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Ch. 14
Ch. 14.
The Milky Way
Ch. 14 OUTLINE
Shorter than book
• 14.1 The Milky Way Revealed
• 14.2 Galactic Recycling (closely related to
Ch. 13)
• 14.3 The History of the Milky Way
• 14.4 The Mysterious Galactic Center
14.1 The Milky Way Revealed
• Our Goals for Learning (not exactly like
book)
• What does our galaxy look like?
• Where do stars form in our galaxy?
What does our galaxy look like?
The Milky Way galaxy appears in our sky as a faint band of light
Dusty gas
clouds obscure
our view
because they
absorb visible
light
This is the
interstellar
medium that
makes new
star systems
All-Sky View at visible wavelengths
plotted in galactic coordinates
What if we could see through most of that dust?
All-Sky View at visible wavelengths
All-Sky View at infrared wavelengths
Remember Extinction and Reddening:
interstellar dust will make stars look fainter and
redder. Dust will affect more the shorter (bluer)
wavelengths and less the longer (redder)
wavelengths. By looking at infrared wavelengths we
can see through most of the dust.
The Shape of our Galaxy: a flattened disk
We see our galaxy edge-on
Primary features: disk, bulge, halo, globular clusters
If we could view the Milky Way from above the
disk, we would see its spiral arms
Andromeda Galaxy: our twin galaxy
Our Galaxy looks like Andromeda
How do we know what our galaxy would look like if viewed
from the top? Infrared and Radio observations penetrate
dark interstellar clouds
Stellar Populations
• Turns out that there are two types of stars in
the Galaxy
– Population I: Relatively young. Similar to the
Sun. Tend to be in the galactic disk. Richer in
heavy elements
– Population II: Few heavy elements, very old
(12-14 billion years), tend to be in the center of
the galaxy or in globular clusters
Remember CH 11 Part III
Star Clusters:
1. Two types of clusters: Open and Globular
Two types of star clusters
• Open clusters: young,
contain up to several
thousand stars and are found
in the disk of the galaxy
(Population I).
• Globular clusters: old,
contain hundreds of
thousands of stars, all
closely packed together.
They are found mainly in
the halo of the galaxy
(Population II).
14.2 Galactic Recycling
• Our Goals for Learning
• How does our galaxy recycle gas into stars?
• Where do stars tend to form in our galaxy?
How does our galaxy recycle gas
into stars?
Star-gas-star
cycle
Recycles gas
from old stars
into new star
systems
14.2 Galactic Recycling
• Where do stars tend to form in our
galaxy?
14.2 Galactic Recycling
• Where do stars tend to form in our
galaxy? In the Disk
Question 4
What happens after an interstellar cloud of gas
and dust is compressed and collapses:
A. It will heat and contract
B. If it gets hot enough (10 million K) it can
produce energy through hydrogen fusion
C. It can produce main sequence stars
Question 4
What happens after an interstellar cloud of gas
and dust is compressed and collapses:
A. It will heat and contract
B. If it gets hot enough (10 million K) it can
produce energy through hydrogen fusion
C. It can produce main sequence stars
D. All of the above
How does our galaxy recycle gas
into stars?
Cycle of Birth and Deaths of Stars
•
•
•
•
Interstellar cloud of gas and dust is
compressed and collapses to form stars
After leaving the main sequence red giants
eject their outer layers back to the
interstellar medium
Supernovas explode and eject their outer
layers back to the interstellar medium
Supernova explosions and other events can
compress an interstellar cloud of gas and
dust that collapses to form stars ………..
Remember the Sun’s Evolutionary Process
Remember mass loss in Intermediate Mass
Stars
Remember Supernova explosions
Star-gas-star
cycle
Recycles gas
from old stars
into new star
systems
Halo: no blue stars  no star formation
Disk: blue stars  star formation
Halo: No emission nebulae, no blue stars
 no star formation
Disk: emission nebulae, blue stars  star formation
Much of star
formation in disk
happens in spiral
arms
Emission Nebulae
Blue Stars
Gas Clouds
Spiral arms are waves of
star formation
The Whirlpool Galaxy
14.3 The History of the Milky Way
• Our Goals for Learning
• What clues to our galaxy’s history do halo stars
hold?
• How did our galaxy form?
What clues to our galaxy’s
history do halo stars hold?
Halo Stars:
0.02-0.2% heavy elements (O, Fe, …),
only old stars
Disk Stars:
2% heavy elements,
stars of all ages
Halo Stars:
0.02-0.2% heavy elements (O, Fe, …),
only old stars
Disk Stars:
2% heavy elements,
stars of all ages
Halo stars
formed first,
then stopped
Halo Stars:
0.02-0.2% heavy elements (O, Fe, …),
only old stars
Halo stars
formed first,
then stopped
Disk Stars:
2% heavy elements,
stars of all ages
Disk stars
formed later,
kept forming
How did our galaxy form?
Our galaxy probably formed from a giant gas cloud
Halo stars formed first as gravity caused cloud to contract
Remaining gas settled into spinning disk
Note: This
model is
oversimplified
Stars continuously form in disk as galaxy grows older
What have we learned?
• What clues to our galaxy’s history do halo
stars hold?
• The halo generally contains only old, low-mass
stars with a much smaller proportion of heavy
elements than stars in the disk. Thus, halo stars
must have formed early in the galaxy’s history,
before the gas settled into a disk.
What have we learned?
• How did our galaxy form?
• The galaxy probably began as
a huge blob of gas called a
protogalactic cloud.
Gravity caused the cloud to
shrink in size, and
conservation of angular
momentum caused the gas to
form the spinning disk of our
galaxy. Stars in the halo
formed before the gas finished
collapsing into the disk.
14.4 The Mysterious Galactic Center
• Our Goals for Learning
• What lies in the center of our galaxy?
What lies in the center of our
galaxy?
Strange radio sources in
galactic center
Stars at galactic center
Stars appear to
be orbiting
something
massive but
invisible … a
black hole!
Orbits of stars
indicate a mass
of about 4
million MSun
What have we learned?
• What lies in the center of
our galaxy?
• Motions of stars near the
center of our galaxy
suggest that it contains a
black hole about 4 million
times as massive as the
Sun. The black hole
appears to be powering a
bright source of radio
emission.