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
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
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
19.
20.
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.
25. 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
26. Assume a spherical cloud with constant density
Gravitational potential energy
Kinetic energy
where
27. In order for the cloud to collapse under its own gravity,
where
Using the equality and solving for M gives a special mass,
MJ, called the Jeans Mass, after Sir James Jeans.
28. Jeans Criterion
When the mass of the cloud contained within radius Rc
exceeds the Jeans mass, the cloud will spontaneously
collapse:
You can also define a Jeans length, RJ
