Much of what we presently know or surmise about the physical processes involved in star formation is derived from the detailed study of a few nearby molecular cloud “nurseries”. Stars in the solar neighborhood are formed from the gravitationally induced collapse of cold molecular gas. Typical molecular gas clouds must contract by a factor of a million in linear dimensions to form a star. Because of this dramatic (and rapid) reduction in size, any small initial rotation of the star-forming cloud is enormously magnified by conservation of angular momentum during collapse. In this way a modestly rotating gas cloud produces a rapidly rotating object – a disk – in addition to a small, stellar core at the end of gravitational collapse. Probably most of the material of a typical star is accreted through its disk, with a small amount left behind to form planetary systems.

Advances in observational techniques spanning the electromagnetic spectrum have been essential in developing our present understanding of star formation. The launch of the Infrared Astronomy Satellite (IRAS) in 1983 led to the recognition that dusty disks are common around young stars. The ISO infrared satellite provided detailed mid-infrared spectra of many bright disks. The Spitzer Space Telescope, the latest in this line of infrared observatories, has now detected mid-infrared disk emission in very large samples of stars spanning a wide range of ages.