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Chapter 9
The Formation and
Structure of Stars
The Interstellar Medium (ISM)
•Gas: ~75% H, 25% He, traces of “metals”
•1% “dust” (silicates, carbon, heavy elements coated with ice,
About the size of the particles in smoke)
•150 m average distance between dust grains
•“Dense” => ~10 to 1000 atoms/cm3
•“Thin” ~ 0.1 atoms/cm3
Structure of the ISM
The ISM occurs mainly in two types of clouds:
• HI clouds:
Cold (T ~ 100 K) clouds of neutral hydrogen (HI);
moderate density (n ~ 10 – a few hundred atoms/cm3);
size: ~ 100 pc
• Hot intercloud medium:
Hot (T ~ a few 1000 K), ionized hydrogen (HII);
low density (n ~ 0.1 atom/cm3);
gas can remain ionized because of very low density.
3 types of nebula
1. Emission
2. Reflection
3. Dark
Q: Why do emission
nebula look red and
reflection nebula blue?
Evidence for the ISM
We see absorption in elements
where the background stars are
too hot to form these lines
Narrow width (low temperature;
low density)
Multiple components (several
clouds of ISM with different
radial velocities)
=> Comes from the ISM
Interstellar reddening
Q: Why do
astronomers
rely heavily on
IR
observations?
Q: How do we know the ISM exists?
The Various Components of the
Interstellar Medium
Infrared observations reveal the
presence of cool, dusty gas.
X-ray observations reveal the
presence of hot gas.
Stellar formation from the ISM:
Must be triggered by
high mass stars –
• Give off intense
radiation
• Explode as SNs
Collapsing cloud
can form 10 to
1000 stars
• Association
• Cluster
The Contraction of a Protostar
Q: Why do you think there’s a lower limit on the mass of a main-seq.
star?
The Contraction of a Protostar
Sun: ~30 million years
15 M: 160,000 years
0.2 M: 1 billion years
From Protostars to Stars
Star emerges
from the
enshrouding
dust cocoon
Ignition of H
 He
fusion
processes
Protostellar Disks and Jets – Herbig-Haro Objects
Q: What are the bipolar flows evidence of?
Herbig-Haro Object HH34
Globules
Bok globules:
~ 10 – 1000
solar masses;
Contracting to
form protostars
Observations of star formation:
Evaporating gaseous globules
(“EGGs”): Newly forming stars
exposed by the ionizing radiation
from nearby massive stars
200 solar mass star
N 11B
V838 Mon
Trifid
Tarantula
N 49
The Source of Stellar Energy
Stars produce energy by nuclear fusion of
hydrogen into helium.
Q: How does the sun fuse H to He?
In the sun, this
happens
primarily
through the
proton-proton
(P-P) chain
The CNO Cycle
Happens in stars
> 1.1 M
More efficient that
the P-P chain.
Requires high T
(>16 million K)
Q: Why does the
CNO require a
higher temp.
than the P-P
chain?
Fusion into Heavier Elements
Fusion into elements
heavier than C, O:
requires high
temperatures (>600
million K);
occurs only in very
massive stars (more
than 8 solar masses).
Stellar structure
Conservation of mass:
Weight of each shell = total weight
Conservation of energy:
E(out) = E(from within)
Hydrostatic equilibrium:
Pressure balances gravity
Energy transport:
Describes flow of energy
dM
 4 r 2 
dr
dL
 4 r 2  e
dr
dP
GM
 2 
dr
r
dT
3  L

dr
16 ac T 3 r 2
Hydrostatic Equilibrium
Imagine a star’s
interior composed of
individual shells
Within each shell, two
forces have to be in
equilibrium with each other:
Outward pressure
from the interior
Gravity, i.e. the
weight from all
layers above
Hydrostatic
Equilibrium (II)
Outward pressure force must
exactly balance the weight of
all layers above, everywhere
in the star.
This is why we find stable stars on
such a narrow strip (main sequence)
in the Hertzsprung-Russell diagram.
Pressure-temperature thermostat
Q: How does the P-T thermostat control
the reactions in stars?
Energy Transport
Energy generated in the star’s center must be transported to the surface.
Inner layers of the sun:
Radiative energy
transport
Outer layers of the
sun (including
photosphere):
Convection
Basically the same
structure for all stars
close to 1 solar mass.
Q: Why is convection in
stars important?
Stellar Models
The structure and evolution of a star is determined by the laws of
• Hydrostatic equilibrium
• Energy transport
• Conservation of mass
• Conservation of energy
A star’s mass (and chemical
composition) completely
determines its properties.
…why stars initially all line up along the main sequence, and
why there’s a mass-luminosity relation….
The Life of Main-Sequence Stars
Stars gradually
exhaust their
hydrogen fuel.
They gradually
becoming brighter,
evolving off the
zero-age main
sequence (ZAMS).
Lifetime of a main-sequence
star (90% of total life is on
main-seq.)

fuel
M
1
 3.5  2.5
rate of consumption M
M
The Lifetimes of Stars
on the Main Sequence
The Orion Nebula:
An Active Star-Forming Region
The Trapezium
less than 2
million years old
The Orion Nebula
Infrared image: ~ 50
very young, cool, lowX-raymass
image:
~ 1000
stars
very young, hot stars
Gas blown
away from
protostars
The BecklinNeugebauer object
(BN): Hot star, just
reaching the main
sequence
IR
Kleinmann-Low
nebula (KL): Cluster
of cool, young
protostars
detectable only in
the infrared
IR + visual
B3 B1
B1
O6
Spectral
types of the
trapezium
stars
Protostars with protoplanetary disks