the interstellar medium. i. visible-wavelength observations a. nebulae b. extinction and reddening...

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The Interstellar Medium

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Page 1: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

The Interstellar Medium

Page 2: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

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

Page 3: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

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.

Page 4: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

Bare-Eye Nebula: OrionOne example of an

interstellar gas cloud (nebula) is visible to the

bare eye: the Orion nebula

Page 5: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

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

Page 6: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

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

Page 7: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

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

Page 8: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

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

Page 9: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

Interstellar Reddening (2)

The Interstellar Medium absorbs light more strongly at shorter wavelengths.

Page 10: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

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)

Page 11: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

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.

Page 12: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

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

Page 13: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

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.

Page 14: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

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

Page 15: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

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

Page 16: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

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.

Page 17: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

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.

Page 18: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

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

Page 19: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

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

Page 20: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

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

Page 21: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

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

Page 22: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

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.

Page 23: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

interstellar mediumnebulaemission nebulaHII regionreflection nebuladark nebulainterstellar dustinterstellar extinctioninterstellar reddeninginterstellar absorption lines

HI cloudsintercloud mediumpressure21-cm radiationmolecular cloud

giant molecular cloudscoronal gas 

New Terms

Page 24: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

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

Page 25: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

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.

Page 26: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

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.

Page 27: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

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.

Page 28: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

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.

Page 29: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

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.

Page 30: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

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.

Page 31: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

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.

Page 32: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

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.

Page 33: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

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.

Page 34: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

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.

Page 35: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

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–).

Page 36: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

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.

Page 37: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

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.

Page 38: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

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

Page 39: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

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

Page 40: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

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.

Page 41: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

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.

Page 42: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

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.

Page 43: The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength

Answers

1. e2. d3. a4. c5. c

7. c8. e9. d10. c

11. e12. b13. d14. d15. e16. b17. b

19. e20. d