Stargazing: How are the Not-So-Bright Stars Named?
Constellations have a lot more stars than the Greek alphabet has letters, so once all the brighter ones have been given a Bayer letter, astronomers have to use a different system for naming stars. As a result, all stars within a constellation are given a number too.
Stars within a constellation are numbered from west to east (right to left in the northern hemisphere, and left to right in the southern). This number is called the Flamsteed number. The bright stars get numbered, too, so they have both a Bayer designation and a Flamsteed number. For example, if you’re looking for the star called Deneb in the constellation of Cygnus the Swan, it may be listed as α Cygni (its Bayer designation) or 50 Cyg (its Flamsteed number).
Very faint stars – those that require binoculars or a telescope to see – are listed with a catalogue number. Many different star catalogues and different designations exist, but two of the more common ones are the Hipparchus (HIP) catalogue and the Henry Draper (HD) catalogue.
Some stars vary in brightness, brightening and then dimming again. To help make these stars stand out in a star chart, some of them are given a capital letter followed by their constellation’s genitive. For example, T Tauri is a variable star in Taurus the Bull.
The letters start with R and proceed to Z before starting again at RR, RS, RT and so on up to ZZ, then AA to AZ, and finally BA to BZ.
Variable stars that are bright enough to have a Bayer designation aren’t given a new letter, so this system applies only to fainter stars.
As well as stars, constellations are home to the faint fuzzies, fixed against the background of stars. Several catalogues list the faint fuzzies:
Messier Catalogue, using M numbers, such as M45, the Pleiades
New General Catalogue, using NGC numbers, such as NGC 4755, the Jewel Box cluster
Index Catalogue, using IC numbers, such as IC 2602, the Southern Pleiades
Caldwell Catalogue, using Caldwell C numbers, such as C6, the Cat’s Eye Nebula