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Neil F. Comins • William J. Kaufmann III
Discovering the Universe
Ninth Edition
CHAPTER 15
The Milky Way Galaxy
Understanding the Universe
The Sombrero Galaxy (also designated M104, the 104th galaxy listed in the
Messier catalog) is 29 million ly from us. By combining infrared, optical, and Xray observations, we can gain insights into its disk of stars, gas, and dust,
along with its central region, and the hot gas surrounding it.
WHAT DO YOU THINK?
1.
2.
3.
What is the shape of the Milky Way Galaxy?
Where is our solar system located in the Milky
Way Galaxy?
Is the Sun moving through the Milky Way
Galaxy and, if so, about how fast?
In this chapter you will discover…
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the Milky Way Galaxy—billions of stars along with gas
and dust bound together by mutual gravitational
attraction
the structure of our Milky Way Galaxy
Earth’s location in the Milky Way
how interstellar gas and dust enable star formation to
continue
that observations reveal the presence of significant mass
in the Milky Way that astronomers have yet to identify
that there is a massive black hole at the center of our
Galaxy
Schematic Diagrams of the Milky Way
(a) This edge-on view shows the
Milky Way’s disk, containing most of
its stars, gas, and dust, and its halo,
containing many old stars. Individual
stars in the halo are too dim to be
visible on this scale, so the bright
regions in the halo represent clusters
of stars.
(b) Our Galaxy has two major arms
and several shorter arm segments,
all spiraling out from the ends of a
bar of stars and gas that passes
through the Galaxy’s center. The
bar’s existence and the presence of
two major arms were confirmed by
the Spitzer Space Telescope.
Lord Rosse’s High-Tech 1.8-m Telescope of 1845
Built in 1845, this structure housed a 1.8-m-diameter telescope, the
largest of its day. The improved resolution it provided over other
telescopes was similar to the improvement that the Hubble Space
Telescope provided over Earthbound optical instruments.
Results of 1845 High-Tech Telescope
Using his telescope, Lord
Rosse made this sketch of the
spiral structure of the galaxy
M51 and its companion galaxy
NGC 5195. Below is a
modern photograph of M51
(also called NGC 5194) and
NGC 5195. The spiral galaxy
M51 in the constellation of
Canes Venatici is known as
the Whirlpool Galaxy because
of its distinctive appearance.
The two galaxies are about 20
million ly from Earth.
A Cepheid Variable Star in Galaxy M100
One of the most reliable ways to determine the distance to moderately
remote galaxies is to locate Cepheid variable stars in them, as discussed
in the text. The distance of 50 million ly to the galaxy M100 in the
constellation Coma Berenices was determined using Cepheids. (Insets)
The Cepheid in this view, one of 20 located to date in M100, is shown at
different stages in its brightness cycle, which recurs over several weeks.
The Period-Luminosity Relation
This graph shows the relationship between the periods and average
luminosities of classical (Type I) Cepheid variables and the closely related
RR Lyrae stars (discussed in Chapter 12). Each dot represents a Cepheid
or RR Lyrae whose luminosity and period have been measured.
Our Galaxy
This wide-angle photograph spans half the Milky Way. The Northern
Cross is at the left and the Southern Cross is at the right. The center of
the Galaxy is in the constellation Sagittarius, in the middle of this
photograph. The dark lines and blotches are caused by hundreds of
interstellar clouds of gas and dust that obscure the light from background
stars, rather than by a lack of stars.
A View Toward the Galactic Center
More than a million stars in the disk of our Galaxy fill this view, which
covers a relatively clear window just 4° south of the galactic nucleus in
Sagittarius. Beyond the disk stars you can see two prominent globular
clusters. Although most regions of the sky toward Sagittarius are thick with
dust, very little obscuring matter appears in this tiny section of the sky.
Electron Spin and the Hydrogen Atom
Due to their spin, electrons and protons both act as tiny magnets. When
an electron and the proton it orbits are spinning in the same direction,
their energy is higher than when they are spinning in opposite directions.
When the electron flips from the higher-energy to the lower-energy
configuration, the atom loses a tiny amount of energy that is radiated as a
radio photon with a wavelength of 21 cm.
A Technique for Mapping the Galaxy
Hydrogen clouds at
different locations along our
line of sight are moving
around the center of the
Galaxy at different speeds.
The component of their
motion away from us varies
with their distance from the
solar system. Radio waves
from the various gas
clouds, therefore, exhibit
slightly different Doppler
shifts, permitting
astronomers to sort out the
gas clouds and map the
Galaxy.
A Map of the Galaxy
This map, based on radio telescope surveys of 21-cm radiation, shows
the distribution of hydrogen gas in a face-on view of the Galaxy. This view
just hints at spiral structure. The galactic nucleus is marked with a dot
surrounded by a circle. Details in the large, blank, wedge-shaped region
toward the upper left of the map are unknown, because gas in this part of
the sky is moving perpendicular to our line of sight and thus does not
exhibit a detectable Doppler shift. (Inset) This drawing, based on visiblelight data, shows that our solar system lies between two arms of the Milky
Way Galaxy.
A Map of the Galaxy
The spiral arms in the Milky Way.
Two Views of a Barred Spiral Galaxy
The galaxy M83 is in the southern constellation of Centaurus, about
12 million ly from Earth. (a) At visible wavelengths, spiral arms are
clearly illuminated by young stars and glowing H II regions. (b) A radio
view at 21-cm wavelength shows the emission from neutral hydrogen
gas. Note that the spiral arms are more clearly demarcated by visible
stars and H II regions than by 21-cm radio emission.
Our Galaxy
As seen from the side, three major
visible components of our Galaxy
are a thin disk, a central bulge, and a
two-part halo system. As noted
earlier, there is also a central bar.
The disk contains gas and dust
along with Population I (young,
metal-rich) stars. The halo is
composed almost exclusively of
Population II (old, metal-poor) stars.
(Inset) The visible matter in our
Galaxy fills only a small volume
compared to the distribution of dark
matter, whose composition is
presently unknown. Dark matter’s
presence is felt by its gravitational
effect on visible matter.
Infrared shows the expected concentration
toward the galactic center
Visible light shows no
such concentration.
Impression of being in center
from visible light.
The Galactic Center
Wide-angle and zooming in on the galactic center. Infrared is used to distinguish
individual stars at the center of our galaxy.
Two Views of the Galactic Nucleus
(a) A radio image taken of the
galactic nucleus and environs.
This image covers an area of
the sky 8 times wider than the
Moon. SNR means supernova
remnant. The Sgr features are
radio-bright objects.
(b) The colored dots show the motion of seven
stars in the vicinity of the unseen massive
object () at the position of the radio source
Sagittarius A*, part of Sgr A. This plot indicates
that the stars are held in orbit by a 4 x 106solar-mass black hole.
Orbits of Stars in Our Galaxy
This disk galaxy, M58, looks very similar to what the Milky Way
Galaxy would look like from far away. The colored arrows show
typical orbits of stars in the central bulge (blue), disk (red), and
halo (yellow). Interstellar clouds, clusters, and other objects in
the various components have similar orbits.
Differential Rotation of the Galaxy
(a) If all stars in the Galaxy had the same angular speed, they would orbit in
lockstep. (b) However, stars at different distances from the galactic center
have different angular speeds. Stars and clouds farther from the center take
longer to go around the Galaxy than do stars closer to the center. As a
result, stars closer to the Galaxy’s center than the Sun are overtaking the
solar system, whereas stars farther from the center are lagging behind us.
The Galaxy’s Rotation Curve
The blue curve shows the orbital speeds of stars and gas in the Galaxy,
and the dashed red curve shows Keplerian orbits that would be caused by
the gravitational force from all the known objects in the Galaxy. Because
the data (blue curve) do not show any such decline, there is, apparently,
an abundance of dark matter that extends to great distances from the
galactic center. This additional mass gives the outer stars higher speeds
than they would have otherwise.
From Newton’s Laws
v = √(G Menclosed / R)
“Kepler” case: Menclosed is constant, v decreases
But Menclosed increases with R – more stars
But the mass of stars is insignificant outside 40,000 Ly
Detailed calculations: not enough stars inside, either
Conclusion: Dark Matter! - to supply the extra gravity
Microlensing by Dark Matter in the Galactic Halo
This technique searches for Massive Compact Halo Objects (MACHOs). It finds
them, but not NEARLY enough of them. The other hypothesis: WIMPS (Weakly
Interacting Massive Particles, meaning subatomic particles).
The mystery: it can’t be any KNOWN subatomic particle, so what IS it?
Summary of Key Ideas
Discovering the Milky Way
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A century ago, astronomers were divided on
whether or not the Milky Way Galaxy and the
universe were the same thing.
The Shapley–Curtis debate was the first major
public discussion between astronomers as to
whether the Milky Way contains all the stars in
the universe.
Cepheid variable stars are important in
determining the distance to other galaxies.
Edwin Hubble proved that there are other
galaxies far outside of the Milky Way.
The Structure of Our Galaxy
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Our Galaxy has a disk about 100,000 ly in diameter and
about 2000 ly thick, with a high concentration of
interstellar dust and gas. It contains around 200 billion
stars.
Interstellar dust obscures our view into the plane of the
galactic disk at visual wavelengths. However, hydrogen
clouds can be detected beyond this dust by the 21-cm
radio waves emitted by changes in the relative spins of
electrons and protons in the clouds, as well as by other
nonvisible emissions.
The Structure of Our Galaxy
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The center, or galactic nucleus, has been studied at
gamma-ray, X-ray, infrared, and radio wavelengths,
which pass readily through intervening interstellar dust
and H II regions that illuminate the spiral arms. These
observations have revealed the dynamic nature of the
galactic nucleus, but much about it remains unexplained.
A supermassive black hole of about 4.3 x 106 solar
masses exists in the galactic nucleus.
The galactic nucleus of the Milky Way is surrounded by a
flattened sphere of stars, called the central bulge,
through which a bar of stars and gas extends.
The Structure of Our Galaxy
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A disk with at least four bright arms of stars, gas, and
dust spirals out from the ends of the bar in the galactic
central bulge.
Young OB associations, H II regions, and molecular
clouds in the galactic disk outline huge spiral arms where
stars are forming.
The Sun is located about 26,000 ly from the galactic
nucleus, between the spiral arms. The Sun moves in its
orbit at a speed of about 878,000 km/h and takes about
230 million years to complete one orbit around the center
of the Galaxy.
The Structure of Our Galaxy
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The entire Galaxy is surrounded by two halos of matter.
The inner halo includes a spherical distribution of
globular clusters and field stars, as well as large
amounts of dark matter. It orbits in the same general
direction as the disk. The outer halo is composed of dark
matter and very old stars, which have retrograde orbits.
Key Terms
central bulge
dark matter (missing mass)
disk (of a galaxy)
distance modulus
galactic cannibalism
galactic nucleus
galaxy
halo (of a galaxy)
microlensing
Milky Way Galaxy
missing mass
nebula (plural nebulae)
nuclear bulge
rotation curve (of a galaxy)
Sagittarius A
Shapley–Curtis debate
spin (of an electron or proton)
spiral arm
synchrotron radiation
21-cm radio radiation
WHAT DID YOU THINK?
What is the shape of the Milky Way
Galaxy?
 The Milky Way is a barred spiral galaxy. A
bar of stars, gas, and dust runs through its
central region. It has two major spiral
arms, several minor arms, and is
surrounded by a complex spherical halo
system of stars and dark matter.
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WHAT DID YOU THINK?
Where is our solar system located in the
Milky Way Galaxy?
 The solar system is between the
Sagittarius and Perseus spiral arms, about
26,000 ly from the center of the Galaxy
(about halfway out to the visible edge of
the galactic disk).
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WHAT DID YOU THINK?
Is the Sun moving through the Milky Way
Galaxy and, if so, about how fast?
 Yes. The Sun orbits the center of the Milky
Way Galaxy at a speed of 878,000 km/h.
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