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The Milky Way Galaxy
A River of Stars
Warm Up
Please complete the following KWL
What I know about the Milky Way is…..
What I would like to know about the Milky
Way is…..
What I learned about the Milky Way is…..
Warm Up
1. Where did the name “Milky Way” most likely originate?
2. What German philosopher proposed that the Milky Way was
a flattened swarm of stars?
3. How is our view of our galaxy like an ant’s inside a fruit
4. What conclusions did Sir William Herschel make about the
Milky Way?
5. How was Harlow Shapley’s model different from Jacobus
6. What did Shapley use to determine the extent of the Milky
Warm Up
What is our solar system’s tilt relative to the galactic disk?
Where is our Sun located relative to the Milky Way’s galactic center?
What is another name for the galactic center?
How quickly is the Sun moving around our galactic center?
How far has the Milky Way rotated since the dinosaurs dies out 65
million years ago?
6. How is galactic density measured?
7. What is the galactic density is the region of our Sun?
8. What is the galactic density near the center of our galaxy?
9. What is the approximate mass of the Milky Way Galaxy?
10. If the mass of the Milky Way is greater, what is responsible for the
Warm Up-12/19/12
1. How is galactic density measured?
2. What is the galactic density is the region of
our Sun?
3. What is the galactic density near the center of
our galaxy?
4. What is the approximate mass of the Milky
Way Galaxy?
5. If the mass of the Milky Way is greater, what
is responsible for the discrepancy?
Warm Up-12/18/12
1. What is our solar system’s tilt relative to the galactic
2. Where is our Sun located relative to the Milky
Way’s galactic center?
3. How quickly is the Sun moving around our galactic
4. How long does it take our galaxy to make one
complete galactic revolution?
5. How far has the Milky Way rotated since the
dinosaurs dies out 65 million years ago?
Warm Up
What are population I stars?
Where are population I stars found?
What are population II stars?
Where are population II stars found?
What is the center of the galaxy called?
What do astronomers believe is at the
center of the Milky Way?
Word Wall Elements
Used correctly in a sentence
Words for Word Wall Elements
Milky Way
Globular cluster
Galactic disk
Galactic halo
Galactic bulge
Spiral arm
Galactic nucleus
Population I stars
Population II stars
Open clusters
12) scattering
13) Dark nebulae
14) Zone of avoidance
15) Emission nebula
16) Reflection nebula
The Milky Way
The origin of the term “Milky Way” has
been lost to antiquity. The Greek word for
galaxy is “galactos” for milk. The Latin
version of Milky Way is “via lactea”, with
“via" meaning “way" or “road" and
“lactea" meaning “milk".
The Milky Way
Our understanding of the Milky Way as a
star system dated back to the 18th century
when English astronomer Thomas Wright
and German philosopher Immanuel Kant
both proposed that the Milky Way was a
flattened swarm of stars
There’s Ants in the Fruit Salad!!
Here’s a good analogy for understanding
the way the Milky Way looks from our
vantage. Imagine that you’re an ant that
has fallen into a round fruit salad. Sitting
there on your berry, you look up and see a
few other berries and down and you see a
few more berries, but when you look
straight you see a sea of berries.
The Milky Way: Sir William Hershel
Late in the 1800’s, Sir
William Hershel
counted the number
of stars that were
visible in different
directions. From this
count he concluded
that stars laid in a
disk and that the disk
was approximately 5
times longer than it
was thick.
The Milky Way: Sir William Hershel
Herschel further concluded that because the
number of stars appeared to be the same in each
direction that our Sun lay roughly in the middle
of this disk.
Sketch of the
Milky Way
Measuring the Galaxy
The Galaxy was first measured in the early
1900’s by Dutch astronomer Jacobus Kapteyn
(cap-tine) and American astronomer Harlow
Measuring the Galaxy
Shapley’s model differed from Kapteyn’s in
that it was larger and the Sun resided about
2/3 of the way from the center. Shapley’s
method was different as well. Instead of
plotting stars, he plotted globular clusters.
Globular Clusters
Globular clusters are groups of up to millions of
stars. Shapley used them because of their great
luminosity. They could be seen at greater
distances than single stars. He noticed that
globular clusters were scattered evenly with the
Measuring the Galaxy
Shapley determined the distances by
methods that we are familiar with:
variable stars in the globular clusters, true
and apparent brightness and the inverse
square law. Shapley believed that the
clusters filled an elliptical region about
100,000 parsecs (300,000 light-years)
Unfortunately for science, each scientist was
partially right and partially wrong. Shipley
correctly placed the Sun within the galaxy, but
he grossly overestimated the size. Kapteyn came
closer to the actual size of our Galaxy. Neither
knew of the attenuating effects of interstellar
gases. Neither knew that there were two types
of variable stars. Looking back at the
controversy, it’s funny that they were wrong
about so much.
The Structure and Content of the
Milky Way Galaxy
The Milky Way is a disk that measures about 30
parsecs (80,000 light-years) across. It consists of
three main parts: the disk, the bulge and the
Where Are We?
Our solar system lies about 8.5 parsecs
(28,000 light-years) from the galactic
center. It is tilted about 600 relative to the
galactic disk. That is why you see the
Milky Way at such a steep angle in the
What Else is Out There?
Along with the stars, there is a lot of gas
and dust. In fact, it makes up about 15%
of the galaxy’s mass. Most of these
regions reveal themselves by blocking the
light of stars.
What Else is Out There?
It is because of this dust and gas that we
are unable to see to the center of our own
galaxy. Optically, we can only see about
3,000 parsecs (10,000 light years) toward
the galactic center known as the galaxy’s
nucleus. However using x-ray, infrared
and radio frequencies, we can “see” that
the core contains a vast number of stars
and a giant black hole.
Forces of the Galaxy
Gravity keeps the galaxy together, but it would
quickly pull the entire thing together if it weren’t
for our galactic spin. The Sun moves around the
center of the galaxy at about 220 km/sec. It is
this rotational velocity that gives the Milky Way
its flat, disk-like structure.
Forces of the Galaxy
Just as a point on the edge on a merry-goround travels faster than one toward the
center, so to do the stars at the outer edge
of the galaxy.
Forces of the Galaxy
While traveling 220 km/sec might seem
fast, it still takes the Sun about 240 million
years to make one complete galactic
revolution. The Sun has only made about
a quarter of a revolution since the
dinosaurs died.
Galactic Density
While the Sun may seem as if it’s lost in a
sea of stars, that is not true. Stars are
relative scattered. Our closest neighbor is
about 4.2 light-years away, comparable to
two pinheads fifteen miles apart.
Galactic Density
Near the core of the galaxy, stars are
packed more densely, some 1500 times
closer together. The analogy would be
like two pinheads at either end of a
football field.
Living in the Burbs
Star density is
measured by stars per
cubic light-year. Near
the core the density is
approximately 10
million stars per cubic
light-year, while in
our region of the
galaxy it falls to
around 0.003 stars per
cubic light-year.
The Mass and Number of Stars in Our
Astronomers can calculate the mass of the solar
system from the gravitational attraction needed
to keep the Sun in orbit. It turns out that the
mass of the Milky Way is about 1011 solar
Dark Matter
However, our gravitational effects on
other galaxies implies that our mass could
be as high as 2 X 1012 solar masses. This
discrepancy implies some missing mass.
Scientist believe that our galaxy is
embedded in a vast halo of material that
we can not see called dark matter.
Stellar Census
By dividing the total number of stars in
our galaxy by its age, astronomers
estimate that every year between 3 and 5
stars are born in our galaxy. This is
several times less than those that were
probably born earlier in our galaxy’s
Dark Matter
Observations indicate that this dark
matter fills a region with a radius of 100
kilo-parsecs (326,000 light years),
dwarfing the observable portion of our
How Many Stars Are in Our Galaxy?
Assuming that most
stars have masses
similar to our Sun, it
is estimated that
about 100 billion
stars reside in the
Milky Way.
Age of the Milky Way
Scientists estimate that the age of our
galaxy is about 13 billion years old.
During that vast expanse of time, the
Milky Way has consumed most of its
original gas. It is believed that in about 10
billion years that the Milky Way will
contain no material to make new stars and
will begin to fade.
Stellar Census
The Milky Way contains all the types of stars that we discussed
during stellar evolution. However, despite the great diversity of
stars, the typical star in the Milky Way is very much like our own
Sun in age and size.
Population I and II Groups
Walter Baade, during the blackouts of
WWII, made a series of photographs at the
Mt. Wilson observatory in California. In
them, he noticed that stars in galaxies
were often segregated by color.
Population I and II Groups
The disks of galaxies contained blue stars.
He called these population I stars.
Population I and II Groups
The galactic bulges and haloes contained
red stars. These he called population II
Population I and II Groups
Population I and II stars differ in almost
every respect: age, color, location, motion
and composition. Population I stars are
younger, averaging from 106 to 109 years
old. Most are blue and lie in the galactic
disk following approximately circular
orbits. Their composition is mainly
hydrogen and helium with only about 3%
of their mass as heavy elements.
Population I and II Groups
Population II stars are generally older and
red whose ages are about 1010 years old.
They lie in the bulge or halo and have
highly elliptical orbits that may be heavily
tilted with respect to the galactic disk.
These stars are mostly hydrogen and
helium as well with only a few 100th’s of a
percent as heavy elements.
Star Clusters
Some of the stars in the Milky Way are
gravitationally bound together into
groups called star clusters.
Star Clusters
Within a star cluster, each star moves along its
own orbit around the center of the mass of the
cluster. One of the most famous is the
constellation Pleiades (daughter of Atlas). This
constellation, visible from August to March, is
located just above the “V” in Taurus.
Types of Clusters
The Milky Way contains two types of star
clusters. The first is the open cluster.
Open clusters generally contain up to a
few hundred stars and typically span 7 to
20 light-years.
Open Clusters
Scientists believe that open clusters form
from interstellar gas caught between the
spiral arms of galaxies. The strong
gravitational fields compress the gas
creating stars that are gravitationally
bound together. These clusters eventually
break apart. Approximately 20,000 star
clusters are believed to exist in the Milky
Way. Our own Sun may have originally
formed this was and become disassociated
with the rest of its cluster over time.
Open Clusters
Globular Clusters
Globular clusters contain many more stars
than open clusters. Their populations can
range from a few hundred thousand stars
to several million per cluster. Their radii
usually range form 40 to 160 light-years.
Because they are more massive, they pull
their stars into a tighter ball. Astronomers
estimate that between 150 to 200 globular
clusters exist within the Milky Way.
Globular Clusters
Measuring the Milky Way
We discussed ways that Shapley used to
determine the size of our galaxy
previously. Using globular clusters
remains one of the best ways.
Astronomers first locate a large number of
bright star clusters. They look for clusters
that are symmetrically positioned around
a central point and assume that this is the
galactic center and measure from that
Measuring the Milky Way
From this, astronomers estimate (and it is
a rough estimate) that the Milky Way has
a diameter of about 40 parsecs with our
Sun lying about 8.5 kiloparsecs (28,000
light-years) from the center.
The Galactic Center
Observation of the galactic center is
difficult due to dust clouds that block the
light. But, using x-ray, infrared, gamma
ray and radio wave telescopes, the
galaxy’s center can be glimpsed.
Astronomers have noticed an intense
source of radio waves known as
“Sagittarius A”.
The Galactic Center
The Galactic Center
Around Sagittarius A, which measures
about 10 AU, can be seen gas and stars
swirling around at an extremely rapid
rate. Some of these bodies travel at
hundred of kilometers per hour.
The Galactic Center
Kepler's Law says that the body around
which these stars orbit has a mass of 4
million solar masses, yet nothing is seen
there. Therefore, many astronomers
almost inescapably conclude that a
massive black hole lies at the heart of our
galaxy. This evidence is perhaps the best
yet that black holes actually exist.
Galactic Center Applet
Galactic center applet.
Galactic Formation applet.
Merging Galaxies Movie.