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Star forming regions in Orion What supports Cloud Cores from collapsing under their own gravity? • Thermal Energy (gas pressure) • Magnetic Fields • Rotation (angular momentum) • Turbulence Gravity vs. gas pressure • Gravity can create stars only if it can overcome the forces supporting a cloud • Molecules in a cloud emit photons: – cause emission spectra – carry energy away – cloud cools – prevents pressure buildup QuickTime™ and a Cinepak decompressor are needed to see this picture. T Tauri : the prototype protostar HH Objects “protoplanetary disks” Debris disks are found around 50% of sunlike stars up to 1 Byr old QuickTime™ and a decompressor are needed to see this picture. Collapse slows before fusion begins: Protostar • • • • • • • Contraction --> higher density --> even IR and radio photons can’t escape --> Photons (=energy=heat) get trapped --> core heats up (P ~ nT) --> pressure increases Protostars are still big --> luminous! Gravitational potential energy --> light! Radiation Pressure • Photons exert a slight amount of pressure when they strike matter • Very massive stars are so luminous that the collective pressure of photons drives their matter into space Upper Limit on a Star’s Mass • Models of stars suggest that radiation pressure limits how massive a star can be without blowing itself apart • Observations have not found stars more massive than about 150MSun Demographics of Stars • Observations of star clusters show that star formation makes many more low-mass stars than high-mass stars Protostellar evolution onto the Main Sequence Protostellar evolution for Different Masses • Sun took ~ 30 million years from protostar to main sequence • Higher-mass stars form faster • Lower-mass stars form more slowly Hayashi Track Physical cause: at low T (< 4000 K), no mechanisms to transport energy out Such objects cannot maintain hydrostatic equilibrium QuickTime™ and a decompressor are needed to see this picture. 4000 K They will rapidly contract and heat until closer to being in hydro. eq. Stromgren sphere: QuickTime™ and a decompressor are needed to see this picture. QuickTime™ and a decompressor are needed to see this picture. What happens when a cloud core collapses? Virial theorem: 2K + U = 0 If 2K < |U|, then • • Force due to gas pressure dominates over gravity Cloud is supported against collapse Assume a spherical cloud with constant density 3 GM U 5 Rc Gravitational potential energy Kinetic energy where 0 3 K NkT 2 Mc N m H 2 c In order for the cloud to collapse under its own gravity, 3M c kT 3GM m H 5Rc 2 c 3M c Rc 4 0 1/3 where Using the equality and solving for M gives a special mass, MJ, called the Jeans Mass, after Sir James Jeans. 5kT 3 M J Gm H 4Gm H 0 3/2 1/2 Jeans Criterion When the mass of the cloud contained within radius Rc exceeds the Jeans mass, the cloud will spontaneously collapse: Mc MJ You can also define a Jeans length, RJ 15kT RJ 4Gm H 0 1/2