If you’re into astronomy, you have seen a combination of these letters with numbers. In short, it’s simply how deep sky objects are cataloged. Read on to find out their origins and what they mean.
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 still well known. Messier was a comet hunter, and was frustrated by fuzzy objects resembling but were not actual comets. He was looking for objects that moved, and since these objects were fixed in the sky, he compiled them into a list so he wouldn’t waste any further time on them.
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. Since Messier observed from France, the list only contains objects visible from the skies he could observe from. While he was able to cover about 2/3 of the sky, he missed objects that are further south.
Many deep sky objects with common names have a Messier number. Here are some examples:
M 31 – Andromeda Galaxy
M 42 – Orion Nebula
M 45 – The Pleiades
NGC objects that also have a Messier number are still usually identified by their Messier number instead.
But I see another catalog with a “C”
That stands for Caldwell, which was made by Patrick Moore. This was made to compliment Messier’s catalog in an attempt to list the brightest deep sky objects that Messier missed. He used his other surname, Caldwell, because Moore had the same initial as Messier. 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. This catalog is not as widely used, but I do know those in the astronomy community that reference it.
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.
Some of my personal favorite NGC objects are:
NGC 869 & NGC 884 – The Double Cluster in Perseus
NGC 6231 – The Scorpius Jewel Box
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.
Why do I see Greek letters next to star names?
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.
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.