Download The Milky Way Galaxy

The Milky Way Galaxy
Structure of the galaxy
• The Milky Way is a
spiral galaxy
• The galactic center
is the thickest part.
• Around a dozen
arms spin around
the center.
• The Solar System is
on the Orion Arm.
Dimensions of the Milky
Way
• Distances in the
galaxy are
measured in
kiloparsecs (kpc).
• Most of the stars are
found near the
galactic plane.
• Globular clusters
are found above and
below the plane.
Parts of the Galaxy
What We Actually See
From Earth
• On Earth, the Milky
Way appears to be a
white band across
the night sky.
• In the constellation
Sagittarius, we look
into the galaxy
center and
thousands of stars
appear.
A Visible View of the Milky Way, pieced
together from many pictures.
Photographs using all parts of
the spectrum:
How Do We Know the Size
of the Galaxy?
• Henrietta Leavitt discovered that the period of
Cepheid variable stars depends on their absolute
magnitudes.
• If we know the magnitudes, we can determine
distances: D = 10(Mapp - Mabs +5)/5
• You can also read the distance off the graph!
Cepheid Variables are Giants
that have left the main sequence.
• Polaris is a good
example of a
Cepheid variable
that is found in our
region of the galaxy.
• Polaris’s magnitude
goes up and down
0.15 in around 3
days on a regular
cycle.
Hubble Records a Cepheid
Leavitt Studied Variables in
the Small Magellanic Cloud
All these stars are
around the same
distance to us, so
those that are
bright are really
bright.
Dr. Leavitt discovered a
variable in the small
Magellanic Cloud with
an apparent magnitude
of +15.5, and its
absolute magnitude is
-4.5.
D = 10(Mapp - Mabs +5)/5
D = 10(15.5- -4.5 +5)/5 = 1025/5 = 105
D = 100,000 pc = 100 kpc
Globular Clusters
• Globular Clusters are
found on the edges of
the galaxy.
• Harlow Shapley found
the distance to these
clusters and he plotted
their positions.
• For them to fit, the Milky
Way must be around 30
kpc across. (Shapley
miscalcuated to around
40 kpc.
Modern Globular Cluster Plot
• Once the globular
clusters are plotted,
using correct
magnitudes, they form a
halo around the galactic
plane.
• This helps us determine
that the center is 8 kpc
from the Solar System
and that the galaxy is
30 kpc in width.
HALO
The Andromeda Galaxy
We can’t see the
entire Milky Way
Galaxy, so we have
to use pictures of
other spiral
galaxies to help us
predict its structure.
2 million light yrs. away
The Rotating Galaxy
• The long spiral arms of
the galaxy tell us that
the galaxy is spinning.
• It takes around 230
million years for one
revolution of the Solar
System.
• We are not anywhere
near the center of the
galaxy; we’re just a
satellite system.
Radio View of the Milky Way
Evidence of Rotation
• The Doppler Shift tells us if an object is moving toward
or away from us.
• Red shift indicates moving away; blue shift towards us.
The Doppler Shift can be caused by
another star’s motion or our motion, or both.
We’ve
We’re
already past catching up
them.
to them.
They’re
catching
up to us.
They’re
already
past us.
Determining Radial Velocity
Vradial = shift x c
rest 
Vradial = 200 x 300,000
5400
Vradial = 10714 km/sec
(Away from us.)
Another Example…
The Solar System is overtaking
a star on the Perseus Arm,
causing a blue shift of 0.14
nanometers.
Vradial = shift x c
rest 
Vradial = 0.14 nm x 300,000
700 nm
Vradial = 60 km/sec
A problem to try for
yourself…
An observer near the galactic center
sees a red shift for the Solar System
of 0.4 nanometers. The rest
wavelength is 600 nm. What is the
velocity of the Solar System through
the galaxy?
Vradial = shift x c
rest 
Vradial = 0.4 x 300,000 km/sec
600
Vradial = 200 km/sec
Center
Motions within the Milky Way
Population I Stars
• The stars of the galaxy
are often divided into two
populations.
• Population I stars are
“metal-rich.”
• This means they have
elements other than
hydrogen and helium
• They are found mostly in
the galactic plane, and are
mostly likely second
generation young stars.
Population II Stars
• These older stars are found mostly in the halo of
the galaxy. They have very eccentric orbits.
• They are “metal-poor.”
• They contain almost only hydrogen and helium.
Summarizing Properties of
the Milky Way
GALACTIC DISK
GALACTIC HALO
GALACTIC BULGE
Highly fla ttened
Roughly spheric alΡ mildly
flat tened
Somewhat fl attened and
elonga ted in the pla ne of the
disk ("football shaped")
Contains both young and
old stars
Contains old stars only
Contains both young and
old stars; more old stars at
greater dista nces from the
center
Contains gas and dust
Contains no gas and dust
Site of ongoing star
formation
No star formation du ring
the last 10 bil lion years
Contains gas and dust,
especially in the in ner
regions
Ongoing star forma tion in
the inner regions
Gas and stars move in
circu lar orbits in the
Galac tic plane
Stars have random or bits in
three d imensions
Stars have lar gely random
orbits but with some net
rotation about the Galactic
center
Spiral arms
No obvious substructure
Galac tic nucleus
Overall whi te coloration,
with blue spir al arms
Reddish in color
Ring of gas and dust near
center
Yellow -white