supernova remnant
A supernova remnant is an expanding shell of ejected material and swept-up interstellar matter. Inside lies a hot, tenuous bubble bounded by a shock wave that heats gas to X-ray emitting temperatures. Several morphological types exist: shell-type (like Cassiopeia A), plerionic (powered by a pulsar wind, like the Crab), and mixed. Supernova remnants enrich the interstellar medium with heavy elements and launch cosmic rays.
History
Supernova remnants have been observed since antiquity: the Crab Nebula is the result of an explosion recorded by Chinese astronomers in 1054. In the 20th century, Fritz Zwicky and Walter Baade linked them to neutron stars.
How it works
The shock wave pushes matter at thousands of kilometers per second, sweeping up interstellar gas. Collisions heat the gas to millions of degrees, making it glow in X-rays — much like a supersonic aircraft's trail turns into a cloud, but much hotter.
💡 The Crab Nebula expands at about 1,500 km/s — while you read this sentence, its diameter increases by a distance comparable to Earth's size.