Published online by Cambridge University Press: 04 August 2017
The massive OB stars in our Galaxy form predominantly in the warm giant molecular clouds which constitute the spiral arms. The clouds are subject to a variety of mechanisms which retard or prevent further contraction, but are nevertheless able to form stable “cores”. In the regime of subsonic internal motions, the cores may be regarded as potential protostars. The formation of massive cores, which then form massive stars, may initially be determined by the statistics of fragmentation, but may then be a feedback process, once underway, due to the steep increase of the minimum Jeans' mass with increasing temperature of the surroundings. This concept is the basis for the model of bi-modal star formation, and its implications for the initial mass function and the distribution of massive stars and metallicity gradients in the Galaxy.