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
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