Key Takeaways:
- Babylonian astronomers created early star catalogs as early as the 12th century BCE, later expanding these to include celestial motions of the Sun, Moon, and planets by 1000 BCE.
- The Messier (M) catalog, compiled by Charles Messier, contains 109 bright, easily mistaken-for-comets objects visible primarily in the Northern Hemisphere, encompassing various deep-sky object types.
- The New General Catalog (NGC) and its Index Catalogues (IC) supplements, compiled by John Louis Emil Dreyer, list 7,840 and 5,386 objects respectively, primarily galaxies, nebulae, and star clusters, expanding upon earlier Herschel catalogs.
- Other significant catalogs mentioned include the Sharpless (Sh 2) catalog focused on HII regions, and Abell catalogs for both rich galaxy clusters and planetary nebulae, showcasing diverse approaches to celestial object classification and cataloging.
Humanity is obsessed with astronomical record keeping. The Babylonians compiled their first listing of the stars in the 12th century b.c.e. Around 1000 b.c.e., they followed it with an expanded catalog that includes familiar constellations and star clusters, as well as motions of the Sun, the Moon, and the planets. The Mayans inscribed into stone and painted onto cloth calendars and almanacs, tracking planets, eclipses, and possibly even the precession of the equinoxes.
Today, telescopes and clocks have allowed for precise records of object locations. Our most prolific surveyor of the cosmos, the European Space Agency’s Gaia satellite, has measured the positions of approximately 1 billion stars and objects. A beginning astronomer can be confused by the plethora of catalogs we use to identify the deep-sky objects we seek in our telescopes, especially when many are listed in multiple catalogs. Let’s explore the more common ones.
The first catalog that beginning astronomers are likely to encounter is the Messier catalog. Named for 18th-century French astronomer Charles Messier, the register contains 109 bright objects (103 original, plus six later additions) that could be mistaken for comets by the unsuspecting observer. Because he was observing from France with a 4-inch refractor, all Messier objects are visible in the Northern Hemisphere, with the southernmost at about –35° declination. Objects range from star clusters and galaxies to nebulae and supernova remnants. Entries are numbered with the prefix M. For instance, the elegant Whirlpool Galaxy in Canes Venatici is M51.
Another common catalog is the New General Catalogue, or NGC. Compiled by Danish astronomer John Louis Emil Dreyer in 1888, it contains 7,840 objects, ordered by right ascension. Many of the objects are galaxies, but it also includes nebulae and star clusters. Dreyer later created two supplements, the Index Catalogues (IC), which list another 5,386 objects. The NGC is descended from William and Caroline Herschel’s catalog first published in 1786.
For nebula lovers, the Sharpless catalog is a close friend. American astronomer Stewart Sharpless published his first catalog in 1953 with 142 objects, then added more in 1959 for a total of 312. This list is almost exclusively HII regions (emission nebulae rich in hydrogen), with a few planetary nebulae and supernova remnants in the mix, surveyed from photographic plates from the Palomar Observatory Sky Survey. Sharpless objects carry the abbreviated label Sh 2, indicating they come from his second catalog. For example, the Lagoon Nebula, also known as M8, is Sh 2–25.
Confusingly, there are two Abell catalogs — one for rich galaxy clusters, and one for planetary nebulae. They were both compiled by the same American astronomer, George O. Abell. The galactic catalog contains a total of 4,073 dense groupings of galaxies (2,712 northern and 1,361 southern), with a minimum of 50 galaxies required to qualify. This is a challenging catalog to explore, requiring large telescopes and dark skies.
The planetary nebulae catalog, published in 1966, has 86 objects. Four were later determined not to be planetary nebulae, but rather other kinds of nebulae, a ring galaxy, and an imaging artifact. Both catalogs use the identifier Abell, encompassing objects such as Abell 21 (the Medusa Nebula) and Abell 1656 (the Coma Galaxy Cluster).
These are a sampling of the catalogs that amateur astronomers most commonly encounter, but advanced astrophotographers tracking down infrequently observed objects may stumble across many more. Let these catalogs be a guide on your astronomical adventures. Happy hunting!
