How Deep-Sky Catalogs Work


Meet the Veil Nebula, one of my favorite deep-sky objects.

The Veil is one of the more common star party requests I get from more experienced participants. Unfortunately, it requires a very powerful telescope. My 11-inch Schmidt-Cassegrain–pretty advanced, as far as intermediate amateur telescopes go–can barely manage it with a nebula filter.

The Veil has several different segments and can’t be viewed all at once. Seriously–the entire Veil Nebula covers an area six times the diameter of the full moon! If it were bright enough to see with the naked eye, it would be a very visible object.

Together, the segments of the veil make up the Cygnus Loop: a ring-shaped phenomenon that is a supernova remnant, formed roughly 10,000 years ago. That’s actually not that long ago, in astronomical terms. But other supernova remnants, such as the Crab Nebula, are much younger.

Those segments have all been observed separately over time and ended up with separate designations in star catalogs, too. The Veil’s components within the NGC star catalog are NGC 6960, NGC 6992, NGC 6995, and IC 1340. It is also known in the Caldwell catalog by Caldwell 34 and 33.

Fainter “knots” of nebulosity that you might not immediately realize are part of a broad, wispy loop are noted as NGC 6974 and NGC 6979.

Different portions of the supernova remnant have also been named the “Witch’s Broom” and “Pickering’s Triangle.” In particular, the Witch’s Broom refers to the same segment as the picture shown above–the Western Veil.

For this post, I thought tell you a bit about how star catalogs work–and share an interesting story about the NGCs!

The oldest star catalog in use today–that I know of–is the Messier catalog, compiled by French astronomer Charles Messier.

Pictured above is my absolute favorite Messier object: M13, the Hercules Cluster. This is a globular cluster, a very particular type of star cluster with fascinating origins. Sometime, I’ll write a post detailing what I know about these objects.

More importantly, however…

If you have good vision, you can probably make out M13 with your naked eye–if you know where to look. The same can be said of such open clusters as M6 and M7, both near the stinger of the constellation Scorpius. The Messier catalog is also home to galaxies, nebulae, and even double stars.

But one thing all Messier objects have in common, even though some are far less visible than others, is that they’re all fairly easy to spot with a telescope. I like them for star parties because it’s always a pretty good bet that I can align my 5-inch refractor to one of them, even though it’s a small manual telescope.

That’s no coincidence, either. The Messier catalog comes from a much earlier time in astronomy, before advancements in telescope optics allowed us to discover fainter objects.

In fact, Charles Messier compiled his catalog in order to discover a comet–because in his day, it was very hard to tell those apart from blurry deep-space objects!

The one surefire way to identify a comet is to observe it moving relative to the surrounding stars. Galaxies and star clusters travel slowly through space, but not nearly as fast as comets–and in Messier’s day, I’m not sure instruments were fine-tuned enough to even discover that deep-space objects move.

So Messier cataloged 110 objects, identifying each one as a blurry not-comet before moving on. And the result was the Messier catalog.

The Messier catalog is also quite the draw for amateur astronomers. Each year during March, astronomers from all over flock to dark-sky locations to marathon observing every single Messier object in one night.

You might wonder how that’s even possible. But it seems the Messiers have a distinct advantage over other catalogs, and I don’t just mean that they’re relatively easy to find. Apparently, if you snag them during a particular time of the year (March), you actually can see them all in one night. Just as long as you view them strategically, before each one sets as the sky rotates above you!

I’ve never actually completed a Messier Marathon yet–but I hope to, just as soon as I can afford to pull an all-nighter again! Working 9-5 shifts of menial labor isn’t exactly conducive to nighttime observing, let alone staying up till morning.

I still enjoy them at star parties, though. It’s always nice to have a set of objects that I can count on being easy to find.

NGC objects, on the other hand, are much more obscure.

“NGC” stands for New General Catalog. (I’ve always thought it was New Galactic Catalog…but I guess not.) And this one doesn’t stop at 110–it has a whopping 7,840 objects!

Except…that’s actually open for debate. Such as in the case of objects such as “NGC 6666.”

I first discovered the mystery surrounding NGC 6666 while scrolling through the r/askastronomy subreddit. Someone had asked what the object was. Weirdly enough, there’s no clear information on whether it’s a galaxy, star cluster, nebula, etc. Usually that’s pretty much the first thing you find when you search up a catalog object.

Even weirder, it doesn’t appear in any star atlases.

But more than that…while NGC 6666 has no clear object description, all sources have a few bits of info in common. It has consistent right-ascension and declination coordinates, a consistent discoverer (Swift), and even consistent shorthand notation! One website I found (which I don’t remember) even mapped out its location in the sky with a little circle drawn where it should be.

But what I found most mysterious was the fact that there was no internet chatter whatsoever about this apparently “nonexistent” object that had specific RA and dec coords.

In light of the history of the NGC catalog, though, it all makes sense.

by Lemuel Francis Abbott, painting, 1785

This is William Herschel, the guy who began compiling the beginnings of the NGC catalog along with his sister Caroline in the 18th century.

It’s important to note that the Herschels did not observe all the objects they compiled. They worked based off of other astronomers’ observations, though they did make plenty of their own. By 1810, the catalog was up to 2,500 objects. In the early 19th century, William’s son John added more objects. By 1864, the catalog was up to 5,079 objects.

Along comes John Louis Emil Dreyer. He gathered data from 50 sources, including the Herschels’ work, to compile his own grand catalog. In 1888, Dreyer published the New General Catalog, complete with 7,840 objects.

In 1895, Dreyer published an extension to the NGC catalog–the IC (Index Catalog). In 1908, he published a second volume of the IC. Together, both volumes indexed another 5,386 objects.

But here’s the catch. Dreyer never managed to visually confirm every object.

I mean, honestly…how could he? There were well over 7,000 objects from more than 50 sources. That would take a huge, combined effort to verify. He just assumed that the objects other people, like the Herschels, had observed, were in fact accurate.

Since then, astronomers haven’t made an effort to fill in the gaps in Dreyer’s work–it would be even harder for them than it was for him! It takes time, effort, and a little bit of prestige for astronomers to win observing time at the major observatories around the world, and it’s far more productive to spend that time making new discoveries than to verify something like 13,000 objects.

Why is there no internet chatter about the mysterious nonexistence of NGC 6666? And why does every source give consistent RA and dec coords?

Because the “object” isn’t special–there are many missing NGC “objects.” And because the initial notes from some guy named Swift were passed down from the Herschels’ work, written into Dreyer’s published catalog and never confirmed.

You’ll notice that earlier deep-sky catalogs like the Messier catalog contain much brighter and more visible objects than later catalogs, like the New General Catalog and the Caldwell catalog.

That’s because–as I mentioned above–human technology has advanced over the centuries. Messier published his catalog in 1774; Dreyer published the NGC in 1888; and Patrick Moore, who apparently had a second surname of Caldwell, published his catalog in 1995.

However, the Caldwell objects are a little different.

The Caldwell catalog was actually compiled as a direct complement to the Messier catalog. The Messier catalog was already an oft used guide to observing deep-sky objects, but Moore had noticed some critical weaknesses.

One was that, as you know, the Messiers weren’t meant as a guide to deep-sky objects at all. Charles Messier was trying to find a comet, so most of them are blurry objects that would easily be confused with comets back in his day. Many of the sky’s brightest deep-sky objects–including some of my star party favorites–were never included.

The second weakness was that, as a French astronomer with a very narrow purpose for his catalog, Messier had only included objects visible from Paris. So deep-sky objects in the southern hemisphere were completely excluded.

And that explains how the Messier Marathon works, I suppose. I suppose one good thing came about from the Messier catalog’s shortcomings?

In compiling the Caldwell catalog, Moore had access to modern-day technology that would let him add obscure objects Messier never would have seen. But the primary purpose of the catalog was to cover the entire sky, north and south, and to focus on including deep-sky objects rather than comet-resembling ones. With a total of 109 objects, the Caldwells closely mirror the Messiers’ breadth.

The nice thing about modern-day technology–primary telescopes, but also digital star charts–is that most of these objects, even the more obscure NGCs and Caldwells, are accessible to the average amateur (and even the confused layman!).

Go-to telescopes, like the one above, are computer-operated. Using a keypad, you align them to several known stars in order to inform the computer of its location and the orientation of the sky. Most go-tos offer one-star, two-star, and three-star align. I usually go with two-star align. It’s far more accurate than one-star align and is much easier than three-star align.

After you orient the telescope, all you have to do is punch the object into the keypad–most keypads will have specific buttons for Messiers, NGCs, and even ICs and Caldwells, as well as a number pad for the object designation.

If you have a manual telescope, such as some refractors and Dobsonians, then a digital star chart on your phone and a good finder scope (like a Telrad) will suffice.

And always remember that lots of objects go by lots of different names. The Veil Nebula is just one example. The Caldwells include some Messiers by virtue of the catalog’s purpose, but there’s also lots of unintentional overlap between Messiers, NGCs, and Caldwells.

So don’t be confused by “conflicting” names and designations for objects–in all likelihood, all the designations are right and you can’t go wrong! Unless, of course, it’s an object like NGC 6666. In that case, there’s a good chance it just doesn’t exist.

2 thoughts on “How Deep-Sky Catalogs Work

  1. I just checked NGC 6666 my Sky Safari app and you are right – it’s listed as a”Nonexistent Deep Sky Object”. It’s not mentioned at all on Wikipedia’s List of NGC Objects.

    It makes me wonder why there isn’t a dedicated catalogue of non-existent deep sky objects. The Revised NGC apparently includes 800 of them. Time for a clean up!

    Liked by 1 person

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