Imagine a crowd in a square. First, children run through it, light and nimble, slipping between people without pushing them. That's how electrons behave in the Crab Nebula. But when adults run through the same square, heavy as they are, they have to shove people aside, and the crowd spreads out. Protons are those adults.

In the nebula left after the star explosion, electrons and protons are accelerated by the dead star at the center. Electrons emit light when they spiral in a magnetic field. Protons emit light differently: crashing into gas, they produce particles that decay into gamma rays. These rays fly outward, and the nebula appears larger in their light.

Previously, scientists couldn't understand why the nebula at high energies looks larger than calculations predicted. Now, adding protons, they got both the right size and the right spectrum. This means that stellar explosions can accelerate not only light particles but also heavy ones, the ones that make up all the matter around us.