  One of the present concepts for the onset of massive star formation is the
Kennicutt criterion. This relates the onset of massive star formation to a
general gravitational instability of the gas disks of spiral galaxies. It is
often overlooked, however that such gravitational instabilities of the gas
disks have severe implications for the overall stability of the gas and star
disks of spiral galaxies. I show by numerical simulations of the evolution of a
combined gas and star disk that the violation of the stability condition
induces violent dynamical evolution of the combined system. In particular the
star disk heats up on time scales less than a Gyr to unrealistic high values of
the Toomre stability parameter Q. The morphologies of both the star and gas
disk resemble then no longer observed morphologies of spiral galaxies. Star
formation of stars on low velocity dispersion orbits would lead to dynamical
cooling of the disks to more realistic states. However, the required star
formation rate is extremely high.
