Astronomy HOMEWORK Chapter 15

1. Where in the Galaxy is the solar system located? a. in the nucleus; b. in the halo; c. in a spiral arm; d. between two spiral arms; e. in the central bulge. d. between two spiral arms. Also, 26,000 LY from the center.

2. What is located in the nucleus of the Galaxy? a. a globular cluster; b. a spiral arm; c. a black hole; d. the solar system; e. a MACHO. c. a black hole. Of 4 million M⊙.

3. Which statement about the Milky Way Galaxy is correct? a. Our Galaxy is but one of many galaxies. b. Our Galaxy contains all stars in the universe. c. All stars in our Galaxy take the same time to complete one orbit. d. Most stars in our Galaxy are in the central bulge. e. None of the stars in our Galaxy move. a. Our Galaxy is but one of many [billions] of galaxies in the universe.

5. How did Edwin Hubble prove that M31 is not a nebula in our Milky Way Galaxy? M31 is the Andromeda Galaxy, called “Andromeda Nebula” at the time. Hubble identified several stars in M31 as Cepheid variables, and measured their pulsation periods and apparent brightness. He then used the known relationship between period and luminosity of Cepheid variables to determine their luminosity. Finally, he used the inverse square law to obtain their distance. Their distance put them many times farther away than the size of our Galaxy, which Shapley had determined in 1915. If they are farther away than the size of our Galaxy, they have to be outside it.

7. What observations led Harlow Shapley to conclude that we are not at the center of the Galaxy? Shapley noted that most globular clusters were in just a few contellations around Sagittarius. He obtained distances by the apparent brightness of RR Lyrae variables (similar to Cepheid variables) and plotted the positions of these globular clusters. They formed a spherical distribution about a point thousands of LY away in the direction of Sagittarius. He correctly reasoned that the globular clusters would form a roughly spherical distribution about the center of the Galaxy.

10. Why do astronomers believe that vast quantities of dark matter surround [and pervade] our Galaxy? First, the dark matter pervades our Galaxy as well as surrounding it. The orbital speeds of stars are too high to be explained by the mass of stars, gas, and dust inside their orbits. In particular, as one goes into the extreme outskirts, orbital speeds continue to climb. Kepler’s third law would require them to go down, if there were not some large mass density in these extreme outskirts. The star/gas/dust density in these extreme outskirts is negligible compared to the mass required to have orbital speed increasing. So, there must be some dark matter.

12. Why are there no massive O or B stars in globular clusters?

Stars of spectral type O or B are high mass, 10 M⊙ and up. Such stars only live 25 million years, or less. Globular clusters are billions of years old, so any O or B stars that formed originally have long finished their life cycles. Furthermore, no stars are forming in globular clusters, nor have any stars formed for billions of years.

13. What evidence indicates that a supermassive black hole is located at the center of our Galaxy? Stars near the center of our Galaxy move in large elliptical orbits at high speeds about a single point in space. Application of Kepler’s third law shows that the mass inside a very small region around this 6 point is 4 × 10 M⊙, which can only be a supermassive black hole. Observations over several years using infrared light established this about 1999. Further observations have confirmed and refined the initial conclusions. 14. Approximately how many times has the solar system orbited the center of the Galaxy since the Sun and planets were formed? The question is, how many orbits are completed in in 4.6 × 109 y if each orbit takes 2.3 × 108 y. Number of Orbits = (4.6 × 109 y)/(2.3 × 108 y/orbit) = 20 orbits.