Astronomy Basics

The Messier and other Catalogues



The Messier Catalogue is a list of interesting objects in the sky, distinguished from ordinary stars. In 1758 French astronomer Charles Messier began compiling a list of nebulae- diffuse objects so that they wouldn't be mistaken for comets. Each object was designated an 'M' number- and astronomers still use this today to identify objects. For instance, the Pleiades are known as M45. There are 110 Messier objects.

The Messier catalogue is of interest to amateur astronomers because, as these objects were discovered in the 18th century, they are usually bright enough to be enjoyed with a modest telescope. Some people claim to have spotted every single one with such equipment.

An excellent site about the Messier Catalogue can be found at, with detailed information and pictures of each object.

Some Highlights of the Messier Catalogue

Star Catalogues

Since ancient times certain stars have been given names. However not all stars can be given a proper name, and such a system would only prove confusing, giving us no information about the whereabouts of the star. Here's some of the ways that you may hear a star referred to, and their meanings.

  • Greek-letter system
    One of the earliest attempts to catalogue the stars was the greek-letter system, devised in the early 17th century. According to this system stars are named according to their constellations, and assigned a greek letter, usually according to it's brightness. For example, the brightest star in Lyra (Vega) is given the name Alpha Lyrae. The second brightest is called Beta Lyrae then Gamma, Delta Lyrae and so on. This is useful to amateur astronomers as it mainly covers the stars we can see, and gives us some idea of it's whereabouts and it's brightness.
  • Flamsteed numbers
    Later on, in the 18th Century astronomer John Flamsteed introduced a numbering system for stars in each constellation, where visible stars are numbered Eastwards, so the Eastern-most star in Centaurus is 1 Centauri, the second 2 Centauri etc.
  • BD system
    BD, the Bonner Durchmunsterung (Bonn Survey) system came next as telescopes were revealing hundreds of thousand of stars. The sky was divided up into 1 degree bands of Declination, and stars numbered according to where they came around the circle. For instance the star BD + 52°1245 is the 1,245th star (counting from Right Ascension 0°) in the area between 51° and 52° declination.
  • SAO catalogue
    SAO, the Smithsonian Astrophysical Observatory star catalogue, is one of the most widely-used catalogues today. It covers most stars down to about the 9th Magnitude, in which stars are numbered according to their Right Ascension co-ordinates.