Chapter 15

1. Chapter 15 The Milky Way: Our Home in the Universe

2. Introduction • We have already described the stars, which are important parts of any galaxy, and how they are born, live, and die. • In this chapter, we describe the gas and dust (small particles of matter) that are present to some extent throughout a galaxy. • Substantial clouds of this gas and dust are called nebulae (pronounced “neb´yu-lee” or “neb´yu-lay”; singular: nebula); “nebula” is Latin for “fog” or “mist.” • New stars are born from such nebulae. • We also discuss the overall structure of the Milky Way Galaxy and how, from our location inside it, we detect this structure.

3. 15.1 Our Galaxy: The Milky Way • On the clearest moonless nights, when we are far from city lights, we can see a hazy band of light stretching across the sky (see figure). • This band is the Milky Way—the gas, dust, nebulae, and stars that make up the Galaxy in which our Sun is located. • All this matter is our celestial neighborhood, typically within a few hundred or a thousand light-years from us. • If we look a few thousand light-years in a direction away from that of the Milky Way, we see out of our Galaxy. • But it is much, much farther to the other galaxies and beyond.

4. 15.1 Our Galaxy: The Milky Way • Don’t be confused by the terminology: The Milky Way itself is the band of light that we can see from the Earth, and the Milky Way Galaxy is the whole galaxy in which we live. • Like other large galaxies, our Milky Way Galaxy is composed of perhaps a few hundred billion stars plus many different types of gas, dust, planets, and so on. • In the directions in which we see the Milky Way in the sky, we are looking through the relatively thin, pancake-like disk of matter that forms a major part of our Milky Way Galaxy. • This disk is about 90,000 light-years across, an enormous, gravitationally bound system of stars.

5. 15.1 Our Galaxy: The Milky Way • The Milky Way appears very irregular when we see it stretched across the sky—there are spurs of luminous material that stick out in one direction or another, and there are dark lanes or patches in which much less can be seen. • This patchiness is due to the splotchy distribution of nebulae and stars. • Here on Earth, we are inside our Galaxy together with all of the matter we see as the Milky Way (see figure). • Because of our position, we see a lot of our own Galaxy’s matter when we look along the plane of our Galaxy. • On the other hand, when we look “upward” or “downward” out of this plane, our view is not obscured by matter, and we can see past the confines of our Galaxy.

6. 15.2 The Illusion That We Areat the Center • The gas in our Galaxy is more or less transparent to visible light, but the small solid particles that we call “dust” are opaque. • So the distance we can see through our Galaxy depends mainly on the amount of dust that is present. • This is not surprising: We can’t always see far on a foggy day. • Similarly, the dust between the stars in our Galaxy dims the starlight by absorbing it or by scattering (reflecting) it in different directions.

7. 15.2 The Illusion That We Areat the Center • The dust in the plane of our Galaxy prevents us from seeing very far toward its center with the unaided eye and small telescopes. • With visible light, on average we can see only one tenth of the way in (about 2000 light-years), regardless of the direction we look in the plane of the Milky Way. • These direct optical observations fooled astronomers at the beginning of the 20th century into thinking that the Earth was near the center of the Universe (see figure).

8. 15.2 The Illusion That We Areat the Center • We shall see in this chapter how the American astronomer Harlow Shapley (pronounced to rhyme with “map´lee,” as in “road map”) realized in 1917 that our Sun is not in the center of the Milky Way. • This fundamental idea took humanity one step further away from thinking that we are at the center of the Universe. • Copernicus, in 1543, had already made the first step in removing the Earth from the center of the Universe.

9. 15.2 The Illusion That We Areat the Center • In the 20th century, astronomers began to use wavelengths other than optical ones to study the Milky Way Galaxy. • In the 1950s and 1960s especially, radio astronomy gave us a new picture of our Galaxy. • In the 1980s and 1990s, we began to benefit from space infrared observations at wavelengths too long to pass through the Earth’s atmosphere. • The latest infrared telescope, launched by NASA in 2003, is the Spitzer Space Telescope. • Infrared and radio radiation can pass through the Galaxy’s dust and allow us to see our Galactic center and beyond. • A new generation of telescopes on high mountains enables us to see parts of the infrared and submillimeter spectrum. • The Atacama Large Millimeter Array, now being built in Chile (see an artist’s concept at the end of this chapter), will give us high-resolution views in the millimeter part of the spectrum. • Giant arrays of radio telescopes spanning not only local areas but also continents and the Earth itself enable us to get crisp views of what was formerly hidden from us.

10. 15.3 Nebulae: Interstellar Clouds • The original definition of “nebula” was a cloud of gas and dust that we see in visible light, though we now detect nebulae in a variety of ways. • When we see the gas actually glowing in the visible part of the spectrum, we call it an emission nebula (see figure). • Gas is ionized by ultraviolet light from very hot stars within the nebula; it then glows at optical (and other) wavelengths when electrons recombine with ions and cascade down to lower energy levels, releasing photons.

11. 15.3 Nebulae: Interstellar Clouds • Additionally, free electrons can collide with atoms (neutral or ionized) and lose some of their energy of motion, kicking the bound electrons to higher energy levels. • Photons are emitted when the excited bound electrons jump down to lower energy levels, so the gas glows even more. • The spectrum of an emission nebula therefore consists of emission lines. • Emission nebulae often look red (on long-exposure images; the human eye doesn’t see these colors directly), because the red light of hydrogen is strongest in them. • Electrons are jumping from the third to the second energy levels of hydrogen, producing the Ha [alpha] emission line in the red part of the spectrum (6563 Å).

12. 15.3 Nebulae: Interstellar Clouds • Other types of emission nebulae can appear green in photographs, because of green light from doubly ionized oxygen atoms. • Additional colors occur as well. • Don’t be misled by the pretty, false-color images that you often see in the news. • In them, color is assigned to some specific type of radiation and need not correspond to colors that the eye would see when viewing the objects through telescopes. • Sometimes a cloud of dust obscures our vision in some direction in the sky. • When we see the dust appear as a dark silhouette (see figure), we call it a dark nebula (or, often, an absorption nebula, since it absorbs visible light from stars behind it).

13. 15.3 Nebulae: Interstellar Clouds • The Horsehead Nebula (see figure) is an example of an object that is simultaneously an emission and an absorption nebula. • The reddish emission from glowing hydrogen gas spreads across the sky near the leftmost (eastern) star in Orion’s belt. • A bit of absorbing dust intrudes onto the emitting gas, outlining the shape of a horse’s head. • We can see in the picture that the horsehead is a continuation of a dark area in which very few stars are visible. • In this region, dust is obscuring the stars that lie beyond.

14. 15.3 Nebulae: Interstellar Clouds • Clouds of dust surrounding relatively hot stars, like some of the stars in the star cluster known as the Pleiades (see figure), are examples of reflection nebulae. • They merely reflect the starlight toward us without emitting visible radiation of their own. • Reflection nebulae usually look bluish for two reasons: (1) They reflect the light from relatively hot stars, which are bluish, and (2) dust reflects blue light more efficiently than it does red light. (Similar scattering of sunlight in the Earth’s atmosphere makes the sky blue. • Whereas an emission nebula has its own spectrum, as does a neon sign on Earth, a reflection nebula shows the spectral lines of the star or stars whose light is being reflected. • Dust tends to be associated more with young, hot stars than with older stars, since the older stars would have had a chance to wander away from their dusty birthplaces.

15. 15.3 Nebulae: Interstellar Clouds • The Great Nebula in Orion (see figure, right) is an emission nebula. • In the winter sky, we can readily observe it through even a small telescope or binoculars, and sometimes it has a tinge of color. • We need long photographic exposures or large telescopes to study its structure in detail. • Deep inside the Orion Nebula and the gas and dust alongside it, we see stars being born this very minute; many telescopes are able to observe in the infrared, which penetrates the dust. • An example in a different region of the sky is shown in the figure (left).

16. 15.3 Nebulae: Interstellar Clouds • They include planetary nebulae (see figure) and supernova remnants. • Thus, nebulae are closely associated with both stellar birth and stellar death. • The chemically enriched gas blown off by unstable or exploding stars at the end of their lives becomes the raw material from which new stars and planets are born. • As we emphasized in Chapter 13, we are made of the ashes of stars!

17. 15.4 The Parts of Our Galaxy • It was not until 1917 that the American astronomer Harlow Shapley realized that we are not in the center of our Milky Way Galaxy. • He was studying the distribution of globular clusters and noticed that, as seen from Earth, they are all in the same general area of the sky. • They mostly appear above or below the Galactic plane and thus are not heavily obscured by the dust. • When he plotted their distances and directions, he noticed that they formed a spherical halo around a point thousands of light-years away from us (see figure).

18. 15.4 The Parts of Our Galaxy • Shapley’s touch of genius was to realize that this point is likely to be the center of our Galaxy. • After all, if we are at a party and discover that everyone we see is off to our left, we soon figure out that we aren’t at the party’s center. • Other spiral galaxies are also shown (see figures) for comparison and to show something of what our Galaxy must look like when seen from high above it.

19. 15.4 The Parts of Our Galaxy • Though Shapley correctly deduced that the Sun is far from our Galactic center, he actually overestimated the distance. • The reason is that dust dims the starlight, making the stars look too far away, and he didn’t know about this “interstellar extinction.” • The amount of dimming can be determined by measuring how much the starlight has been reddened: Blue light gets scattered and absorbed more easily than red light, so the star’s color becomes redder than it should be for a star of a given spectral type. • This is the same reason sunsets tend to look orange or red, not white.

20. 15.4 The Parts of Our Galaxy • Our Galaxy has several parts: • 1. The nuclear bulge. Our Galaxy has the general shape of a pancake with a bulge at its center that contains millions of stars, primarily old ones. This nuclear bulge has the Galactic nucleus at its center. The nucleus itself is only about 10 light-years across. • 2. The disk. The part of the pancake outside the bulge is called the Galactic disk. It extends 45,000 light-years or so out from the center of our Galaxy. The Sun is located about one half to two thirds of the way out. The disk is very thin—2 per cent of its width—like a phonograph record, CD, or DVD. It contains all the young stars and interstellar gas and dust, as well as some old stars. The disk is slightly warped at its ends, perhaps by interaction with our satellite galaxies, the Magellanic Clouds. Our Galaxy looks a bit like a hat with a turned-down brim.

21. 15.4 The Parts of Our Galaxy • It is very difficult for us to tell how the material in our Galaxy’s disk is arranged, just as it would be difficult to tell how the streets of a city were laid out if we could only stand on one street corner without moving. • Still, other galaxies have similar properties to our own, and their disks are filled with great spiral arms—regions of dust, gas, and stars in the shape of a pinwheel (see figure). • So, we assume the disk of our Galaxy has spiral arms, too. • Though the direct evidence is ambiguous in the visible part of the spectrum, radio observations have better traced the spiral arms.

22. 15.4 The Parts of Our Galaxy • The disk looks different when viewed in different parts of the spectrum (see figure). • Infrared and radio waves penetrate the dust that blocks our view in visible light, while x-rays show the hot objects best.

23. 15.4 The Parts of Our Galaxy • 3. The halo. Old stars (including the globular clusters) and very dilute interstellar matter form a roughly spherical Galactic halo around the disk. The inner part of the halo is at least as large across as the disk, perhaps 60,000 light-years in radius. The gas in the inner halo is hot, 100,000 K, though it contains only about 2 per cent of the mass of the gas in the disk. As we discuss in Chapter 16, the outer part of the halo extends much farther, out to perhaps 200,000 or 300,000 light-years. Believe it or not, this Galactic outer halo apparently contains 5 or 10 times as much mass as the nucleus, disk, and inner halo together—but we don’t know what it consists of! We shall see in Section 16.4 that such “dark matter” (invisible, and detectable only through its gravitational properties) is a very important constituent of the Universe.

24. 15.5 The Center of Our Galaxy • We cannot see the center of our Galaxy in the visible part of the spectrum because our view is blocked by interstellar dust. • Radio waves and infrared, on the other hand, penetrate the dust. • The Hubble Space Telescope, with its superior resolution, has seen isolated stars where before we saw only a blur (see figure, right). • In 2003, NASA launched an 0.85-m infrared telescope, the Spitzer Space Telescope (Section 3.8c, also see figure, left). • Its infrared detectors are more sensitive than those on earlier infrared telescopes. • Spitzer completes NASA’s series of Great Observatories, including the Compton Gamma Ray Observatory (now defunct), the Chandra X-ray Observatory, and the Hubble Space Telescope.

25. 15.5 The Center of Our Galaxy • One of the brightest infrared sources in our sky is the nucleus of our Galaxy, only about 10 lightyears across. • This makes it a very small source for the prodigious amount of energy it emits: as much energy as radiated by 80 million Suns. • It is also a radio source and a variable x-ray source. • High-resolution radio maps of our Galactic center (see figure) show a small bright spot, known as Sgr A* (pronounced “Saj A-star”), in the middle of the bright radio source Sgr A. • The radio radiation could well be from gas surrounding a central giant black hole (as shown in the image opening Chapter 14).

26. 15.5 The Center of Our Galaxy • Extending somewhat farther out, a giant Arc of parallel filaments stretches perpendicularly to the plane of the Galaxy (see figure, right). • As we discuss further in Chapter 17, adaptive optics techniques in the near-infrared have allowed very rapid motions of stars to be measured much nearer the Galactic center than was previously possible (see figures, left & below). • The orbits measured show the presence of a supermassive black hole that is about 3.7 million times the Sun’s mass. • One of the stars comes within an astonishing 17 light-hours of Sgr A*.

27. 15.5 The Center of Our Galaxy • Observations of the Galactic center with the Chandra X-ray Observatory and the European Space Agency’s INTEGRAL gamma-ray spacecraft (see figures) reveal the presence of hot, x-ray luminous gas and stars there.

28. 15.6 All-Sky Maps of Our Galaxy • The study of our Galaxy provides us with a wide range of types of sources to study. • Many of these have been known for decades from optical studies (see figure on next slide, and the figure at top). • The infrared sky looks quite different (see figure, middle), with its appearance depending strongly on wavelength. • The radio sky provides still different pictures, depending on the wavelength used (see figure, below).

29. 15.6 All-Sky Maps of Our Galaxy

30. 15.6 All-Sky Maps of Our Galaxy • Maps of our Galaxy in the x-ray region of the spectrum (see figure, above) show the hottest individual sources (such as x-ray binary stars) and diffuse gas that was heated to temperatures of a million degrees by supernova explosions. • The Compton Gamma Ray Observatory produced maps of the steady gamma rays (see figure, below), most of which come from collisions between cosmic rays (see our discussion in Section 13.2f ) and atomic nuclei in clouds of gas.

31. 15.6 All-Sky Maps of Our Galaxy • A different instrument on the Compton Gamma Ray Observatory detected bursts of gamma rays that last only a few seconds or minutes (see figure). • These gamma-ray bursts, which were seen at random places in the sky roughly once per day, are especially intriguing. • NASA’s Swift satellite, mentioned in Sections 3.7a and 14.10a, was sent aloft in 2004 specifically to study them in detail.

32. 15.6 All-Sky Maps of Our Galaxy • Though some models suggested that the gamma-ray bursts were produced within our Galaxy (either very close to us or in a very extended halo), more recent observations have conclusively shown that most of them are actually in galaxies billions of light-years away. • As we discussed in Chapter 14, these distant gamma-ray bursts may be produced when extremely massive stars collapse to form black holes, or when a neutron star merges with another neutron star or with a black hole. • The Chandra X-ray Observatory is producing more detailed images of x-ray sources than had ever before been available. Studies of the highest-energy electromagnetic radiation like x-rays and gamma rays, and of rapidly moving cosmic-ray particles (Section 13.2f ) guided to some extent by the Galaxy’s magnetic field, are part of the field of high-energy astrophysics. • Riccardo Giacconi received a share of the 2002 Nobel Prize in Physics for his role in founding this field.

33. 15.7 Our Pinwheel Galaxy • It is always difficult to tell the shape of a system from a position inside it. • Think, for example, of being somewhere inside a maze of tall hedges; we would find it difficult to trace out the pattern. • If we could fly overhead in a helicopter, though, the pattern would become very easy to see (see figure). • Similarly, we have difficulty tracing out the spiral pattern in our own Galaxy, even though the pattern would presumably be apparent from outside the Galaxy. • Still, by noting the distances and directions to objects of various types, we can determine the Milky Way’s spiral structure.

34. 15.7 Our Pinwheel Galaxy • Young open clusters are good objects to use for this purpose, for they are always located in spiral arms. • We think that they formed there and that they have not yet had time to move away (see figure). • We know their ages from the length of their main sequences on the temperature-luminosity diagram (Chapter 11). • Also useful are main-sequence O and B stars; the lives of such stars are so short we know they can’t be old. • But since our methods of determining the distances to open clusters, as well as to O and B stars, from their optical spectra and apparent brightnesses are uncertain to 10 per cent, they give a fuzzy picture of the distant parts of our Galaxy. • Parallaxes measured from the Hipparcos spacecraft do not go far enough out into space to help in mapping our Galaxy. • We need new astrometric satellites.

35. 15.7 Our Pinwheel Galaxy • Other signs of young stars are the presence of emission nebulae. • We know from studies of other galaxies that emission nebulae are preferentially located in spiral arms. • In mapping the locations of emission nebulae, we are really again studying the locations of the O stars and the hottest of the B stars, since it is ultraviolet radiation from these hot stars that provides the energy for the nebulae to glow. • It is interesting to plot the directions to and distances of the open clusters, the O and B stars, and the clouds of ionized hydrogen known as H II (pronounced “H two”) regions as seen from Earth. • When we do so, they appear to trace out bits of three spiral arms, which are relatively nearby.

36. 15.7 Our Pinwheel Galaxy • Interstellar dust prevents us from using this technique to study parts of our Galaxy farther away from the Sun. • However, another valuable method of mapping the spiral structure in our Galaxy involves spectral lines of hydrogen and of carbon monoxide in the radio part of the spectrum. • Radio waves penetrate the interstellar dust, allowing us to study the distribution of matter throughout our Galaxy, though getting the third dimension (distance) that allows us to trace out spiral arms remains difficult. • We will discuss the method later in this chapter.

37. 15.8 Why Does Our GalaxyHave Spiral Arms? • The Sun revolves around the center of our Galaxy at a speed of approximately 200 kilometers per second. • At this rate, it takes the Sun about 250 million years to travel once around the center, only 2 per cent of the Galaxy’s current age. (Our Galaxy, after all, must be older than its globular clusters, whose age we discussed in Chapter 11.) • But stars at different distances from the center of our Galaxy revolve around its center in different lengths of time. (As we will see in Chapter 16, the Galaxy does not rotate like a solid disk.) • For example, stars closer to the center revolve much more quickly than does the Sun. • Thus the question arises: Why haven’t the arms wound up very tightly, like the cream in a cup of coffee swirling as you stir it?

38. 15.8 Why Does Our GalaxyHave Spiral Arms? • The leading current solution to this conundrum says, in effect, that the spiral arms we now see do not consist of the same stars that would previously have been visible in those arms. • The spiral-arm pattern is caused by a spiral density wave, a wave of increased density that moves through the gas in the Galaxy. • This density wave is a wave of compression, not of matter being transported. • It rotates more slowly than the actual material and causes the density of passing material to build up. • Stars are born at those locations and appear to form a spiral pattern (see figure), but the stars then move away from the compression wave.

39. 15.8 Why Does Our GalaxyHave Spiral Arms? • Think of the analogy of a crew of workers fixing potholes in two lanes of a four-lane highway. • A bottleneck occurs at the location of the workers; if we were in a traffic helicopter, we would see an increase in the number of cars at that place. • As the workers continue slowly down the road, fixing potholes in new sections, we would see what seemed to be the bottleneck moving slowly down the road. • Cars merging from four lanes into the two open lanes need not slow down if the traffic is light, but they are compressed more than in other (fully open) sections of the highway. • Thus the speed with which the bottleneck advances is much smaller than that of individual cars.

40. 15.8 Why Does Our GalaxyHave Spiral Arms? • Similarly, in our Galaxy, we might be viewing only some galactic bottleneck at the spiral arms. • The new, massive stars would heat the interstellar gas so that it becomes visible. • In fact, we do see young, hot stars and glowing gas outlining the spiral arms, providing a check of this prediction of the density-wave theory. • This mechanism may work especially well in galaxies with a companion that gravitationally perturbs them (as seen in the opening image in Chapter 16).

41. 15.9 Matter Between the Stars • The gas and dust between the stars is known as the interstellar medium or “interstellar matter.” • The nebulae represent regions of the interstellar medium in which the density of gas and dust is higher than average. • For many purposes, we may consider interstellar space as being filled with hydrogen at an average density of about 1 atom per cubic centimeter. (Individual regions may have densities departing greatly from this average.) • Regions of higher density in which the atoms of hydrogen are predominantly neutral are called H I regions (pronounced “H one regions”; the Roman numeral “I” refers to the neutral, basic state). • Where the density of an H I region is high enough, pairs of hydrogen atoms combine to form molecules (H2). • The densest part of the gas associated with the Orion Nebula might have a million or more hydrogen molecules per cubic centimeter. • So hydrogen molecules (H2) are often found in H I clouds.

42. 15.9 Matter Between the Stars • A region of ionized hydrogen, with one electron missing, is known as an H II region (from “H two,” the second state—neutral is the first state and once ionized is the second). • Since hydrogen, which makes up the overwhelming proportion of interstellar gas, contains only one proton and one electron, a gas of ionized hydrogen contains individual protons and electrons.

43. 15.9 Matter Between the Stars • Wherever a hot star provides enough energy to ionize hydrogen, an H II region (emission nebula) results (see figures).

44. 15.9 Matter Between the Stars • Studying the optical and radio spectra of H II regions and planetary nebulae tells us the abundances (proportions) of several of the chemical elements (especially helium, nitrogen, and oxygen). • How these abundances vary from place to place in our Galaxy and in other galaxies helps us choose between models of element formation and of galaxy evolution. • Tiny grains of solid particles are given off by the outer layers of red giants. • They spread through interstellar space, and dim the light from distant stars. This “dust” never gets very hot, so most of its radiation is in the infrared. • The radiation from dust scattered among the stars is faint and very difficult to detect, but the radiation coming from clouds of dust surrounding newly formed stars is easily observed from ground-based telescopes and from infrared spacecraft. • They found infrared radiation from so many stars in our Galaxy that we think that about one star forms in our Galaxy each year.

45. 15.9 Matter Between the Stars • Since the interstellar gas is often “invisible” in the visible part of the spectrum (except at the wavelengths of certain weak emission lines), different techniques are needed to observe the gas in addition to observing the dust. • Radio astronomy is the most widely used technique, so we will now discuss its use for mapping our Galaxy.

46. 15.10 Radio Observations of Our Galaxy • The first radio astronomy observations were of continuous radiation; no spectral lines were known. • If a radio spectral line is known, Doppler-shift measurements can be made, and we can tell about motions in our Galaxy. • What is a radio spectral line? • Remember that an optical spectral line corresponds to a wavelength of the optical spectrum that is more intense (for an emission line) or less intense (for an absorption line) than neighboring wavelengths. • Similarly, a radio spectral line corresponds to a wavelength at which the radio radiation is slightly more, or slightly less, intense. • A radio station is an emission line on a home radio.

47. 15.10 Radio Observations of Our Galaxy • Since hydrogen is by far the most abundant element in the Universe, the most-used radio spectral line is a line from the lowest energy levels of interstellar hydrogen atoms. • This line has a wavelength of 21 cm. • A hydrogen atom is basically an electron “orbiting” a proton. • Both the electron and the proton have the property of spin, as if each were spinning on its axis. • The spin of the electron can be either in the same direction as the spin of the proton or in the opposite direction. • The rules of quantum physics prohibit intermediate orientations. • The energies of the two allowed conditions are slightly different.

48. 15.10 Radio Observations of Our Galaxy • If an atom is sitting alone in space in the upper of these two energy states, with its electron and proton spins aligned in the same direction, there is a certain small probability that the spinning electron will spontaneously flip over to the lower energy state and emit a bundle of energy—a photon (see figure, left). • We thus call this a spin-flip transition (see figure, below). • The photon of hydrogen’s spin-flip transition corresponds to radiation at a wavelength of 21 cm—the 21-cm line. • If the electron flips from the higher to the lower energy state, we have an emission line. • If it absorbs energy from passing continuous radiation, it can flip from the lower to the higher energy state and we have an absorption line.

49. 15.10 Radio Observations of Our Galaxy • If we were to watch any particular group of hydrogen atoms in the slightly higher energy state, we would find that it would take 11 million years before half of the electrons had undergone spin-flips; we say that the “half-life” is 11 million years for this transition. • Thus, hydrogen atoms are generally quite content to sit in the upper state! • But there are so many hydrogen atoms in space that enough 21-cm radiation is given off to be detected. • The existence of the line was predicted in 1944 and discovered in 1951, marking the birth of spectral-line radio astronomy.

50. 15.11 Mapping Our Galaxy • The 21-cm hydrogen line has proven to be a very important tool for studying our Galaxy (see figure) because this radiation passes unimpeded through the dust that prevents optical observations very far into the plane of our Galaxy. • It can even reach us from the opposite side of our Galaxy, whereas light waves penetrate the dust clouds in the Galactic plane only about 10 per cent of the way to the Galactic center, on average.