For years, astronomers have been fascinated and perplexed by what appears to be a massive cloud of gas and dust hurtling towards the black hole at the center of the galaxy. A team from UCLA now say they've finally figured out what it is.

The object, when it was first discovered in 2002, appeared to be headed straight towards the supermassive black hole at the core of the Milky Way. The cloud, called G2, made its closest approach earlier this year, attaining a perilous distance of just over 3,000 times the radius of the event horizon itself (~260 AU, or 36 light-hours).


(Max Planck Institute for Extraterrestrial Physics)

By studying this encounter, astronomers from UCLA have a good idea what it is: G2 is most likely a pair of binary stars that had been orbiting the black hole in tandem — and then merged together forming an extremely large star cloaked in gas and dust. Remarkably, G2 appears to be just one of an emerging class of stars near the supermassive black hole.


The astronomers ruled out the possibility that it was just a hydrogen cloud, which they say would have simply been torn apart by the black hole. G2 survived and continued on its orbit; a gas cloud would not have reacted in the same way. The star appears to have suffered an abrasion to its outer layer, but it's fine for the most part. (gif at left: a 3D animation of some stars orbiting the central black hole. Via MPIEP)

According to team leader Andrea Ghez, when two stars near the black hole merge into one, the star expands for more than one million years before it settles back down.


"This may be happening more than we thought," she says. "The stars at the center of the galaxy are massive and mostly binaries. It's possible that many of the stars we've been watching and not understanding may be the end product of mergers that are calm now."

G2 is currently in the midst of that inflated stage. It's undergoing what Ghez calls "spaghetti-fication" — a common phenomenon near black holes in which large objects become elongated. Also, the gas at G2's surface is being heated by the stars around it, creating an enormous cloud of gas and dust that has shrouded most of the massive star.

Read the entire study at The Astrophysical Journal: "Detection of Galactic Center Source G2 at 3.8 μm during Periapse Passage".


Top visualization: ESA