07Lab_The-Milky-Way

M ONTGOMERY C OLLEGE – R OCKVILLE A STRONOMY 101 ASTR101 Laboratory 7 - The Milky Way 1 Name: 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. (The sky should be dark.) Zoom out and move the ground around until you can see the whole sky. We want to display only the stars- no star names, no planets, no constellations or constellation names, and no nebulae.  Open the Sky and viewing options menu. In the Sky submenu, under the Stars section on the right, uncheck the Labels and Markers box.  In the Sky submenu, under the Stars section on the right, 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.  In the SSO submenu, uncheck the Solar System objects box.  In the Starlore submenu, uncheck all boxes under Options .  Close the menu and change the Date & Time to July 6, 2021 , 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? It looks like the sky has more starts in one part of the sky specifically around the south 1 Last edit Spring 2022. 1

ASTR101 L ABORATORY 7 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? Yes, it does 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 below at least 8 of the constellations that the Milky Way passes through. You may want to let time pass so that you catch all the constellations through which the Milky Way passes: - Cygnus - Sagittarius - Scutum - Cassiopeia - Sagitta - Aquilla - Scorpius 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.  Use the View menu and the Markings submenu to enable the Galactic Grid and Galactic Equator .  Now click anywhere along the disk of 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 bottom of the screen.  Count the number of stars on the screen. Enter the value in the Table 1 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 near North Galactic Pole (NGP). The NGP is toward the constellation Coma Berenice s. 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 (SGP). The SGP is toward the constellation Sculptor . 2

ASTR101 L ABORATORY 7 scattered around the sky, we should be able to determine where the center of the Milky Way galaxy is.  Turn on the constellation boundaries and labels if they aren’t on already.  Use Stellarium's Search function to find each of the Globular Clusters in Table 2 below. Record the constellation in which each cluster is located.  At the same time, or after you complete the table, mark the location of each cluster on the star map with a small circle. It may be helpful to use a colored pencil or pen. You do not have to get the location exactly, but the clusters need to be in the correct boundaries. For example, if you have 5 clusters in Sagittarius, you should have 5 circles in the Sagittarius boundary. Table 2 Cluster Constellation Cluster Constellation M2 Aquarius M72 Aquarius M3 Canes Venatici M75 Sagittarius M4 Scorpius M79 Lepus M5 Serpens M80 Scorpius M9 Ophiuchus M92 Hercules M10 Ophiuchus M107 Ophiuchus M12 Ophiuchus NGC28 8 Sculptor M13 Hercules NGC36 2 Tucana M14 Ophiuchus NGC12 61 Horologium M15 Pegasus NGC18 51 Columba M19 Ophiuchus NGC22 98 Puppis M22 Sagittarius NGC24 19 Lynx M28 Sagittarius NGC28 08 Carina M30 Capricornus NGC32 01 Vela M53 Coma Berenices NGC41 47 Coma Berenices M55 Sagittarius NGC43 72 Musca M56 Lyra NGC48 33 Musca M62 Ophiuchus NGC50 Coma Berenices 4

ASTR101 L ABORATORY 7 53 M68 Hydra NGC52 86 Centaurus M69 Sagittarius NGC62 29 Hercules M70 Sagittarius NGC63 97 Ara M71 Sagitta NGC67 12 Scutum 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 your star map? The dots would be scattered all over the place instead of being just near the right side. Now look at the clusters you've plotted on the star map. Are the globular clusters you plotted evenly distributed around the sky? No 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!) We can conclude that we are not in the center of the Milky Way What is the approximate “center” of the distribution of Globular Clusters on your graph? (In other words, what constellation appears to be at the center of the globular cluster distribution?) Because there are so many clusters near Ophiuchus and Sagittarius, the center seems to be there. 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. 5

ASTR101 L ABORATORY 7 7