Physics 5
Introduction to Astronomy

H. E. Smith                                                                                                       Spring 2007

Quiz 7 Answer Key

Physics 5 – Spring Quarter 2007

Prof. Smith

 

1. How was Edwin Hubble able to use his discovery of a Cepheid in Andromeda to prove that the "spiral nebulae" were actually entire galaxies?

A. He used main-sequence fitting to determine the distance to Andromeda and show that it was far outside the Milky Way Galaxy.

B. From the period-luminosity relation for Cepheids, he was able to determine the distance to Andromeda and show that it was far outside the Milky Way Galaxy.

C. He measured the stellar parallax of the Cepheid in Andromeda, was able to determine the distance to it, and showed that it was far outside the Milky Way Galaxy.

D. There are no Cepheids in the Milky Way, so his discovery proved that it had to be in another galaxy.

E. Since a Cepheid is a type of luminous galaxy, when he found it in Andromeda he was able to prove that Andromeda was a separate galaxy from the Milky Way.

 

2. Dr. X believes that the Hubble constant is H0 = 20 km/s/Mly. Dr. Y believes it is H0 = 24 km/s/Mly. Which statement below automatically follows?

A. Dr. X believes that the universe is expanding, but Dr. Y does not.

B. Dr. X believes that the universe is older than Dr. Y believes.

C. Dr. X believes that the universe has a much higher density than Dr. Y believes.

D. Dr. X believes that the Andromeda Galaxy (a member of our Local Group) is moving away from us at a slower speed than Dr. Y believes.

E. Dr. X believes that the universe will someday stop expanding, while Dr. Y believes it will expand forever.

 

3. I observe a galaxy that is 100 million light years away: what do I see?

A. the light from the galaxy as it is today, but it is blueshifted

B. the light from the galaxy as it was 100 million years ago and it is redshifted

C. the light from the galaxy as it is today, but it is redshifted

D. the light from the galaxy as it was 100 million years ago and it it blueshifted

E. Nothing: the galaxy lies beyond the cosmological horizon.

 

4. Why should galaxy collisions have been more common in the past than they are today?

A. Galaxies were much bigger in the past since they had not contracted completely.

B. Galaxy collisions shouldn't have been more common in the past than they are now.

C. Galaxies attracted each other more strongly in the past because they were more massive; they had not yet turned most of their mass into stars and light.

D. Galaxies were closer together in the past because the universe was smaller.

E. Galaxies were more active in the past and therefore would have collided with each other more frequently.

NOTE: D was option E on your quiz - there was a numbering mistake.

 

5. Does Hubble's law work well for galaxies in the Local Group? Why or why not?

A. Yes, it works every so well that we have never detected any measurable deviations from its predictions.

B. No, because we do not know the precise value of Hubble's constant.

C. No, because Hubble did not know the Local Group existed when he discovered his law.

D. No, because galaxies in the Local Group are gravitationally bound together.

 

6. What is the most accurate way to determine the distance to a very distant irregular galaxy?

A. using Cepheid variables

B. Hubble's law

C. the Tully-Fisher relation

D. using a white-dwarf supernova as a standard candle

E. main-sequence fitting

 

7. How do we know that there are insignificant (little to none) amounts of dark matter in the solar system? You may have to think about this (and why we think there is dark matter in galaxies). Explain your reasoning!

We can measure the distribution of mass in the solar system through analysis of the rotation curve, i.e. the velocity at which planets at different distances from the Sun rotate, in an analogous way to the rotation curves of galaxies. Since the rotation curve of the solar system decreases with increasing distance in the same way that we would predict for a central mass (the Sun), we infer that there are no ÒhiddenÓ sources of gravitational mass in the solar system. Dark Matter must be distributed over much larger scales.