1 - Optical observations of nebulae

Summary

Discovery

In Ptolemy's Almagest, six objects are listed as ‘stella nebulosa’, hazy, luminous spots on the Celestial Sphere. Once viewed telescopically, these six objects were resolved into clusters of stars; however, other nebulous objects were noticed. The first, in Orion, appears to have been discovered by Fabri de Peiresc in 1610. Two years later, Simon Marius noted a nebula in Andromeda that had also been recorded by Al Sufi in the tenth century. The discoveries continued and, in 1781, Charles Messier compiled a list of nebulae and star clusters. The 105 objects he catalogued are still identified by their Messier number; the Orion Nebula is M42 while the Andromeda Nebula is M31.

An all-sky survey carried out by the Herschels at the turn of the nineteenth century resulted in a General Catalogue containing over 5000 nebulous objects. In 1888 a New General Catalogue was published by J. L. E. Dreyer; later editions included Index Catalogues and tabulated more than 13000 objects. The Orion Nebula, M42, is also identified as NGC 1976; the Andromeda Nebula, M31, is NGC 224.

About the middle of the nineteenth century, Lord Rosse constructed a sixfoot reflector in Ireland with which he made numerous visual sketches of nebulae and applied names to them by which they are still referred. M97 (NGC 3587) is called the ‘Owl’, while M51 (NGC 5194) is known as the ‘Whirlpool’, indicative of its spiral form.

During the initial discovery period of nebulae, a debate was ongoing as to whether or not all nebulae could be resolved into stars if a telescope of sufficient light-gathering power and resolution were available.