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Chapter 11
The Formation of Stars
Guidepost
Previous chapters have used the basic principles of physics
as a way to deduce things about stars and the interstellar
medium. All of the data we have amassed will now help us
understand the life stories of the stars in this chapter and
those that follow.
In this chapter, we use the laws of physics in a new way. We
develop theories and models based on physics that help us
understand how stars work. For instance, what stops a
contracting star and gives it stability? We can understand this
phenomenon because we understand some of the basic laws
of physics.
Throughout this chapter and the chapters that follow, we
search for evidence. What observational facts confirm or
contradict our theories? That is the basis of all science, and it
must be part of any critical analysis of what we know and
how we know it.
Outline
I. Making Stars from the Interstellar Medium
A. Star Birth in Giant Molecular Clouds
B. Heating By Contraction
C. Protostars
D. Evidence of Star Formation
II. The Source of Stellar Energy
A. A Review of the Proton-Proton Chain
B. The CNO Cycle
III. Stellar Structure
A. Energy Transport
B. What Supports the Sun?
C. Inside Stars
D. The Pressure-Temperature Thermostat
Outline (continued)
IV. The Orion Nebula
A. Evidence of Young Stars
The Life Cycle of Stars
Dense, dark
clouds, possibly
forming stars in
the future
Aging
supergiant
Young stars, still
in their birth
nebulae
Giant Molecular Clouds
Barnard 68
Infrared
Visible
Star formation collapse of the cores of giant molecular clouds:
Dark, cold, dense clouds obscuring the light of stars behind them.

(More transparent in infrared light.)
Parameters of Giant Molecular Clouds
Size: r ~ 50 pc
Mass: > 100,000 Msun
Temp.: a few 0K
Dense cores:
R ~ 0.1 pc
M ~ 1 Msun
Much too cold and too low density
to ignite thermonuclear processes
Clouds need to contract and
heat up in order to form stars.
Contraction of Giant Molecular Cloud Cores
Horse
Head
Nebula
• Thermal Energy (pressure)
• Magnetic Fields
• Rotation (angular momentum)
• Turbulence
 External
trigger required to
initiate the collapse of clouds
to form stars.
Shocks Triggering Star Formation
Trifid
Nebula
Globules = sites where stars
are being born right now!
Sources of Shock Waves
Triggering Star Formation (1)
Previous star formation can trigger
further star formation through:
a) Shocks from
supernovae
(explosions of
massive stars):
Massive stars die
young =>
Supernovae tend
to happen near
sites of recent
star formation
Sources of Shock Waves
Triggering Star Formation (2)
Previous star formation can trigger
further star formation through:
b) Ionization
fronts of hot,
massive O or B
stars which
produce a lot of
UV radiation:
Massive stars die
young => O and
B stars only exist
near sites of
recent star
formation
Sources of Shock Waves
Triggering Star Formation (3)
Giant molecular clouds are very
large and may occasionally
collide with each other
c) Collisions
of giant
molecular
clouds.
Sources of Shock Waves
Triggering Star Formation (4)
d) Spiral arms
in galaxies like
our Milky Way:
Spirals’ arms
are probably
rotating shock
wave patterns.
Protostars
Protostars =
pre-birth
state of
stars:
Hydrogen to
Helium fusion
not yet
ignited
Still enshrouded in opaque “cocoons” of dust =>
barely visible in the optical, but bright in the infrared.
Heating By Contraction
As a protostar contracts, it heats up:
Protostellar Disks
Conservation of angular
momentum leads to the
formation of
protostellar disks 
birth place of planets
and moons
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 (2)
Herbig Haro Object HH34
Protostellar Disks and Jets – Herbig
Haro Objects (3)
Herbig Haro Object HH30
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.
Evidence of Star Formation (2)
Smaller,
sunlike
stars,
probably
formed
under
the
influence
of the
massive
star
Young, very
massive star
Infrared
Optical
The Cone Nebula
Evidence of Star Formation (3)
Star Forming Region RCW 38
Globules
Bok
Globules:
~ 10 to
1000
solar
masses;
Contracting
to form
protostars
Globules (2)
Evaporating Gaseous
Globules (“EGGs”): Newly
forming stars exposed by
the ionizing radiation from
nearby massive stars
Open Clusters of Stars
Large masses of
Giant Molecular
Clouds => Stars
do not form
isolated, but in
large groups,
called Open
Clusters of Stars.
Open Cluster M7
Open Clusters of Stars (2)
Large, dense
cluster of
(yellow and red)
stars in the
foreground; ~
50 million years
old
Scattered
individual (bright,
white) stars in
the background;
only ~ 4 million
years old
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.
Energy Transport
Energy generated in the star’s center must be
transported to the surface.
Inner layers:
Radiative energy
transport
g-rays
Outer layers (including
photosphere):
Convection
Cool gas
Gas particles
sinking down
of solar
interior
Bubbles of hot
gas rising up
Conduction, Convection, and Radiation
(SLIDESHOW MODE ONLY)
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
The Sun
(SLIDESHOW MODE ONLY)
Hydrostatic Equilibrium
Imagine a star’s
interior composed of
individual shells.
Within each shell, two forces
have to be in equilibrium with
each other:
Gravity, i.e. the
weight from all
layers above
Outward pressure
from the interior
Hydrostatic Equilibrium (2)
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.
H-R Diagram (showing Main Sequence)
Energy Transport Structure
Inner convective,
outer radiative
zone
Inner radiative,
outer convective
zone
CNO cycle dominant
PP chain dominant
Summary: Stellar Structure
Convective Core,
radiative envelope;
Energy generation
through CNO Cycle
Sun
Radiative Core,
convective envelope;
Energy generation
through PP Cycle
The Orion Nebula: An Active StarForming Region
In the Orion Nebula
The BecklinNeugebauer
Object (BN):
Hot star, just
reaching the
main sequence
Kleinmann-Low
nebula (KL):
Cluster of cool,
young
protostars
detectable only
in the infrared
B3 B1
B1
O6
Visual image of
the Orion Nebula
Protostars with
protoplanetary disks
New Terms
shock wave
free-fall contraction
protostar
cocoon
protostellar disk
birth line
T Tauri star
Bok globule
Herbig–Haro object
bipolar flow
association
T association
O association
CNO (carbon–nitrogen–
oxygen) cycle
opacity
hydrostatic equilibrium
Discussion Questions
1. Ancient astronomers, philosophers, and poets
assumed that the stars were eternal and unchanging. Is
there any observation they could have made or any line
of reasoning that could have led them to conclude that
stars don’t live forever?
2. How does hydrostatic equilibrium relate to hot-air
ballooning?
Quiz Questions
1. In which component of the interstellar medium do new stars
form?
a. In the HI clouds.
b. In the HII intercloud medium.
c. In the hot coronal gas.
d. In molecular clouds.
e. Both a and d above.
Quiz Questions
2. What force causes the contraction of a cloud of interstellar
matter to form a star?
a. The electrostatic force.
b. The strong nuclear force.
c. The weak nuclear force.
d. The gravitational force.
e. All of the above.
Quiz Questions
3. Which factor resists the contraction of a cloud of interstellar
matter?
a. Thermal energy.
b. The interstellar magnetic field.
c. Rotation.
d. Turbulence.
e. All of the above.
Quiz Questions
4. What triggers the gravitational collapse of material inside a
molecular cloud?
a. Collisional cooling.
b. Shielding of the interstellar magnetic field.
c. Tidal forces slow the rate of rotation.
d. A subsidence in turbulence due to internal friction.
e. A passing shock wave.
Quiz Questions
5. What is the source of a shock wave that passes through a
molecular cloud and triggers star formation?
a. A supernova explosion.
b. The ignition of hot stars within the cloud.
c. A collision of molecular clouds.
d. A spiral wave pattern within a galaxy.
e. All of the above.
Quiz Questions
6. What happens to the temperature and density inside a
collapsing protostar?
a. Temperature and density both increase.
b. Temperature and density both decrease.
c. Temperature increases and density decreases.
d. Temperature decreases and density increases.
e. The product of temperature and density remains constant.
Quiz Questions
7. What is a protostar's energy source?
a. Nuclear fusion.
b. Gravitational energy.
c. Chemical energy.
d. Both a and b above.
e. All of the above.
Quiz Questions
8. What characteristic of the collapsing cloud that forms a
protostar allows it to also form a protostellar disk?
a. Thermal energy.
b. The interstellar magnetic field.
c. Rotation.
d. Turbulence.
e. All of the above.
Quiz Questions
9. At what wavelengths can we observe the early stages of
protostar formation?
a. Infrared.
b. Visible.
c. Ultraviolet.
d. Both a and b above.
e. Both a and c above.
Quiz Questions
10. What eventually halts the slow contraction of a newly
forming star?
a. A second shock wave.
b. Electrostatic repulsion.
c. The Coulomb barrier.
d. Nuclear fusion.
e. Gravity.
Quiz Questions
11. The gestation period for humans is 40 weeks. What was
the gestation period for our Sun; that is, how much time passed
between the onset of gravitational collapse and the Sun's
arrival on the main sequence?
a. About 40 weeks.
b. About 30,000 years.
c. About 30 million years.
d. About 1 billion years.
e. About 5 billion years.
Quiz Questions
12. According to Figure 11-5, the Protosun was cooler yet much
more luminous than the Sun is now. How can this be true?
a. The Protosun had more mass.
b. The Protosun was much larger.
c. The rate of nuclear fusion was higher inside the Protosun.
d. Both a and c above.
e. Both b and c above.
Quiz Questions
13. What evidence do we have that the Orion region is actively
forming stars?
a. Protostars are seen here at infrared wavelengths inside their
cocoons.
b. Some stars here are between the birth line and the main
sequence.
c. Some visible stars in the Orion region have disks.
d. Some short-lived stars are located in this region.
e. All of the above.
Quiz Questions
14. How does the CNO cycle differ from the proton-proton
chain?
a. The CNO cycle requires a higher temperature than the
proton-proton chain.
b. The rate of the CNO cycle is more temperature sensitive
than the proton-proton chain.
c. The energy produced by one sequence through the CNO
cycle is greater than for one sequence through the protonproton chain.
d. Both a and b above.
e. All of the above.
Quiz Questions
15. Which stars produce most of their energy by the CNO
cycle?
a. Protostars.
b. Upper main sequence stars.
c. Lower main sequence stars.
d. Both a and b above.
e. Both a and c above.
Quiz Questions
16. Which method of energy transport is NOT important inside
most stars?
a. Conduction.
b. Convection.
c. Radiation.
d. Both a and b above.
e. Both a and c above.
Quiz Questions
17. How does the extreme temperature sensitivity of the CNO
cycle affect a star's interior?
a. The CNO cycle generation zone occupies a very small
region.
b. CNO cycle stars have radiative cores and convective
envelopes.
c. CNO cycle stars have convective cores and radiative
envelopes.
d. Both a and b above.
e. Both a and c above.
Quiz Questions
18. What prevents the enormous amount of energy released
from the fusion reactions at a star's core from blowing the star
apart?
a. Gas pressure.
b. Density.
c. Opacity.
d. Gravity.
e. All of the above.
Quiz Questions
19. What would happen in the interior of a normal star if gravity
were to shrink the star's size a small amount?
a. The interior temperature would increase.
b. The rate of fusion would increase.
c. The gas pressure would increase.
d. Both a and b above.
e. All of the above.
Quiz Questions
20. Where in the Sun is the law of hydrostatic equilibrium at
work?
a. At the visible surface.
b. At the outer boundary of the energy-generating core.
c. At the convective zone/radiative zone boundary.
d. About halfway between the center and visible surface.
e. At every point inside the Sun.
Answers
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
d
d
e
e
e
a
b
c
a
d
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
c
b
e
d
b
a
e
d
e
e