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Assignment #12 – The Milky Way
For thousands of years people assumed that the stars they saw at night were the entire universe.
Even after telescopes had been invented, the concept of a “galaxy” was a difficult one for people to grasp.
The idea that we live in one of billions of “island universes” was so mind-blowing that it took many
decades to be accepted. Understanding first that we live in something called the Milky Way Galaxy, and
then that there billions of other galaxies like ours in the universe was a huge task!
PART A
The first step to understanding our universe is noticing that the stars are not uniformly distributed
across the sky. There are certain areas of the sky that have more stars than others. Let's see if we can see
where that part of the sky is located.
Start Stellarium. Turn off the Atmosphere and Fog. Zoom out and move the ground around
until you can see the whole sky. Display only the stars – no star names, no planets, no constellations or
constellation names, and no nebulae. Also, in the View Window, under the Stars section on the left,
change the Absolute scale to 1.5 and the Relative scale to 0.35. This makes all the stars appear to be the
roughly the same size, as they really do in the sky. Finally, change the Date & Time to July 6, 2006, at
Midnight. You are looking at the sky as it would appear in a VERY dark location in the middle of
summer at Midnight.
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Do the stars look like they are evenly distributed throughout the sky, or do they look like there are
more stars in one part of the sky than another? _______________________________________
Since the beginning of time, people have noticed that there is a faint, milky-white band of light
that spreads across the sky. Do you see the Milky Way in the sky? It's difficult to see, both in real life,
and in Stellarium, unless you are looking in very dark conditions. Try turning off the lights in the room
where you're doing this assignment, if possible.
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Does the Milky Way correspond to the part of the sky where you saw more stars? ___________
The band across the sky where you see many more stars corresponds to the Milky Way. You can
see this band in the sky on a dark night away from city lights.
Display the constellations (and their labels) and list the constellations that the Milky Way passes
through – be sure to let time pass so that you catch all the constellations through the which the Milky way
passes:
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Now let's compare the number of stars along the Milky Way with the number far away from it, to
see if the stars are evenly distributed throughout the sky.
Click anywhere along the Milky Way, and center on that spot by pressing the Space bar. Zoom
in until the Field of View is 1°. Remember, the Field of View (FOV) is listed in the Information Bar at
the top of the screen. Count the number of stars on the screen. Enter the value in the table below next to
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“Milky Way 1.” Now zoom back out and click on a different random spot along the Milky Way and do
the same thing. Enter this star count as “Milky Way 2.” Do the same for a total of five random Milky
Way star counts.
Now click on an area in the sky as far from the Milky Way as possible. These places in the sky
are called the North and South Galactic Poles. Again, zoom in to a 1° field of view and count the stars at
the North Galactic Pole. Enter this Star Count in the table. Do the same for the South Galactic Pole.
Location
Star Count
Milky Way 1
Milky Way 2
Milky Way 3
Milky Way 4
Milky Way 5
North Galactic Pole
South Galactic Pole
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What is the average star count for your five Milky Way locations? _______________________
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Is this average star count along the Milky Way greater than, less than or equal to the counts at the
NGP and the SGP? _________________________________
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What does this answer lead you to believe about the way stars are distributed in the Milky Way
galaxy? _____________________________________________________________________
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A good guess would be that this Milky Way is the light from the billions of stars that make up our
home galaxy, also called the Milky Way. Since this light is brighter along one smooth band in the sky,
we might guess that the Milky Way itself is not spherically symmetrical. In fact, the only shape for our
Milky Way that is consistent with what we see in the sky is that of a flat disk, so we can assume the
Milky Way is a flat disk. But where are we in that disk? At the center? Near the edge? Let's try and find
out.
PART B
Astronomers believe that gigantic spherical clusters of stars, called Globular Clusters, each with
hundreds of thousands of stars, formed evenly around the center of the Milky Way many billions of years
ago. These globular clusters are so large and bright that they can be seen from very far way – much
farther away than individual stars would be visible. By looking at the way these globular clusters are
scattered around the sky, we should be able to determine where the center of the Milky Way galaxy is.
Use Stellarium's Search function to find each of the Globular Clusters in Table 1 on the next
page. Once you have found each cluster, record its Right Ascension and Declination in the table.
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Table 1 - Globular Clusters in the Sky
Cluster
RA
Dec
Cluster
M2
M72
M3
M75
M4
M79
M5
M80
M9
M92
M10
M107
M12
NGC288
M13
NGC362
M14
NGC1261
M15
NGC1851
M19
NGC2298
M22
NGC2419
M28
NGC2808
M30
NGC3201
M53
NGC4147
M55
NGC4372
M56
NGC4833
M62
NGC5053
M68
NGC5286
M69
NGC6229
M70
NGC6397
M71
NGC6712
RA
Dec
Now plot, with a dot, the position of each cluster on the following graph: The x-axis is Right
Ascension, and the y-axis is Declination:
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If we were at the center of the Milky Way galaxy, and the globular clusters were distributed
randomly throughout our Milky Way, what would the distribution of globular clusters look like in
the sky to us? _______________________________________________________
Now look at the graph you've plotted. Remember, each dot shows the position of one of the
Globular Clusters.
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Are the globular clusters you plotted evenly distributed around the sky? ___________________
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What can you conclude from the answer to the previous two questions? (Remember, we are
assuming that the Globular Clusters are indeed spread out evenly throughout the Milky Way!)
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What is the approximate Right Ascension and Declination of the “center” of the distribution of
Globular Clusters on your graph? In other words, where are the clusters most densely packed
together? RA ________________ Dec _____________________
Now look at the sky in Stellarium. Let time pass until you can see the spot where the globular
clusters are most closely packed. You may want to turn on the Equatorial Grid by pressing the E key to
help see the RA and Dec addresses in the sky.
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What constellation appears to be at the center of the globular cluster distribution? ____________
The Milky Way is a flat disk of stars, with a bulge at the center, and we are not at the center of it!
The center of the Milky Way is toward the constellation you just named.
Discerning the shape and structure of a galaxy without ever leaving it is a complex, tricky job.
In the case of the Milky Way, years of painstaking measurement and analysis have determined that we
live about 2/3 of the way out from the center of a barred spiral galaxy that is quite flat, is surrounded by a
symmetrical halo of globular clusters, and turns out to be only one of billions of other galaxies.
Write a brief conclusion explaining what you learned in this exercise ______________________
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