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Chapter 10
The Interstellar Medium
Guidepost
You have begun your study of the sun and other stars, but
now it is time to focus on the thin gas and dust that drifts
through space between the stars. This chapter will show
you how important this material is to the story of stars
and will help you answer three important questions:
• How do astronomers study the gas and dust between the
stars, called interstellar medium?
• What kinds of material make up the interstellar medium?
• How does the interstellar medium interact with the stars?
The gas and dust between the stars is the starting point
for the life story of the stars. The next four chapters will
trace the birth, life, and death of stars.
Outline
I. Studying the Interstellar Medium
A. Nebulae
B. Extinction and Reddening
C. Interstellar Absorption Lines
D. Interstellar Emission Lines
E. Infrared Radiation from Dust
II. Components of the Interstellar Medium
A. Cool Clouds
B. The Intercloud Medium
C. Molecular Clouds
D. Coronal Gas
III. The Gas-Stars-Gas Cycle
A. Gas and Dust from Aging Stars
B. A Preview of Star Formation
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
Bare-Eye Nebula: Orion
One example of an
interstellar gas cloud
(nebula) is visible to the
bare eye: the Orion nebula
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 Fox Fur Nebula
The Trifid
NGC 2246Nebula
Three Kinds of Nebulae (2)
Star illuminates a gas
and dust cloud;
star light is reflected
by the dust;
reflection nebulae appear
blue because blue light is
scattered by larger angles
than red light;
the same phenomenon
makes the day sky
appear blue (if it’s not
cloudy)
2) Reflection Nebulae
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;
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.
Barnard 68
Visible
Infrared radiation
is hardly
absorbed at all.
Interstellar
clouds make
background
stars appear
Infrared redder.
Interstellar Reddening (2)
The Interstellar Medium absorbs light
more strongly at shorter wavelengths.
Interstellar Reddening (3)
Nebulae that appear as dark nebulae in the
optical, can shine brightly in the infrared due to
blackbody radiation from the warm dust.
Interstellar Absorption Lines
These can be
distinguished from
stellar absorption
lines through:
The interstellar medium
produces absorption lines in
the spectra of stars.
a) Absorption from
wrong ionization
states
Narrow absorption lines from Ca II: Too low
b) Small line width
ionization state and too narrow for the O
(too low
star in the background; multiple components
temperature; too
low density)
c) Multiple
components
(several clouds of
ISM with different
radial velocities)
Observing Neutral Hydrogen:
The 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 =>
Lower energy
Magnetic field
due to proton spin
21 cm line
Magnetic field
due to electron
spin
Equal magnetic
fields repel =>
Higher energy
The 21-cm Line of Neutral Hydrogen (II)
Transitions from the higher-energy to the lowerenergy spin state produce a characteristic 21-cm
radio emission line.
=> Neutral
hydrogen
(HI) can be
traced by
observing
this radio
emission.
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
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)
Interstellar Dust
Probably formed in the atmospheres of cool stars
Mostly observable through infrared emission
Spitzer Space
Telescope (infrared)
image of interstellar
dust near the center
of our Milky Way
(Right:) Infrared
Emission from
interstellar dust and
gas molecules in the
“Whirlpool Galaxy”
M51
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
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
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 there’s H2, there’s also CO
Use CO as a tracer for H2 in the ISM!
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
Molecules
nearby stars destroys
survive
molecules in the outer
parts of the cloud; is Cold, dense
molecular
absorbed there.
cloud core
Diameter ≈ 15 – 60 pc
HI Cloud
Temperature ≈ 10 K
Largest molecular
clouds are called
“Giant Molecular
Clouds”:
Total mass ≈ 100 – 1 million solar masses
Molecular Clouds (2)
The dense
cores of
Giant
Molecular
Clouds are
the birth
places of
stars.
The Coronal Gas
Additional component
of very hot, low-density
gas in the ISM:
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
Probably originates in supernova explosions (near the edge of) a
coronal gas bubble.
and winds from hot stars
The Four Components of the
Interstellar Medium
The Gas-Star-Gas Cycle
All stars are constantly blowing gas out
into space (recall: Solar wind!)
The more luminous the star is, the
stronger is its stellar wind.
These winds
are particularly
strong in aging
red giant stars.
The Gas-Stars-Gas Cycle
Stars, gas, and dust are in constant interaction with each other.