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 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 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