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The Milky Way Model
Student Handout
I. Background
Thousands of years ago, people
peering up into the sky from anywhere on
Earth couldn’t possibly miss the broad, hazywhite streak stretching across the sky. This
cloudy band of light was about 5 degrees
wide - 10 times the width of the Moon in the
sky - bulging brightest in the direction of the
constellation Sagittarius, where it plumped
out into a sphere. In some places, particularly
around the Mediterranean, the cloudy band
came to be known as the Milky Way. For
centuries astronomers and philosophers
debated the true nature of this cosmic smear: was it a cloud of gas and dust or a collection of
closely packed stars, was it part of the Earth’s atmosphere or at great astronomical distances?
These days, looking up at the sky from even a small city or suburb, the Milky Way isn’t even
visible anymore due to light pollution and has lost its hold on imagination of most people on
Earth. Yet at the same time scientists know more about it than ever before.
Galileo is the first person credited with uncovering the most basic property of the Milky
Way in the 17th century. Pointing his new invention - the telescope - in the direction of the
Milky Way, he observed the seemingly uniform cloud dissolve into a great number of individual
stars. He was able to separate (resolve) the stars that otherwise blended together when observed
by the naked eye; much like putting on a pair of glasses helps a near-sighted person separate the
letters on a blurry road sign. So it was revealed that what was once hypothesized to be a cloud of
gas was actually an immense collection of tightly packed stars. By counting the stars in different
directions across the sky it wasn’t too hard to determine that the Milky Way must have an edge
with the Solar System located closer to it than the center (though the first attempt by William
Herschel in 1785 wasn’t quite successful, see the diagram to the left, the sun is the dark spot near
the center). Furthermore, since the Milky Way appeared only in a relatively thin band on the
sky, it couldn’t be round like a sphere
but must be flat like a disk.
Astronomers in the centuries
following Galileo’s initial observation
found a variety of fuzzy cloud like
objects
(nebulae)
scattered
throughout the sky. The most famous
of these was known as the Great Andromeda Nebula, named for the constellation it is found in.
Andromeda, like many of the “spiral nebulae”, appeared to be a swirling cloud of gas; something
like a storm cloud or tornado. A new debate came to the fore: were these cloudy nebulae located
outside the Milky Way, so far away that the stars within them could not be resolved, or were
these actually clouds of gas located within the Milky Way itself? Perhaps more provocatively,
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astronomers posed the cosmological
question: was the Milky Way the whole
entire universe or was it just one of many
“island universes” located within a much
greater system? Eventually a famous
astronomer, Edwin Hubble was able to
use a powerful telescope to resolve the
stars in these spiral nebulae - ending the
debate - and measure the great distances
to them using Cepheid-variable stars.
These nebulae were renamed Galaxies:
the Milky Way being just one rather
standard example.
Through
more
detailed
observations with conventional optical
(visible wavelength) telescopes and across the entire electromagnetic spectrum of light - from
the radio to x-rays - astronomers have determined that the Milky Way Galaxy is actually a
complicated and busy collection of stars, gas and dust. Measurements of the distance and
velocity of a variety of astronomical objects within the Galaxy have revealed that the bulk of the
Milky Way appears to rotate around a central point. Perhaps most astonishingly, the buzzing
hive of stars that populate the Galactic center reveal an incredibly massive but invisible object
in the middle of it all: a super-massive black hole!
II. Intro to Galactic Coordinates
In this activity you will assemble a 3-D scale model of the Milky Way based on a
handful of interesting objects, which have been observed at different wavelengths of light: radio,
infra-red, visible, and x-ray. You will be given pictures of these objects along with their
observation wavelength, galactic coordinates and distance, to place them appropriately in your
model. Only two angular coordinates are needed to specify the location of any object on the
sky, for instance common coordinates used in astronomy are:
1. Azimuth (az) - the angle the object can be found along the horizon
2. Elevation (el) - the angle the object can be found above the horizon
These two coordinates are perpendicular to each other just like left-right (az) and up-down (el)
are.
The Milky Way does not lie along or perpendicular to the horizon, so az/el coordinates
are not convenient when describing the position of objects in the Galaxy. This is why
astronomers often use coordinates aligned with plane of the Galaxy; galactic coordinates. These
are specified by:
1. Galactic Longitude (l) - the angle along the plane of the Milky Way.
2. Galactic Latitude (b) - the angle away from the plane of the Milky Way.
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These two coordinates are left-right and up-down
in the Galaxy as seen from the Earth. Now, since
the Milky Way is a pretty much just a flat disk,
most of the objects we observe that are
associated with it lie in the plane, i.e. zero
Galactic latitude (b = 0). So we can ignore the
up-down and get away with only using the leftright longitude coordinate.
Finally, for the three-dimensional space
we live in, these two-dimensional coordinates are
incomplete. The angular coordinates only tell us
which direction; the final coordinate that locates
an object in space (not just on the sky) is its
distance. Astronomical distances are most often
specified in light-years, which are very large.
Galactic Center
III. Setting Up the Model Galactic Coordinate System
Before building your model of the Milky Way Galaxy you must determine the dimensions of
your workspace and the appropriate scale - i.e. feet per light-years - to use in your model; the
entire room you are working in will be filled with the Milky Way model.
1. Begin by measuring the size of your room. Locate and mark the center: this will be the
location of the Galactic Center (you can place those pictures here now).
2. The narrowest width of the room is the widest you can make the model. Record this
width; round down to the nearest meter or foot: _________________________
The Milky Way is approximately 100,000 light-years across (but only about 1,000 light years
thick, hence why we can neglect the up-down dimension). You will need to fit these 100,000
light years into the width you recorded above.
3. Calculate the scale in feet (or inch, meter, centimeter, etc.) per light-year; divide the
width the narrowest width of your room by the width of the Galaxy:
_________________________
This scale will now tell you how to convert a distance given in light-years to the model units
you have chosen. Now, galactic coordinates are not measured from the Galactic Center (GC),
they are measured from the location of the Earth, or rather the Sun (Sol). So you must first place
the Sun in its proper location before you can proceed to locate any other objects.
4. Calculate the distance between the Sun and the GC in the units of your model. Find the
distance to the GC on its picture and multiply it by the scale you determined in step 2:
(distance to GC in light-years) x (scale) = ________________________
5. Place the Sun in its proper position. It doesn’t matter where around the center you
choose, only the correct distance is important (since the Galaxy is a round disk).
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Now there’s only one last step left before we can populate the rest of the galactic disk. The
scale calculated in step 2 allows us to work with the distance coordinate, but we must establish
the angular coordinate as well.
6. Place a large sheet of poster board at the location of the Sun. Make sure to secure it to
the floor with copious amounts of tape; the paper should never move!
7. Place a protractor on the paper so that:
a. the center point of the protractor is located at the exact location of the Sun.
b. the 0-degree line points directly from the Sun to the GC; this is the definition of
zero galactic longitude (l = 0)
8. Use the protractor to draw out a circle around the position of the Sun and mark the angle
at intervals of 5 degrees from 0 to 360; remember 0 points to the GC.
Congratulations, you have successfully established your very own galactic coordinate system
and distance scale! Now you can read off the objects’ galactic longitude and distance
coordinates to place them on the floor and assemble your model. Go ahead and put together your
very own Milky Way.
Galactic
Center
Obscuring
Dust
Center
The
Plane
The
Bulge
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The Milky Way Model
Teacher Guide
Much of the information presented here has been adapted from “The Milky Way Model”, by
R.B. Friedman, AER 7, 176 (2009) (http://aer.noao.edu/cgi-bin/article.pl?id=289). Please see
that article for additional information.
Make certain you visit http://kicp.uchicago.edu/nsta09 to download accompanying resources!
See the attached source list for suggested objects to use in the model.
I. Basic Setup and Supplies
The nice thing about the Milky Way model is that it is rich in information and teachable
concepts, but cheap on resources! In principle, you can do this entire exercise on graph paper or
with Excel and displaying the images on a computer screen or with a projector. However, a
large-scale (e.g. gym sized) model is much more compelling and engaging for students. Here are
some tips for constructing your BIG model:
1. The Milky Way is vast but most of the beautiful objects we have imaged with high
enough resolution to be impressive to use here are relatively nearby and clustered. For
this reason a lot of space is necessary to properly setup and view the model. A
gymnasium floor, parking lot, or playing field is ideal. For example we used the 60-foot
diameter floor in the dome at Yerkes Observatory.
2. To hold up the astronomical images in this model, we attached wooden clothespins to the
ends of ~3 foot wooden dowels using hot glue and cable ties. The images can then be
easily clipped to the dowels as the model is assembled. To use these indoors, we
fashioned a small wooden block base (drilling out a hole for the dowel); modeling clay or
playdoh may also work well. If constructing this model in a field, stick the dowels right
into the ground.
3. To measure out the large distances, just measure the appropriate length of twine, string or
yarn using standard rulers or yardsticks.
4. Print out the images and laminate them! This isn’t cheap, but definitely worth it. Make
sure to either print or hand-write all the important information about each object on the
back of the image. These can be found at: http://kicp.uchicago.edu/nsta09
(left) Students measuring the Galactic Longitude from the center of the coordinate system (Sol) and the
distance with string. (right) Instructor leading a discussion of the model. The dowel, clothespin and block
base display setup should be clearly seen in these photos.
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II. Milky Way Talking Points
The Milky Way model can simply be used as unique way to teach coordinate systems or the
concept of scale, but it can also be used as a springboard for an exploration of astrophysical
concepts. There are a number of important astronomical and physical that this model can be
used to illustrate: 1) coordinate systems, 2) morphology of the Milky Way, 3) distribution of
objects in the Milky Way, 4) the electromagnetic spectrum, 5) the life cycle of stars, etc.
While this isn’t intended to be an extensive review, below are three examples:
ON THE INSIDE LOOKING OUT: The most important thing to get across to students is that we
have (and will) NEVER photograph or observe the Milky Way from outside of it. We are
forever bound within our Galaxy and getting an exterior perspective would take many thousands
of light-years, traveling at the speed of light of course. Everything we know about the Milky
Way has been learned sitting right here on Earth looking through it. Any exterior “image” of the
Milky Way Galaxy is an artist’s conception based on interior data and comparisons with other
galaxies we can see.
THE SPIRAL ARMS: Once the model is constructed, the spiral arm structure of the Milky Way
Galaxy should be visible in the distribution of objects - this was intended to be so. The number
of Milky Way arms is still a hot topic of astronomical research and under some debate. The
current convention is that there are 5: Orion, Perseus, Sagittarius, Scutum-Crux and Norma - in
that order from outside in. The sun is believed to reside in the Sagittarius arm.
THE LIFE CYCLE OF STARS: This is a very rich astrophysical discussion topic that can be nicely
coupled to the Milky Way model. Dividing the Galaxy into 3 components representing stages of
the life cycle - birth, life and death - correctly frames it as a living, evolving, causally connected
system. Furthermore, each of the 3 components discussed below is observed in some signature
electromagnetic spectral range, or band. This is a convenient way to demonstrate the existence
and necessity of multi-frequency observation in astronomy. Broadly speaking, higher energy
phenomena produce higher energy emission both in intensity and spectral properties, i.e. higher
frequency (shorter wavelength) light.
Birth: The first component - stuff that will be stars - is
mainly cold gas in the form of molecular clouds
(composed of molecules) and dust; observed in the radio
to far infrared or as an obstruction to optical radiation.
Examples of these objects are W 44 and W 51, though it
is very important to note that the star forming regions
(second component) are embedded within molecular
clouds. The dark regions surrounding and obscuring
many of the nebulae - such as M16 - are regions of
molecular gas.
Life: The second component - stars themselves - is
represented by star forming regions containing protostars
or open and globular clusters; observed from radio to
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M16
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ultraviolet. There are many examples of star forming regions (most of the nebulae) and star
clusters in the galactic object set. The star cluster M16, embedded (center-left) within the Eagle
Nebula is a particularly nice example. The dark surrounding regions are composed of cold
molecular gas. The dark fingers (popularly known as the Pillars of Creation) protruding into the
nebula are also composed of molecular gas. In both cases the cold gas is being stripped away by
powerful ionizing radiation from the young stars in the cluster
M1
Death: The third and most dramatic component - the
stuff that was once stars - is populated by supernova
remnants, planetary nebulae, neutron stars and black
holes; observed in the visible to x-ray range. The best
representative example in the object set is M1 - the
Crab Nebula. The Crab Nebula is a famous
supernova remnant - the diffuse remains of a star’s
atmosphere - produced by the violent explosive end
of the star’s life; actually observed by Chinese
astronomers in 1054. At the center of the remnant is
the Crab Pulsar - the remains of the star’s interior - a
compact neutron star that emits radio waves in a focused pulsating beam, much like a lighthouse.
III. Suggested Questions
1. If the Milky Way is approximately 1,000 light years wide and 100,000 light years across and
the Earth is located approximately 25,000 light years from the center, what are the
approximate angular dimensions of the Milky Way on the sky?
2. While the extent of the solar system is just barely accessible to humans using space probes, it
is insignificant compared to the scale of the Milky Way. The solar system is approximately
100 AU wide; use your model scale to estimate how big it would be in the Milky Way
model. (1 AU = 1.5 x 1011 meters, 1 light year = 9.5 x 1015 meters). For comparison a
human hair is 10-4 meters wide, a red blood cell is 10-6 meters wide and an atom is 10-11
meters wide (approximately).
3. The objects in the Milky Way model are preferentially clustered about the location of the
Sun/Earth, why? The answer is NOT that the sun is located in some particularly interesting
location, or that the Milky Way has some crazy shape. What kinds of objects are seen
farthest from the sun, in which observations frequencies?
4. Without measuring the distances to any objects in the sky, how might you determine the
relative location of the sun in the Galaxy? What would the sky look like if we were in the
center? What would it look like if we weren’t located in a galaxy?
IV. Recommended Sources for Milky Way Information
If you’d like to learn more about the topics above, consider consulting the following references.
Even if you think you know everything, think again! The astronomical community is constantly
developing a more thorough picture of the Milky Way and new discoveries are made regularly.
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•
•
•
•
•
•
The Wikipedia Milky Way entry is actually quite thorough with great references:
http://en.wikipedia.org/wiki/Milky_Way.
For a history of Galactic astronomy, check out "Coming of Age in the Milky Way" by
Timothy Ferris.
For up-to-date Milky Way news, which is actually rapidly developing, see space.com
(http://www.space.com/milkyway/).
A now outdated but still very cool page dedicated to multi-wavelength observations of
the Milky Way: http://mwmw.gsfc.nasa.gov/
Wikipedia entries on star formation and evolution are also a good place to start:
http://en.wikipedia.org/wiki/Star_formation and
http://en.wikipedia.org/wiki/Stellar_evolution.
For detailed information on galactic and stellar astronomy, consult a general astronomy
textbook; “Universe” by Freedman and Kaufman is an excellent example.
V. Finding More Astronomical Data
Finding the appropriate objects to use - those which best represent the components of the Galaxy
you wish to highlight and have interesting or publicly available images to accompany them - is
no easy task. Here are some resources that may help you:
•
•
•
•
•
•
Hubble Heritage (http://heritage.stsci.edu/):
Hubble is the go to for beautiful
astronomical images. This site features the cream of the crop and is by no means an
exhaustive gallery of Hubble images or a thorough survey of the Milky Way.
Astronomy Picture of the Day (http://antwrp.gsfc.nasa.gov/apod/astropix.html): If you’re
not already checking this with your cup of coffee in the morning, start. Use their index
link at the bottom of the page to search by astronomical object type. The best part is you
get a short astronomy lesson with the images!
Chandra Image Gallery (http://chandra.harvard.edu/photo/): Chandra is to X-ray what
Hubble is to Optical; search for “images by category”.
NRAO Image Gallery (http://images.nrao.edu/): There is no Hubble for radio
observations, but there is the National Radio Astronomy Observatory, which is really a
US association of radio telescopes. They seem to know enough to default to a category
search page.
IPAC Missions Image Gallery
(http://coolcosmos.ipac.caltech.edu/image_galleries/missions_gallery.html): these guys
work on a number of different Infra-Red telescopes and have an extensive collection of
Infra-Red images for you to peruse.
SIMBAD Astronomical Database (http://simbad.u-strasbg.fr/simbad/): come here once
you’ve found some nice images; you can find an extensive collection of astronomical
information for a vast number of objects. You can search for the position (and available
data) by object name: go to SIMBAD, click on queries - by identifier and enter “Crab
Nebula” into the identifier field (as an example). Doing so actually brings up the Crab
pulsar, located in the center of the nebula (see the essential note below the coordinates).
There’s plenty to play with here. You can also launch the Aladin application from here
to view an image of this object.
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Object
Band
Type
Arm
Distance (ly)
Longitude (deg)
Sgr A*
IR
SMBH
Sgr A*
Radio
SMBH
Galactic Center
26,000
0
Galactic Center
26,000
0
Sgr A*
X-Ray
SMBH
Galactic Center
26000
0
M76
M36
Optical
PNe
Perseus
4000
131
Optical
OC
Perseus
5100
175
M38
Optical
OC
Perseus
5300
172
M1
Optical
SNR
Perseus
6500
185
M103
Optical
OC
Perseus
8000
128
NGC 7538
Radio
GMC
Perseus
9100
112
M52
Optical
OC
Perseus
9500
113
Sol
Optical
Star
Orion
0
0
M45
Optical
OC
Orion
380
167
M44
Optical
OC
Orion
577
206
NGC 1333
IR
Reflection Neb
Orion
720
158
M78
Optical
Reflection Neb
Orion
1600
205
M42
Optical
HII
Orion
1600
209
M97
Optical
PNe
Orion
2100
149
M57
Optical
PNe
Orion
2300
63
IC 1396
IR
Emission Neb
Orion
2450
99
IC 1396
IR
Emission Neb
Orion
2450
99
NGC 7129
IR
Reflecction Neb
Orion
3300
105
RCW 38
IR
SFR
Orion
6000
268
M29
Optical
OC
Orion
6000
77
DR 21
IR
SFR
Orion
6200
82
Carina
Optical
SFR
Orion
7500
288
RCW 49
IR
SFR
Orion
13,700
284
M18
Optical
OC
Sagittarius
4900
14
M26
Optical
OC
Sagittarius
5000
24
M20
IR
Diffuse Neb
Sagittarius
5200
7
M8
IR
Diffuse Neb
Sagittarius
5200
6
M20
Optical
Diffuse Neb
Sagittarius
5200
7
M8
Optical
Diffuse Neb
Sagittarius
5200
6
M17
Optical
Diffuse Neb
Sagittarius
5500
15
M16
Optical
OC
Sagittarius
7000
17
M16
Optical
Emission Neb
Sagittarius
7000
17
W 44
Radio
GMC
Sagittarius
9000
35
W 51
Radio
GMC
Sagittarius
22,000
58
M24
Optical
OC
Scutum-Crux
12,000
13
M22
Optical
GC
Scutum-Crux
12,400
10
M55
Optical
GC
Norma
17,600
9
M28
Optical
GC
Norma
18,600
8
S201
Radio
SFR
Unknown
7500
138.2
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Object
Band
Type
Arm
Distance (ly)
Longitude (deg)
S206
Radio
SFR
Unknown
11,400
150.7
W30
Radio
SNR
Unknown
14,700
8.6
K3-35
Radio
PNe
Unknown
16,300
56.1
W50
Radio
SNR
Unknown
17,900
39.7
GS62.1+0.2-18
Radio
Galcatic Shell
Unknown
30,000
62.8
M80
Optical
GC
Unknown
32,000
353
M56
Optical
GC
Unknown
32,900
63
W49A
Radio
GMC
Unknown
37,200
43.2
Object Type Legend:
• SFR - Star Forming Region
• SNR - Supernova Remnant
• PNe - Planetary Nebula
• GC - Globular Cluster (largest group of stars, gravitationally bound together)
• OC - Open Cluster (smaller group of unbound stars)
• GMC - Giant Molecular Cloud
• SMGH - Super Massive Black Hole
• Neb - Nebulae (Reflection, Emission, Diffuse, etc.)
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