The Milky Way:

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 nebyu-lee or
  nebyu-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 Waythe 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

• Dont 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 skythere 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
  Galaxys 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 cant 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 maplee, 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 Earths atmosphere.
• The latest infrared telescope, launched by NASA
  in 2003, is the Spitzer Space Telescope.
• Infrared and radio radiation can pass through the
  Galaxys 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 artists 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 doesnt 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.
• Dont 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).

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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 Orions belt.

• A bit of absorbing dust intrudes onto the
  emitting gas, outlining the shape of a horses
  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.

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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 Earths 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).

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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

• Shapleys 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 arent at the partys 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 didnt
  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 stars 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.

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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
  thin2 per cent of its widthlike 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 Galaxys 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 armsregions 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.

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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 togetherbut we dont 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 NASAs 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 Suns 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
  Agencys 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.
• NASAs 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 Galaxys 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 Ways 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
  cant 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 Galaxys 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 havent 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 stateneutral 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 energya
  photon (see figure, left).
• We thus call this a spin-flip transition (see
  figure, below).
• The photon of hydrogens spin-flip transition
  corresponds to radiation at a wavelength of 21
  cmthe 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.

51
15.11 Mapping Our Galaxy

• Astronomers have ingeniously been able to find
  out how far it is to the clouds of gas that emit
  the 21-cm radiation.
• They use the fact that gas closer to the center
  of our Galaxy rotates with a shorter period than
  the gas farther away from the center.
• Though there are substantial uncertainties in
  interpreting the Doppler shifts in terms of
  distance from the Galaxys center, astronomers
  have succeeded in making some maps.
• These maps show many narrow arms but no clear
  pattern of a few broad spiral arms like those we
  see in other galaxies (Chapter 16).
• The question emerged Is our Galaxy really a
  spiral at all?
• With the additional information from studies of
  molecules in space that we describe in the next
  section, we finally made further progress.

52
15.12 Radio Spectral Linesfrom Molecules

• Radio astronomers had only the hydrogen 21-cm
  spectral line to study for a dozen years, and
  then only the addition of one other group of
  lines for another five years.
• Then radio spectral lines of water (H2O) and
  ammonia (NH3) were found.
• The spectral lines of these molecules proved
  surprisingly strong, and were easily detected
  once they were looked for.
• Over 100 additional types of molecules have since
  been found.
• The earlier notion that it would be difficult to
  form molecules in space was wrong.
• In some cases, atoms apparently stick to
  interstellar dust grains, perhaps for thousands
  of years, and molecules build up (see figure).
• Though hydrogen molecules form on dust grains,
  most of the other molecules may be formed in the
  interstellar gas, or in the atmospheres of stars,
  without need for grains.

53
15.12 Radio Spectral Linesfrom Molecules

• Studying the spectral lines provides information
  about physical conditionstemperature, densities,
  and motion, for examplein the gas clouds that
  emit the lines.
• Studies of molecular spectral lines have been
  used together with 21-cm line observations to
  improve the maps of the spiral structure of our
  Galaxy (see figure).
• Observations of carbon monoxide (CO), in
  particular, have provided better information
  about the parts of our Galaxy farther out from
  the Galaxys center than our Sun.
• We use the carbon monoxide as a tracer of the
  more abundant hydrogen molecular gas, since the
  carbon monoxide produces a far stronger spectral
  line and is much easier to observe molecular
  hydrogen emits extremely little.

54
15.13 The Formation of Stars

• We have already discussed (in Chapter 12) some of
  the youngest stars known and how stars form.
• Here we will discuss star formation in terms of
  the gas and dust from which stars come.
• Astronomers have found that giant molecular
  clouds are fundamental building blocks of our
  Galaxy.
• Giant molecular clouds are 150 to 300 light-years
  across.
• There are a few thousand of them in our Galaxy.
• The largest giant molecular clouds contain about
  100,000 to 1,000,000 times the mass of the Sun.
• Since giant molecular clouds break up to form
  stars, they only last 10 million to 100 million
  years.

55
15.13 The Formation of Stars

• Most radio spectral lines seem to come only from
  the molecular clouds. (Carbon monoxide is the
  major exception, for it is widely distributed
  across the sky.)
• Infrared and radio observations together have
  provided us with an understanding of how stars
  are formed from these dense regions of gas and
  dust.
• Carbon-monoxide observations reveal the giant
  molecular clouds, but it is molecular hydrogen
  (H2) rather than carbon monoxide that contains a
  vast majority of the mass.

56
15.13 The Formation of Stars

• Many radio spectral lines have been detected only
  in a particular cloud of gas, the Orion Molecular
  Cloud.
• It is located close to a visible part, which we
  call the Orion Nebula, of a larger cloud of gas
  and dust.
• The Orion Molecular Cloud contains about 500
  thousand times the mass of the Sun.
• It is relatively accessible to our study because
  it is only about 1500 light-years away.
• Even though less than 1 per cent of the Clouds
  mass is dust, that is still a sufficient amount
  of dust to prevent ultraviolet light from nearby
  stars from entering and breaking the molecules
  apart.
• Thus molecules can accumulate.

57
15.13 The Formation of Stars

• The properties of the molecular cloud can be
  deduced by comparing the radiation from its
  various molecules and by studying the radiation
  from each molecule individually.
• The average density is a few hundred to a
  thousand particles per cubic centimeter, but the
  cloud center may have up to a million particles
  per cubic centimeter.
• This central region is still billions of times
  less dense than our Earths atmosphere, though it
  is much denser than the typical interstellar
  density of about 1 particle per cubic centimeter.

58
15.13 The Formation of Stars

• We know that young stars are found in the center
  of the Orion Nebula (see figures, left and
  middle).
• The Trapezium (see figure, right), a group of
  four hot stars readily visible in a small
  telescope, is the source of ionization and energy
  for the Orion Nebula.
• The Trapezium stars are relatively young, about
  100,000 years old.

59
15.13 The Formation of Stars

• The Orion Nebula, though prominent at visible
  wavelengths, is but an H II region located along
  the near side of the much more extensive
  molecular cloud (see figure).

60
15.13 The Formation of Stars

• The Near-Infrared Camera and Multi-Object
  Spectrometer (NICMOS) on the Hubble Space
  Telescope is able to record infrared light that
  had penetrated the dust, bringing us images of
  newly formed stars within the Orion Molecular
  Cloud (see figure).

61
15.14 At a Radio Observatory

• What is it like to go observing at a radio
  telescope?
• First, you decide just what you want to observe,
  and why.
• You have probably worked in the field before, and
  your reasons might tie in with other
  investigations underway.
• Then you decide with which telescope you want to
  observe, usually the most suitable one accessible
  to you let us say it is the Very Large Array
  (VLA) of the National Radio Astronomy
  Observatory.
• You send in a written proposal describing what
  you want to observe and why.
• Your proposal is read by a panel of scientists.
• If the proposal is approved, it is placed in a
  queue to wait for observing time.
• You might be scheduled to observe for a five-day
  period to begin six months after you submitted
  your proposal.

62
15.14 At a Radio Observatory

• At the same time, you might apply (usually to the
  National Science Foundation) for financial
  support to carry out the research.
• Your proposal possibly contains requests for some
  salary for yourself during the summer, and salary
  for a student or students to work on the project
  with you.
• You are not charged directly for the use of the
  telescope itselfthat cost is covered in the
  observatorys overall budget.

63
15.14 At a Radio Observatory

• You carry out your observing at the VLA
  headquarters at Socorro, New Mexico.
• A trained telescope operator runs the mechanical
  aspects of the telescope.
• You give the telescope operator a computer
  program that includes the coordinates of the
  points in the sky that you want to observe and
  how long to dwell at each location.

• The telescopes (see figure) operate around the
  clockone doesnt want to waste any observing
  time.

64
15.14 At a Radio Observatory

• The electronics systems that are used to treat
  the incoming signals collected by the radio
  dishes are particularly advanced.
• Computers combine the output from the 27
  telescopes and show you a color-coded image, with
  each color corresponding to a different
  brightness level (see figure).
• Standard image-processing packages of programs
  are available for you to use back home, with the
  radio community generally using a different
  package from that used in the optical community.

• You are expected to publish the results as soon
  as possible in one of the scientific journals,
  often after you have given a presentation about
  the results at a professional meeting, such as
  one of those held twice yearly by the American
  Astronomical Society.

65
15.14 At a Radio Observatory

• Astronomy has become a very collaborative
  science.
• Many consortia of individual scientists, such as
  those studying distant supernovae, have dozens of
  members.
• Telescope projects have also become so huge that
  collaboration is necessary.

• The Atacama Large Millimeter Array (ALMA), to be
  built in Chile on a high plain where it hasnt
  rained in decades (see figure), will use at least
  50 high-precision radio telescopes as an
  interferometer to examine our Galaxy and other
  celestial objects with high resolution.
• It is a joint project of the United States
  National Science Foundation, the European Space
  Agency, and Chile.