Sh2-115 - wonderfuluniverse.de

Sh2-115 from DSW

In 2018, I received a deep SHO narrowband dataset of Sh2-115, including the faint planetary nebula Abell 71, from the Deep Sky West (DSW) Observatory. Located beneath the exceptionally dark skies of New Mexico, the observatory provided high-quality S II, Hα, and O III data that reveal both the extensive hydrogen emission of Sh2-115 and the subtle shell of the much smaller planetary nebula Abell 71. The combination of these two very different nebular objects within a single field offers a fascinating view of both the birth and death of stars.

Located in the constellation Cygnus, Sh2-115 is a diffuse H II region situated approximately 7,500 light-years from Earth. It forms part of the vast Cygnus X complex, one of the richest star-forming regions in the Milky Way. This enormous concentration of molecular clouds, young stellar clusters, and emission nebulae provides astronomers with an outstanding laboratory for studying the formation and evolution of massive stars.

Sh2-115 is an H II region catalogued by Stewart Sharpless in 1959, located a couple of degrees northwest of Deneb in Cygnus. It lies on the ionized edge of a giant molecular cloud in the Orion Arm of the Milky Way, at a distance of roughly 2.3–3 kpc, i.e. about 7,500 light‑years.

The nebula’s glow is powered mainly by hot, massive O‑ and B‑type stars in the open cluster Berkeley 90, which sits in the eastern part of the nebula and is physically associated with it. Structurally, Sh2‑115 appears as a chaotic, oval patch of emission threaded by dark lanes of dust, with filaments and knots that show up well in narrowband Hα and often very faintly in [O III] and [S II].

Abell 71 is the small, nearly circular patch of emission near Sh2‑115, often seen as a bluish or teal disk in narrowband or RGB images depending on the mapping. It carries several designations (Abell 71, Sh2‑116, PK 085+04.1, PN G084.9+04.4) because it was originally classified as a planetary nebula by George Abell in the 1950s.

Later work has questioned this classification because Abell 71 shows very weak green [O III] emission compared to typical planetary nebulae, and its morphology includes small extensions or “arms” more reminiscent of irregular H II patches. Some authors therefore treat it as a compact H II region, while others still argue for a very evolved planetary nebula, pointing to spectroscopic evidence of expansion and the presence of a faint central star around magnitude 18.9.

The coexistence of Sh2-115 and Abell 71 within the same field provides an excellent illustration of the stellar life cycle. While Sh2-115 represents the birthplace of massive stars still embedded within their natal clouds, Abell 71 records the final chapter in the evolution of a star with a mass similar to that of our Sun. The contrast between these two objects emphasizes the continuous recycling of matter within the Galaxy, as material expelled by dying stars eventually becomes incorporated into new generations of stellar systems.

The SHO dataset from Deep Sky West Observatory beautifully captures the rich complexity of this region. Brilliant hydrogen clouds, subtle sulfur and oxygen structures, and the delicate shell of Abell 71 are all rendered with exceptional detail, demonstrating how narrowband imaging can reveal faint structures that remain largely invisible in broadband observations. Together, these two nebulae create a remarkable portrait of both stellar birth and stellar death, unfolding side by side against the spectacular backdrop of the Milky Way.

Data calibration, registration, and final image processing were all performed using PixInsight. The result was as follows: