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Chapter 4: Formation of stars
Insterstellar dust and gas
• Viewing a galaxy edge-on, you see a
dark lane where starlight is being
absorbed by dust.
An all-sky map of neutral
hydrogen in the Milky Way.
The plane of the galaxy is
highly obscured by absorbing
gas and dust.
Looking toward the Galactic
centre, in visible 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
• In supernova explosions, most of the star is dispersed throughout the
ISM.
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.
Properties of interstellar dust
• Dust grains form by condensing out of a cooling cloud
of interstellar gas.
• Facilitate many chemical reactions
 They provide the only mechanism known for forming H2
• Radiate efficiently in the infrared, and therefore
provide an effective means of cooling
• Makes up ~10% of the
ISM by mass
• Composition: graphite,
SiC, silicates, H2, H2O
Types of molecular clouds
Translucent clouds
T=15-50 K
n~5x108-5x109 m-3
M~3-100 MSun
R~ 1-10 pc
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
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
It can be shown that a uniformdensity cloud will collapse if the
mass exceeds the Jeans mass (or,
equivalently, if the radius
exceeds the Jeans length)
 375k 3
M J  
3 3
4

G
mH

 15k
RJ  
 4GmH
1/ 2



1/ 2



1/ 2
 T3 
 3 
 
1/ 2
 T 


  
Example: Diffuse HI clouds
What is the Jeans mass for a typical diffuse cloud?
M  100 M Sun
T  50 K
n  5 108 m 3
Example: molecular cloud cores
Typical conditions in molecular cloud cores:
10  M / M Sun  1000
T  150 K
n  5 10 m
14
3
The sites of star formation
• Could occur in giant molecular clouds with masses up to
~3x106Msun, in core regions where T≤30K
 Additional support provided by turbulence, magnetic fields, rotation
 need a trigger to start formation of small, dense cores where
gravity can dominate
 possible triggers: supernova shock wave; stellar winds, spiral arm
density waves
Break
Star formation
A slowly-rotating, Jeans-unstable core of a molecular cloud can
start to collapse
Evolution of a solar mass protostar
1.
Initially the clump is able to
radiate all its gravitational
energy efficiently, and
collapses quickly.
2. As the core density increases
the energy goes into heating
the cloud. The core reaches
approximate hydrostatic
equilibrium, with a radius of
~5 AU. This is the protostar.
Evolution of a solar mass protostar
3.
4.
5.
Above the protostar, the rest of
the cloud is still in free-fall.
Rotation of the cloud means this
collapsing material forms a disk.
Eventually T becomes high enough
that molecular hydrogen
dissociates; this absorbs some of
the energy supporting the
protostar, so the core begins to
collapse further, until it becomes
~30% larger than the present
Solar radius (but still much less
massive).
The protostar continues to
accrete material from the
infalling cloud.
Stellar disks
Young main sequence stars often still have disks, even after the
molecular cloud has been dispersed.
Infrared-emitting dust disk around
b-Pic. The central star has been
subtracted.
The dust disk around Vega. At least
one large planet is known to exist
within this disk.
Disk shapes
We can see that many
disks are thinner in the
centre than the edges.
Why is this?
Evolution of a solar mass protostar
•
When the star begins nuclear fusion it releases a
large amount of energy in a bipolar jet, which:
 Prevents further collapse of material?
 Disperses gas disk?
 Gets rid of angular momentum?
•
As dust agglomerates
into planetesimals, or is
ejected by the jet, the
central star becomes
visible.
Here we can actually see the
stellar disk, illuminated by the
central, obscured, star
Herbig-Haro objects
• Jets associated with star formation interact with the surrounding ISM,
exciting the gas and forming bright, emission line objects. These are HH
objects.
Observations: T Tauri stars
• T Tauri stars are a class of pre-main sequence stars with M<3
MSun with unusual spectra




Strong Balmer emission lines
Emission lines of Ca II and Fe
Absorption lines of Li
Forbidden lines [OI] and [SII]: indicate low gas densities
• First view of the star as the cocoon is blown away.
Pre-main sequence evolution
Next lecture
Planet formation
• Condensation of solids from the solar nebula
• Planet growth by:
 Collisional accretion
 Gravitational attraction
• Final fate of planetesimals