for G2 not to be absorbed despite being a gas cloud if it was once part of a bigger gas cloud.
Having broken up, the bigger gas cloud would leave a trail of smaller gas clouds, looking very much like beads on a piece of string. Using a computer model, the Max Planck group retraced the path of another object called G1 (first observed more than 10 years ago) and showed that it followed an orbit highly similar to G2's, concluding that the two were previously part of the same bigger gas cloud.
Finding other "beads in the string" will undoubtedly further prove that G2 is a gas cloud, but for now the object is back to being classified as such. As for the bigger gas cloud from where the two objects came from, Max Planck scientists say it's possibly wind clumps released by a massive disk star about 100 years ago near the farthest point of G2's orbit.