3.4.2 Denoting nebulae

The two nebula catalogues NGC and IC are complementary, i.e. they don't have¹ any nebulae in common. For example, the North America Nebula is called `NGC 7000' and the Orion Nebula is 'NGC 1976'. The IC catalogue usually contains fainter objects.

Parallel to that there is the much older and much smaller Messier catalogue, abbreviated `M'. For example, the Orion Nebula can also be called 'M 42', whereas the North America Nebula isn't included in Messier's catalogue.

Pay attention to

• the catalogue abbreviation in all uppercase and
• the space between the catalogue abbreviation and the number.