Name Date Class 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. 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. 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: 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 9/9/2012 1
the top of the screen. Count the number of stars on the screen. Enter the value in the table below next to 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 Milky Way 1 Star Count Milky Way 2 Milky Way 3 Milky Way 4 Milky Way 5 North Galactic Pole South Galactic Pole What is the average star count for your five Milky Way locations? Is this average star count along the Milky Way greater than, less than or equal to the counts at the NGP and the SGP? Are stars distributed randomly in our Milky Way Galaxy? How are they distributed? 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. 9/9/2012 2
Use Stellarium's Search function to find each of the Globular Clusters in Table 1 below. Once you have found each cluster, record its Right Ascension and Declination in the table. ROUND EACH MEASUREMENT TO THE NEAREST DEGREE OR HALF-HOUR. Table 1 - Globular Clusters in the Sky Cluster RA Dec Cluster RA Dec M2 M3 M4 M5 M9 M10 M12 M13 M14 M15 M19 M22 M28 M30 M53 M55 M56 M62 M68 M69 M70 M71 M72 M75 M79 M80 M92 M107 NGC288 NGC362 NGC1261 NGC1851 NGC2298 NGC2419 NGC2808 NGC3201 NGC4147 NGC4372 NGC4833 NGC5053 NGC5286 NGC6229 NGC6397 NGC6712 Now plot, on the next page, the position of each cluster on the graph with a dot: The x-axis is Right Ascension, and the y-axis is Declination: 9/9/2012 3
Globular Cluster Location in the Sky 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 on this map? Now look at the graph you've plotted. Remember, each dot shows the position of one of the Globular Clusters. Are the globular clusters you plotted evenly distributed around the sky? What can you conclude about our Solar System s position within the Milky Way 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!) 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. What constellation appears to be at the center of the globular cluster distribution? 9/9/2012 4
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 9/9/2012 5