Wednesday, Jul 2, 2025

This image captures SH2-91, a faint and filamentary structure located in the constellation Cygnus. It represents a part of an ancient supernova remnant known as G65.3+5.7, believed to be over 20,000 years old and spanning more than 3 degrees across the sky. What we see here is just a thin section of its intricate filament network. SH2-91 is especially remarkable in the OIII emission line, which reveals a sinuous, glowing arc tracing the shock front where the expanding blast wave from the supernova continues to plow through the interstellar medium. The ionized oxygen, energized by the remnant’s shock, emits strongly in blue-green light, producing the elegant structure reminiscent of a cascading cosmic waterfall—hence the nickname “The OIII Falls.”