What was Messier actually hunting? Comets — and by 1773 he was keeping a running list of the things that weren't comets, so he'd stop being fooled by the same smudge twice. Number 51 on that list turned out to be a spiral galaxy about 31 million light-years out. A small scope will give you two soft grey blobs and no arms at all; the dust lanes and those pink knots of gas in the picture want a camera. Under the handle of the Dipper, off Alkaid.
A hundred and ten objects, and the organising principle is basically "not that again." Lyra, it's the only catalogue in science assembled entirely out of mild irritation (and we've been using his grudge list as a sightseeing guide for 250-odd years).
Messier was working with small achromats, a few inches of aperture, and he logged M51 as a nebula without structure. The arms had to wait for Lord Rosse's 72-inch speculum mirror at Parsonstown in 1845 — polished metal, not glass, and it tarnished, so they kept a spare.