We recently discovered that gigantic gamma ray bubbles were balanced above and below the center of the Milky Way, stretching out 25,000 light-years in each direction. And now we might finally know where these mysterious bubbles come from.
When we first reported on the bubbles back in November, astronomers couldn't yet explain their existence, although the supermassive black hole at the center of our galaxy was seen as a likely culprit. Now, four months later, physicists have been able to fine-tune that idea into a working hypothesis.
University of Hong Kong researcher Kwong Sang Cheng and his team started with the fact that there are about 100,000 stars located within a light-year of the galactic black hole. Although most can survive in this extreme environment pretty much indefinitely, they calculate that the black hole's gravity will rip apart one of the stars once every 30,000 years. Half of the stellar mass goes straight into the black hole, while the other half is ejected back out. From there, the stellar remnants fiercely interacts with the gas that's right around the galactic disc until gamma rays are emitted. And that, over time, is how the bubbles were formed.
However, not everyone buys into that specific explanation, even if the basic mechanism appears sound. Harvard astrophysicist Douglas Finkbeiner, for instance, argues that the edges of the bubble are too sharp to be formed by the slow accumulation of stellar masses. Instead, he proposes that an entire star cluster or perhaps a huge cloud of gas falls in once every one to ten million years, and the most recent of these is what formed the bubbles we can now see. He explains:
"To make a sharp edge, the mechanism really needs to turn on and turn off. My money is on the explanations that involve something more dramatic and more rare."