Questions for Chapter 20

1. What is this? This is a spectrum of one of the galaxies. What’s up?

2. What is a galaxy? Think of this in general terms, especially what you would include and what you would exclude.

3. Why do we need to understand the evolution of the universe in order to understand the lives of galaxies?

4. What are the three major types of galaxies, and how do they look different relative to one another?

5. Describe the differences between normal spiral galaxies, barred spiral galaxies, and lenticular galaxies.

6. Classify the following galaxies? Justify your answers.

a. Galaxy NGC 4594 b. Galaxy NGC 6744 c. Galaxy NGC 4414 d. Galaxy NGC 1300 e. Galaxy M87

7. Distinguish between the disk component and the spheroidal component of a spiral galaxy. Which component includes cool gas and active star formation?

8. What is the major difference between spiral and elliptical galaxies? Answer in terms of the presence or absence of disk and spheroidal components. How does this difference explain the lack of hot, young stars in elliptical galaxies?

9. In which type of galaxy would you be most likely to observe a massive star supernova: in a giant elliptical galaxy or in a large spiral galaxy? Explain your reasoning.

10. How are galaxy types different in clusters of galaxies than among smaller groups and isolated galaxies?

11. What do we mean by a standard candle? Explain how we can use standard candles to measure distances.

12. Summarize each of the major links in the distance chain. Why are Cepheid variable stars so important? Why are white dwarf supernovae so useful, even though they are quite rare?

13. Explain how Hubble used Cepheid variable stars to prove that the Andromeda Galaxy lies beyond the bounds of the Milky Way.

14. There are several ways to measure distances to clusters in our galaxy and distances to other galaxies. How does each work? What are the pros and cons of each

15. What is Hubble's law? What is Hubble's constant? Explain what we mean when we say that Hubble's constant is between 20 and 24 kilometers per second per million light-years.

16. What is the Cosmological Principle, and how is it important to our understanding of the universe?

17. What do we mean by the lookback time to a distant galaxy? Briefly explain why lookback times are less ambiguous than distances when discussing objects very far away.

18. What is the cosmological horizon, and what determines how far away it lies?

19. What do we mean by a cosmological redshift?

Decide whether the statement makes sense (or is clearly true) or does not make sense (or is clearly false). Explain clearly; not all these have definitive answers, so your explanation is more important than your chosen answer.

20. If you want to find a lot of elliptical galaxies, you'll have better luck looking in clusters of galaxies than elsewhere in the universe.

21. Cepheid variables make good standard candles because they all have exactly the same luminosity.

22. If the standard candles you are using are less luminous than you think they are, then the distances you determine from them will be too small.

23. Galaxy A is moving away from me twice as fast as galaxy B. That probably means it's twice as far away.

24. After measuring the galaxy's redshift, I used Hubble's law to estimate its distance.

25. The center of the universe is more crowded with galaxies than any other place in the universe.

26. I'd love to live in one of the galaxies near our cosmological horizon, because I want to see the black void into which the universe is expanding.

27. If someone in a galaxy with a lookback time of 4.6 billion years had a super powerful telescope, they could see our solar system in the process of its formation.

28. We can't see galaxies beyond the cosmological horizon because they are moving away from us faster than the speed of light.