Exploring the Sharpless Catalogue of Bright Nebulae: A Fantastic Journey Through the Nebulae of the Milky Way

Author | Wang Siliang

Review | Zheng Chengzhuo

Editor | Zhao Jingyuan

Among the vast sea of ​​stars, nebulae are one of the most fascinating celestial bodies. They are not only the birthplace of stars, but also the active display of chemical elements in the universe. Their colorful forms and mysterious properties attract the attention of countless astronomers and astronomy enthusiasts. As a star catalog covering many important nebulae, the Sharpless Catalog reveals many amazing nebula regions in the Milky Way and is an important data for recording these cosmic wonders.

Elephant's Trunk Nebula Sh 2-131 (photo by the author)

1. Origin and Development

In 1953, American astronomer Stewart Sharpless used images from the Palomar Sky Survey (POSS) to conduct an extensive survey and catalog of regions of ionized hydrogen (H II) in the Milky Way. H II regions are nebulae formed by young, hot stars energizing the surrounding gas and are often hotbeds of star formation. Based on this work, Sharpless published the first edition of the Sharpless Catalog of Bright Nebulae in 1953, which included 142 objects. This was followed by a second and final edition in 1959, which included 313 objects. The catalog covers H II regions as comprehensively as possible to the north of declination -27°, although some nebulae further south are also included. A complete image of the Sharpless Nebulae can be viewed in reference 8.

American astronomer Stuart Sharpless. Image source: Rochester Review, 29, 7 (1966)

The coordinates in the Sharpless Catalog were based on the Bonn and Cordoba catalogs, but the epoch of the second edition was adjusted to 1900. In the second edition, the coordinates of some southern celestial regions had an uncertainty of more than 1 arc minute. Therefore, the BFS (Blitz, Fich and Stark) revision was later published, adding 65 new regions and removing about 20 objects.

Sh 2-190 contains the Heart Nebula IC 1805 and the Fish Head Nebula NGC 896 (photo by the author)

2. Content, Features and Significance

Sharpless used a very broad definition of the term "H II region", which resulted in the inclusion of some planetary nebulae, supernova remnants, and reflection nebulae. In his catalog, Sharpless classified H II regions based on several parameters, such as diameter, morphology (round, elliptical, irregular), structure (amorphous/linear), brightness, and the stars associated with them. The paper also analyzed the distribution of H II regions and used these regions to determine the direction of the galactic plane. In addition to the true H II regions, the catalog also includes some "misclassified" objects, such as reflection nebulae (often associated with H II emission nebulae), large planetary nebulae, supernova remnants, Wolf-Rayet bubbles (such as the Dolphin Nebula Sh 2-308), and two Maffei galaxies.

Supernova remnant Veil Nebula Sh 2-103 (photo by the author)

The Sharpless Catalog of Bright Nebulae also includes some objects that already exist in other famous star catalogs [such as the Messier Catalog (M), the New General Catalog of Nebulae and Clusters and its Supplement (NGC and IC), and the RCW Catalog, etc.], including many nebulae familiar to astronomy enthusiasts, including classic objects in the Messier Catalog [such as the Swan Nebula (Sh 2-45) of M17 and the Great Nebula of Orion (Sh 2-281) of M42], as well as famous NGC objects [such as the Veil Nebula (Sh 2-103, above, composed of NGC 6960 and NGC 6992), and the California Nebula (Sh 2-220) of NGC 1499]. Of course, there are also many objects that have not been cataloged before, such as Barnard's Ring Sh 2-276 (in the direction of Orion, it can be regarded as one of the most famous objects in the catalog), and the Eridanus Ring Sh 2-245, which has a similar name but a larger area.

Although the Sharpless Catalog mainly includes objects in the northern sky, in order to fully understand the nebulae in the Milky Way, astronomers have also created similar catalogs covering the southern sky, such as the contemporary RCW and Gum catalogs. The RCW catalog was published by Australian astronomers Alex Rodgers, Colin Campbell, and John Whiteoak in 1960. It mainly covers the southern sky and is an expansion of Colin Gum's earlier catalog. Some nebulae in the Sharpless Catalog also appear in the RCW catalog, and many of them overlap with each other - at least 50 objects in the Sharpless Catalog also appear in the RCW catalog. There are many famous southern nebulae in the RCW catalog, such as RCW 53 Great Nebula in Carina (below). The Gum Nebula Catalog is less frequently used, but Gum 12 (often called the Gum Nebula) is very famous as the largest emission nebula in the sky. It is located in the direction of the Vela and Puppis constellations and spans about 36° (the area it occupies overlaps with the Vela supernova remnant), which is equivalent to the width of 72 full moons!

RCW 53 Carina Nebula (photo by the author)

The importance of the Sharpless Catalog lies not only in the fact that it provides astronomers with a systematic list of nebulae, but also in the fact that it provides valuable data for the study of star-forming regions. These nebulae are the cradles of star birth and are crucial for our understanding of the life cycle of stars, the structure and evolution of the Milky Way, and the physical and chemical properties of the interstellar medium.

3. Amateur Observation

For astrophotographers, the Sharpless Catalog is a pretty perfect list of nebulae to photograph, even in strong light pollution environments. Many experienced deep sky photographers have begun to photograph the Sharpless Nebulae one by one, producing Hubble-colored masterpieces of nebulae. Using a 3nm bandwidth SHO narrowband filter, these nebulae still have very high transmittance and contrast even under the interference of strong moonlight and urban light pollution. However, the size of the objects in the Sharpless Catalog varies greatly. Some objects, such as the Tianjin Nebula Sh 2-109, span more than 1000 arc minutes, while others are only a few arc minutes. This means that no matter what equipment you use, no matter how large your telescope and imaging sensor CCD camera are, you can always find an object that suits your equipment. However, this also means that if you hope to photograph many of the objects in the Sharpless Catalog, you will need a range of different equipment. The smallest targets may require large telescopes with focal lengths of more than 1000mm, while larger targets may require equipment such as very short focal length camera lenses.

Supernova remnant Sh 2-105 Crescent Nebula and part of the Cygnus Wide Nebula (photo by the author)

For astronomy enthusiasts who are keen on visual observation, the Sharpless Catalog of Bright Nebulae is also a treasure trove, and its challenge and difficulty far exceed the Messier Catalog and the Caldwell Catalog. Although the Sharpless Catalog of Bright Nebulae includes many Messier objects and NGC/IC objects, it also contains many objects that are not covered by other catalogs, especially low-excitation large H II clouds. These targets are huge in area and are therefore suitable for observation with small telescopes. Barnard's Ring Sh 2-276 can even be observed with the naked eye through an OIII filter. However, many of these nebulae are difficult for visual observers. Unlike CCD observers who can use H α filters, our eyes are very insensitive to the red H α emission line. This emission line is mainly detected by some cones that are sensitive to red, but the dark adaptation ability of these cones is much lower than that of rods that are sensitive to green. Therefore, we must rely on the O III emission line (496nm and 501nm) and the H β emission line (486nm). O III filters or narrow-band UHC filters covering O III and H β emission lines can be used; for low-excitation objects in this catalog, the intensity of the O III emission line is weak, and their best observation band is usually the H β band. For these objects, H β filters are recommended. But even with appropriate filters, many Sharpless objects are still difficult to observe visually due to their low brightness, large area and low contrast. Visual observation of these objects often requires dark, clear sky conditions. However, the night vision goggles that have become popular in the amateur astronomy circle in recent years have gradually smoothed out these visual gaps, especially the use of F1-2.8 large-aperture telephoto fast lens with H α+O III dual narrow-band filter. The very fine details of these nebulae can be displayed in real time in the night vision goggles of more than three generations, providing a full immersive experience.

Nebulae photographed through the night vision eyepiece, involving 13 Sharpless Nebulae (photographed by the author)

The Sharpless Catalog is not only an important tool for astronomical research, but also provides a rich resource for astronomy enthusiasts to explore. It promotes the study of star formation and interstellar medium by systematically recording and classifying H II regions and related celestial bodies in the Milky Way, and has become a challenging and fascinating observation and photography list in the field of amateur astronomy. In the future, the understanding of these nebulae will be more comprehensive and in-depth, and the Sharpless Catalog will continue to shine in the development of astronomy.

References

1. Sharpless catalog, Wikipedia, https://en.wikipedia.org/wiki/Sharpless_catalog

2. Stewart Sharpless, A Catalog of H II Regions, Astrophysical Journal Supplement, 1959, Vol. 4, P.257-279

3. sharpless HII regions, Telescopes and Deep Sky by Reiner Vogel, https://www.reinervogel.net/index_e.html?/Sharpless/Sharpless_e.html

4. Reiner Vogel, The Sharpless Observing Guide, 2012, https://www.reinervogel.net/pdf/Sharpless.pdf

5. Sharpless Catalogue Observers Guide, Astronomy, Astrophotography and Telescope Blog, http://astro.neutral.org/astronomy_blog/blog/item/2010/08/sharpless-catalogue-observers-guide

6. Sharpless HII Regions, Adventures in Deep Space, https://adventuresindeepspace.com/sharp.htm

7. Alan Goldstein, Gum Nebula, Astronomy, https://www.astronomy.com/science/gum-nebula/

8. The Sharpless Catalog, Galaxy Map, http://galaxymap.org/cat/list/sharpless/1

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