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Class 12 : Star formation Interstellar gas Gravitational collapse; the Jeans Mass Protostars and T-Tauri stars I : The Interstellar Medium (ISM) Galactic disk has significant dust and gas between stars But the gas not uniform… Hot ISM (T=106-107K, n=10-4-10-2cm-3; ionized) Warm ISM (T=6000-10000K; n=0.2-0.5cm-3; ionized) Cold ISM (50-100K; n=20-50cm-3; neutral/dusty) HII regions (8000K; n=102-104cm-3; ionized) Molecular clouds (10-20K; n=102-106cm-3; molecular) Hot and Warm ISM fill most of the volume Cold ISM fills about 1-5% of volume HII regions and molecular clouds are distinct “clouds” that fill <<1% of volume Stars are born in molecular clouds 1 2 3 II : Gravitational collapse and the Jeans Mass Star formation is driven by the gravitational collapse of interstellar gas clouds A gas cloud of density ρ and temperature T collapses if its mass is above a critical amount given by… This is called the Jeans Mass A very large gas cloud will fragment into small pieces, each of which is equal to the Jeans Mass As a gas cloud collapses and increases in density, the Jeans Mass decreases… the gas fragments into yet smaller pieces 4 III : Protostars Gravitational collapse leads to formation of a protostar Gravitational contraction releases energy Energy is transported to surface via convection Energy then radiated as IR radiation Protostar evolves into a main sequence star T-Tauri stars… So we have a young star surrounded by a remnant accretion disk Disk (and star) sheds excess angular momentum via formation of a jet Disk is the site for planet formation 5 6 7