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ž  Homework # 6 is due on Tuesday, November
22nd.
ž  Homework # 7 starts on Tuesday, November
22nd. It is due on Thursday, December 1st.
ž  As we approach the end of the semester:
ž  Quiz # 6 takes place on Tuesday, November
29th;
ž  The third mid-term exam will take place on
Tuesday, December 6th;
ž  Quiz # 7 will take place on Thursday, Dec. 8th;
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ž  Chapters:
70, 71, 72, 73.2, 73.3
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Our Galaxy looks like
a flattened `pancake
of stars, gas, and dust
with a `bulge in the
center.
It is about 12 kpc in
radius (1 kpc = 1000
pc; 1 pc = 3.26 lys)
We are located about
8 kpc from the center.
Many other galaxies look like the Milky Way
The Galaxy is a
gravitationally-bound
structure, containing
about
100,000,000,000
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stars.
ž 
We cannot see it
from `outside so
we either use similar
galaxies or models
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About 2 million light
years away.
Same size as the Milky
Way.
Another island universe!
Many galaxies, including
the Milky Way are slowly
rotating disks
Galaxies are the building blocks of the Universe
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What is the shape of the Milky Way galaxy?
ž  How do we know where we are in the
Galaxy?
ž  What wavelengths of radiation effectively
penetrate the dusty interstellar medium?
ž  How do we know the rotating structure of
the Galaxy?
ž 
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ž  What
is a galaxy:
§  Galaxies as the building blocks of the Universe
ž  The three basic infos about the Galaxy:
§  The Milky Way is an ecosystem for stars.
§  The Milky Way is mostly empty space (about 1 star for
10 pc3; 1 pc = 3.26 lys) … but it is rather dusty.
§  The Milky Way barely moves at all on the scale of a
human lifetime (however, it spins at 220 km/s; but it is so
huge that 1 rotation takes 240,000,000 years).
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To measure mass, we need orbital motions
Sun orbits Center of Galaxy at v~220 km/s and distance
r~8.5 kpc
Circumference is 2π r; at that speed period of rotation is
240 million years
Thus, mass inside the Sun orbit is (Third Kepler Law):
ž  MR-Sun = (2 π)2 r3 / G P2 ~ 0.9 x 1011 Mo
Including the mass outside the Sun, one actually finds:
ž  MTot ~ 4 x 1011 Mo
However, mass in star (from star counting) is much less:
ž  MStar ~ 0.8 x 1011 Mo
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Key Parts:
Globular
clusters
- disk: flattened,
rotating; stars rotate
along circular orbits
- halo: non-rotating;
however, stars and
globular clusters still
orbit, and orbits are
elliptical
Halo
Bulge
Disk
- bulge: around the
center of the Galaxy
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By counting stars along various directions,
astronomers understood that the Galaxy had the
shape of a disk.
ž  But the globular clusters in the halo (around) the
Milky Way have told us two important things
about our own galaxy:
ž 
§  The Sun is not at the center of the galaxy
§  The galaxy is a much larger system than it appeared
based on early observations
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1920: Harlow Shapley
Observed that the globular star clusters were centered
about a point that was displaced from the Sun. Shapley
proposed that the point was the center of the Milky Way.
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Basic components. I.
Disk:
Young blue stars (Pop I, a few billion
years old), gas, dust, metal rich young
clusters; disk contains spiral arms
Bulge:
Mainly old red stars (Pop II, many billion
years old), some gas, and a supermassive black hole
Halo:
Old stars (Pop II), metal poor, globular
clusters
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(A Reminder)
Survey Question
What characteristic of a star would imply that many
star generations preceded it?
1)
2)
3)
4)
5)
a
a
a
a
a
high hydrogen abundance
high helium abundance
high metal abundance
low metal abundance
low helium abundance
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(A Reminder)
Survey Question
What characteristic of a star would imply that many
star generations preceded it?
1)
2)
3)
4)
5)
a
a
a
a
a
high hydrogen abundance
high helium abundance
high metal abundance
low metal abundance
low helium abundance
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The Milky Way disk contains 4-5 spiral arms
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Disk supported by
rotation
Halo supported by
random motion
Bulge supported by
random motion and
small rotation
Formation (over many 108 years):
• Halo/Bulge old: rapid fragmentation
and collapse of gas
• Disk young: progressive collapse of
gas, dissipation and formation of
rotating disk, star formation
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M33
Dust – a hindrance to our study of the Milky Way
A view at visible wavelengths of the galactic plane.
Dust is generated in the late stages of low and high mass
stars, when carbon and silicon is dredged up from the cores
and ejected in stellar winds, planetary nebulae, and possibly
supernova remnants.
The blocking of visible light by dust is called dust extinction.
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If the dust is thick enough, visible
light is absorbed (or scattered) and
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only the longer wavelengths get through.
Radio
Microwave
longer wavelength
(redder)
Blocked by
Infrared Interstellar
Visible Dust
UV
X-ray
shorter wavelength
(bluer)
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Very Large Array
Chandra X-ray Observatory
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The Halo doesn t have too much dust,
so we can see right through it with optical
telescopes.
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The Halo doesn t have too much dust,
so we can see right through it with optical
telescopes.
In fact, looking through the halo we can
see other galaxies.
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Main Sequence
turn-off point
gives age of
cluster
Our Galaxy is
about 13
billion years
old!
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Very Large Array
• As a result of
dust extinction,
most of what we
know about the
disk of our galaxy
has been learned
from observations
at radio and IR
wavelengths.
Interstellar hydrogen emits strongly at 21cm wavelengths.
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Survey Question
Why do we expect to find hydrogen in the interstellar
medium?
1) hydrogen is produced in stars
2) hydrogen is the most abundant
element in the universe.
3) hydrogen is the only thing that emits at
radio wavelengths.
4) hydrogen is preferentially expelled by
stars
5) hydrogen is preferentially attracted by
large masses
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Survey Question
Why do we expect to find hydrogen in the interstellar
medium?
1) hydrogen is produced in stars
2) hydrogen is the most abundant
element in the universe.
3) hydrogen is the only thing that emits at
radio wavelengths.
4) hydrogen is preferentially expelled by
stars
5) hydrogen is preferentially attracted by
large masses
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ž 
You are here
Looking for 21-cm
wavelengths of light …
§  emitted by interstellar
hydrogen
§  as we look along the
disk of the Milky Way
(from inside), we see
21-cm photons
Doppler shifted
varying amounts
§  this allows the
velocity and mass of
interstellar hydrogen
to be mapped
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The Structure of the Milky Way Disk
The dominant
structures in the disk
are the spiral arms.
Spiral arms are density
waves that move at
different velocities from
the stars.
What is a density wave?
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The gas and stars in the galaxy rotate at a different rate than
the spiral arms (density waves)
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The gas and stars in the galaxy rotate at a different rate than
the spiral arms (density waves)
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The gas and stars in the galaxy rotate at a different rate than
the spiral arms (density waves)
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The gas and stars in the galaxy rotate at a different rate than
the spiral arms (density waves)
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The gas and stars in the galaxy rotate at a different rate than
the spiral arms (density waves)
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Survey Question
We find mostly hot, massive stars in the spiral arms
of galaxies because
1) hot, massive stars are preferentially
produced in the spiral arms
2) less massive stars live long enough to rotate out of
the spiral arms
3) supernovae destroy the less massive stars in the
spiral arms
4) there is too high a density in the spiral arms to
create low-mass stars
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Hot, bright, blue highmass stars light-up the
spiral density waves.
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There is a continuous reprocessing of gas in the
galaxy into stars.
ž Stars form from dense gas in molecular clouds.
ž Stars age and eject their outer layers.
ž The ejected gas eventually finds its way back into
an overly dense region and become part of the
next generation of stars.
ž This process is repeated as long as there is enough
hydrogen around to create new stars.
§  Our Galaxy contains sufficient gas to live another ~10
billion years; it forms stars at a pace of a few per year.
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Chandra survey of
the Galactic center
Red: 1-3 keV Green: 3-5 keV Blue: 5-8 keV
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Wang et al. (2002)
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It is the site of energy
`flares
It contains a supermassive black-hole
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•  More than 5000 km/s at
a mere 17 light hours
distance -- about 3x the
size of our solar system.
• 4 million solar masses
within this distance.
MPE: www.mpe.mpg.de/www_ir/GC/gc.html
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•  Most other galaxies have such `monster’
objects in their centers
•  Sometimes these objects are very
active: the material falling into the
black hole `lights up’, producing an
Active Galactic Nucleus (AGN) or, in the
most extreme cases (and at the
beginning of the Universe’s life), a
Quasar.
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