In 1054, a star flared up in the constellation Taurus. The light of its death was so bright that it was visible in daylight for nearly a month, and Chinese astronomers recorded it in their chronicles. Today, the Crab Nebula is spreading over that spot, and its claws are clouds of gas still being whipped up by the dead star at its center. That star spins thirty times a second and hurls particles into space with such force that they glow like a white-hot wire.

For a long time, it was thought that all this light comes from electrons, light particles spiraling in the nebula's magnetic field. But when telescopes measured the size of the glow at different energies, a strange thing emerged: the higher the energy, the larger the nebula appears. Electrons do not behave that way. They should stay in a narrow beam, not spread out like batter on a griddle.

To explain the swelling, protons were added to the calculations, particles two thousand times heavier than electrons. A proton accelerated to enormous speed slams into the nebula's ordinary matter and creates an intermediary particle that instantly decays into two gamma rays. These rays fly off in opposite directions, making the nebula look bigger in their light. The result is a hybrid model: electrons account for low energies, protons for high ones.

The test was striking. A purely electronic model predicts a size at the highest energies six times smaller than the hybrid one. If future telescopes see the Crab swelling to one and a half arcminutes at such energies (about one twentieth of the Moon's disk), that means protons are there. And that means stellar explosions can accelerate not only light particles but also the heavy building blocks of matter.

The Crab is spreading its claws again. This time, in gamma rays. And by how wide they stretch, we can tell which particles burst from the heart of the explosion.