Science Lab – $10 – Shapely

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Where in the Milky Way Are We?

Description:

In the 21st century, most of us are aware that Earth is one of several planets in our Solar System, which is a system of planets that orbit a star we call the Sun. The Sun is one of several hundred billion stars in our Galaxy, which we call the Milky Way, but where in the Milky Way are we?

At night, all the stars you see are part of the Milky Way. You may also know that the Milky Way is a spiral galaxy. How could we find out where Earth and the solar system are located within this spiral galaxy? This exercise gives students the tools to design and execute an experiment to determine our location within the Milky Way.

Introduction:

Figuring out where we are in the universe is an important part of understanding why we observe what we do. From the beginning of recorded history, humans have tried to determine their place in the universe. The ancient Mayans thought that the universe consisted of Earth, the Sun, the Moon, the planets (Venus and Mercury) and the stars. Of course, Earth was at the center of their universe because it did not appear to move. Separate from Earth were the Sun, the Moon, Venus, Mercury and the stars.

In the beginning of the 20th century, astronomers began to realize that the universe was much bigger than previously thought. This is when astronomers began to understand that we live in an enormous galaxy that is just one of billions of other galaxies in the universe. Until this realization, models of the universe did not go very far beyond the solar system. Most models assumed that outside the solar system there were stars. The rest of the universe was just more stars.

Now we know that stars exist primarily within structures known as galaxies. Galaxies contain stars (from tens of millions in dwarf galaxies to several trillions in the largest galaxies), gas and dust. Our Sun is just one of several hundred billion stars in the Milky Way galaxy. But, like a tree in the forest, it is hard to tell where we are. Given our history as humans, we would probably be satisfied with the premise that the Sun must be at the center of the Milky Way. But, there is observational evidence to the contrary.

In this exercise, you will determine the (three-dimensional) shape of the Galaxy and our position within it (or at least, relative to its center).

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.

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.)

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.

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.

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.

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.

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).

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.

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 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

Sp10
Name: ________________________________

Lab Report for Lab #9: Where in the Milky Way Are We?

Part I: The shape of the Milky Way

[Type answer here]

[Type answers here]

Part II: Mapping the Milky Way with Globular Clusters

[Type answers here]

Notes:

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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