Milky Way Galaxy 11

Figure 1 Figure 1. Use this graph to map the distribution of globular clusters in the galaxy. It is important to note that the Sun is at the center of the graph. Note on the scale of thi the same point in the center of the graph.

Question 1: Place an “X” on the graph at the center of the distribution of globular clusters you plotted. You might do this in pencil so you can examine your result and make adjustments. Question 2: | Now using the graph and the position that you marked for “X”, then determine the right ascension (h : min) and distance in kilo-light-years and put your answers in'the boxes below. Remember again that there is 60 minutes in an hour, not 100! Galactic Center Position [Ty 25 Kly Right Ascension (include units) Distance (incluée units) Question 3: Now that you have a graph of the distribution of globular clusters, use Harlow Shapley’s assumption that the center of the Milky Way galaxy is located at the center of the distribution of globular clusters. Do you think that the Sun is in the center of the Milky Way galaxy? Explain how you know using a complete sentence. Figure 2 A Figure 2. An artist’s impression of the Milky Way galaxy as seen from the outside. 68

Question 8: Using your result from Question 2 concerning the distance to the galactic center, and estimating that the distance from the center of the galaxy to the edge is three times the distance from the Sun to the galactic center, determine the diameter of the Milky Way galaxy. Note that you're requested to calculate the diameter, and not the radius, of the Milky Way galaxy. It might help to sketch a picture in the space provided below. Milky Way galaxy 150.000 N Diameter (include ‘(mits) 70

ame: ) o Cado The Sun’s Location in the Milky Way Galaxy A globular cluster is a group of stars roughly in a spherical shape. There are roughly 200 globular clusters in the Milky Way galaxy. : : Objective: In this exercise you will recreate a famous observation made by the astronomer Harlow Shapley. It is important because the observation of the distribution of globular clusters in the sky enabled him, and will enable you, to determine the location of the Sun in the galaxy. The essential assumption is made (shown to be correct) that the globular clusters are distributed around in the halo of the Milky Way galaxy. Nearly one hundred years after Shapley’s work we know that at the center of this halo is the galactic nucleus, and that in the center of the galactic nucleus is a super massive black hole. Harlow Shapley assumed that if he could find the center of the distribution of globular clusters in the galaxy he would find the center of the galaxy. In order to do this he needed to determine the direction and the distance to each globular cluster. You will find this information in Table 1. Process: Table 1 lists the right ascension, declination and the distance for a sample of globular clusters. You will plot these data on a graph (Figure 1) called a polar graph. It is strongly advised that the student plots points on the graph using a pencil so that mistakes can be more easily corrected. With the polar graph we can plot the distance of each globular cluster measured from the center of the graph in the direction specified by the right ascension of the cluster. The right ascension is plotted around the graph in a similar way as a clock hand moves around a clock. In fact, we will use time as a coordinate system starting at the top of the graph in Figure 1. The polar graph is the sky coordinate system analogous to longitude and latitude on Earth. Right Ascension may be measured from the top at 0 hours clockwise to 24 hours. The given graph allows you to record hours and minutes of Right Ascension around the graph with 12 hours at the bottom and 24 hours again at the top. The distance to each globular cluster is then plotted by using the circles moving outward from the center of Figure 1. Note the Sun is located at the center of Figure 1. The unit of distance is kilo- light-years (thousands of light-years). A light-year is the distance light travels in one year. You will plot right ascension in hours vs. the distance in kilo-light-years on Figure 1. The data is located in Table 1. Notice that the graph will be in terms of right ascension and distance only, so the declination is not plotted on the graph. 65