Star Formation in Spiral Arms
  The origin and types of spiral arms are reviewed with an emphasis on the
connections between these arms and star formation. Flocculent spiral arms are
most likely the result of transient instabilities in the gas that promote dense
cloud formation, star formation, and generate turbulence. Long irregular spiral
arms are usually initiated by gravitational instabilities in the stars, with
the gas contributing to and following these instabilities, and star formation
in the gas. Global spiral arms triggered by global perturbations, such as a
galaxy interaction, can be wavemodes with wave reflection in the inner regions.
They might grow and dominate the disk for several rotations before degenerating
into higher-order modes by non-linear effects. Interstellar gas flows through
these global arms, and through the more transient stellar spiral arms as well,
where it can reach a high density and low shear, thereby promoting
self-gravitational instabilities. The result is the formation of giant spiral
arm cloud complexes, in which dense molecular clouds form and turn into stars.
The molecular envelops and debris from these clouds appear to survive and drift
through the interarm regions for a long time, possibly 100 Myr or more, with
lingering spontaneous star formation and triggered star formation in the pieces
that are still at high-pressure edges near older HII regions.
