Astronomy Blog

Star naming

Our galaxy contains about 100 thousand million stars. Of these, we can only see a few thousand with our own eyes and pretty much all of these visible stars have proper names that were given to them by Greek, Roman and Arab astronomers. Once the telescope was invented, it became obvious that there were many thousands of un-named stars out there and it was slightly impractical to give them all proper names. Not only was it difficult to come up with so many new names, but it was difficult to remember where a star was; imagine trying to memorise addresses for everyone in London by name. So, several systems or catalogues were devised to make it easier to find a particular star on the sky.

One approach was to assign a Greek letter to each star in a constellation starting with the brightest and adding the constellation name on the end. So, the brightest star in Centaurus was named Alpha Centauri and the third brightest star in Orion was Gamma Orion (proper name: Bellatrix). This system was created by Johann Bayer in 1603 and is still used today even though the ordering wasn't always right and some of the constellation designations are now incorrect.

Since Bayer's time other methods have been adopted and these, generally, tend to assign catalogue numbers to each star. These names usually come with an abbreviation for the particular catalogue before the number. So, Bellatrix is also known as

HD 35468 - the 35,468th star in the Henry Draper catalogue. That catalogue is organised in order of the Right ascension coordinate meaning that stars with a similar catalogue number won't be too far away from each other on the sky. That can be quite useful, as you can then tell where a star is roughly by its number. Some catalogues go even further in this regard by putting the star coordinates as part of the name. For example 1RXS J052507.7+062103 - a star in the 1st ROSAT X Survey - tells you the star has a Right ascension of 5 hrs, 25 mins, 7.7 secs and a declination of 6° 21' 3" even if it does look a bit unweildly.

The only drawback to catalogue names is that, over the years, many catalogues have been compiled for different research projects. As a result, many stars actually have several different catalogue numbers. For instance Bellatrix is also known as HIP 25336 (Hipparcos catalogue), IRC +10084 (Infrared catalogue) and SAO 112740 (Smithsonian Astrophysical Obs.) amongst others. Although having several names could be confusing, computer-based tools such as Simbad are great at finding the alternatives, so the situation isn't too bad.

The only down side to number-based names is that most people find memorising strings of numbers quite difficult. But, just as people used to remember telephone numbers of friends and relatives (that doesn't happen so much anymore with mobile phones), astronomers start to recognise their favourite objects by catalogue numbers too. Personally, for very large catalogues, I prefer names created from the star's coordinates as they are more useful. They also have less ambiguous pronunciations than names such as Betelgeuse or Zubenelgenubi.