Sh2-305

Sh2-305 is one of the many intriguing emission nebulae found in the Milky Way. Located in the southern constellation of Puppis, this H II region is a vast cloud of ionized gas, where the energy from newly formed, hot stars excites the surrounding hydrogen making it glow in its characteristic reddish hue. The nebula is also known under alternative designations such as RCW 11 and LBN 1084.

First Discovery

Sh2-305 was included as entry number 305 in the Sharpless Catalogue, a landmark 1959 compilation by Stewart Sharpless of optical H II regions based on photographic survey data from Palomar and other observatories. Sharpless’s work laid the foundation for understanding star-forming regions throughout the Milky Way.

Sh2-305 is now increasingly studied due to its active star formation and location in a relatively under-imaged region of the Galaxy. Early observations, made using photographic plates, laid the groundwork for later detailed studies using modern telescopes and advanced imaging techniques. 

Structure and Composition

Sh2-305 is primarily composed of ionized hydrogen gas, with traces of helium, oxygen, nitrogen, and other elements that contribute to its emission spectrum. Observations using infrared (2MASS, WISE) and radio wavelengths have revealed filamentary structures, dense clumps, and bubble-like morphologies, often shaped by the stellar winds from nearby massive stars.

The ionizing source of Sh2-305 is believed to be the open cluster Dolidze 25, located within the nebula itself. This cluster contains several O- and B-type stars, which provide the intense ultraviolet radiation that energizes the surrounding gas.

According to a 2010 study by Deharveng et al. in Astronomy & Astrophysics, “Sh2-305 shows morphological evidence of triggered star formation at its edges”, supporting the idea that stellar feedback—winds and radiation from hot stars—can compress nearby gas and initiate further star formation.

Star Population

The area surrounding Sh2-305 hosts a population of young stellar objects (YSOs) and pre-main-sequence stars, many of which have been identified via their infrared excess in data from Spitzer and WISE. These stars are still embedded in their natal clouds and are in early evolutionary stages.

In a study of Galactic star-forming regions, Russeil et al. (2007) highlighted Sh2-305 as part of a larger OB association complex in the Outer Galaxy, emphasizing its role in the ongoing buildup of stellar populations in spiral arms.

_{Sh2-305 shows morphological evidence of triggered star formation at its edges}

Offering a helpful photometric reference in this image, just southeast of Sh2-305, lies HD 59674, a relatively F-type main-sequence star (spectral type F5) with an apparent magnitude of 7.9. Located just 370 light-years from Earth—vastly closer than the nebula—it is unrelated to Sh2-305 but appears superimposed due to our line of sight.

Scientific Significance

The nebula’s scientific significance extends beyond its role in star formation. Studies of Sh2-305 help understanding the feedback effects of massive stars on the interstellar medium. Its bubble-like structures and ionization fronts are of particular interest to researchers studying triggered star formation, molecular cloud dynamics, and the evolution of H II regions.

A 2020 paper by Samal et al. (A&A) examining similar H II regions notes: “The study of large-scale star formation triggered by expanding H II regions is critical for understanding the role of feedback in shaping galactic structure”.

Like many H II regions, Sh2-305 is a transient phase in the lifecycle of interstellar clouds. Over the next few million years, radiation pressure and stellar winds will likely continue to erode the surrounding molecular cloud, halting star formation in some areas while possibly triggering it in others.

Eventually, the most massive stars within Dolidze 25 will reach the end of their lives and may explode as supernovae, injecting heavy elements into the surrounding interstellar medium and leaving behind neutron stars or black holes. These events contribute to the chemical evolution of the galaxy and the enrichment of future stellar generations.

Planetary Nebula Drechsler 14

Located far from Sh2-305 (right ascension 19h13m45.30s and declination +13°22'09.99”), Drechsler 14 appears in the wider imaging field captured by the team. Also catalogued as StDr 14, it has an apparent size of approximately 1 arcminute and has been classified as a likely planetary nebula. ​

The designation “StDr” refers to the collaborative efforts of amateur astronomers Xavier Strottner and Marcel Drechsler, who have contributed to the identification and cataloguing of numerous planetary nebula candidates. 

Unfortunately, specific details about Drechsler 14's morphology, central star or distance remain limited. Its identification as a probable planetary nebula suggests it is the remnant of a dying star that has expelled its outer layers, leaving behind a glowing shell of ionized gas. Continued observations and studies are necessary to confirm its nature and to gather more comprehensive data about its characteristics.

ShaRA Project #12.1—A scientific collaboration

This image of Sh2-305 marks the Project #12.1 of the Team. The data was acquired with the Planewave CDK24 telescope of Vikas Chander, an internationally recognised photographer and member of the team. The final image, a.k.a. the “superstack”, includes the contribution of each member of the team who took part in the project. This reflects the core spirit of the ShaRA project: bringing together passionate, skilled astrophotographers from around the world to explore the infinite beauties of the cosmos.

In a testament to the growing impact of the ShaRA Project, this image was commissioned by Dr. Mark McCaughrean—former Senior Advisor for Science & Exploration at ESA, now at the Max Planck Institute for Astronomy—for inclusion in his upcoming book “111 Places in Space You Must Not Miss” (Emons Verlag, Cologne).

Dr. McCaughrean shared that the James Webb Space Telescope will soon observe the core of Sh2-305, focusing on its embedded stellar cluster and potential protostars. His proposal includes examining jets, outflows, and the low-mass stellar population within the region.

He contacted the team after struggling to find a suitable wide-field image of Sh2-305, noting that “Sh 2-305 really hasn't been imaged often at optical wavelengths”. He invited us to consider producing additional wide-field optical context imagery to complement the JWST survey—potentially opening the door to a joint scientific collaboration. Our image will appear alongside a chapter on the nebula, helping to introduce a broader audience to this often-overlooked region of the Milky Way.

This recognition and outreach reinforce the ShaRA Project’s mission: to combine artistic vision, scientific rigour, and international collaboration—and highlight the growing appreciation for the quality of our work.

Observing Sh2-305

Sh2-305 lies at approximately RA 07h 32m, Dec -18° 45′, in Puppis. It is best viewed from southern latitudes, though it becomes accessible to observers in the southern U.S., Canary Islands, and Mediterranean regions in the late winter to early spring (January to March).

The nebula is a rewarding target for narrowband imaging, especially in H-alpha, and benefits from dark skies and long integration times due to its relative faintness. While not easily visible visually, astrophotographers using telescopes with moderate to long focal lengths can capture its reddish glow and surrounding structures, particularly with the help of H-alpha and OIII filters.

Reference List

1. Sharpless, S. (1959). A Catalogue of H II Regions. Astrophysical Journal Supplement Series, 4, 257.
2. Deharveng, L., et al. (2010). Triggered star formation on the borders of Galactic H II regions. A&A, 523, A6.
3. Russeil, D., et al. (2007). Star formation in the outer Galaxy. A&A, 470, 161.
4. Samal, M. R., et al. (2020). Star formation around expanding H II regions. A&A, 641, A28.
5. SIMBAD Astronomical Database. http://simbad.u-strasbg.fr
6. WISE & Spitzer Data Archives (NASA/IPAC Infrared Science Archive)