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Lecture Slides
CHAPTER 15: Our Galaxy: The Milky Way
Understanding Our Universe
SECOND EDITION
Stacy Palen, Laura Kay, Brad Smith, and George Blumenthal
Prepared by Lisa M. Will,
San Diego City College
Copyright © 2015, W. W. Norton & Company
Our Galaxy: The Milky Way
 Describe the Milky
Way’s structure.
 Explain the history of
star formation and
chemical composition
in the Milky Way.
 Discuss how the Milky
Way teaches us about
galaxy formation.
Structure
 The Milky Way is visible
as a band of light across
our sky.
 This is similar to our view
of some distant galaxies.
 Comparing these
observations will teach us
about the Milky Way.
Structure (Cont.)
Structure (Cont.)
Structure: Vast Clouds
 The dark regions visible
in the Milky Way are now
known to be vast clouds
of interstellar gas and
dust, obscuring the light
from the stars behind
them.
Structure: Vast Clouds (Cont.)
Structure: Vast Clouds (Cont.)
Structure: Interstellar Dust
 Interstellar dust blocks shorter wavelengths more
efficiently: interstellar reddening.
 Long wavelengths (infrared and radio) penetrate dust.
Structure: Neutral Hydrogen
 Neutral hydrogen emits radio waves with  = 21 cm.
 Light of this wavelength penetrates the dust.
 Radio and infrared surveys have helped us map the
structure of the Milky Way.
Structure: Neutral Hydrogen (Cont.)
 Observations of the
distribution of interstellar
hydrogen and young
stars indicate that the
Milky Way galaxy has a
spiral structure.
Structure: Spiral Galaxy
 Surveys of the motions
of stars near the
center of the Milky
Way indicate that the
central region is not
spherical, but is rather
an elongated bulge.
 Milky Way is a barred
spiral galaxy.
Size: Globular Clusters
 Globular clusters are
tightly gravitationallly
bound collections of up
to a million stars.
 Because globular
clusters reside within a
spherical halo around
the Milky Way,
astronomers use them
to map out the galaxy.
Size: Globular Cluster Stars
 Main sequence turnoffs
show globular clusters
contain very old stars.
 Globular clusters contain
RR Lyrae and Cepheid
variable stars that can be
used as standard candles.
Size: Distance Between Galaxies
 Globular clusters are
very bright, so they can
help find distances in
our galaxy and to other
galaxies.
 They were used to
determine the size of the
Milky Way and that the
Sun is not at the center.
Size: Galactic Center
 The distance to the
galactic center cannot
be directly measured
because it is blocked
by interstellar dust.
 The gravitational
center of the globular
clusters is the
gravitational center of
the entire Milky Way:
27,000 ly from our
solar system.
Size: Milky Way Disk
 We now have a sense of
the size of the Milky Way
galaxy.
 The disk is about
100,000 ly across.
 The solar system exists
within the disk.
 The center of the Galaxy
forms a bulge.
 The halo extends across
300,000 ly.
Composition & Age
 We find globular
clusters in the
galactic halo.
 Globular clusters
are amongst the
oldest objects
known – up to 13
billion years old!
 We see no young
globular clusters.
Composition & Age: Open Clusters
 Open clusters are loosely
bound collections of dozens to
thousands of stars found in the
disk of a spiral galaxy.
 Open clusters have a wide
range of ages, from far
younger to a little older than
our Sun.
Composition & Age: Open Clusters (Cont.)
Composition & Age: Open Clusters (Cont.)
Composition & Age: Galactic Halo
 All open clusters are much
younger than globular clusters.
 Difference in ages is a clue!
 Stars in the halo must have
formed first.
Class Question
Where in the Milky Way galaxy is the solar System
located?
A. Bulge
B. Disk
C. Halo
Composition & Age: Hydrogen and Helium Stars
 Stars are mostly hydrogen and helium.
 Heavier elements must have been formed in stars.
 The more massive elements found in a star, the more
prior star formation took place.
Composition & Age: Heavy Elements in Stars
 We observe younger stars to have more heavy
elements than older stars.
 Residing in the disk of our galaxy, the Sun has about
a 2% concentration of heavy elements, a relatively
high amount.
 Even globular cluster stars have a small amount of
heavy elements, but much less than disk stars. =>
There must have been at least one generation of
stars that existed prior to globular clusters.
Composition & Age: Star Formation
 Higher heavy element amounts correspond to more
abundant star formation in the inner disk, but other
effects may be present.
 Galactic fountain model: gas is ejected from the disk
by hot young stars and supernovae and then falls
back onto the disk.
Structure: Halo Formation
 Molecular clouds in
the plane of the Milky
Way form a thin disk.
 Newly formed stars
are also concentrated
this thin disk.
 Older stars make a
thicker disk, above
and below the plane
of the Galaxy.
Structure: Cosmic Rays
 Molecular
clouds anchor
Milky Way’s
magnetic field.
 Cosmic rays—
high-energy,
charged
particles
moving at near
the speed of
light—can be
trapped by the
magnetic field.
Mass & Dark Matter: Doppler Velocities
 The Doppler shifts of
the 21-cm line tell us
the motions of neutral
hydrogen in the Milky
Way.
 Redshifts and
blueshifts appear on
opposite sides of the
galactic center.
Mass & Dark Matter: Pattern of Rotation
 This is the pattern of the
rotation velocity of a disk.
 The observed velocities
allow us to measure our
galaxy’s rotation curve and
determine structure within
the disk.
Mass & Dark Matter: Rotation Velocity
 The rotation curve for the Milky Way is similar to
those we saw for other spiral galaxies.
 The Milky Way is mostly dark matter. Visible matter
dominates the inner part of our galaxy, and dark
matter dominates its outer parts.
Supermassive Black Hole
 X-ray, infrared, and radio observations reveal the
center of the Milky Way.
 The images show hundreds of thousands of stars, as
well as wisps and loops of material distributed
throughout the region.
Supermassive Black Hole (Cont.)
Supermassive Black Hole (Cont.) (this image is from
Dr. Stolovy’s research combining Spitzer and Hubble
data in infrared wavelengths)
Supermassive Black Hole (Cont.)
Supermassive Black Hole: Galactic Center
 The stars near the
galactic center have
orbital periods on
order of a decade.
 Using Kepler’s third
law with these stars,
we estimate the black
hole at the galactic
center has a mass of
about 4 million M⊙.
=> supermassive!
Supermassive Black Hole: Gamma Ray Bubbles
 Gamma ray bubbles
extend 25,000 ly
above and below the
galactic plane.
 The bubbles may have
formed material fell
into the supermassive
black hole, releasing
enough energy to
power the bubbles.
Galaxy Formation
 The Milky Way offers clues to galaxy formation.
 The stars in globular clusters in the halo have some
heavy elements, so at least one prior generation of
stars must have existed.
 Halo objects were formed before interstellar gas was
all concentrated into the disk.
 Later star formation has been concentrated in the disk.
Galaxy Formation: The Milky Way
 Galaxies do not exist
in isolation.
 The Milky Way is one
of three large galaxies
amongst a few dozen
dwarf galaxies.
=> Local Group
Galaxy Formation: Dwarf Companion Galaxies
 The Milky Way
probably formed
by mergers of
many smaller
proto-galaxies in a
dark matter “cloud.”
 Several of these
are still orbiting the
Milky Way as
satellite galaxies.
Chapter Summary
 Our solar system is in the disk of the Milky Way, a
barred spiral galaxy that is 100,000 light-years across.
 There is a supermassive black hole at the center of the
galaxy, as well as an extended dark matter halo.
 The Milky Way offers many clues about the way
galaxies form.
Astronomy in Action
Size of Active Galactic Nuclei
Click the image to launch the Astronomy in Action Video
(Requires an active Internet connection)
Nebraska Applet
Milky Way Rotational Velocity Explorer
Click the image to launch the Nebraska Applet
(Requires an active Internet connection)
Nebraska Applet
Milky Way Habitability Explorer
Click the image to launch the Nebraska Applet
(Requires an active Internet connection)
Understanding Our Universe
SECOND EDITION
Stacy Palen, Laura Kay, Brad Smith, and George Blumenthal
Prepared by Lisa M. Will,
San Diego City College
This concludes the Lecture slides for
CHAPTER 15: Our Galaxy:
The Milky Way
wwnpag.es/uou2
Copyright © 2015, W. W. Norton & Company