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Chapter 9
The Formation and
Structure of Stars
The Interstellar Medium (ISM)
The space between the stars is not completely
empty, but filled with very dilute gas and dust,
producing some of the most beautiful objects
in the sky.
We are interested in the interstellar
medium because:
a) Dense interstellar clouds are the
birth place of stars.
b) Dark clouds alter and absorb the light
from stars behind them.
Compression of the ISM by Winds
from Hot Stars
The Contraction of a Protostar
From Protostars to Stars
Star emerges
from the
enshrouding
dust cocoon
Ignition of H
→ He fusion
processes
Factors in Nuclear Fusion
• Hydrogen atoms are ionized (bare nuclei)
• Nuclei repel each other (Coulomb barrier)
• High enough temperature means a small
percentage will have a high enough energy to
get close enough for strong interaction to
occur (Maxwell distribution of velocities)
• Sufficiently high pressure ensures that enough
reactions occur to supply energy needs of star
Evidence of Star Formation
Nebula around
S Monocerotis:
Contains many
massive, very
young stars,
including T Tauri Stars:
strongly variable; bright
in the infrared.
T Tauri Stars
Very young stars, still in the forming stage
Typically 100,000 – 10 million years old
Protostellar Disks and Jets –
Herbig Haro Objects
Disks of matter accreted onto the protostar (“accretion
disks”) often lead to the formation of jets (directed
outflows; bipolar outflows): Herbig Haro Objects
Globules
Bok Globules:
~ 10 – 1000
solar masses;
Contracting to
form protostars
Globules
Evaporating Gaseous
Globules (“EGGs”): Newly
forming stars exposed by
the ionizing radiation from
nearby massive stars.
Winds from Hot Stars
Very young, hot stars produce massive stellar winds,
blowing parts of it away into interstellar space.
Eta Carinae
The Orion Nebula
An Active Star-Forming Region
The Trapezium
Only one of the
Infrared image:
~ hot
50
trapezium
stars is
very
young,tocool,
lowenough
ionize
X-ray
mass
image:
1000
hydrogen
instars
the~ Orion
very young,
hot stars
nebula.
The Orion Nebula
KleinmannLow nebula
(KL): Cluster
of cool, young
protostars
detectable
only in the
infrared
The BecklinNeugebauer Object
(BN): Hot star, just
reaching the main
sequence
B3 B1
B1
O6
Spectral
types of the
trapezium
stars
Visual image of the Orion Nebula
Protostars with protoplanetary disks
The Source of Stellar Energy
Recall from our discussion of the sun:
Stars produce energy by nuclear fusion of
hydrogen into helium
In the sun, this
happens
primarily
through the
proton-proton
(PP) chain.
Energy Source
• 1H + 1H  2H + e+ + ν
– 2H moving fast
– e+ annihilates an electron producing Gamma rays
– Neutrino escapes from sun
• 2H + 1H  3He + γ
• 3He + 3He  4He + 1H + 1H
The CNO Cycle
In stars slightly
more massive
than the sun, a
more powerful
energy generation
mechanism than
the PP chain
takes over.
The CNO
Cycle
Fusion into Heavier Elements
Fusion into heavier
elements than C, O:
requires very high
temperatures; occurs
only in very massive
stars (more than 8
solar masses)
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
Outward pressure force
must exactly balance the
weight of all layers
above everywhere in
the star.
This condition uniquely
determines the interior
structure of the star.
This is why we find stable
stars on such a narrow strip
(Main Sequence) in the
Hertzsprung-Russell diagram.
Regulation of Energy Production
• If the energy production were to be
insufficient then temp of core would decrease.
• Pressure would decrease which would cause
star to contract causing temp to increase again
because of energy release from gravity.
• If energy production were to be too much
then the steps would occur in reverse.
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
Stellar Structure
Flow of energy
Energy transport
via convection
Sun
Energy transport
via radiation
Energy
generation via
nuclear fusion
Basically the same
structure for all stars
with approx. 1 solar
mass or less.
Temperature, density
and pressure decreasing
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.
That’s why stars initially all line up along the main sequence.
Interactions of Stars and their
Environment
Supernova explosions
of the most massive
stars inflate and blow
away remaining gas of
star forming regions.
Young, massive stars excite
the remaining gas of their
star forming regions,
forming HII regions.
The Life of Main Sequence Stars
Stars gradually
exhaust their
hydrogen fuel.
In this process of
aging, they are
gradually
becoming brighter,
evolving off the
zero-age main
sequence.
The Lifetimes of Stars on
the Main Sequence