Molecular Cloud Evolution
  I describe the scenario of molecular cloud (MC) evolution that has emerged
over the past decade or so. MCs can start out as cold atomic clouds formed by
compressive motions in the warm neutral medium (WNM) of galaxies. Such motions
can be driven by large-scale instabilities, or by local turbulence. The
compressions induce a phase transition to the cold neutral medium (CNM) to form
growing cold atomic clouds, which in their early stages may constitute thin CNM
sheets. Several dynamical instabilities soon destabilize a cloud, rendering it
turbulent. For solar neighborhood conditions, a cloud is coincidentally
expected to become molecular, magnetically supercritical, and gravitationally
dominated at roughly the same column density, $N \sim 1.5 \times 10^21 \psc
\approx 10 \Msun$ pc$^{-2}$. At this point, the cloud begins to contract
gravitationally. However, before its global collapse is completed ($\sim 10^7$
yr later), the nonlinear density fluctuations within the cloud, which have
shorter local free-fall times, collapse first and begin forming stars, a few
Myr after the global contraction started. Large-scale fluctuations of lower
mean densities collapse later, so the formation of massive star-forming regions
is expected to occur late in the evolution of a large cloud complex, while
scattered low-mass regions are expected to form earlier. Eventually, the local
star formation episodes are terminated by stellar feedback, which disperses the
local dense gas, although more work is necessary to clarify the details and
characteristic scales of this process.
