Charting the heavens

For centuries, astronomical catalogues have helped bring order to the boundless cosmos. Pete Lawrence takes a deep dive into the essential databases every astronomer should know

- Pete Lawrence

Star and deep-sky catalogues are crucial resources for making sense of the night sky. There are a lot of them, though – so many, in fact, that you almost need a catalogue of catalogues to keep track of them all!

There's a rich history wrapped up in the creation of these databases, and many astronomical objects appear in multiple catalogues, each with its own naming conventions and quirks. In this guide, we'll explore some of the widely used catalogues and the types of celestial objects they cover, from nearby stars to distant galaxies.

With massive amounts of high-precision data now streaming into mainstream planetarium apps, what role do older catalogues play? Stars and deep-sky objects may not change rapidly, but when it comes to pinpoint accuracy, even small shifts can matter. We'll look at which catalogues are being actively maintained and refined.

If you've ever wondered what those odd suffixed numbers on star maps mean, read on and be impressed by how human beings have brought structure to the cosmos.

Star catalogues we all start with

We'll begin with the star catalogues that any new or experienced astronomer is likely to encounter.

The Bayer star labelling system was compiled by German astronomer Johann Bayer and published in 1603 as part of his Uranometria star atlas. Bright constellation stars were given a Greek letter in order of descending brightness. When all 24 Greek alphabet characters were exhausted, Roman letters a-z were used, then A-Q. Beyond Q, we find the beginnings of the variable star labelling system, variables identified with R-Z then RR, RS, RT, and so on, to ZZ. Similar brightness levels and long-term variability mean that some Bayer labels appear out of sequence. For example, Betelgeuse (Alpha (α) Orionis) is currently dimmer than Rigel (Beta (β) Orionis).