Download Ch.15 star formation

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
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T Tauri : the prototype protostar
HH Objects
“protoplanetary disks”
Debris disks are found around 50% of sunlike stars
up to 1 Byr old
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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
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4000 K
They will rapidly
contract and heat until
closer to being in
hydro. eq.
Stromgren sphere:
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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  
 

Gm H  4Gm 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  

4Gm H 0 
1/2
