SDSO1, a large glowing arc seen near the Andromeda galaxy (M31), is not part of that distant galaxy but instead lies much closer, within our own Milky Way.

Using very deep, narrowband images, the researchers show that SDSO1 is caused by a “ghost planetary nebula,” which is the faint, expanded remains of gas once expelled by a dying star.

The source of this gas is the binary star system EG Andromedae, which includes a white dwarf. EG And is moving extremely fast—over 100 kilometers per second—through the thin gas between stars. This rapid motion creates a powerful bow shock, similar to a wave forming in front of a fast-moving boat, which heats and excites the surrounding gas so it glows, especially in oxygen light.

Although the original planetary nebula has faded and become nearly invisible, the shock it produces remains bright enough to detect. The team also finds a long, turbulent tail of gas trailing behind SDSO1, showing where material has been stripped away over hundreds of thousands of years.

The study identifies SDSO1 as the first clear example of a new stage in planetary nebula evolution, revealing that many old nebulae may still be detectable through their interaction with interstellar gas rather than by starlight alone.

Image Credit:
Mark Petersen
Tim Schaeffer
Carl Björk
Steeve Body
Tarun Kottary
Patrick Sparkman
Sendhil Chinnasamy
Yann Sainty
Marcel Drechsler
Xavier Strottner
Dr. Patrick Ogle
Dr. R. Michael Rich
Dr. Lewis McCallum
Dr. Alberto Noriega-Crespo
Dr. Biny Sebastian

Research Paper:
SDSO1 is a Ghost Planetary Nebula Bow Shock in Front of M31