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The Milky Way Galaxy
• We live on the quiet outskirts
of a galaxy of approximately
100 Billion stars.
• This galaxy, the Milky Way,
is roughly disk-shaped and is
about 100,000 light years in
diameter.
• Besides stars the Milky Way
also contains enormous
clouds of gas and dust
The Milky Way as seen from
some place on Earth far from city lights.
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Parts of Our Galaxy
• Our Galaxy is a spiral
galaxy.
• It is composed of:
– a central bulge
– a flat disk containing the
spiral arms and
– a halo of globular clusters
• there may also exist an
extended halo of some
unknown material
commonly known as
“dark matter”.
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More Properties of the Milky Way
The Milky Way as seen in
infrared light. Dust and gas in our
galaxy prevent us from seeing
very far into the galaxy in visible
light. In the infrared and radio we
can see much farther. Using this
information helps us to recognize
that we live in a spiral galaxy.
• Gas and Dust in our galaxy
account for about 15% of its
mass.
• The spiral arms are
composed mostly of clusters
of bright young stars.
• The Sun orbits the center of
the galaxy with a speed of
about 220 kilometers per
second.
• It takes the Sun 240 million
years to go around the
galaxy one time.
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Learning About the Structure of the Milky Way
• William Herschel in 1784
recognized that the Sun was in
a disk-shaped galaxy of stars.
• Because of dust and gas
blocking the light of distant
stars it appeared that the Sun
was near the center of the
galaxy.
• This same dust and gas makes
it more difficult to observe
other galaxies too especially
along the disk of our galaxy
and towards the center.
Top. Herschel’s Map of the Milky Way
Bottom. A picture of a dark dust cloud and a star cluster
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Globular Clusters and the Center of the Galaxy
Globular
Clusters
contain some
of the oldest
stars in the
galaxy.
• In the 1920s an estimate was
made for the size of the galaxy
and the position of the Sun
within it.
• It was found that certain dense
clusters of stars known as
globular clusters tended to be
distributed in an elliptical
region of space. The center
would indicate the center of the
galaxy.
• The Sun was found not to be at
the center of this region.
• Later estimates placed this
center at about 28,000 light
5
years away.
Mass of the Milky Way
• Using the same method used
to measure the mass of planets
and binary stars we can also
use the modified form of
Kepler’s 3rd Law to measure
the mass of the Milky Way.
• Using the distance to the
There are several methods of determining
center of the galaxy and the
the mass of the Milky Way. Astronomers
orbital period of the Sun the
comparing the estimated mass of all visible
mass of the galaxy is at least
matter in the galaxy with the
1011 Solar masses.
gravitational mass of the galaxy have
found that most of the mass is not visible.
• This method only measures
This is where the theory that the galaxy
the mass of the galaxy
must contain some kind of “dark matter”
contained within the orbit of
comes from.
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the Sun.
Spiral Arms - Density Waves
• The arms are traced out by
massive bright, young stars. The
arms do not “carry” stars with
them but are regions of higher
density that stars and gas pass
through.
• An excess of mass in the arm
exerts a slightly stronger
gravitational pull. Clouds pulled
into these arms collapse and form
stars.
• Smaller stars live long enough to
pass out of the arm as it sweeps
by. Massive stars die before
leaving the arm
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Spiral Arms - SSF
• Another way that spiral arms
form is due to supernova
explosions of large stars.
• These explosions trigger the
collapse of gas clouds
causing new stars to form.
• The process continues as new
large stars explode causing
more clouds to collapse.
• As these stars orbit the galaxy
the stars closer in orbit faster
and form a spiral shape.
• This is called selfpropagating star formation
or SSF.
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What is Lurking in the Center of
Our Galaxy?
• Near the center of our galaxy
there are roughly 10 million
stars packed into one cubic
light year.
• In the very center is an object
smaller than the size of
Jupiter’s orbit but with a
This is called a supermassive
mass several million times
black hole. Most other galaxies
the mass of the Sun.
are also thought to have these huge • Since so little light is visible
black holes in their centers.
from this object it is thought
to be a very large black hole.
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