1. These questions concern the size and structure of the Milky Way, our home galaxy.
a) Explain why astronomers at first failed to realize the true size of the Milky Way galaxy. Why did they only begin to understand its size when they estimated distances to globular clusters?
b) In science fiction stories people often live on planets at the "rim" or extreme edge of the Milky Way's stellar disk. Suppose you were transported to such a planet located 7000 parsecs from the Sun, in the direction opposite to the center of the Galaxy. Would you expect many of the stars that we see from Earth still to be visible to your unaided eye? Would you expect to see a difference when you looked at the Milky Way in the sky? Explain why in both cases.
c) The Milky Way has two major "dark" mass components that emit little or no light. One of these is the "super massive black hole" (SMBH in astro-speak) in the Galactic center. The other is the dark matter halo. Explain how we know of the existence of these two components of our Galaxy even though they are not detectable in optical light.
2. Now let's turn to properties of galaxies beyond the Milky Way:
a) How are Cepheid variable stars used as "standard candles" to determine distances to galaxies? Explain how the recognition of this property of Cepheid variables led to the discovery that galaxies are huge stellar systems located outside of the Milky Way.
b) Describe at least two ways by which the structures of spiral and elliptical types of galaxies differ from one another.
c) Which kinds of galaxies, spiral or elliptical, show the most direct evidence for active star formation? Recently we have found that a large fraction of the stars in the universe are located in elliptical and S0 types of galaxies. What is this telling us about the evolution of elliptical galaxies?
3. Although black holes behave in strange ways, it is clear that they can exist in nature.
a) Black holes have been described as holes in spacetime where nature can exile material away from the normal universe. Do you agree with this summary? Explain your answer in a few sentences.
b) Astronomers traditionally found candidates for black holes formed in the supernova explosions of massive stars by studying binary stars. What did they want to measure that could distinguish a black hole from a neutron star? Briefly explain your answer.
c) Suppose you were able to orbit a black hole from a safe distance but are sufficiently close to see details in the region around the black hole. As you observe the black hole projected against a distant field of stars you will seedistortions. Explain why the background star fields have an abnormal pattern when observed close to a black hole.