Abstract:
This article explores the astrophysical context of the emission nebula Sh2-129 and the embedded bipolar outflow Ou4, also known as the Giant Squid Nebula. Sh2-129 is a large H II region in Cepheus, shaped and ionized by massive stars within the Cepheus OB2 association, most notably the early-type B-star system HD 202214 (HR 8119).

Ou4, discovered in 2011 by amateur astrophotographer Nicolas Outters, is a vast and faint [O III]-emitting structure with a striking bipolar morphology. Initially considered a planetary nebula, recent spectroscopic studies suggest it is more likely a shock-excited outflow, not associated with a typical hot central star. Observational evidence indicates a true spatial and physical connection between Ou4 and Sh2-129, with shared distance, radial velocity, and signs of dynamic interaction, including bow shocks and a central dust cavity.

1. Introduction
In my previous post, I focused on the image processing techniques used to reveal the structures of Sh2-129 and the bipolar outflow named „Ou4“. This time, the spotlight shifts to the astrophysical background of this remarkable region. The large H II region Sh2-129 is a hydrogen emission and was shaped by the influence of massive stars. Embedded within lies a faint and mysterious outflow: „Outters 4“
The true nature of  Ou4 remains a subject of ongoing research. In this article, I explore the structural characteristics and current scientific understanding of this fascinating object as well as the story of its discovery by an amateur astrophotographer in 2011.

2. Discovery of Ou4

French amateur astronomer Nicolas Outters cataloged the object only in 2011 as the fourth entry in his list of personal discoveries—hence „Ou“ stands for Outters and „4“ for the fourth nebula he identified. During deep narrowband imaging of Sh2-129, Ou4 turned out to be an enormous and previously unknown bipolar structure. Its faint O-III emission made it invisible in traditional broadband images, but with the right filters and long exposures, its dramatic shape emerged—earning it the nickname „Giant Squid Nebula“.

Really interesting to read the report on the discovery. Here is a link to Nicolas‘ document and the dedicated page of the discovery:

3. Sources of Ionization

How is such a shaped double-ionized oxygen (O-III) structure formed and how is its formation related to the larger H-II emission nebula? Which stars or star systems were involved in its formation and how does Sh2-129 interact with the Squid Nebula (Ou4)? Is Ou4 maybe a planetary nebula and if so, where is its hot and compact central star?

The H-II emission nebula Sh2-129 is ionized primarily by the ultraviolet radiation from the massive triple star system HD 202214 (HR 8119), located near its center. This system consists of early-type B0–B0.5 main-sequence stars and is part of the Cepheus OB2 association, at a distance of approximately 2,320 light-years.

Ou4 is a large bipolar structure rich in [O III] emission, located within the H-II nebula. Although initially classified as a possible planetary nebula, spectroscopic studies (especially Corradi et al. 2014) show that:

• Ou4 is not a classical planetary nebula, but rather a shock-excited bipolar outflow.
• It was likely ejected by the same central stars (especially HR 8119) that ionize Sh2-129.
• Ionization of Ou4 occurs through a combination of shocks (from the outflow interacting with surrounding gas) and UV radiation from the OB stars.
• No separate hot central star typical of planetary nebulae has been found in Ou4.

All current evidence supports also a true physical association between the H-II and O-III regions:

• Matching velocity, distance, and extinction strongly suggest Ou4 lies within Sh2-129, not merely in projection.
• The bipolar lobes of Ou4 are centered on the same star cluster, aligning morphologically and dynamically with Sh2-129’s structure.
• Shock fronts and a dust bubble observed in the same region point to active interaction—Ou4 is likely carving into the interior of Sh2-129.