Secular Evolution of the Galactic Disk
  In the solar-neighbourhood, older stars have larger random velocities than younger ones. It is argued that the increase in velocity dispersion with time is predominantly a gradual process rather than one induced by discrete events such as minor mergers. Ephemeral spiral arms seem to be the fundamental drivers of disk heating, although scattering by giant molecular clouds plays an important moderating role. In addition to heating the disk, spiral arms cause stars' guiding centres to diffuse radially. The speed of this diffusion is currently controversial.   Data from the Hipparcos satellite has made it clear that the Galaxy is by no means in a steady state. This development enormously increases the complexity of the models required to account for the data. There are preliminary indications that we see in the local phase-space distribution the dynamical footprints of long-dissolved spiral waves. 
James Binney (Oxford)