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.
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.
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!
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 Lettes 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.
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?
Comets are 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.
For example, what does C/2019 Y4 (ATLAS) mean? The 2019 should first tell you that it was discovered that year.
Now 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…
So for 2019 Y4, that means in 2019 it was the fourth comet discovered in the second half of December. The C/ means it’s considered a non-periodic comet.
Okay.. What About the Names?
Individuals who discover the comet can still have it named after them. There are also plenty of comets named after the same individual, such as Robert H. McNaught who has 82 comets named after him!
Nowadays, new comets are discovered much more frequently, that they’re just named after the project observatory, which often have long names abbreviated into a simple name. 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
So while you can simply call it “Comet Atlas” and use that name in a google search for a well known comet getting mainstream attention, just remember that there exist plenty of other “Comet Atlas’,” hence why it’s important to use the other designations to specify it when talking to astronomers.
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