If you’re into astronomy, you have seen a combination of these letters with numbers. Many have possibly heard names as M42 or NGC 861, and of course don’t know what they are or what they mean. In short, it’s simply how deep sky objects are cataloged. Read on to find out their origins and what they\ stand for.

You’ll find that many objects have more than one catalog designation!

As For Comet Names and Designations, Scroll Down to The Bottom!

M stands for Messier, after Charles Messier

Charles Messier, a french astronomer, wasn’t the first astronomer to catalog deep sky objects, but the catalog he published in 1771 is well known and still very much used.

Messier was a comet hunter, and was frustrated by sighting many fuzzy looking objects that resembled but were not actual comets.  As he was looking for objects that moved, he compiled these objects that stayed fixed into a list so he wouldn’t waste any further time on them. Go to this website for Messier’s actual accounts as he cataloged the objects. 

There are a total of 110 Messier objects in the night sky. They are among the most well known and brightest of the nebulae, star clusters, and galaxies – They can all be observed through small telescopes under dark skies.

Since Messier observed from France, the list only contains objects visible from the skies he could observe from. While he was still able to cover about 2/3 of the sky, he missed objects that are further south.

Many famous deep sky objects with common names have a Messier number. Here are some well known examples:

M 1 – Crab Nebula
M 7 – Ptolemy Cluster
M 8 – Lagoon Nebula
M 13 – Hercules Globular Cluster
M 31 – Andromeda Galaxy
M 33 – Triangulum Galaxy
M 42 – Orion Nebula
M 44 – Beehive Cluster
M 45 – The Pleiades
M 57 – Ring Nebula
M 81 – Bode’s Galaxy
M 82 – Cigar Galaxy

All the Messier objects have an NGC designation as well, but astronomers will usually use their Messier names when identifying them.

Click Here For a Full List of Messier Objects

But I see Another Catalog with a “C”

That stands for Caldwell, which was made by Patrick Moore. He used his other surname, Caldwell, because Moore had the same initial as Messier. This was made to compliment Messier’s catalog in an attempt to list the brightest deep sky objects that Messier missed. Because Messier was simply listing objects that could be confused as comets, Moore wanted a catalog that actually was meant for deep sky objects!

It’s simply another catalog like Messier’s, only this one is listed in order from north to south, and also includes objects in the southern hemisphere. Here are some of my favorites:

C13 – Owl Cluster
C14 – The Double Cluster
C49 & C50 – The Rosette Nebula and Cluster
C63 – The Helix Nebula
C76 – The Scorpius Jewel Box
C80 – Omega Centauri Cluster

I sometimes use it to identify respective objects, but most people identify them by their NGC catalog number instead.

Click Here to Check out the Caldwell Catalog

NGC stands for New General Catalog

The New General Catalog is much larger and much more comprehensive. It was compiled in the 1880s by John Louis Emil Dreyer, and it was essentially an update to existing catalogs that were published by William Hershel and his son decades earlier. The New General Catalog, with later modifications and updates, is still widely used by astronomers today.

There are 7,840 NGC objects. No, I do not know them all, especially because only a fraction of them have names, and most of them are much dimmer in general than the Messier or Caldwell objects. Often times, I only need to identify them when I see them in long exposures when they previously weren’t visible with just my eyes through a telescope!

You can check out the list by clicking here, but good luck!

What about “IC?”

“IC” stands for Index Catalog, which were supplements to the New Galactic Catalog. These were mainly clusters, nebulae, and galaxies that were discovered thanks to photography after the initial NGC was published. Since most of these were discovered with long exposure photography, chances are you may not see them very well even in large telescopes from a dark sky!

Why Do I see Greek Letters or Numbers Next to Star Names?

They’re simply ways to identify and catalog the notable stars in the sky.

Under a system constructed by German astronomer Johann Bayer, familiar stars in a constellation are designated from brightest to dimmest using the Greek alphabet, with α (Alpha) being the brightest, β (Beta) being the next brightest and so on. For example, while a star may have a formal name, like Aldeberan in Taurus, it will also be designated α Tauri (Alpha Tauri) with Tauri being the genitive Latin name for Taurus. This is just a simple way to catalog the familiar stars you see, especially when they don’t have a formal name. When the Greek letters ran out, Latin letters were used

What about numbers in the case of stars like 61 Cygni? That comes from the Flamsteed designation, named after John Flamsteed. Some, but not all constellations use these designations, but when they do, they’re merely another way of identifying stars. It can be a little confusing, as the numerical order is not from brightest to dimmest (For example, the star Deneb, the brightest in Cygnus, has a Bayer designation of α Cygni, but under Flamsteed’s, it’s given the name 50 Cygni). They instead are listed in ascending order according to their position in Right Ascension…

There are other catalogs as well, but I won’t be going over them. After reading this post, you now have a quick rundown on the most common catalogs that astronomers use when referencing deep sky objects.

If you purchase a computerized go-to telescope, more than likely your hand controller software will have every Messier, Caldwell, NGC, and IC object available at the push of a button!

So… Why Do We See These Numbers and Odd Names for Comets Nowadays? Can’t I just use a Simple Name?

Comets are listed by their type and designated by the year of their discovery followed by a letter indicating the half-month of the discovery and a number indicating the order of discovery. The actual name is in parentheses.

Let’s go over that first letter:

P/ – periodic comet: a shorter orbital period less than 200 years.
C/ – non – periodic comet: longer, more eccentric orbits lasting over 1,000 years.
X/ – a comet that doesn’t have an orbit that was calculated.
D/ – a comet that has broken apart, or been lost.
A/ – once thought to be a comet but is now a minor planet
I/ – an interstellar object from outside our solar system!

Now for that second letter and number.

When it comes to half months, each letter corresponds to either the first half or second half of the month it was discovered. (A and B for January, C and D for February, and so on…) The letters not used in these designations are “I” and “Z,” so after April uses “G” and “H,” May uses “J” and “K”  and finally, December uses “X” and “Y.” After that, the number used is the order it was discovered – first to be discovered gets number 1, and so on…

C/2020 F3? A non-periodic comet/ third comet discovered in the second half of March 2020.

Okay.. What About the Names?

Amateur astronomers and major research observatories are ALWAYS pointing their telescopes to the sky and looking for them, whether as part of a team, or on their own. Individuals who discover the comet can still choose to have it named after them, or if I ever have that chance, I’ll name it after my late brother.

There are plenty of comets named after the same individual, such as Robert H. McNaught who has discovered 82 comets, thus many are named after him, even though the most famous “Comet McNaught” will always be C/2006 P1 – the Great Comet of 2007.

One comet got named the way it was because it was discovered by two people independently!

Alan Hale and Thomas Bopp both happened to chance upon a new comet on the same night in July 1995 while viewing from different USA locations. By the time Thomas Bopp had notified the the clearing house for astronomical discoveries via a telegram of all things, Alan Hale had emailed them three times with updated coordinates. Because both men were confirmed to discover it on the same night, it was given the name “Hale-Bopp.”

Nowadays, new comets are discovered much more frequently, that they’re just named after the project observatory, which often have long names abbreviated into simpler acronyms. Here are the names you’ve most likely have seen:

ATLAS – Asteroid Terrestrial-impact Last Alert System
PANSTARRS – Panoramic Survey Telescope and Rapid Response System
ISON –  International Scientific Optical Network
LINEAR – Lincoln Near-Earth Asteroid Research
NEOWISE – Near Earth Object Wide-field Infrared Survey Explorer

Yes, you can always use simple names like “Comet Atlas” or “Comet Neowise” when referring to a comet that you heard about. Google searches will most likely get you to that respective comet if you look for it without necessarily using the designation numbers. However, it’s important to remember that there exists more than one NEOWISE, or more than one PANSTARRS, etc…

And if you are presenting about a specific comet that isn’t very well known or because there exists more than one with the same name,  then it helps to use the designations to make it easier to identify. Otherwise it can turn into an Abbot and Costello routine of “here’s Comet McNaught, no not that McNaught, this McNaught…”