Observation and Theory of The Initial Stellar Mass Function

Observations of normal galactic star-forming regions suggest there is widespread near-uniformity in the initial stellar mass function (IMF) in spite of diverse physical conditions. Fluctuations may come largely from statistical effects and observational selection. There are also tantalizing, but uncertain reports that the IMF shifts systematically in peculiar regions, giving a low mass bias in quiescent gas, and a high mass bias in active starbursts. Theoretical proposals for the origin of the IMF are reviewed. The theories generally focus on a combination of four physical effects: wind-limited accretion of stellar mass, coalescence of protostellar gas clumps, mass limitations at the thermal Jeans mass, and power-law cloud structure. Hybrid theories combining the best of each may be preferred.