What Shapes the Structure of Molecular Clouds: Turbulence or Gravity?
  We revisit the origin of Larson's scaling relations, which describe the
structure and kinematics of molecular clouds, based on recent observations and
large-scale simulations of supersonic turbulence. Using dimensional analysis,
we first show that both linewidth-size and mass-size correlations observed on
scales 0.1-50 pc can be explained by a simple conceptual theory of compressible
turbulence without resorting to the often assumed virial equilibrium or
detailed energy balance condition. The scaling laws can be consistently
interpreted as a signature of supersonic turbulence with no need to invoke
gravity. We then show how self-similarity of structure established by the
turbulence breaks in star-forming clouds through development of gravitational
instabilities in the vicinity of the sonic scale, l_s~0.1 pc, leading to the
formation of prestellar cores.
