TL;DR

This image combines Sh2-129, the red HII region known as the Flying Bat, Ou4, the extremely faint OIII Squid Nebula, and Barnard 150, the dark foreground Seahorse cloud. Ou4 is the main focus of this project. In the current literature, the Squid is described as a fast bipolar outflow likely associated with the massive triple star HR 8119, expanding inside Sh2-129 and glowing most strongly where the moving gas has compressed and heated the surrounding material. While once believed to be a planetary nebula, this is no longer the case. Barnard 150 is much closer to us and only appears aligned with the Sh2-129/Ou4 system because of projection. The final image is the result of nearly 1,900 hours of collaborative data from 28 contributors, combining many different imaging systems into one deep view of emission, extinction, and a very strange-looking outflow in Cepheus.

Overview of the Field of View

This image contains three distinct features of the interstellar medium: recombination emission for ionized hydrogen in the central red HII region known as Sh2-129 (The Flying Bat), shock-excited OIII emission in the bipolar structure of Ou4 (The Squid), and dust extinction from the cold molecular cloud Barnard 150 (The Seahorse). Near the apparent centre of Ou4 lies HR 8119, a massive triple star system associated with Sh2-129 and favoured as the possible source of the fast, collimated outflow that shapes the Squid. B150, by contrast, is a foreground molecular cloud seen in silhouette against the wider background field. There is also a compact infrared source / planetary-nebula candidate in the field, IRAS 21034+5941, although it is difficult to distinguish at this image scale.

Line-of-Sight Geometry and Distances

The alignment of Barnard 150 with the Sh2-129/Ou4 system is a projection effect to us here on Earth, while Sh2-129 and Ou4 may be physically associated. Corradi et al. determined a distance of 712 pc for HR 8119 and Sh 2-129, and argued that Ou4’s position, reddening, radial velocities, and morphology are consistent with a shock-excited outflow located inside the Sh2-129 H II region. That interpretation is favoured, but not final. However, bipolar planetary nebula or an eruptive event from an unidentified asymptotic giant branch (AGB) or post-AGB star is not fully excluded.

Raw OIII data of the Ou4 region.

The OIII detail in Ou4 helps explain why the nebula has such an unusual shape. According to Corradi et al., the Squid formed when a fast, narrow flow of gas from the region of the central star system pushed outward into the surrounding gas of Sh2-129, creating two large opposing lobes. The whole volume of the object does not glow evenly because the OIII signal is brightest where the moving gas has swept up, compressed, and heated material along the the edges and lobe tips. That is what gives Ou4 its bright rims, curved internal arcs, and stronger terminal regions. The uneven structure inside the lobes likely comes from the flow moving through a non-uniform HII region, where some areas are denser than others. The Squid looks strange, but its traces how a fast bipolar outflow interacted with the surrounding gas inside Sh2-129.

Barnard 150 is foreground to that system. Published estimates for the associated GF9 region include 270 ± 10 pc from Gaia DR2 and polarimetric analysis, while broader outreach summaries of B150 often quote roughly 1,200 light-years, or about 368 pc. Either way, B150 lies in front of the Sh 2-129/Ou4 system and is not physically part of it.

A plot of the relative distance between of major features from an observer on Earth

Sh 2-129 and the Ou4 Outflow

Ou4 was discovered by @Nicolas OUTTERS in June 2011 during deep narrowband imaging of Sh2-129. The discovery used 12.5 hours of OIII exposure with a 106 mm f/5 refractor, a useful reminder that with persistence and dedication amateurs can make scientifically interesting discoveries. When a team of professional astrophysicists published a 2014 paper in Astronomy & Astrophysics detailing the physical conditions of Ou4, they specifically highlighted Outters' gear and methods in their introduction. They wrote:

"It is a clear example of the relevant science that can be done with very small telescopes: the long integrations that can be obtained by dedicated amateur astronomers allow very faint detection levels to be reached, which are sometimes difficult to achieve with professional telescopes given the limited amount of time available."

Acker et al. initially presented Ou4 as a planetary-nebula candidate with an angular extent greater than one degree, making it an unusually large and physically awkward object under that interpretation. The authors considered the possibility that Ou4 could be one of the nearest planetary nebulae known, while noting that its PN nature still required confirmation.

Subsequent spectroscopy and kinematic modelling by Corradi et al. argued that Ou4 is better explained as a shock-excited, fast, collimated bipolar outflow associated with HR 8119 given the latest research. At the adopted 712 pc distance, Ou4 has a projected physical size of 14.4 × 2.5 pc. Their bow-shock model gives velocities of approximately 112 km/s for the southern lobe and 83 km/s for the northern lobe, with a dynamical age near 90,000 years. The estimated kinetic energy is about 4 × 10⁴⁷ erg. That is well below a supernova, but still enough to suggest that the Squid did not enter the field quietly.

Barnard 150 and Low-Mass Star Formation

Barnard 150, also associated with LDN 1082 and GF9, is a dark molecular cloud visible in silhouette against the background stellar field and diffuse galactic emission. First catalogued by E. E. Barnard as one of the early dark markings in the sky, B150 removes photons from the line of sight through absorption and scattering by cold interstellar dust grains.

In B150, the high dust column density produces a large optical depth at visible wavelengths. The transmitted flux is attenuated approximately according to the radiative transfer equation, reducing the background optical starlight and diffuse emission and producing the observed dark silhouette.

The GF9 region within this dark cloud is also an active site of low-mass star formation. GF9-2, also known as IRAS 20503+6006, is identified as an extremely young, low-luminosity Class 0 protostar. Furuya et al.identified a cold protostellar envelope around GF9-2 in 350 μm dust-continuum emission, and used molecular-line profiles to infer inward gas motions, indicating that the core is in a very early stage of gravitational collapse.

Additional NH₃ mapping of the GF9 filament revealed at least seven dense cores, plus three possible additional cores, with kinetic temperatures below about 10 K and masses consistent with low-mass star-forming sites. The spacing and masses of these cores are consistent with gravitational fragmentation along the natal filament.

Additional Small-Scale Structures in the Field

There is an interesting feature inside the Sh2-129 emission nebula in Cepheus, near RA 21h 17m 53.3s, Dec +60° 07′ 30″. While the object is unlikely to be of scientific interest, we performed a rudimentary analysis to better understand it. In RGB and Hα it appears primarily as a compact dark absorption region projected against the surrounding ionized gas. The same area shows weak, patchy OIII structure and a diffuse WISE W3 signal, while W1, W2, and W4 do not show a convincing compact source at the area itself.

The multi-band appearance favours a small externally illuminatd dust clump or photodissociation-region feature. Catalogue checks show no obvious SIMBAD planetary nebula or compact nebula at the position. A nearby 2RXS/SIMBAD X-ray source appears offset from the dark globule and is more likely associated with a separate infrared/stellar point source.

A conservative interpretation is that this is an uncatalogued small dust/PDR substructure embedded in the Sh2-129 field, with local Hα/OIII emission and W3-bright dust emission tracing material around the obscuring dark dust.

PDR substructure mentioned above
WISE W3 of the region

GN 21.01.6 is also visible in the RGB data as a small reflection nebula. In the supporting figure, GN 21.01.6 is shown at top, with the PDR substructure mentioned above shown at bottom.

GN 21.01.6 -- Reflection Nebula (RGB)

Positions of additional objects.

Collaboration Details
This collaboration began in May 2025 after being proposed by @Chris Todd .Since then, 28 contributors have gathered nearly 1,900 hours of data on this field. One interesting aspect of this project is that there were no strict equipment requirements. Any system regardless of focal length, focal ratio, or camera type, could contribute.

That openness made the project more accessible, but it also made the processing far less straightforward. Combining data from many different systems meant dealing with large differences in image scale, field rotation, optical quality, filter response, sky conditions, signal depth, and gradient behaviour. The registration, normalization, rejection, and stacking stages all required careful tuning so that the final integration could benefit from the full data set without being dominated by its most uneven parts.

We ultimately cropped to a smaller region of the full field because the widest combined footprint did not have enough uniform coverage for even processing across the entire frame. The crop used for the final image preserves the strongest and most consistent data coverage while still retaining the most interesting objects in the region.The exposure map below shows the non-uniform coverage across the full data set and illustrates why the final framing was chosen.

The uncropped very wide field provided by @Nick Fritz

The image subs were stacked and the image processed by @adon, who also wrote this description and created the figures. A big thank you to all contributors for the time, data, and patience required to make a project of this scale work, and to Starfront and its Discord community (esp. ip) for providing the structure that made the collaboration possible.

Renditions Provided by Other Contributors

Version by @NE-FL-Astro

Version by @Nick Fritz

References

Acker et al.
Corradi et al. 2014
Furuya et al. 2006
Furuya et al. 2008
Clemens et al. 2018
NASA APOD 2023-04-20