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In 2020, the eROSITA X-ray telescope discovered a pair of enormous bubbles looming above and below the disk of our Galaxy. These clouds of gas heated to millions of degrees are the aftermath of a colossal energy release from the center.

Astronomers examined the southeastern edge of one bubble, where matter slams into the sparse background. By breaking X-ray light into colors with spectroscopy, they found the shock wave racing at 700 km/s, surrounded by a near-vacuum: the gas is mostly hydrogen, with less than 10% of the heavy elements found in the Sun. Although the gas is scorching, an astronaut inside wouldn't feel the heat — the density is trillions of times lower than air, and particles zip by hardly colliding. This shock wave acts as a natural particle accelerator: it revs up particles to such speeds that they become cosmic rays bombarding Earth. Understanding these bubbles links the history of the galaxy to the mystery of the most energetic particles in the universe.