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The Milky Way
Announcements
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Assigned reading: Chapter 15.1
Please, follow this final part of the course
with great care
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It is the most difficult one, less intuitive one
Lots of new notions
The Milky Way, Our Galaxy
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What is a galaxy
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Galaxies as the building blocks of the Universe
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The bottom line about galaxies: visible matter and
dark matter
The basic structure of the Milky Way Galaxy
The-star-gas-star cycle
The motion of the Milky Way
Super-massive black hole in the center
How Galaxies are really made
Dark matter outweighs
visible matter by 1 to 10
It is the dominant source
of gravity in the Universe
The Universe is made
of Dark Matter
Visible Matter is only
the tip of the iceberg
Yet, even if we detect
Its presence, we still do
not know what Dark
Matter is made of!
The Big Questions
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What is the shape of the Milky Way galaxy?
How do we know where we are in the Galaxy?
What wavelengths of radiation effectively
penetrate the dusty interstellar medium?
How do we know the rotating structure of the
Galaxy?
Another Galaxy: the
Andromeda Galaxy
About 2 million light
years away.
Angular size (about 2
degrees) ---> the size of
the Milky Way.
Another island universe!
To study the structure of the Milky Way, we
need to measure distances to stars
There are well-tested methods for
measuring distances over short length
scales:
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Radar ranging - good for measuring distances
in the solar system (up to about 0.0001 light
years)
Parallax - good for measuring distances to a
few hundred light years
But what do we do about
objects too far away to use
radar ranging or parallax?
Standard Candles
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If we know an
object’s true
Luminosity
Brightness 
luminosity, we can
2
4  distance
measure its distance
by measuring its
apparent brightness.
An object that has a known luminosity
is called a standard candle.
Standard Candle #1 - Cepheid
Variable Stars
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Cepheid variable stars
have variable brightness
that is very regular.
The period of the
variation can be from
days to weeks – and it
seems to be a reliable
indication of the star’s
luminosity!
Important!! Go refresh
Capter 12, Section 4.
Cepheid Variable Stars
Henrietta Leavitt
Henrietta Leavitt
(1868-1921).
Luminosity=4D2B
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.
Harlow Shapley's diagram of the distances of
the globular clusters from the Sun.
Globular Clusters and
Understanding our Galaxy
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The globular clusters in the halo of the Milky
Way have told us two important things about
our own galaxy:
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The Sun is not at the center of the galaxy
The galaxy is a much larger system than it
appeared based on early observations
The Anatomy of (the light-emitting
part of) the Galaxy
Basic components:
Thin disk
Bulge
Spiral arms
halo
The Galaxy
Globular
clusters
Halo
Key Parts:
- disk: supported by
rotation
- halo: supported by
motion pressure
- bulge: supported by
both rotation and
motion pressure
Bulge
Disk
Rotation of the disk
Disk supported by
rotation
Halo supported by
random motion
Bulge supported
by random motion
and small rotation
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.
A Reminder About Scattering
If the dust is thick enough, visible
light is absorbed (or scattered) and
only the longer wavelengths get through.
Radio
Microwave
longer wavelength
(redder)
Blocked by
Infrared Interstellar
Visible Dust
UV
X-ray
shorter wavelength
(more blue)
So, to examine our own galaxy, we must
use Radio, mm-wavelength, infrared, and Xray telescopes to peer through the
interstellar medium.
Very Large Array
Chandra X-ray Observatory
Infrared view of the sky
Survey Questions
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What is the shape of the Milky Way galaxy?
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How do we know where we are in the Galaxy?
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What wavelengths of radiation effectively
penetrate the dusty interstellar medium?
Survey Questions
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How do we measure the motion of the Milky
Way?
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How is the disk of the Milky Way supported?
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How is the bulge supported?
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How do we know there is a super-massive black
hole in the center of the Milky Way?
Radio Observations are key to
understanding the Disk.
Very Large Array
Interstellar hydrogen emits strongly at 21cm wavelengths.
A full sky image of hydrogen (21 cm emission)
By looking at the Doppler Shift of the 21 cm emission, we can reconstruct
the distribution of objects in the galaxy.
Radio observations help map the galactic disk
You are here
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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
A Map of the Milky Way Based on
21-cm wavelength light mapping
Spiral Galaxy M83 observed in both
visible light and radio wavelengths.
The Nature of the Spiral Arms
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?
The gas and stars in the galaxy rotate at a different rate than
the spiral arms (density waves)
The gas and stars in the galaxy rotate at a different rate than
the spiral arms (density waves)
The gas and stars in the galaxy rotate at a different rate than
the spiral arms (density waves)
The gas and stars in the galaxy rotate at a different rate than
the spiral arms (density waves)
The gas and stars in the galaxy rotate at a different rate than
the spiral arms (density waves)
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
The Star-Gas-Star Cycle
There is a continuous reprocessing of gas in
the galaxy into stars.
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Stars form from dense gas in molecular clouds
Stars age and then give up their outer layers
(via solar wind, planetary nebula, or supernova)
The ejected gas eventually finds its way back
into an overly dense region and become part of
the next generation of stars.
This process is repeated as long as there is
enough hydrogen around to create new stars.
What characteristic of a star would imply
that many star generations preceded it?
1) a high hydrogen abundance
2) a high helium abundance
3) a high “metal” abundance
4) a low “metal” abundance
5) a low helium abundance
Chandra survey of
the Galactic center
Red: 1-3 keV Green: 3-5 keV Blue: 5-8 keV
Wang et al. (2002)
X-ray Flare from Sgr A*
Baganoff et al. (2003)
Super Massive Black Holes
Our Galactic Center
• 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
Three key things to keep in
mind about the Milky Way
1) The Milky Way is an ecosystem for
stars.
2) The Milky Way is mostly empty
space … but it is rather dusty.
3) The Milky Way barely moves at all on
the scale of a human lifetime.
Discussion questions
• 1) What wavelengths of radiation effectively
penetrate the dusty interstellar medium?
• Radio and Infrared
• what velocity and what period?
– vel. = 220 km/s, period = 240 million years
• True/False - The same stars move along with
the spiral density waves all the time.
– False, the stars orbit the galactic center at a
different rate than the spiral density waves.
Survey Questions
• How do we measure the motion of the Milky
Way?
• How is the disk of the Milky Way supported?
• How is the bulge supported?
• How do we know there is a super-massive
black hole in the center of the Milky Way?