The term ‘deep-sky objects’ is used by astronomers to refer to galaxies, nebulae or star clusters, which, as the name suggests, lie well beyond the limits of our solar system.
Galaxies were originally classed as nebulae, until they were shown, by Edwin Hubble in the early 1920s (see Hubble’s Law), to be enormous collections of stars, at distances far beyond the limits of our own galaxy.
It is now estimated that the observable universe contains at least 100 billion galaxies.
Quasars are extremely luminous and highly-redshifted galaxies, visible at the furthest limits of the observable universe, the brightest of which appear as faint, star-like points of light through the largest amateur telescopes.
Charles Messier was the first to catalogue the positions of around 100 of the brightest nebulae and star clusters, in 1771. However, he did so to distinguish them from comets, for which he was hunting, in collaboration with his assistant Pierre Méchain. There are now 110 objects in the Messier Catalogue, which are assigned numbers from M1 – the Crab Nebula – to M110 – a dwarf elliptical galaxy, also known as ‘The Edward Young Star’.
The New General Catalogue (NGC), charted the positions of around 1,000 deep-sky objects, when it was first published by John Louis Emil Dreyer, in 1888. However, through subsequent editions, the NGC now contains 7,840 objects.