- How do we name objects? -
So... how do we name all of the things we find in the sky? This may sound trivial but is in fact quite important. In case you hadn’t realised, there are an awful lot of them and if you try to find anything celestial in a book or sky atlas you very quickly run across a confusing jumble of names, numbers and combinations of both. Although it isn’t necessarily important to know and remember the individual names of all the objects, it is important to at least recognise what it might be referring to when an obscure name or number is thrown at you. Hopefully that way it will prevent too much confusion occurring.
Planets, moons, asteroids and comets
All of the planets, the sun and the moon are named after Roman and Greek gods. As new planets were discovered this naming system was continued.
Most of the moons are also named after people or creatures associated with the god they happen to be circling. Unfortunately there are a few exceptions. For some reason the names of the moons around Uranus were given names (by John Herschel in 1852) from works by Shakespeare and Pope. When new moons were discovered in 1986 by Voyager 2 the International Astronomical Union (IAU) decided to continue this system.
Whenever a new asteroid is discovered the discoverer gets to nominate a name for it. The name has to then be agreed upon by the IAU before it becomes official. You are not allowed to name an asteroid after yourself or political and military identities. Initially most of the asteroids also followed the mythological naming system but recently names of prominent scientists, famous people and places can be found given to asteroids.
Comets are the only objects in space that can be named after you. Generally speaking if you discover a comet and are the first person to let the rest of the world know about it, then it is called after you (eg. Comet Encke). Even spacecraft can have comets named after them (eg. Comet SOHO). If more than one person discovers it at the same time then both names are associated with it (eg. Comet Hale-Bopp). Up to three people can have their names to a comet (eg. Comet Tuttle-Giacobini-Kresak). If you or perhaps a pair of people working as a team discover more than one comet then it is simply your name(s) and the running tally of how many you have found (eg. Comet Machholz 2, Comet Shoemaker-Levy 9).
Non solar system and non stellar objects
Some objects have common names, as you might expect (eg. the Andromeda galaxy, the Swan nebula and the Jewel Box). But there are an awful lot of objects, so it isn't really practical to give them all common names. An alternative is to assign them a catalogue number.
Back in 1780 a French amateur astronomer, Charles Messier, was looking for comets. At the time it was what all people of noble birth were doing. But the method by which you find a comet is to look for a fuzzy object. If you find one you then keep observing it over a period of a few nights and see if it moves. If it does, chances are you have a comet. If it doesn't, well, tough luck, it’s something else. Messier keep getting annoyed at a lot of fuzzy objects that would turn out not to be comets, so he made a catalogue of them to tell people not to bother looking. His catalogue had 109 entries. The first was called M1, the 22nd was called M22 and so on. As it turns out these 109 objects are much more interesting than the comets he was after, since they turn out to be galaxies, nebulae and star clusters.
M1 – the Crab nebula
M22 – (a globular cluster)
M31 – the Andromeda galaxy
M42 – the Orion nebula
M78 – the Horsehead nebula
In 1888, J.L.E. Dreyer created another catalogue. This time it had approximately 8,000 objects listed, including all of the Messier objects. It was called the New General Catalogue. The objects therefore had an NGC number depending their listing in the catalogue.
NGC 1952 – the Crab nebula
NGC 6656 - (a globular cluster)
NGC 224 - the Andromeda Galaxy
NGC 1976 – the Orion nebula
NGC 2068 – the Horsehead nebula
Some other famous, but definitely southern hemisphere examples are :
NGC 5139 – Omega Centauri
NGC 4755 – the Jewel Box
NGC 5128 – Centaurus A
In about 1900 an addendum was made to it and the Index Catalogue was created. Objects listed in this have an IC number. This catalogue has about 15,000 entries (eg. IC 434 – the Horsehead nebula).
Another, later catalogue was the Barnard Catalogue. These had B numbers (eg. B 33 – the Horsehead nebula).
One of the problems is that as each catalogue was made the previous ones were never abandoned. So any particular object may literally have dozens of names and catalogue numbers. Fortunately the main ones used and the ones most people will ever come across are the Messier and NGC catalogues.
Stars
Obviously some stars have common names. These are mainly the brighter ones and are limited to a couple of thousand at most (eg. Sirius, Canopus, Arcturus, Vega, Capella, Rigel, Betelgeuse).
Some are named after the people that discovered them (eg. Barnard's star, Kapteyn's star, Wolf's star).
But once again, with about 6,000 individual stars visible to the naked eye alone, there are just far too many to give a normal name to. So how do we name them?
One way is to use the constellation in which they lie. If you call the brightest star in the constellation "alpha" and then a derivative of the constellation name you have a naming system. The next brightest would be "beta" and so on. Of course the obvious limitation here is that there are only 24 letters in the Greek alphabet (eg. a Centauri, b Crucis, e Eridani).
One way of getting around this problem is to again use the constellations but this time starting from left to right number the stars as you bump into them. So the first one would be 1 then the derivative of the constellation name. The second would be 2, the third 3 and so on (eg. 61 Cygni, 40 Eridani, 47 Tucana). This gives an unlimited number of names but what happens if you get through naming them all and some conscientious astronomer finds another star back in the middle? Do you constantly rename them? No.
Today there exists a slightly different system. All stars are now basically located and listed by their co-ordinate in the sky. Until recently they may have had an SAO number (named after the Smithsonian Astrophysical Observatory) but as with all other things there are an enormous number of different catalogues to choose from. Perhaps the two most common and popular ones today are the Hubble Guide Star Catalogue (or simply the Guide Star Catalogue or GSC for short) and the Tycho Catalogue created by the spacecraft Hipparcos when it mapped the locations of over a million stars to an unprecedented accuracy.