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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, 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 The Fox Fur Nebula emission lines. The Trifid NGC 2246 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

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: a) Absorption from wrong ionization states The interstellar medium produces absorption lines in the spectra of stars. Narrow absorption lines from Ca II: Too low b) Small line width ionization state and too narrow for the O star (too low 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/cm 3); size: ~ 100 pc • Hot intercloud medium: Hot (T ~ a few 1000 K), ionized hydrogen (HII); low density (n ~ 0. 1 atom/cm 3); 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: • H 2 = Molecular Hydrogen Ultraviolet absorption and emission: Difficult to observe! But: Where there’s H 2, there’s also CO. Use CO as a tracer for H 2 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 Largest molecular clouds are called “Giant Molecular Clouds”: Temperature ≈ 10 K Total mass ≈ 100 – 1 million solar masses HI Cloud

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/cm 3 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/cm 3] 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 (H 2). 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 - 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 - 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 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