Why an old supernova remnant radiates like a stellar nursery

A supernova remnant called Cygnus Loop is old enough that some of its shock regions have cooled into clouds of hot gas. Using the VLA at 1 and 5 GHz, the team mapped radio emission from filaments previously studied in optical light, finding that the oldest, radiative filaments emit via thermal bremsstrahlung—the same mechanism as around newborn stars—rather than the nonthermal particle acceleration typical of younger SNRs. This suggests remnants evolve into thermal sources as they age and expand.