In previous episodes .. Stars are formed in the spiral arms of the Galaxy, in the densest and coldest regions of the interstellar medium, which are called Molecular Clouds. Image: portion of the galactic plane: darker colours indicate higher extinction regions (due to dust in MCs). Kandori 2007 Star Formation clustered isolated Orion Taurus Perseus 0.1 pc Observationally, two modes of star formation in the Galaxy: isolated mode, in dark clouds clustered mode, in giant molecular clouds Dark clouds (10 3 Msun, 5 pc, 10 K) Dark clouds (10 3 Msun, 5 pc, 10 K) Giant Molecular Clouds (10 5 Msun, 100 pc, 30 K) Giant Molecular Clouds (10 5 Msun, 100 pc, 30 K) Star Formation: must convert dense cores into stars: temperature ~10100 K to >10 7 K -> 6 o. m. size ~0.5 pc to 2x10 -8 (1 Rsun) -> 7 o.m. density cm -3 to ~10 26 cm -3 -> 22 o.m. Young stars are associated with the dense cores in molecular clouds Young stars are associated with the dense cores in molecular clouds The properties of these dense cores differ for regions of high and low mass The properties of these dense cores differ for regions of high and low mass star formation Low Mass Star Formation... Blackbody Spectrum SPECTRAL ENERGY DISTRIBUTION Spectral Energy Distribution (SED) Class 0 > 0, peak ~100 m, BB at K, Lsmm/Lbol > 0.5%, faint IR cold envelope opt thick in free-fall. powerful and collimated outflows, protostar = d log F / d log, between 2 and 25 micron Class I >~ 0, flat SED, superposition of BB at different T,IR, faint optical envelope part. thick + disk. Moderate outflowsevolved protostar Moderate outflows, evolved protostar Class II -1.5 < < 0, mid-IR excess compared to BB, optical envelope being disrupted faint outflow, classical T Tauri, FU Ori Class III < -1.5, similar to BB, optical stellar photosphere reddened by circumstellar extinction debris disk, weak-line T Tauri, PMS Spatial Distribution Hartmann 2003 Class II ~ Class III spatial distribution. Class I: higher density regions