ESO 217-25 The Mermaid Nebula

A sea creature in Centaurus made not of water, but of the ghostly remains of a stellar explosion

ESO 217-25, often called the Mermaid Nebula or the Beta Fish Nebula, is the brightest optical portion of the much larger supernova remnant G296.5+10.0 in the constellation Centaurus. What appears here as an elegant web of luminous blue filaments and red shock fronts is only part of the full expanding debris field left behind by a star that exploded thousands of years ago.

The remnant is estimated to lie about 4,500 light-years away. In radio and X-ray observations, the full supernova remnant spans roughly 90 × 65 arcminutes, giving it an immense physical size of about 120 × 85 light-years. The optically prominent section captured here is much smaller, with the bright visible filamentary structure in this field covering roughly 26 × 19 arcminutes, or about 34 × 25 light-years at that distance.

What makes ESO 217-25 so striking is its extraordinary texture. Rather than forming a smooth shell, the remnant has broken into shock-compressed sheets, twisted streamers, and translucent oxygen-rich veils. The luminous blue filaments trace ionized oxygen, while the thin red edges mark hydrogen emission where the blast wave is still colliding with surrounding interstellar gas. The bright central arc is one of the most active visible sections of the shock front, while the fainter detached wisps reveal how fragile and diffuse the structure has become as the remnant continues to expand.

This is an object where subtlety matters more than brightness. ESO 217-25 is not a dense nebula but a tenuous, aging shell of stellar debris, stretched across space into one of the sky’s most delicate and haunting supernova remnants

Imaged in HOO RGB OTA CDK 1000 at Observatorio El Sauce, Chile

Image acquisition and processing: Mike Selby