1. Chapter 11 The Interstellar Medium

2. The Eagle Nebula
Interstellar space is the place both where stars are “born” and to which they return at “death.” Rich in gas and dust, yet extraordinarily thinly distributed throughout the vast, dark regions among the stars, interstellar matter occasionally glows as nebulae or contracts to form new stars (and sometimes planets). Here, we see a billowing tower of cold, loose matter extending for nearly 10 light-years from a stellar nursery known as the Eagle Nebula. Radiation from its young stars excites the surrounding gas, which then re-emits colorful light characteristic of abundant elements
(red is hydrogen, blue is oxygen). (STScI)

3. Interstellar Medium Gas - atoms and small molecules
Dust - clumps of atoms and molecules

4. Figure 11.1 Milky Way Mosaic
The Milky Way photographed almost from horizon to horizon, spanning nearly 180°. This band contains high concentrations of stars as well as interstellar gas and dust. The grey box shows the field of view of Figure (Axel Mellinger)

5. Dust Typical dust grains are size of wavelength of visible light
Opaque to short wavelengths: optical, UV and X-ray
Transparent to long wavelengths: radio and infrared radiation
Shorter wavelength reduced - “reddening”

6. (a) Starlight passing through a dusty region of space is both dimmed and reddened, but spectral lines are still recognizable in the light that reaches Earth. Note that this reddening has nothing to do with the Doppler effect—the frequencies of the lines are unchanged, although their intensities may be substantially reduced. (b) This dusty interstellar cloud, called Barnard 68, is opaque to visible light, except near the edges, where some light from background stars can be seen. Because blue light is more easily scattered or absorbed by dust than is red light, stars seen through the cloud appear red. The cloud spans about 0.2 pc and lies about 160 pc away. Frame (c) illustrates (in false color) how infrared radiation can penetrate Barnard 68, although it too is preferentially stripped of its shorter wavelengths. (ESO)
Figure 11.2 Reddening

7. Figure 11.3 Reddening in Earth’s Atmosphere
The phenomenon of reddening is especially evident as sunlight passes through Earth’s atmosphere at the end of a long, hot summer day. Airborne dust particles and evaporated seawater molecules most easily scatter the Sun’s blue light, leaving only its dimmed, red light—a spectacular sunset. (NCAR)

8. Composition of Interstellar Medium
90% of gas is atomic or molecular H
9% is He
1% is heavier elements
Dust composition not well known

9. Density of Interstellar Medium
106 atoms per cubic meter
Best laboratory vacuum is 109 atoms per cubic meter
About 1 dust particle for every trillion or so atoms
Vast distances cause absorption

10. Figure 11.4 Milky Way in Sagittarius
Enlargement of the central portion of Figure 11.1, showing regions of brightness (vast fields of stars) as well as regions of darkness (where interstellar matter obscures the light from more distant stars). The field of view is roughly 30° across. The four red emission nebulae discussed in the text are labeled. (Palomar Observatory/Caltech)

11. Star-forming regions Emission nebula (nebulae plural)
Glowing clouds of hot interstellar matter
Messier objects (18th century system)
Contain newly formed hot O- or B-type star
UV light from star ionizes gas
Electrons recombine, causing glow

12. Figure 11.5 Galactic Plane
A photograph of a smaller portion (about 12° across) of the region of the sky shown in Figure 11.4, showing stars, gas, and dust as well as several distinct fuzzy patches of light known as emission nebulae. The plane of the Milky Way is marked with a white diagonal line. (Harvard College Observatory)

13. Figure 11.6 - M20 top and M8 bottom

14. Figure 11.7 Trifid Nebula
(a) Further enlargement of the top of Figure 11.6, showing only M20 and its interstellar environment. The nebula itself (in red) is about 6 pc in diameter. It is often called the Trifid Nebula because of the dust lanes (in black) that trisect its midsection. (b) A false-color infrared image taken by the Spitzer Space Telescope reveals bright regions of star-forming activity mostly in those lanes of dust. (AURA; NASA)

15. Figure 11.8 Nebular Structure
An emission nebula results when ultraviolet radiation from one or more hot stars ionizes part of an interstellar cloud. The nebula’s reddish color is produced as electrons and protons recombine to form hydrogen atoms. Dust lanes may be seen if part of the parent cloud happens to obscure the emitting region. If some starlight happens to encounter another dusty cloud (or perhaps another part of the cloud harboring the emission nebula), some of the radiation, particularly at the shorter wavelength blue end of the spectrum, may be scattered back toward Earth, forming a reflection nebula.

16. Figure 11.9 Emission Nebulae
Enlargements of selected portions of Figure (a) M16, the Eagle Nebula. (b) A Hubble Space Telescope image of huge pillars of cold gas and dust inside M16 shows delicate sculptures created by the action of stellar ultraviolet radiation on the original cloud. (c) M8, the Lagoon Nebula. (d) A high-resolution view of the core of M8, a region known as the Hourglass. Notice the irregular shape of the emitting regions, the characteristic red
color of the light, the bright stars within the gas, and the patches of obscuring dust. The varied colors of the insets result from observations at different wavelengths. (AURA; NASA)

17. Figure 11.10 Nebular Spectrum
The visible spectrum of the hot gases in a nearby star-forming region known as the Omega nebula (M17). Shining by the light of several very hot stars, the nebula produces a complex spectrum of bright and dark lines (bottom), also shown here as an intensity trace from red to blue (center). (ESO)

18. Dark Dust Clouds
More than 99% of space is devoid of emission nebulae and stars
Such dark regions are 100 K temperature
Dark dust clouds found in dark regions
Cooler and more dense than surroundings
Made of dust and primarily gas

19. Figure 11.11 Obscuration and Emission
(a) At optical wavelengths, this dark dust cloud (known as L977) can be seen only by its obscuration of background stars. (b) At radio wavelengths, it emits strongly in the CO molecular line, with the most intense radiation coming from its densest part. (C. and E. Lada)

20. Figure Dark Dust Cloud
(a) The dark dust cloud Rho Ophiuchi is “visible” only because it blocks light coming from stars behind it. The dashed line indicates the cloud’s approximate outline. (b) Another view of the region on a slightly enlarged scale, showing fainter foreground objects and more subtle colors. To orient (a) and (b), note the “pentagon” of bright objects visible in the lower half of each image. The bright star Antares is at the bottom. Up and to its right, near the edge of the image, is a star cluster called M4. Rho Ophiuchi itself is the bright object near the top, surrounded by a blue reflection nebula. (c) An infrared map of the same region, to roughly the same scale as parts (a) and (b). The very bright source near the top of the cloud is a hot emission nebula, also visible in the optical images. The “streamers” at left are the dark dust lanes evident in parts (a) and (b). (The black diagonal streak at right is an instrumental effect.) (Harvard Observatory; D. Malin; NASA)

21. Figure 11.13 Horsehead Nebula
(a) The Horsehead Nebula in Orion is a striking example of a dark dust cloud, silhouetted against the bright background of an emission nebula. (b) A stunning image of the Horsehead, taken at highest resolution by the Very Large Telescope (VLT) in Chile. •(Sec. 3.2) The “neck” of the horse is about 0.25 pc across. The nebular region lies roughly 1500 pc from Earth. (Royal Observatory of Belgium; ESO)

22. Hydrogen 21-cm emission Much of interstellar gas is atomic H
H has proton and electron
Two possible quantum states:
Spins same direction (higher energy)
Spins opposite direction (lower energy)
Energy difference between states small
Emits photon of 21 cm wavelength

23. Figure 11.14 Hydrogen 21-cm Emission
A ground-level hydrogen atom changing from a higher-energy state (electron and proton spinning in the same direction) to a lower-energy state (spinning in opposite directions). The emitted photon carries away an energy equal to the energy difference between the two spin states.

24. 21-centimeter line Radio wavelength Much larger than dust particles
Passes right through dust clouds

25. Molecular Clouds Cold regions - 20 K Gas is molecules, not atoms
Molecular Hydrogen, H2, is common but doesn’t emit radio wavelengths
Use tracer molecules, CO, HCN, NH3, H2O, H2CO

26. Figure 11.15 Molecules near M20
Contour map of the amount of formaldehyde near the M20 nebula, demonstrating how formaldehyde is especially abundant in the darkest interstellar regions. Other kinds of molecules have been found to be similarly distributed. The contour values increase from the outside to the inside, so the maximum density of formaldehyde lies just to the bottom right of the visible nebula. The green and red contours outline the intensity of formaldehyde lines at different frequencies. (AURA; contours by the authors)

27. Figure 11.16 Molecular Cloud Complexes - CO emission
This radio map shows the outer portion of the Milky Way (that is, looking in the opposite direction from Figure 11.1) as it appears in CO emission. The bright regions are molecular cloud complexes, dense regions of interstellar space where molecules abound and, apparently, stars are forming. The map extends over a full quadrant of the Galaxy, or 90°, along the Galactic plane and was made from 1,696,800 observations of CO spectra. (Five College Radio Astronomy Observatory)

28. Formation of sun-like stars
Seven stages of formation
Fight between gravity and heat
Gravity pulls particles inward
Heat (via pressure) pushes particles outward

29. Figure Atomic Motions
The motions of a few atoms within an interstellar cloud are influenced by gravity so slightly that the atoms’ paths are hardly changed (a) before, (b) during, and (c) after an accidental, random encounter.

30. Triggering a clump Possibilities: O and B stars form a shock wave
Supernova forms a shock wave
Interstellar cloud cools and collapses
For a 100 K cloud, need 1057 atoms to make clump permanent
Roughly mass of sun

31. Table 11.2 Seven Stages of Prestellar Evolution of a Sun-like Star

32. Stage 1 - Interstellar cloud
Up to 10’s of parsecs across
10 K
109 particles/m3
Thousands of times mass of sun
Collapse and fragmentation

33. Figure 11.18 Cloud Fragmentation
As an interstellar cloud contracts, gravitational instabilities cause it to fragment into smaller pieces. The pieces themselves continue to contract and fragment, eventually forming many tens or hundreds of individual stars.

34. Stage 2 - Cloud fragment Fragment contains about 1 to 2 M
About 100X size of solar system
1012 particles/m3 at center
Radiation escapes except at center
Center might be 100 K

35. Stage 3 - Fragment/protostar
About size of solar system
1018 particles/m3 at center
Radiation escapes except at center
Center might be 10,000 K
Center is a protostar

36. Figure 11.19 Orion Nebula, Up Close
(b) Enlargement of the framed region of part (a), suggesting how the nebula is partly surrounded by a vast molecular cloud. Various parts of this cloud are probably fragmenting and contracting, with even smaller sites forming protostars. The three frames at the right show some of the evidence for those protostars: (c) false-color radio image of some intensely emitting molecular sites, (d) real-color visible image of embedded nebular “knots” thought to harbor protostars, and (e) high-resolution image of several young stars surrounded by disks of gas and dust where planets might ultimately form. (J. Sanford/Astrostock-Sanford; AURA; Harvard-Smithsonian Center for Astrophysics; NASA)

37. Stage 4 - Protostar About size of Mercury’s orbit
1000X luminosity of sun (large area)
Center around 1,000,000 K
Surface is 3000 K
Can track on H-R diagram

38. Figure 11.20 Protostar on the H-R Diagram
The red arrow indicates the approximate evolutionary track followed by an interstellar cloud fragment before becoming a stage-4 protostar. The boldface numbers on this and subsequent H–R plots refer to the prestellar evolutionary stages listed in Table 11.2 and described in the text.

39. Stage 5 - Evolving Protostar
About 10X size of sun
10X luminosity of sun (large area)
Center around 5,000,000 K
Surface is about 4000 K
Violent activity with bipolar flow
T-Tauri phase

40. Figure 11.21 Interstellar Cloud Evolution
Artist’s conception of the changes in an interstellar cloud during the early evolutionary stages outlined in Table (Not drawn to scale.) The duration of each stage, in years, is indicated.

41. Figure 11.23 Protostellar Outflow
(a) This remarkable image shows two jets emanating from the young star system HH30, the result of matter accreting onto an embryonic star near the center. The system is viewed roughly edge-on to the disk. (b) This view of the Orion molecular cloud shows the outflow from a newborn star, still surrounded by nebular gas. The inset shows a pair of jets called HH1/HH2, formed when matter falling onto another protostar (still obscured by the dusty cloud fragment from which it formed) creates a pair of high-speed jets of gas perpendicular to the flattened protostellar disk. The jets are nearly 1 light-year across. Several more Herbig–Haro objects can be seen at top right. One of them, the oddly shaped “waterfall” at top right, may be due to an earlier outflow from the same protostar responsible for HH1 and HH2. (AURA; NASA)

42. Figure Protostars
(a) An infrared image of a planetary system–sized dusty disk in the Orion region, showing heat and light emerging from its center. On the basis of its temperature and luminosity, this unnamed source appears to be a low-mass protostar around Stage 5 in the H–R diagram. (b) An optical image of a slightly more advanced circumstellar disk surrounding an embedded protostar in Orion. (NASA)

43. Figure 11.22 Newborn Star on the H-R Diagram
The changes in a protostar’s observed properties are shown by the path of decreasing luminosity, from stage 4 to stage 6. At stage 7, the newborn star has arrived on the main sequence.

44. Stage 6 - Newborn star About 1,000,000 km radius
Surface is about 4500 K
Center around 10,000,000 K
Hot enough to ignite nuclear burning

45. Stage 7 - Main sequence Reaches size & luminosity of sun
Center around 15,000,000 K
Surface is about 6000 K
Pressure and gravity now balance
Reaches Main sequence
million years from stage 1 - 7
On Main sequence 10 billion years

46. Figure 11.25 Prestellar Evolutionary Tracks
Prestellar evolutionary paths for stars more massive and less massive than our Sun.

47. Zero-age main sequence
The most massive fragments contract into O type stars in a million years
M type stars contract in a billion years
Start “clock” when star reaches main sequence

48. Failed stars
Some cloud fragments too small to become stars (< 0.08 M)
Form brown dwarfs (no fusion)
Hard to observe
Perhaps as many brown dwarfs as stars in our galaxy

49. Figure Brown Dwarfs
(a) This Hubble Space Telescope image shows Gliese 623, a binary system that may contain a brown dwarf (marked by an arrow). Astronomers hope that continued observations of this system will allow the companion’s mass to be measured with sufficient accuracy to determine whether or not it really is a brown dwarf. The “rings” in the image are instrumental artifacts. (b) A ground-based image of the binary-star system Gliese 229. The two objects are only 7” apart; the fainter “star” (arrow) has a luminosity only a few millionths that of the Sun and an estimated mass about 50 times that of Jupiter. (c) A Hubble image of the same system. (The bright diagonal streak in the latter image is caused by a hardware problem in the CCD chip used to record it.) (NASA; S. Kulkarni; NASA)

50. Star clusters Cloud collapse forms a group of stars
Star clusters useful to study
Formed at same time out of same cloud

51. Open clusters & Associations
Several 100 to several 1000 stars
Association (more extended)
Few 100 stars

52. Figure 11.27 Newborn Cluster
The star cluster NGC 3603 and a portion of the larger molecular cloud in which it formed. The cluster contains about 2000 bright stars and lies some 6000 pc from Earth. The field of view shown here spans about 20 light-years. Radiation from the cluster has cleared a cavity in the cloud several light-years across. The inset shows the central area more clearly, including the most massive star in the region (called Sher 25, above and to the left of the cluster), which is already near the end of its lifetime, having ejected part of its outer layers and formed a ring of gas. Many low-mass stars, less massive than the Sun, can also be seen. (ESO; NASA)

53. Figure 11.28 Pleiades Open Cluster
(a) The Pleiades cluster (also known as the Seven Sisters or M45) lies about 120 pc from the Sun. (b) An H–R diagram for the stars of this well-known open cluster. (NOAO)

54. Globular clusters Roughly spherical 100,000s to millions of stars
Generally found away from plane of Milky Way galaxy
Lack upper main sequence stars
At least 10 billion years old
150 globulars around Milky Way survivors of much larger original population

55. Figure 11.29 Globular Cluster
(a) The globular cluster Omega Centauri is approximately 5000 pc from Earth and some 40 pc in diameter. (b) An H–R diagram for some of its stars. (J. Lodriguss)

56. Figure 11.30 Young Stars in Orion
(a) Visible-light and (b) infrared Hubble views of the central part of the Orion nebula. Note the four bright stars of the Trapezium. The visible image is dominated by the emission nebula and shows few stars. However, the infrared image shows an extensive star cluster containing stars of many masses, possibly including many brown dwarfs. (NASA)

57. Figure 11.31 Protostellar Collisions - Supercomputer simulation
In the congested environment of a young cluster, star formation is a competitive and violent process. Large protostars may grow by “stealing” gas from smaller ones, and the extended disks surrounding most protostars can lead to collisions and even mergers. The cluster environment can play a crucial role in determining the types of stars that form. This frame from a supercomputer simulation shows a small star cluster emerging from an interstellar cloud that originally contained some 50 solar masses of material, distributed over a volume 1 light-year across. Remnants of the cloud are shown here in red. (M. Bate, I. Bonnell, and V. Bromm)