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Our understanding of the processes involved in the formation of low- and high-mass stars has made great progress within the last years. Stars generally form through a fragmented collapse of a molecular cloud as a group or cluster. This simple picture seems to be generally true but bears many unsolved details. The complete initial mass function of such groups is still unknown. The formation of low-mass stars is presently unclear. What are the lower and upper mass cut-offs? What role does the environment of the collapsing clouds play, for example, its radiation field and winds? How d oes the formation of the high-mass stars in a group influence the ongoing star formation. What is the role of metallicity in the star formation process? Unfortunately, the early stages of star formation, where all these processes run and stellar masses are determined, cannot be observed easily. The reason is that the spectral emission of warm and hot gas clouds with temperatures between 20K and 200K peaks at wavelengths between 150µm and 15µm, mostly inaccessible from the ground. Airborne or space borne experiments, on the other hand, are or have been limited in their spatial resolution to several 10arcsec and are thus not able to spatially resolve star forming regions well enough. In addition, many of these star forming regions are heavily obscured by dense foreground dust clouds with Av > 100. Such foreground extinctions make observation difficult even in the near infrared (e.g. Orion KL region). The MIR & FIR instruments foreseen for SOFIA will be sensitive enough and provide a high enough spatial resolution (about 5arcsec at 60µm) to solve these issues. SOFIA will allow us to spatially separate the protostars and study them individually. Mass functions of different regions, diverse in geometry and metallicity, will be compared. Deeply embedded high-mass stars will be searched for and their environment studied. Cameras onboard SOFIA will be able to map out the environment of pre-main-sequence objects like TTauri, HH-objects, and Ae/Be stars to study their expanding shells, envelopes or outflows which are probably remnants from the parent molecular clouds. The spectral energy distribution of such shells will be determined with low-resolution spectroscopy. These studies are crucial for our understanding of how stars and planetary systems form, and how our solar system may have formed. Circumstellar concentrations of dust have been discovered around several evolved stars, Beta Pic being the most well known example. It may well be possible that circumstellar dusty disks are fairly common since they have also been discovered around such ordinary stars as Alpha Lyrae. The study of such disks or concentrations, their shape and statistics, is essential for the understanding of the origin and formation of planetary systems which is linked to the question of our own existence.
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