What is the New General Catalogue?

There have been hundreds of deep sky catalogues compiled over the millennia, with a handful remaining popular with both professional and amateur astronomers alike. One of these is the New General Catalogue* (NGC), and it’s fair to say there probably isn’t a serious astronomer alive who hasn’t observed objects listed in it. That said, if you’re new to the hobby, you may have heard of it but may not fully understand what it is. So what types of objects are in the catalogue? And why is it so popular?

*There are, of course two spellings that could be used here – catalogue or catalog. Most sources use Catalogue in reference to the NGC, and for the sake of consistency, we’ll use that spelling here.

The Catalogues of the Ancient Astronomers

Astronomers have been compiling catalogues of the stars for thousands of years. For example, the Greek astronomer Hipparchos published the earliest known catalogue in 129 BCE. His work provided the positions of 850 stars, while later works expanded on this by adding more stars and more detailed information.

Of particular note was the catalogue compiled by the Danish astronomer Tycho Brahe. The first edition was published in 1598 and listed the positions and magnitudes of 1004 stars, but it became a landmark for a different reason: it was the first to assign Greek letters (eg, Alpha, Beta, Gamma, etc) to the stars, according to their brightness.

Despite the variety of authors and their locations across the globe, each of these catalogues had something in common: they only contained stars. This was due to the fact that the telescope had yet to be invented, and wouldn’t come into popular astronomical use until the early 17th century. As such, astronomers could only record what they could see with their eyes, and almost all of the fainter objects (for example, globular star clusters and planetary nebulae) remained unseen and undiscovered.

(There were a few exceptions. For example, the Pleiades and Praesepe star clusters  – as well as a few others – were easily visible to the naked eye, as were the Great Orion Nebula and the Andromeda Galaxy. However, since their true nature was usually unknown, these objects were often either ignored or listed as being non-stellar.)

A New Catalogue for Telescopic Times

With the invention and widespread adoption of the telescope, astronomers were finally able to explore the night sky in much greater detail. This, in turn, led to a sudden burst of discoveries, the likes of which had never been seen before (or, arguably, since.) Now, it was no longer a matter of listing all the stars in the sky (a seemingly impossible task in itself) but also of listing all the other newly discovered objects too.

Obviously, any attempt to produce a catalogue that included both the stars and all the deep sky objects would be an enormous undertaking, and so astronomers tended to focus on one or the other.

For example, one of the earliest (and certainly the most famous) catalogue of deep sky objects is the Messier catalogue. Compiled by the French astronomer Charless Messier and first published in 1771, Messier included all the non-stellar objects that could easily be confused for a potential new comet discovery.

However, when he published his final version ten years later, it only included 103 objects, and there were some notable omissions. For example, the famous Double Cluster, easily visible to the naked eye, was not a part of Messier’s list. Additionally, since Messier was observing from Paris, France, he was unable to see much of the southern hemisphere, and while Messier never intended his list to be a fully comprehensive catalogue, it was apparent that something better was needed.

The Herschels and Dreyer Dive Deep

Starting in 1782 – the year after Messier published the final edition of his list – the British astronomer William Herschel embarked on a series of deep sky surveys that would ultimately become the backbone of the New General Catalogue. Over the next twenty years, he discovered over 2,500 objects and published his findings in a series of three catalogues, with the first being published in 1786. The second came along three years later, in 1789, with the final catalogue published in 1802.

Following William’s death in 1822, his son John took up the challenge and relocated to South Africa in 1833, where he spent the next five years cataloguing the stars and deep sky objects of the southern hemisphere. In all, he is credited with discovering another 1,754 objects, bringing the total discoverd by father and son to more than 4,200.

However, it wasn’t until the 1880’s that the New General Catalogue first came into being. The Danish astronomer Johan (often listed as John) Dreyer had published a supplement to William Herschel’s catalogue and intended to publish a second. The Royal Astronomical Society of England asked Dreyer to compile a completely new work instead, and hence, the first edition of the New General Catalogue was published in 1888.

Following its publication, Dreyer produced the first supplemental catalogue, the Index Catalogue of Nebulae and Clusters of Stars (IC) in 1895. The Revised New Catalogue of Nonstellar Astronomical Objects (RNGC) was then published in 1973, and included additional data and descriptions of objects.

What the New General Catalogue Contains and How It Works

In all, there are 7,840 objects in the New General Catalogue. The vast majority of these are galaxies, with star clusters and nebulae accounting for the rest. However, more than 30 have since been found to be merely stars, while a number of others are duplicates or errors. For example, NGC 1429, once thought to be a nebula, is now known not to exist.

In all, there are:

• 6,317 Galaxies
• 495 Open star clusters
• 247 Diffuse nebulae
• 133 Globular star clusters
• 96 Planetary nebulae

All of Messier’s objects are included in the NGC, but while every Messier object has a corresponding NGC number (for example, M45, the Pleiades, is also known as NGC 1432), astronomers will always defer to an object’s Messier number. For one thing, it’s much easier to remember a list of 110 objects, compared to a list of nearly 8,000. For example, if you talk about M45, almost everyone will know you’re referring to the Pleiades, whereas if you mention NGC 1432, you’ll most likely be met with blank stares!

Unlike Messier’s catalogue, which listed objects in an apparently random order, William Herschel ordered his catalogue by Right Ascension, and the Dreyer elected to keep that format for the NGC. At the time of the catalogue’s publication, NGC 1 had an RA of 00h 00m 07s, but in the years since the catalogue’s initial publication, the effects of precession have altered each object’s coordinates a little. As a result, according to Epoch 2000 coordinates, NGC 7800 and NGC 7801 are the two objects closest to having an R.A. of 00h 00m 00s, with NGC 7801 being slightly closer.

Compare this to the Caldwell catalogue. Compiled by the amateur astronomer Sir Patrick Moore, it was designed to be a supplement to Messier’s catalogue and therefore contains 109 of the best objects that Messier either omitted or that simply weren’t visible to him. As such, whereas the majority of Messier’s objects were located in the northern celestial hemisphere, like the NGC, the Caldwell catalogue also included objects located in the south.

However, whereas the NGC orders objects by Right Ascension, the Caldwell catalogue orders them by declination, from north to south. Caldwell 1 (NGC 188) therefore has a declination of +85° 15′ 18″ while Caldwell 109 (NGC 3195) has a declination of -80° 51′ 31″.

The Top 10 Best of the New General Catalogue

While Messier’s list certainly has some outstanding objects, there’s plenty more to see in the NGC. Here are just ten of the best you won’t find in Messier’s list.

1. NGC 104 – 47 Tucanae – This wonderful globular is bright enough to be visible to the naked eye and was thought to be a star in pre-telescopic times. Binoculars will show its bright core, while a small telescope will start to resolve the cluster into its individual stars.
2. NGC 292 – The Small Magellanic Cloud – It may be the lesser of the two satellite galaxies easily visible to naked eye, but the Small Magellanic Cloud has its own NGC number, and is still large enough to hold several dozen other NGC objects within it.
3. NGC 869/884 – The Double Cluster – Here’s one sight that would have been easily visible to Messier and yet somehow escaped his list. The Double Cluster can be seen with the naked eye from reasonably dark skies and looks great through binoculars and telescopes.
4. NGC 2392 – The Clown Face Nebula – Unlike many other planetary nebulae, the Clown Face is comparatively bright, making it an easy target for small telescopes. The nebula shows a bluish disc, but a magnification of over 150x is needed to see any detail
5. NGC 3372 – The Eta Carinae Nebula – Another easy naked eye target, the Eta Carinae Nebula is large and shows a wealth of detail through a telescope at low power. Look out for the Keyhole Nebula, a dark cloud of dust located near the nebula’s center.
6. NGC 4755 – The Jewel Box Cluster – Widely regarded as a contender for the title of “best open star cluster”, the Jewel Box is easily spotted with the naked eye beside Beta Crucis. While most are blue-white stars, you’ll also find several dozen stars of yellow and orange.
7. NGC 5128 – Centaurus A – Certainly one of the brightest galaxies in the sky, Centaurus A can be spotted with binoculars and telescopes. It has a distinct, broad dark band across its center that’s readily apparent through even small telescopes.
8. NGC 5139 – Omega Centauri – Like 47 Tucanae, Omega Centauri is bright enough to be seen with the naked eye and is easily mistaken for a star – hence its designation. Binoculars and telescopes will show it to be slightly flattened.
9. NGC 7000 – The North America Nebula – If you live under truly dark skies, you may be able to spot the North America Nebula with just your eyes, but light pollution can easily render it invisible. That said, it’s a favorite target for astrophotographers.
10. NGC 7293 – The Helix Nebula – The Helix can be notoriously fickle, especially if your skies suffer from a little light pollution. The problem is that its light is spread over a large area, making it difficult to see. While potentially visible in binoculars and a small scope, an aperture of 250mm or more and a filter will start to show some detail.