The east arc of the Cygnus Loop, where the blast wave from a star that died around ten thousand years ago is still ramming into interstellar gas at well over a hundred kilometres a second. The filaments look like rope, but they’re thin sheets of shocked gas that happen to be folded toward us, so we see them edge-on and they turn bright. Blue is oxygen at the leading edge, red is hydrogen behind it. Everything here is about 2,400 light-years away, which means the wave you’re looking at moved on a long time before the photons left.
