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Chapter 15
The Milky Way:
Our Home in the
Universe
Introduction
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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.
15.1 Our Galaxy: The Milky Way
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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.
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But it is much, much farther to the other galaxies and beyond.
15.1 Our Galaxy: The Milky Way
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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.
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This disk is about 90,000 light-years across, an
enormous, gravitationally bound system of stars.
15.1 Our Galaxy: The Milky Way
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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.
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Here on Earth, we are inside our Galaxy together with all of the matter
we see as the Milky Way (see figure).
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This patchiness is due to the splotchy distribution of nebulae and stars.
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.
15.2 The Illusion That We Are
at the Center
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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.
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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.
15.2 The Illusion That We Are
at the Center
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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.
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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).
15.2 The Illusion That We Are
at the Center
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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.
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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.
15.2 The Illusion That We Are
at the Center
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In the 20th century, astronomers began to use wavelengths other than
optical ones to study the Milky Way Galaxy.
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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.
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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 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.
15.3 Nebulae: Interstellar Clouds
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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).
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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.
15.3 Nebulae: Interstellar Clouds
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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.
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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.
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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).
15.3 Nebulae: Interstellar Clouds
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Other types of emission nebulae can appear
green in photographs, because of green
light from doubly ionized oxygen atoms.
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Don’t be misled by the pretty, false-color
images that you often see in the news.
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Additional colors occur as well.
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.
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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).
15.3 Nebulae: Interstellar Clouds
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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.
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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.
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In this region, dust is
obscuring the stars that lie
beyond.
15.3 Nebulae: Interstellar Clouds
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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.
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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.
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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.
15.3 Nebulae: Interstellar Clouds
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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.
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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.
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An example in a different region of the sky is shown
in the figure (left).
15.3 Nebulae: Interstellar Clouds
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They include planetary nebulae
(see figure) and supernova
remnants.
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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.
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As we emphasized in Chapter 13,
we are made of the ashes of
stars!
15.4 The Parts of Our Galaxy
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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.
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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).
15.4 The Parts of Our Galaxy
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Shapley’s touch of genius was to realize that this point is likely to be the
center of our Galaxy.
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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.
15.4 The Parts of Our Galaxy
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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.”
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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.
15.4 The Parts of Our Galaxy
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Our Galaxy has several parts:
 1. The nuclear bulge. Our Galaxy has the general shape of
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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.
15.4 The Parts of Our Galaxy
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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.
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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.
15.4 The Parts of Our Galaxy
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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.
15.4 The Parts of Our Galaxy
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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.
15.5 The Center of Our Galaxy
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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).
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In 2003, NASA launched an 0.85-m infrared
telescope, the Spitzer Space Telescope (Section 3.8c,
also see figure, left).
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Its infrared detectors are more sensitive than those
on earlier infrared telescopes.
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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.
15.5 The Center of Our Galaxy
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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.
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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.
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The radio radiation could well be from
gas surrounding a central giant black
hole (as shown in the image opening
Chapter 14).
15.5 The Center of Our Galaxy
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Extending somewhat farther out, a giant Arc of
parallel filaments stretches perpendicularly to the
plane of the Galaxy (see figure, right).
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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).
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The orbits measured show
the presence of a
supermassive black hole
that is about 3.7 million
times the Sun’s mass.
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One of the stars comes
within an astonishing 17
light-hours of Sgr A*.
15.5 The Center of Our Galaxy
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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.
15.6 All-Sky Maps of Our Galaxy
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The study of our Galaxy provides us with a wide range of types of
sources to study.
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Many of these have been
known for decades from
optical studies (see figure
on next slide, and the
figure at top).
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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).
15.6 All-Sky Maps of Our Galaxy
15.6 All-Sky Maps of Our Galaxy
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Maps of our Galaxy in the x-ray region of the spectrum (see
figure, above) show the hottest individual sources (such as xray 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.
15.6 All-Sky Maps of Our Galaxy
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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.
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NASA’s Swift satellite, mentioned in Sections 3.7a and 14.10a, was
sent aloft in 2004 specifically to study them in detail.
15.6 All-Sky Maps of Our Galaxy
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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.
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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.
15.7 Our Pinwheel Galaxy
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It is always difficult to tell the shape of a
system from a position inside it.
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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.
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Still, by noting the distances and directions
to objects of various types, we can
determine the Milky Way’s spiral structure.
15.7 Our Pinwheel Galaxy
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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).
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Also useful are main-sequence O and B stars; the
lives of such stars are so short we know they can’t
be old.
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We know their ages from the length of their main
sequences on the temperature-luminosity diagram
(Chapter 11).
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.
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We need new astrometric satellites.
15.7 Our Pinwheel Galaxy
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Other signs of young stars are the presence of emission
nebulae.
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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.
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When we do so, they appear to trace out bits of three spiral
arms, which are relatively nearby.
15.7 Our Pinwheel Galaxy
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Interstellar dust prevents us from using this technique to
study parts of our Galaxy farther away from the Sun.
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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.
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We will discuss the method later in this chapter.
15.8 Why Does Our Galaxy
Have Spiral Arms?
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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.)
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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.
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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?
15.8 Why Does Our Galaxy
Have Spiral Arms?
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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.
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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.
15.8 Why Does Our Galaxy
Have Spiral Arms?
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Think of the analogy of a crew of workers fixing potholes
in two lanes of a four-lane highway.
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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.
15.8 Why Does Our Galaxy
Have Spiral Arms?
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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.
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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).
15.9 Matter Between the Stars
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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.
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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).
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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.)
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.
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So hydrogen molecules (H2) are often found in H I clouds.
15.9 Matter Between the Stars
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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.
15.9 Matter Between the Stars
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Wherever a hot star provides enough energy to ionize hydrogen, an H II
region (emission nebula) results (see figures).
15.9 Matter Between the Stars
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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).
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Tiny grains of solid particles are given off by the outer layers of
red giants.
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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.
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.
15.9 Matter Between the Stars
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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.
15.10 Radio Observations of Our Galaxy
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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.
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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.
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.


A hydrogen atom is basically an electron “orbiting” a
proton.




This line has a wavelength of 21 cm.
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.
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.
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 “halflife” 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.
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.
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.


Though there are substantial uncertainties in interpreting
the Doppler shifts in terms of distance from the Galaxy’s
center, astronomers have succeeded in making some
maps.


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.
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.
15.12 Radio Spectral Lines
from 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.
15.12 Radio Spectral Lines
from Molecules

Studying the spectral lines provides
information about physical conditions—
temperature, densities, and motion, for
example—in 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 Galaxy’s 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.
15.13 The Formation of Stars

We have already discussed (in Chapter 12) some of the
youngest stars known and how stars form.


Astronomers have found that giant molecular clouds
are fundamental building blocks of our Galaxy.



Here we will discuss star formation in terms of the gas and
dust from which stars come.
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.
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.
15.13 The Formation of Stars

Many radio spectral lines have been detected only in a
particular cloud of gas, the Orion Molecular Cloud.


The Orion Molecular Cloud contains about 500 thousand
times the mass of the Sun.


It is located close to a visible part, which we call the Orion
Nebula, of a larger cloud of gas and dust.
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 Cloud’s 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.
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 Earth’s atmosphere, though it is much
denser than the typical interstellar density of about
1 particle per cubic centimeter.
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.
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).
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).
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.



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.


You have probably worked in the field before, and your
reasons might tie in with other investigations underway.
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.
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 itself—that cost is covered in the
observatory’s overall budget.
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 clock—one
doesn’t want to waste any
observing time.
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.
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 hasn’t rained in decades
(see figure), will use at least 50 highprecision 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.