Dark clouds within a few hundred pc of the Sun contain hundreds of condensations with typical size 0.1 pc, density 104 molecules per cubic cm, mass 1 M⊙, and temperature 10 K. These “dense cores” are defined by maps of molecular lines, such as the (J,K)=(1,1) line of ammonia at 1.3 cm wavelength. They are associated with regions of opaque visual obscuration, groups of T Tauri stars, and other cores. They are closely correlated with steep-spectrum, low-luminosity (1-10 L⊙) IRAS sources! of about 60 cores with ammonia maps, half have an IRAS source within one map diameter. Thus cores form low-mass stars, which are probably precursors of T Tauri stars. Simple models indicate that time for a core to wait before collapsing, to collapse and form a star, and to disperse are each of order 105 yr. Cores with stars have broader lines and bigger velocity gradients than cores without stars, suggesting interaction between the star and the core due to gravity and/or outflow. Stars in cores have about 30 mag greater circumstellar extinction, and greater likelihood of CO outflow, than stars near, but not in, cores. Models of the 1-100 μm spectra of stars in cores suggest that inside of ∼100 A.U., the typical star suffers relatively little line-of-sight extinction but is accompanied by a source of significant luminosity at 5-25 μm. Models involving circumstellar disks provide good fits to the observed spectra.
Dynamics of wind and the dusty environments in the accreting T Tauri stars RY Tauri and SU Aurigae