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This year, astronomers traced high-energy particles called cosmic rays back to their birthplaces in the debris clouds of supernovae—a feat that Science's editors chose as a runner-up for Breakthrough of the Year. Theorists had long suspected that most cosmic rays are accelerated in the shock waves from massive exploding stars called supernovae. But magnetic fields in space scramble the particles' trajectories, making it impossible to trace them back to their sources. Instead, astronomers looked for the radiation signatures of reactions that cosmic rays trigger when they crash into thinly scattered atoms in interstellar space. This year, the Fermi Gamma-ray Space Telescope found them—right where they were supposed to be.