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B.G. Elmegreen (IBM Watson)
Star formation has been found to require a threshold gas surface density in a galaxy disk, and to have a rate that scales with a power law dependence on this surface density once the threshold is exceeded. These large-scale empirical laws suggest that the self-gravity of the gas is important, as if spiral and cloud-forming instabilities are involved. However, the scales for gravitational instabilities are much larger than the scales of individual cloud cores, where most stars form. One wonders how the cores know about the large scale properties of the galaxy around them? Stars also form in a hierarchical fashion on scales ranging from parsecs to kiloparsecs, revealing fractal patterns in young star fields that are similar to the fractal patterns in the gas. Young cluster ages are also correlated with their positions in a manner reminiscent of turbulence. These are different empirical laws, and they suggest that turbulence regulates star-formation instead of gravitational instabilities, which tend to have a characteristic length. This situation is complicated by the common observation that most stars actually form in dense clusters, and that most clusters in the Solar neighborhood were directly triggered by adjacent HII regions. Evidently, there are three unique processes of star formation, and each can independently account for most young stars, depending on how star formation is observed. This talk will address a unifying model in which all of the empirical laws and morphologies can be explained, and it will suggest some important challenges for the future.