Cosmic rays are particles that have been arriving on Earth from interstellar space for billions of years, like messages from the past and distant corners of the Universe. Their flow works like a giant train station: first come the commuter trains from nearby suburbs — particles from the remnants of supernovae in our Galaxy. Then, trains from more distant regions — a second, more powerful type of accelerator within the Milky Way. And then, suddenly, international express trains burst onto the schedule — particles born beyond our galaxy, where giant black holes accelerate matter to unimaginable energies.

Using spectroscopy — a method that sorts particles by energy, like mail into pigeonholes — astrophysicists studied the nuclei of hydrogen, helium, and iron. It turned out that the sharp break in the flow (the so-called 'knee') is precisely the moment of changing trains, when the local ones end and the external ones begin. Back in the 1930s, Fritz Zwicky suggested that the remnants of exploded stars are natural accelerators, and now this data seals the deal.

But the most amazing thing: iron nuclei, which are 56 times heavier than hydrogen, behave in the stream exactly like the light particles. The acceleration mechanism is indifferent to mass — like a truck and a motorcycle racing down the highway at the same speed. Future observations of carbon and oxygen nuclei will finally confirm that the Galaxy truly can't do more.