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 T Tauri : the prototype protostar HH Objects protoplanetary disks Debris disks are found around 50% of sunlike stars up to 1 Byr old Collapse slows before fusion begins: Protostar Contraction --> higher density --> even IR and radio photons cant 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 Stars 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 150M Sun 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 4000 K Hayashi Track Physical cause: at low T (< 4000 K), no mechanisms to transport energy out Such objects cannot maintain hydrostatic equilibrium They will rapidly contract and heat until closer to being in hydro. eq. Stromgren sphere: Virial theorem:2K + U = 0 What happens when a cloud core collapses? If 2K < |U|, then Force due to gas pressure dominates over gravity Cloud is supported against collapse Assume a spherical cloud with constant density Gravitational potential energy Kinetic energy where In order for the cloud to collapse under its own gravity, where Using the equality and solving for M gives a special mass, M J, called the Jeans Mass, after Sir James Jeans. Jeans Criterion When the mass of the cloud contained within radius R c exceeds the Jeans mass, the cloud will spontaneously collapse: You can also define a Jeans length, R J