These letters, when followed by numbers, refer to the specific entries in an archive of observations made by the European Southern Observatory telescopes.

Since the Hubble Space Telescope (HST) is a joint venture between NASA and the European Space Agency, ESO numbers are used to designate HST discoveries.

Galaxies with ESO numbers are generally not included in earlier catalogs, such as those with numbers starting with “NGC” (designating John Dreyer’s New General Catalogue of Nebulae and Clusters of Stars) or “M” (designating Charles Messier’s Catalog of Nebulae and Star Clusters).