Lab 7 Finshed

ASTR 264 Name _________ Lab Activity 7 : The Milky Way Galaxy 1. Investigation of mass in the Milky Way. The tables below list orbital speeds and distances for objects in our solar system and our galaxy. a. Plot the data on the graphs on the last page of the activity and connect the dots. Planet Distance from Sun (AU) Orbital Speed (km/s) Mercury 0.4 48 Earth 1.0 29 Jupiter 5.2 13 Saturn 9.5 9 Uranus 19 7 Neptune 30 5 Star or Gas Cloud Distance from center of Milky Way (10 3 ly) Orbital Speed (km/s) 1 5 110 2 10 250 3 20 200 Sun 28 220 4 40 240 5 50 235 6 60 240 b. Describe some differences between the general shapes of the two graphs. Do not worry about how wiggly your lines are – just focus on the overall shapes (as you go to larger distances, do the speeds of the objects rise or fall? Quickly or gradually? Fall off or plateau? Etc.). The first graph which is for the planets show that as you go to larger distances, the orbital speed decreases or it falls quickly based on the graph. On the other hand, for the stars, there is a rise, then fall, and plateau of the line. From the first to second star, there is an observed quick increase in orbital speed, from second to third, there is a decrease in the speed, from Sun to the fourth one there is a slight increase, and the fourth to sixth stars have the same speed. c. The distribution of mass determines how fast objects will move depending on their distances (i.e., how they orbit). In the solar system , where is most of the mass concentrated? (Mass distributed in this manner means objects will move according to Keplerian orbits.) In the solar system most of the mass is concentrated in our Sun, almost 99.8 to 99.9 percent. The rest of it account for the planets and other heavenly bodies in the solar system. d. Do the stars/gas clouds in the galaxy follow Keplerian orbits like the planets in the solar system do? What does this tell us about where most of the mass is concentrated in the galaxy? The stars/gas clouds in the galaxy do not follow Keplerian orbits like the planets in the solar system. This shows that the mass in the galaxy is not concentrated on its center unlike with the solar system. In the solar system, the concentration of the mass is at the center, Sun. It also shows that the more mass might be found far from the center of the galaxy.

ASTR 264 Act 13 e. What is the explanation for this mass that makes up most of the matter in the galaxy? This is where the concept of dark matter comes. Most of the mass in our galaxy is invisible. They cannot be seen directly or cannot be detected by looking or observing the electromagnetic radiation. Scientists try to discern its existence by observing its effects on its neighboring objects that are observable. 3. a. How are the orbits of the halo/bulge stars different from the orbits of disk stars? Halo/bulge stars are older stars that are found in the central bulge and the surrounding halo of the galaxy. These stars have more random and elliptical orbits because they formed during a time when the galaxy was less structured. On the other hand, disk stars are younger stars that are found in the flat disk of the galaxy. These stars have more circular orbits because they formed after the galaxy had settled into a disk-like structure. b. What does this tell us about how the galaxy formed? (Which formed first, etc.?) Knowing the difference in the orbits of these stars provides clues about the formation and evolution of the galaxy. The random and elliptical orbits of the halo/bulge stars suggest that they formed first, during a time when the galaxy was more chaotic. As the galaxy evolved and settled into a disk, the gravitational forces became more ordered, leading to the formation of newer stars with more circular orbits. This suggests that the disk of the galaxy formed later. 4. a. In what part of the star-gas-star cycle are iron atoms formed? Iron atoms are formed during the supernova stage of the star-gas-star cycle. b. In what part of the star-gas-star cycle is most of the gas in the galaxy found? Most of the gas in the galaxy is found in the interstellar medium stage of the star-gas-star cycle. c. What types of objects do we most commonly find in the galaxy’s spiral arms? What part of the star-gas-star cycle do they indicate? We most commonly find young stars and gas clouds in the galaxy’s spiral arms. This indicates the star formation part of the star-gas-star cycle. d. How are the spiral arms maintained? The spiral arms are maintained through a process known as density wave theory.

ASTR 264 Act 13