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1
Note that the following lectures include
animations and PowerPoint effects such as fly ins
and transitions that require you to be in
PowerPoint's Slide Show mode (presentation mode).
2
The Interstellar Medium

• Chapter 10

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Guidepost
In a discussion of bread baking, we might begin
with a chapter on wheat and flour. In our
discussion of the birth and death of stars, the
theme of the next five chapters, we begin with a
chapter about the gas and dust between the stars.
It is the flour from which nature bakes stars.
This chapter clearly illustrates how astronomers
use the interaction of light and matter to learn
about nature on the astronomical scale. That
tool, which we developed in Chapter 7, Starlight
and Atoms, is powerfully employed here,
especially when we include observations at many
different wavelengths. We also see in this
chapter the interplay of observation and theory.
Neither is useful alone, but together they are a
powerful method for studying nature, a method
generally known as science.
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Outline
I. Visible-Wavelength Observations A.
Nebulae B. Extinction and Reddening C.
Interstellar Absorption Lines II. Long- and
Short-Wavelength Observations A. 21-cm
Observations B. Molecules in Space C. Infrared
Radiation from Dust D. X Rays From the
Interstellar Medium E. Ultraviolet Observations
of the Interstellar Medium III. A Model of the
Interstellar Medium A. Four Components of the
Interstellar Medium B. The Interstellar Cycle
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A World of Dust
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
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Bare-Eye Nebula Orion
One example of an interstellar gas cloud (nebula)
is visible to the bare eye the Orion nebula
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Three Kinds of Nebulae (1)
1) Emission Nebulae
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 emission lines.
The Trifid Nebula
The Fox Fur Nebula
NGC 2246
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Three Kinds of Nebulae (2)
2) Reflection Nebulae
Star illuminates gas and dust cloud
star light is reflected by the dust
reflection nebula appear blue because blue light
is scattered by larger angles than red light
Same phenomenon makes the day sky appear blue (if
its not cloudy).
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Scattering in Earths Atmosphere
(SLIDESHOW MODE ONLY)
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Three Kinds of Nebulae (3)
Dense clouds of gas and dust absorb the light
from the stars behind
3) Dark Nebulae
appear dark in front of the brighter background
Bernard 86
Horsehead Nebula
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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
Interstellar clouds make background stars appear
redder
Infrared
Visible
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Interstellar Reddening (2)
The Interstellar Medium absorbs light more
strongly at shorter wavelengths.
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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
Narrow absorption lines from Ca II Too low
ionization state and too narrow for the O star in
the background multiple components
b) Small line width (too low temperature too low
density)
c) Multiple components (several clouds of ISM
with different radial velocities)
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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.
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Observing Neutral HydrogenThe 21-cm (radio)
line (I)
Electrons in the ground state of neutral hydrogen
have slightly different energies, depending on
their spin orientation.
Opposite magnetic fields attract gt Lower energy
Equal magnetic fields repel gt Higher energy
Magnetic field due to proton spin
21 cm line
Magnetic field due to electron spin
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The 21-cm Line of Neutral Hydrogen (II)
Transitions from the higher-energy to the
lower-energy spin state produce a characteristic
21-cm radio emission line.
gt Neutral hydrogen (HI) can be traced by
observing this radio emission.
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Observations of the 21-cm Line (1)
G a l a c t i c p l a n e
All-sky map of emission in the 21-cm line
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Observations of the 21-cm Line (2)
HI clouds moving towards Earth
HI clouds moving away from Earth
Individual HI clouds with different radial
velocities resolved
(from redshift/blueshift of line)
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Molecules in Space
In addition to atoms and ions, the interstellar
medium also contains molecules.
Molecules also store specific energies in their
a) rotation
b) vibration
Transitions between different rotational /
vibrational energy levels lead to emission
typically at radio wavelengths.
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The Most Easily Observed Molecules in Space

• CO Carbon Monoxide ? Radio emission
• OH Hydroxyl ? Radio emission.

The Most Common Molecule in Space

• H2 Molecular Hydrogen ? Ultraviolet absorption
  and emission

Difficult to observe!
But Where theres H2, theres also CO.
Use CO as a tracer for H2 in the ISM!
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Molecular Clouds

• Molecules are easily destroyed (dissociated) by
  ultraviolet photons from hot stars.

? They can only survive within dense, dusty
clouds, where UV radiation is completely absorbed.
?Molecular Clouds
UV emission from nearby stars destroys molecules
in the outer parts of the cloud is absorbed
there.
Largest molecular clouds are called Giant
Molecular Clouds
Molecules survive
Cold, dense molecular cloud core
Diameter 15 60 pc
Temperature 10 K
HI Cloud
Total mass 100 1 million solar masses
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Interstellar Dust
Probably formed in the atmospheres of cool stars.
Mostly observable through infrared emission.
Infrared and radio emissions from molecules and
dust are efficiently cooling gas in molecular
clouds
IRAS (infrared) image of infrared cirrus of
interstellar dust.
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The Coronal Gas
Additional component of very hot, low-density gas
in the ISM
X-ray image of the Cygnus region
T 1 million K
n 0.001 particles/cm3
Observable in X-rays
Called Coronal gas because of its properties
similar to the solar corona (but completely
different origin!)
Our sun is located within (near the edge of) a
coronal gas bubble.
Probably originates in supernova explosions and
winds from hot stars
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The Four Components of the Interstellar Medium
Component Temperature K Density atoms/cm3 Main Constituents
HI Clouds 50 150 1 1000 Neutral hydrogen other atoms ionized
Intercloud Medium (HII) 103 - 104 0.01 Partially ionized H other atoms fully ionized
Coronal Gas 105 - 106 10-4 10-3 All atoms highly ionized H
Molecular Clouds 20 - 50 103 - 105 Neutral gas dust and molecules
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The Interstellar Cycle
Stars, gas, and dust are in constant interaction
with each other.
Stars are formed from dense molecular cloud cores.
Supernovae trigger shock waves in the ISM that
lead to the compression of dense clouds and new
star formation.
Hot stars ionize gas, producing HII regions.
Young star clusters illuminate the remnants of
their mother clouds, producing reflection
nebulae
Supernovae of massive stars produce coronal gas
and enrich the ISM with heavier elements.
Young star clusters leave trails of rarefied ISM
behind.
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New Terms
interstellar medium nebula emission nebula HII
region reflection nebula dark nebula forbidden
line metastable level interstellar
dust interstellar extinction interstellar
reddening interstellar absorption lines HI
clouds intercloud medium pressure 21-cm
radiation molecular cloud
giant molecular clouds infrared cirrus coronal
gas local bubble or void  
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Discussion Questions
1. When we see distant streetlights through smog,
they look dimmer and redder than they do
normally. But when we see the same streetlights
through fog or falling snow, they look dimmer but
not redder. Use your knowledge of the
interstellar medium to discuss the relative sizes
of the particles in smog, fog, and snowstorms
compared to the wavelength of light. 2. If you
could see a few stars through a dark nebula, how
would you expect their spectra and colors to
differ from similar stars just in front of the
dark nebula?
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Quiz Questions
1. Which of the following is evidence that the
spaces between the stars are not totally
empty? a. The interstellar extinction of
starlight. b. The presence of absorption lines of
singly-ionized calcium in the spectra of hot
stars. c. Absorption lines in stellar spectra
that are much thinner than the other spectral
lines. d. Some stars appear redder than they
should, based on their spectral types. e. All of
the above.
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Quiz Questions
2. What is responsible for the extinction and
reddening of starlight? a. Gas atoms and
molecules. b. Dust grains with diameters near the
wavelength of light. c. Dust grains the size of
olives. d. Both a and b above. e. All of the
above.
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Quiz Questions
3. Which wavelengths of starlight ionize the cool
hydrogen atoms in the interstellar medium? a.
Ultraviolet. b. Visible light. c. Infrared. d.
Microwave. e. Radio.
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Quiz Questions
4. What type of spectra is obtained from a
reflection nebula? a. Continuous spectra. b.
Emission line spectra. c. Absorption line
spectra. d. Both b and c above. e. All of the
above.
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Quiz Questions
5. Why are interstellar absorption lines so much
thinner than stellar absorption lines? a. The
interstellar medium contains many chemical
elements not found in stars. b. Most interstellar
gas is at a lower temperature than that of
stellar atmospheres. c. The density of
interstellar gas is less than that of stellar
atmospheres. d. Both b and c above. e. All of the
above.
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Quiz Questions
6. What do forbidden lines tell us about the gas
in the interstellar medium? a. Interstellar gas
contains chemical elements not found anywhere
else. b. Most interstellar gas is at low
temperature. c. The density of interstellar gas
is very low. d. Both a and b above. e. All of the
above.
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Quiz Questions
7. The abundances of chemical elements in the
interstellar medium, based on absorption lines,
are the same as that of the Sun for hydrogen,
carbon, and oxygen. However, calcium and iron
have a lower abundance in the interstellar medium
than on the Sun. Why? a. The Sun is producing
calcium and iron. b. The Sun is consuming
hydrogen, carbon, and oxygen. c. The heavier
elements on the Sun have settled toward its
center. d. The absorption lines of calcium and
iron are difficult to detect at low
temperature. e. Calcium and iron are in dust
grains of the interstellar medium.
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Quiz Questions
8. Hot emission nebulae are somewhat red, and
cool reflection nebulae are blue. Why are these
colors different from what Wien's law tells us
about the radiation emitted by a blackbody? a.
The gases in an emission nebula do not emit light
like a blackbody. b. We see reflection nebulae by
reflected light, not emitted light. c. The dust
grains in reflection nebulae scatter shorter
wavelengths of visible light better than longer
wavelengths. d. Both a and b above. e. All of the
above.
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Quiz Questions
9. How can the HII intercloud medium be much
hotter than neutral HI clouds, and yet have about
the same pressure? a. Gas pressure and
temperature are not related in the near vacuum of
space. b. The HI clouds have a greater abundance
of heavy elements. c. The HII intercloud medium
has a greater abundance of heavy elements. d. The
HI clouds have greater density. e. The HII
intercloud medium has greater density.
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Quiz Questions
10. What wavelength band is observed to map the
distribution of carbon monoxide (CO)
molecules? a. Visible. b. Infrared. c. Radio. d.
Ultraviolet. e. X-ray.
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Quiz Questions
11. Why is locating the tracer CO molecule
important in the study of the interstellar
medium? a. It gives the location of poisonous
gas that is to be avoided. b. It gives the
location of hot coronal gas. c. It gives the
location of cool atomic hydrogen. d. It gives the
location of ionized hydrogen. e. It gives the
location of molecular hydrogen.
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Quiz Questions
12. What type of hydrogen emits 21-cm
radiation? a. Hot atomic hydrogen. b. Cool
atomic hydrogen. c. Ionized hydrogen (HII). d.
Molecular hydrogen (H2). e. The hydroxyl radical
(OH).
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Quiz Questions
13. At what wavelength can we observe the hot
coronal gas component of the interstellar
medium? a. X-ray. b. Ultraviolet. c.
Infrared. d. Both a and b above. e. All of the
above.
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Quiz Questions
14. What effect do dust grains have on the gas in
a giant molecular cloud? a. Dust grains shield
molecules from destructive ultraviolet
radiation. b. Gas atoms can find partners on the
surfaces of dust grains and form molecules. c.
Dust grains shield molecules from destructive
radio waves. d. Both a and b above. e. All the
above.
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Quiz Questions
15. Which of the following lists the four
components of the interstellar medium in order
from low TEMPERATURE to high? a. HII intercloud
medium - molecular cloud - HI cloud -
coronal gas b. Coronal gas - HII intercloud
medium - HI cloud - molecular cloud c. HI
cloud - molecular cloud - coronal gas - HII
intercloud medium d. HII intercloud medium -
molecular cloud - coronal gas - HI cloud e.
Molecular cloud - HI cloud - HII intercloud
medium - coronal gas
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Quiz Questions
16. Which of the following lists the four
components of the interstellar medium in order
from low DENSITY to high? a. HII intercloud
medium - molecular cloud - HI cloud -
coronal gas b. Coronal gas - HII intercloud
medium - HI cloud - molecular cloud c. HI
cloud - molecular cloud - coronal gas - HII
intercloud medium d. HII intercloud medium -
molecular cloud - coronal gas - HI cloud e.
Molecular cloud - HI cloud - HII intercloud
medium - coronal gas
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Quiz Questions
17. Carbon monoxide (CO) molecules absorb thermal
energy through collisions with other molecules
inside giant molecular clouds. Each CO molecule
de-excites by emitting a radio photon with a
wavelength of 2.6 mm. What effect does this
process have on the giant molecular cloud? a. It
decreases the density of the cloud. b. It cools
the cloud. c. It warms the cloud. d. Both a and b
above. e. Both a and c above.
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Quiz Questions
18. What does the infrared cirrus that was
discovered by IRAS tell us about the interstellar
medium? a. Dust is distributed in patches along
the galactic plane. b. Dust is distributed
uniformly along the galactic plane. c. The
interstellar medium is turbulent. d. Both a and c
above. e. Both b and c above.
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Quiz Questions
19. What effect does a supernova event have on
the interstellar medium? a. Such events are the
sources of the hot coronal gas. b. Material is
injected into the interstellar medium. c. They
create low-density expanding bubbles in the
interstellar medium. d. Both a and c above. e.
All of the above.
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Quiz Questions
20. The best vacuum chambers on Earth can reach
densities of about 1,000,000 atoms per cubic
centimeter. Which of the four components of the
interstellar medium has lower densities than such
a chamber? a. The hot coronal gas. b. The hot
coronal gas and HII intercloud medium. c. The hot
coronal gas, HII intercloud medium, and HI
clouds. d. The hot coronal gas, HII intercloud
medium, HI clouds, and molecular clouds. e. None
of the above.
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Answers
1. e 2. b 3. a 4. c 5. d 6. c 7. e 8. e 9. d 10. c
11. e 12. b 13. d 14. d 15. e 16. b 17. b 18. d 19
. e 20. d