Help keep this category in order, modify [[Category:Messier objects]] to add a sort key:

Articles with the Messier number in the title

use [[Category:Messier objects|###]], dropping the M prefix, and using only the number. The number should be padded up to 3 digits using zeroes.

Articles without the Messier number in the title

use [[Category:Messier objects]]. A redirect containing the Messier number should also be added to the category, sorted as above.