For sale – a star name! Unfortunately, if you read the fine print you will find out that the name you have just given “your star” has no formal validity. Bright stars already have names – in some cases more than one. The very brightest stars generally have names that derive from Arabic – Deneb means tail, so there is Deneb in Cygnus, the tail of the swan and Denebola in Leo, from Deneb Alased, the tail of the lion. Deneb is also called alpha Cygni, since it is the brightest star in Cygnus, Denebola is also beta Leonis, the second brightest star in Leo. The alpha, beta designations come from 1603, when a German lawyer, Johann Bayer started naming stars in the order of their brightness within each constellation. He was a bit inconsistent on this, so within Ursa Major, the brighter stars are named in order of their position from one end of the Plough to the other. This leaves Delta Ursae Majoris (the dim star at the join between the handle and the plough share) with a name that implies it is the fourth brightest star in the constellation, even though it is considerably dimmer than Epsilon, Zeta and Eta in the handle.
Besides these Bayer names, stars can have Flamsteed numbers. The Astronomer Royal John Flamsteed reckoned that it was better to number the stars within a constellation, ignoring the often hard to determine brightness of a star. He published his catalogue in 1725, listing each star from west to east within a constellation. Later astronomers added numbers to this, so Deneb is known as 50 Cygni and Denebola is 94 Leonis . Flamsteed numbers worked fine for a while – in my 19th edition Norton’s Star Atlas there are only 8800 stars brighter than 6.49 magnitude. But there are a lot more than 8800 stars in the sky! As telescopes uncovered fainter stars, these faint stars were given just a catalogue number. The most used tends to be the Henry Draper catalogue, stars numbered up to HD 225300 are from the first catalogue, from HD225301 to HD359083 they are in the extension published in 1949. Many other catalogues have been produced for a specific purpose, so 17180 double stars have a number in the New General Catalogue of Double Stars and the Guide Star Catalog II has positions and magnitudes for over 445 million stars.
Variable stars can have a number in the General Catalogue of Variable Stars or the New Catalogue of Suspected Variable Stars. Variable stars actually have their very own naming system. For each constellation they are given a capital letter from R to Z, then a double form from RR to RZ, SS to SZ and so on to ZZ. This is good enough to name the first 54 variable stars in a constellation. Then you have to go back to AA and so on to AZ, repeat for BB and continue on to get up to 334 variable stars names. After that they are just numbered V335 and so on per constellation. Some of the more well-known variable stars include RR Lyrae, T Tauri and the amusingly named FU Orionis – also known as fuors.
If you do want to name a star yourself and are determined to pay money for it, then consider the website of one company. They are currently suggesting that you purchase a star for Father’s day, and have in tiny, pale letters near the bottom of the home page of their website: “Company X star naming is not recognized by the scientific community. Your stars [sic] name is reserved in Company X records only.” I wonder how many people have named a star “caveat emptor”?
This episode of “What is the Stars?” was broadcast the week of June 11, 2012. Listen live on RTÉ’s lyricfm Mondays and Fridays at 22:45. 
