Star forming regions in Orion

1. Star forming regions in Orion

2. What supports Cloud Cores from collapsing under their own gravity? • Thermal Energy (gas pressure) • Magnetic Fields • Rotation (angular momentum) • Turbulence

3. 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

9. T Tauri : the prototype protostar

10. HH Objects

11. “protoplanetary disks”

12. Debris disks are found around 50% of sunlike stars up to 1 Byr old

13. 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!

14. 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

15. 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

16. Demographics of Stars • Observations of star clusters show that star formation makes many more low-mass stars than high-mass stars

17. Protostellar evolution onto the Main Sequence

18. 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

21. 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.

22. Stromgren sphere:

25. 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

26. Gravitational potential energy Kinetic energy where Assume a spherical cloud with constant density

27. where Using the equality and solving for M gives a special mass, MJ, called the Jeans Mass, after Sir James Jeans. In order for the cloud to collapse under its own gravity,

28. You can also define a Jeans length, RJ Jeans Criterion When the mass of the cloud contained within radius Rc exceeds the Jeans mass, the cloud will spontaneously collapse: