Supernova Remnantとは何か？
A supernova remnant is what a star leaves in the sky after it explodes: a shell of ejected material expanding into interstellar space at thousands of kilometres per second, glowing for tens of thousands of years as it ploughs up everything in its path.
The explosion itself lasts seconds. A massive star's iron core collapses, rebounds, and drives a shockwave through the overlying layers, releasing on the order of 10^44 joules of kinetic energy — more than the Sun will emit in its entire lifetime. For a few weeks the single star can outshine its whole host galaxy. Then the light fades and the remnant takes over the story.
The expanding shell sweeps up interstellar gas ahead of it, heating it to millions of degrees and lighting it up in X-rays, while a reverse shock travels back inward and heats the ejecta. Over centuries the wreckage develops the filamentary, tangled structure seen in the Crab Nebula, left by the supernova Chinese astronomers recorded in 1054, or Cassiopeia A, which exploded around 1680 and is now the brightest radio source in the sky beyond the Sun.
These shells are where the periodic table comes from. Elements heavier than iron cannot be made by ordinary fusion; they are forged in the explosion and in the neutron-rich material it flings outward, then mixed into the interstellar clouds that form the next generation of stars and planets. The calcium in your bones and the iron in your blood were both assembled in a shell like this one.