Download Where in the Milky Way Are We

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Sp10
Lab Report for Lab #9: Where in the Milky Way Are We?
Part I: The shape of the Milky Way
1. Look up the morphological classification of the Milky Way. There is a bit more to it
than “spiral”. Look for information on morphological classification in text books and/or
online. Find out the shape of the Milky Way galaxy.
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2. Identify the parts the Milky Way. Some possible parts of a galaxy are: bulge, disk, halo,
bar, or arm. Knowing the type of galaxy the Milky Way is, you should be able to identify
the types of features the Milky Way has. Create a physical model of the Milky Way and
identify the features it has. (A globe is an example of a physical model of Earth.)
Describe your model and label all possible parts.
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3. Use your planetarium software to observe the Milky Way from the northern and
southern hemispheres. Based on your observations (without the aid of telescopes or
other wavelengths of light outside the visible range) explain how these observations
support the conclusion that we live in a galaxy with the shape of your model. Further,
determine in what part of the galaxy the Sun must be located to see the Milky Way as we
do.
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Part II: Mapping the Milky Way with Globular Clusters
Galaxies like the Milky Way have objects, called “globular clusters” that exist in the outer
regions of the galaxy (or halo). Globular clusters are dense clusters of stars that are generally
free of gas and dust. In galaxies like the Milky Way, the gas and dust are located primarily in
the disk. If we want to look for objects that we can see clearly (with no interference from gas
and dust) we must observe objects in the halo. In the late 19th century an astronomer named
Harlow Shapley devised an experiment to determine the position of Earth in the universe
using globular clusters. He chose these objects because of their apparent lack of interaction
with the gas and dust in the Milky Way.
Step 4. Explain how Shapley’s experiment could be used to determine the position of
Earth within the Milky Way. Given what you already know about the part of the galaxy
in which the Sun is located and the information about globular clusters, what information
would Shapley need to figure out the position of Earth? For about how many globular
clusters do you think he would need to get this information to determine the position of
Earth? Describe the experiment and list the steps one would need to take.
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Step 5. As a group, devise a method for determining the position of Earth within the
Milky Way using globular clusters. Decide how many globular clusters you want to
observe. Decide what information you need to do the analysis. There may be many
different ways of analyzing the data, there does not need to be group consensus for the
analysis, but each method that is used should be discussed with the group.
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6. Gather the data you need from sources on the internet. You may use planetarium
software to aid in the identification of globular clusters. Share the responsibility for
gathering data with every classmate (No one should be exempt from gathering data.)
Each individual class member should analyze the data set on their own, but gathering the
data can be done much more quickly if everyone gets the data for a number of globular
clusters, and then the data are shared. In fact, this is often how data are gathered in
astronomy since no one individual can get all the data they need all the time, and
certainly not all at once.
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7. Analyze the data. Use the entire data set gathered by the class and do the analysis you
choose. Determine the position of Earth within the Milky Way. You should be able to
determine distance from the center of the galaxy and angular position from some
reference point (a measure similar to longitude on the surface of Earth which is relative to
the prime meridian).
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Step 8. Create a scientific “poster presentation” via whole class discussion board. Your
final product will be a discussion board presentation. You will need to outline the
procedures you used to gather information (and data), display (upload) or describe your
physical model, show your data, describe your analysis process and present your result.
Compare your results with known values (use a textbook or some other reliable resource
to check your result). Explain any differences between your result and the accepted
values in terms of the data you used, the analysis you used and the assumptions you
made.
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Notes:
Students will gather RA and Dec for galactic coordinate data as well as distances from Earth
for a number (at least 25) of globular clusters. These data will be plotted on an x-y or polar
graph (using Excel or some other graphing software or using graph paper). Earth’s position
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is at the center of the graphic. The center of the Milky Way will be the center of the
distribution of globular clusters plotted.
The following website gives the RA and Dec along with distance from Sun and from the
galactic center: http://seds.org/~spider/spider/mwgc/mwgc.html.
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