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Lecture 14
Star formation
Insterstellar dust and gas
• Dust and gas is mostly found in galaxy
disks, and blocks optical light
The interstellar medium
• Stars are born from this gas and dust, collectively
known as the interstellar medium.
• During their lifetime, stars may return some material
to the ISM through surface winds or explosive events
Composition of the ISM
• Hydrogen is by far the most
common element in the ISM
 Molecular (H2)
 Neutral (HI)
 Ionized (HII)
• Also contains helium and other
elements. The solid component is in
the form of dust.
Neutral hydrogen
• HI can emit radiation if the electron flips its spin angular
momentum vector.
• This is a very small energy difference of only 5.9 meV,
corresponding to a wavelength l=21 cm.
- corresponds to radio frequencies, 1420 MHz
The Milky Way in optical light
A map of neutral H in the Milky Way
Neutral Hydrogen in the Milky Way
HI gas in the Milky Way clearly reveals spiral structure
Properties of interstellar dust
• Grain sizes: 1nm-10 mm
(i.e. similar to visible
light)
• Composition: graphite,
SiC, silicates, H2, H2O
Interstellar dust
• Interstellar dust is likely produced in the envelopes
around red supergiant stars.
• Radiate in the infrared (cooling mechanism)
• Are easily destroyed by collisions
Interstellar extinction
Dust scatters starlight. Thus a star behind a dust cloud will
appear fainter. The apparent magnitude of a star is therefore:
ml  M l  5 log 10 d  5  al
where d is measured in parsecs, and al is the number of
magnitudes of extinction along the line of sight. How is this
related to the optical depth?
al  1.086 l
Molecular clouds
When hydrogen becomes dense enough, molecules of H2
form:
• H2 is nearly impossible to observe: there are no
emission or absorption lines at visible or radio
wavlengths
 Thus we rely on tracer molecules, most commonly CO but also
CH, OH, CS and C3H2.
Types of molecular clouds
Translucent clouds
T=15-50 K
n~5x108-5x109 m-3
M~3-100 MSun
R~ 1-10 pc
aV~1-5
Giant molecular clouds
T~20 K
n~1x108-3x108 m-3
M~106 MSun
R~50 pc
Giant molecular cloud cores
T~100-200 K
n~1x1013-3x1015 m-3
M~10 – 1000 MSun
R<1 pc
aV~50-1000
The sites of star formation
The cores of molecular clouds are likely sites of new
star formation
The formation of protostars
There are many unanswered questions about the
formation of protostars
 Since they form in very dense, opaque clouds of dust and gas
they are very difficult to observe in detail
Break
The Jeans mass
A simple energetic argument can give a rough approximation for the
conditions required for a molecular cloud to collapse and form
stars.
2K U  0
The virial theorem relates (time-averaged) kinetic
to potential energy, for a stable, gravitationally
bound system:
This indicates a stability criterion: if the kinetic
energy is too low, the cloud will collapse under
the force of gravity
This defines a critical mass, known as
the Jeans mass:
It can also be expressed as a radius,
in terms of the Jeans length:
 375k 3
M J  
3 3
4

G
mH

 15k
RJ  
 4GmH
1/ 2



1/ 2



1/ 2
 T3 
 3 
m 
1/ 2
 T 


 m 
 5kT

M

J
The two are related by:
 GmmH

 RJ

Example: molecular cloud cores
What is the Jeans mass for a
molecular cloud core?
10  M / M Sun  1000
T  150 K
n  5 1014 m 3
M J  4.5M Sun
Thus these cores should be collapsing under the weight of their own
gravity, consistent with their association with the sites of star
formation.
Cloud collapse
A collapsing molecular cloud starts off simply:
 In free-fall, assuming the pressure gradients are too small to have much effect
 The gas is approximately isothermal, if gas is optically thin so energy can be
efficiently radiated away.
 8G 0 r02  r0 
dr
 
  1
dt
3
r


The time it takes for
the shell containing
mass Mr to collapse
to r=0 is the freefall time scale:
1/ 2
 3 1 

t ff  
 32 G 0 
Example: cloud collapse
Notice the collapse starts off slowly, but the density increases sharply
during the final stages.