Transcript The Milky Way - University of North Texas

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Chapter 10
The Interstellar Medium
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.
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
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 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 it’s not
cloudy).
2) Reflection Nebulae
Scattering in Earth’s Atmosphere
(SLIDESHOW MODE ONLY)
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
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 Reddening (2)
The Interstellar Medium absorbs light
more strongly at shorter wavelengths.
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)
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.
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)
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
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.
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
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
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
The Interstellar Cycle
Stars, gas, and dust are in constant interaction with each other.
Stars are formed from dense
molecular cloud cores.
Hot stars
ionize gas,
producing
HII regions.
Young star clusters
illuminate the remnants
of their “mother”
clouds, producing
reflection nebulae
Supernovae
trigger shock
waves in the ISM
that lead to the
compression of
dense clouds and
new star
formation.
Supernovae of
Young star clusters leave
trails of rarefied ISM behind. massive stars
produce coronal
gas and enrich
the ISM with
heavier elements.
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
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?
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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–).
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.
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.
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
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
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.
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.
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.
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.
Answers
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
e
b
a
c
d
c
e
e
d
c
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
e
b
d
d
e
b
b
d
e
d