The Andromeda Galaxy and the SDSO 1 anomaly
The Andromeda galaxy (Messier 31) lies roughly 2.5 million light‑years from the Milky Way and is the closest major external galaxy. For centuries its sheer size and brightness have made it a key object for understanding how galaxies evolve.
That changed in 2021 with the discovery of SDSO 1, a massive, apparently random patch of [O III] emission close to M31. Amateur astrophotographers and professional astronomers alike were intrigued by SDSO 1 and various explanations were proposed, ranging from an inter‑galactic shock between our Galaxy and M31 to a foreground object aligned by chance.
A deeper look | evidence for a “ghost” planetary nebula
A new wide and deep image of M31, combining data from multiple observatories, has solved the mystery. By identifying the central star and analysing the shock structure, the research team concluded that SDSO 1 is a shock wave in the Milky Way driven by an otherwise invisible planetary nebula moving at Mach 7 relative to the surrounding gas. The shock compresses gas to ≈100,000 K, producing the bright [O III] emission, while surrounding H‑α waves are formed from material blown off the nebula’s shell. Without this shock front and trail, the nebula would be too tenuous and faint to observe. The object represents the first recognised member of a new class of very late‑stage “ghost planetary nebulae” (GPNe), in which the expanding shell has thinned so much that only a shock can reveal it. Please refer to the image analysis at the bottom of this page for further details and for an in‑depth discussion of these findings, please see the published research paper, available here: https://arxiv.org/pdf/2507.15834
Collaboration and data collection
Beginning in December 2024, members of the Deep Sky Collective, the Polaris Imaging Group and the initial discoverers Marcel Drechsler, Xavier Strottner and Yann Sainty teamed up with scientist Patrick Ogle to produce the most detailed image of M31 to date. Lead photographer Mark Petersen and observatory director Michael Rich supplied fresh data from Takahashi FSQ 106 telescopes operating at f/3.0 and f/3.6. The Deep Sky Collective contributed deep‑field datasets from Tarun Kottary, Patrick Sparkman and Sendhil Chinnasamy while Tim Schaeffer Coordinated the projected, Carl Bjork did all the pre processing and myself Steeve Body did the image processing. Yann Sainty also added further exposures to extend the integration time.
The final dataset represents a staggering 555 hours of integration, captured from Bortle 1–2 sites. Approximately 312 hours of the data are [O III] exposures and 184 hours are H‑α; the remainder comprises broadband RGB. All images were captured with high‑quality filters and combined using PixInsight.
Credits & acknowledgements
The science team included Dr Patrick Ogle (Space Telescope Science Institute), Dr Lewis McCallum (St Andrews), Dr Alberto Noriega‑Crespo (STScI), Dr Michael Rich (UCLA) and Dr Biny Sebastian. The Polaris Imaging Group’s Mark Petersen (lead photographer), Sendhil Chinnasamy (photographer) and the initial discoverers Marcel Drechsler, Yann Sainty and Xavier Strottner were instrumental. Special thanks go to Patrick for his scientific insight, to Carl for data stacking and problem‑solving, and to Tim for the coordination.
