  The M51 galaxy is one of best examples of grand design structure in galaxies.
The most likely origin of the spiral arms in M51 is the ongoing interaction
with NGC 5195. Here we report recent calculations which model the orbit of M51
and NGC 5195. These calculations, which for the first time focus on the gas
dynamics, are able to reproduce even detailed features in M51, as well as
demonstrate the nature of the spiral structure of M51.
