The faint, filamentary arcs of SH2-91 trace the expanding remains of a massive star that exploded thousands of years ago in the constellation Cygnus. This supernova remnant is part of a larger, diffuse structure of glowing gas that stretches across the star-rich fields of the Milky Way. Delicate tendrils of hydrogen and oxygen emission form the faint ribbons that mark the shock fronts moving through interstellar space.

Although far less well-known than the nearby Veil Nebula, SH2-91 offers a glimpse of the same cosmic process: the recycling of stellar material into the galaxy. The remnants of the long-gone star continue to shape the interstellar medium, seeding it with heavier elements that will one day become part of new stars and planets.

This ethereal arc of glowing gas marks part of the supernova remnant Sh2-91, located in the rich star fields of the constellation Cygnus. Likely the result of a massive star that exploded tens of thousands of years ago, the remnant stretches across space in tangled, luminous filaments of ionized hydrogen (red) and oxygen (blue-green). Though not as well-known as the Veil Nebula, Sh2-91 is believed to be part of the same vast Cygnus Loop complex, now fading into the interstellar medium.

Captured in exquisite detail, this deep exposure reveals the shock fronts sweeping through the surrounding interstellar material. The arcing structure and radiant hues convey the immense energy of the original explosion, now diffused across light-years of space.

H2-91 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.”
Capturing this image has required 19h of exposure in Ha and OIII filters, plus 1.5h of exposure in R, G and B filters for the stars. Total exposure time has been about 40h.

Sh2-91 is a faint and intricate supernova remnant located in the constellation Cygnus, also known as the Network Nebula. This remnant is part of a larger supernova event that occurred thousands of years ago, scattering stellar material across the region. It is characterized by its delicate filaments of gas and dust, glowing predominantly in hydrogen-alpha light, which creates a striking visual of interwoven tendrils against the backdrop of the Milky Way.

The distance to Sh2-91 is estimated to be around 2,600 light-years from Earth. This remnant is particularly interesting to astronomers due to its complex structure and the insights it provides into the aftermath of supernova explosions. The remnants of the progenitor star interact with the surrounding interstellar medium, offering clues about the life cycle of stars and the dynamics of supernova remnants. Sh2-91, with its ethereal appearance, is a testament to the powerful and violent processes that shape our galaxy.