The more we explore our Galaxy the more difficult it becomes to find static initial states. This leads us to study molecular clouds as dynamic in which initial conditions are neither prescribed or necessary. And this leads us to a new emerging paradigm which solves many of the nagging problems of the last two decades. The birth of stars is regulated by supersonic turbulence and its interplay with gravity.
In our discussion of cloud formation we attempted to isolate physical processes and develop them as competing theories. Each may have its own realm of importance, although probably in combination with others. Comparison of diverse regions, however, demonstrates ubiquitous behaviour: self-similar or fractal structure and power-law distributions and correlations are remarkably similar.
There is one means to link these observations: by taking a multi-physics approach in which the macroscopic properties of fluid dynamics unites the system properties. The nature of supersonic turbulence is defining or supervising the formation, structure and development of molecular clouds.
Finally, having made progress, we step back to ask: do clouds actually exist? The classical idea was that the observed Gaussian velocity distributions correspond to the superimposed spectral lines of many small discrete components within a cloud. The deviations from the classical Gaussian profile is attributed to effects on the periphery of individual clumps.
In the turbulent picture, discrete objects don't exist. Layers of gas are constantly dispersing and new layers forming as a sea of weak shock waves pervades the entire region. The high-speed deviations are then very naturally interpreted as due to intermittency. Intermittency is the name given to the spatial and temporal variations that accompany turbulence due to the fact that most of the energy is contained on the largest scales: any variations to this structure are not veiled by the random contributions of many other regions. The clouds we detect using any tracer molecule are just the tips of the iceberg viewed in the light of that molecule.
It is often stated that whatever the initial conditions were, they will be washed over and made untrackable if we let turbulence evolve. Analogous to the macroscopic fluid approximation to the microscopic particle motions, turbulence can be described by a statistical theory in which initial conditions cease to have any effect. We cannot predict the final locations of the young stars although we can determine a probability.
Quiescent clouds in dynamical equilibrium may prove quite common because they live quiet stress-free lives. Like a family seen from the outside, all get on peacefully, working in unison to raise the next generation of young stars. It turns out that the active family is extremely violent: true star-forming clouds don't wait to be observed and, as we will see, the protostars which form do their best to quickly cover up their origins
Last Revised: 2009 November 6th