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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
The Various Appearances of the ISM
Three kinds of nebulae
1) Emission Nebulae (HII Regions)
Hot star illuminates
a gas cloud;
excites and/or
ionizes the gas
(electrons kicked
into higher energy
states);
electrons
recombining, falling
back to ground
state produce
The Fox Fur Nebula
emission lines.
NGC
2246
The
Trifid
Nebula
2) Reflection Nebulae
Star illuminates gas and
dust cloud;
star light is reflected by
the dust;
reflection nebula appears
blue because blue light is
scattered by larger angles
than red light;
Same phenomenon makes
the day sky appear blue (if
it’s not cloudy).
Emission and Reflection Nebulae
3) Dark Nebulae
Dense clouds
of gas and
dust absorb
the light from
the stars
behind;
appear dark
in front of the
brighter
background;
Barnard 86
Horsehead Nebula
Interstellar Reddening
Blue light is strongly scattered and
absorbed by interstellar clouds
Red light can more easily
penetrate the cloud, but is
still absorbed to some extent
Infrared
radiation is
hardly absorbed
at all
Barnard 68
Visible
Interstellar
clouds make
background
stars appear
Infrared
redder
Interstellar Absorption Lines
The interstellar medium produces
absorption lines in the spectra of stars.
These can be
distinguished from stellar
absorption lines through:
a) Absorption from wrong
ionization states
b) Small line width (too
low temperature; too
low density)
c) Multiple components
(several clouds of ISM
with different radial
velocities)
Narrow absorption lines from Ca II: Too low
ionization state and too narrow for the O
star in the background; multiple components
Structure of the ISM
The ISM occurs in two main 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.
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.
Shocks Triggering
Star Formation
Henize 206
(infrared)
The Contraction of a Protostar
From Protostars
to Stars
Star emerges
from the
enshrouding
dust cocoon
Ignition of H
 He
fusion
processes
Evidence of Star Formation
Nebula around
S Monocerotis:
Contains many massive,
very young stars,
including T Tauri Stars:
strongly variable; bright
in the infrared.
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
Protostellar Disks and Jets –
Herbig-Haro Objects (II)
Herbig-Haro Object HH34
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
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
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 (II)
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.
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
Flow of energy
Stellar Structure
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
The Orion Nebula:
An Active Star-Forming Region
The Trapezium
The 4 trapezium stars:
Only one of the
Brightest, very young
trapezium
Infrared image:
stars is
~ hot
50
(less than 2 million
very
enough
young,tocool,
ionize
lowyears
old)
stars
in
the
X-ray
image:
~
1000
hydrogen
massinstars
the Orion
central
region
ofstars
the
very young,
hot
nebula
Orion nebula
The Orion Nebula
Kleinmann-Low
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
Protostars with protoplanetary disks
Visual image of the Orion Nebula