The pulsar in the middle
Not every supernova leaves nothing. Some leave something almost incomprehensible — a city-sized object spinning dozens of times per second, and still going.
What is left of the core, spinning.
The corpse that broadcasts
When a massive star collapses and explodes, the outer layers become the wreck still expanding — the glowing filaments observers spend hours chasing. But the core is a different story entirely. Compressed beyond any ordinary matter, the iron core crushes down to a sphere perhaps twenty kilometres across, where protons and electrons are forced together into neutrons. What is left is a neutron star, and if its magnetic axis and rotation axis are misaligned — which they usually are — it sweeps a beam of radiation around the sky like a lighthouse. From Earth, we call that a pulsar.
The name comes from "pulsating radio source," coined in 1968 after Jocelyn Bell Burnell and her supervisor Antony Hewish detected the first one at Cambridge. The pulses were so regular that the team briefly labelled the signal LGM-1 — Little Green Men — before the natural explanation became clear. Bell Burnell had found one of the fundamental objects of the universe using a radio telescope she had helped build by hand.
The archetypal example sits at the heart of the Crab Nebula, M1 in Messier's catalogue, the remnant of a supernova recorded by Chinese astronomers in 1054. The Crab Pulsar spins roughly thirty times per second and is energetic enough to drive the entire nebula's visible glow — the synchrotron radiation from electrons spiralling in its magnetic field lights the filaments from the inside. Without the pulsar, the Crab would fade. With it, the nebula is actively powered, not merely coasting.
The name comes from "pulsating radio source," coined in 1968 after Jocelyn Bell Burnell and her supervisor Antony Hewish detected the first one at Cambridge.
For visual observers, the pulsar itself is irrelevant. At visible wavelengths it does pulse — but thirty times per second is far beyond the eye's ability to detect flicker. What you see through the eyepiece is the surrounding nebulosity, not the rotating star inside it. The Vela Pulsar, embedded in a much larger and older remnant, is fainter still; its gossamer loops in Puppis are a genuine southern-sky challenge, with no hope of spotting the neutron star visually.
The pulsar is ultimately the reason remnants are worth watching at all. It marks the moment a star ran out of options — and kept broadcasting anyway.