We have OBAFGKM to denote the strength of hydrogen lines.
Apparently we added S, N, C, and the W classes when we learned of new kinds of stars.
We use T, L, and Y to denote brown dwarfs.
The D classes refer to electron-degenerate stars (white dwarfs).
I've looked around quite a bit and I haven't seen anything regarding neutron stars and black holes. Does this mean there's no common symbol or abbreviation? What do they use in star catalogs? (I've never found a catalog with stars of all degeneracies in it.)