The Interstellar Medium
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Transcript of The Interstellar Medium
The Interstellar Medium
I. Visible-Wavelength ObservationsA. NebulaeB. Extinction and ReddeningC. Interstellar Absorption Lines
II. Long- and Short-Wavelength ObservationsA. 21-cm ObservationsB. Molecules in SpaceC. Infrared Radiation from DustD. X Rays From the Interstellar MediumE. Ultraviolet Observations of the Interstellar Medium
III. A Model of the Interstellar MediumA. Four Components of the Interstellar MediumB. The Interstellar Cycle
Outline
A World of Dust
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 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.
Bare-Eye Nebula: OrionOne 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 NGC 2246
The Trifid Nebula
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
Three Kinds of Nebulae (3)Dense clouds of gas and dust absorb the light from the stars behind;
Bernard 86
Horsehead Nebula
appear dark in front of
the brighter background;
3) Dark Nebulae
Red light can more easily penetrate the cloud, but is still absorbed to some extent
Interstellar Reddening
Infrared
Blue light is strongly scattered and absorbed by interstellar clouds
Infrared radiation is hardly absorbed at all
Interstellar clouds make background stars appear redderVisible
Barnard 68
Interstellar Reddening (2)
The Interstellar Medium absorbs light more strongly at shorter wavelengths.
Interstellar Absorption LinesThe interstellar medium
produces absorption lines in the spectra of stars.
These can be distinguished from stellar absorption
lines through:
a) Absorption from wrong ionization
statesNarrow 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)
Structure of the ISM
• HI clouds:
• Hot intercloud medium:
The ISM occurs in two main types of clouds:
Cold (T ~ 100 K) clouds of neutral hydrogen (HI);
moderate density (n ~ 10 – a few hundred atoms/cm3);
size: ~ 100 pc
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.
Magnetic field due to proton spin
Magnetic field due to electron
spin
Opposite magnetic fields attract => Lower energy
Equal magnetic fields repel => Higher energy
21 cm line
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.
=> Neutral hydrogen
(HI) can be traced by observing this radio emission.
Observations of the 21-cm Line (1)
All-sky map of emission in the 21-cm line
G a l a c t i c p l a n e
Observations of the 21-cm Line (2)
HI clouds moving towards Earth
(from redshift/blueshift of line)
HI clouds moving away from Earth
Individual HI clouds with different radial velocities resolved
Molecules in SpaceIn 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:
Difficult to observe!
Use CO as a tracer for H2 in the ISM!
• H2 = Molecular Hydrogen Ultraviolet absorption and emission:
But: Where there’s H2, there’s also CO.
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”:
Largest molecular clouds are called “Giant Molecular Clouds”:
Diameter ≈ 15 – 60 pcTemperature ≈ 10 K
Total mass ≈ 100 – 1 million solar masses
Cold, dense molecular cloud core
HI Cloud
UV emission from nearby stars destroys molecules in the outer parts of the cloud; is
absorbed there.
Molecules survive
Interstellar DustProbably formed in the atmospheres of cool stars.
Mostly observable through infrared emission.
IRAS (infrared) image of infrared cirrus of
interstellar dust.
Infrared and radio emissions from molecules and dust are efficiently cooling gas in molecular clouds
The Coronal GasAdditional component of very hot, low-density gas in the ISM:
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!)
Probably originates in supernova explosions and winds from hot stars
X-ray image of the Cygnus region
Our sun is located within (near the edge of) a coronal gas bubble.
T ~ 1 million K
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 CycleStars, 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
Young star clusters leave trails of rarefied ISM behind.
Supernovae of massive stars
produce coronal gas and enrich the ISM with
heavier elements.
Supernovae trigger shock
waves in the ISM that lead to the compression of
dense clouds and new star
formation.
interstellar mediumnebulaemission nebulaHII regionreflection nebuladark nebulainterstellar dustinterstellar extinctioninterstellar reddeninginterstellar absorption lines
HI cloudsintercloud mediumpressure21-cm radiationmolecular cloud
giant molecular cloudscoronal gas
New Terms
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?
Discussion Questions
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
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 gasb. 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 cloude. 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 gasb. 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 cloude. 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
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. e2. d3. a4. c5. c
7. c8. e9. d10. c
11. e12. b13. d14. d15. e16. b17. b
19. e20. d