Hot gas is exploding out of the center of the galaxy. It forms giant bubbles that extend more than 30,000 light-years above and below the galaxy’s hub. The bubbles emit gamma rays — the most powerful form of energy. Fermi Gamma-ray Space Telescope has traced these expanding bubbles, which have been named Fermi bubbles in its honor. The observations have revealed how fast the gas is moving. In turn, that reveals when the outflow began. The astronomers used Hubble Space Telescope to observe a distant quasar that’s behind the bubbles. As the quasar’s light passes through the Fermi bubbles, atoms of gas absorb some of the energy. These observations indicate that the gas is rushing out of the Milky Way’s central regions in all directions — at about two million miles per hour. That speed reveals that the outflow started about two-and-a-half million to four million years ago. So that’s when something happened to trigger the eruption. One possibility is that a cloud of gas fell toward the supe
