Chapter 26 Galaxies

26.8 Questions and Exercises

Review Questions

1: Describe the main distinguishing features of spiral, elliptical, and irregular galaxies.

2: Why did it take so long for the existence of other galaxies to be established?

3: Explain what the mass-to-light ratio is and why it is smaller in spiral galaxies with regions of star formation than in elliptical galaxies.

4: If we now realize dwarf ellipticals are the most common type of galaxy, why did they escape our notice for so long?

5: What are the two best ways to measure the distance to a nearby spiral galaxy, and how would it be measured?

6: What are the two best ways to measure the distance to a distant, isolated spiral galaxy, and how would it be measured?

7: Why is Hubble’s law considered one of the most important discoveries in the history of astronomy?

8: What does it mean to say that the universe is expanding? What is expanding? For example, is your astronomy classroom expanding? Is the solar system? Why or why not?

9: Was Hubble’s original estimate of the distance to the Andromeda galaxy correct? Explain.

10: Does an elliptical galaxy rotate like a spiral galaxy? Explain.

11: Why does the disk of a spiral galaxy appear dark when viewed edge on?

12: What causes the largest mass-to-light ratio: gas and dust, dark matter, or stars that have burnt out?

13: What is the most useful standard bulb method for determining distances to galaxies?

14: When comparing two isolated spiral galaxies that have the same apparent brightness, but rotate at different rates, what can you say about their relative luminosity?

15: If all distant galaxies are expanding away from us, does this mean we’re at the center of the universe?

16: Is the Hubble constant actually constant?

Thought Questions

17: Where might the gas and dust (if any) in an elliptical galaxy come from?

18: Why can we not determine distances to galaxies by the same method used to measure the parallaxes of stars?

19: Which is redder—a spiral galaxy or an elliptical galaxy?

20: Suppose the stars in an elliptical galaxy all formed within a few million years shortly after the universe began. Suppose these stars have a range of masses, just as the stars in our own galaxy do. How would the color of the elliptical change over the next several billion years? How would its luminosity change? Why?

21: Starting with the determination of the size of Earth, outline a sequence of steps necessary to obtain the distance to a remote cluster of galaxies. (Hint: Review the chapter on Celestial Distances.)

22: Suppose the Milky Way Galaxy were truly isolated and that no other galaxies existed within 100 million light-years. Suppose that galaxies were observed in larger numbers at distances greater than 100 million light-years. Why would it be more difficult to determine accurate distances to those galaxies than if there were also galaxies relatively close by?

23: Suppose you were Hubble and Humason, working on the distances and Doppler shifts of the galaxies. What sorts of things would you have to do to convince yourself (and others) that the relationship you were seeing between the two quantities was a real feature of the behavior of the universe? (For example, would data from two galaxies be enough to demonstrate Hubble’s law? Would data from just the nearest galaxies—in what astronomers call “the Local Group”—suffice?)

24: What does it mean if one elliptical galaxy has broader spectrum lines than another elliptical galaxy?

25: Based on your analysis of galaxies in Table 26.1, is there a correlation between the population of stars and the quantity of gas or dust? Explain why this might be.

26: Can a higher mass-to-light ratio mean that there is gas and dust present in the system that is being analyzed?

Figuring for Yourself

27: According to Hubble’s law, what is the recessional velocity of a galaxy that is 108 light-years away from us? (Assume a Hubble constant of 22 km/s per million light-years.)

28: A cluster of galaxies is observed to have a recessional velocity of 60,000 km/s. Find the distance to the cluster. (Assume a Hubble constant of 22 km/s per million light-years.)

29: Suppose we could measure the distance to a galaxy using one of the distance techniques listed in the earlier chapters on distances — for example using parallax or Leavitt’s Law  — and it turns out to be 200 million light-years. The galaxy’s redshift tells us its recessional velocity is 5000 km/s. What is the Hubble constant?

30: Calculate the mass-to-light ratio for a globular cluster with a luminosity of 106LSun and 105 stars. (Assume that the average mass of a star in such a cluster is 1 MSun.)

31: Calculate the mass-to-light ratio for a luminous star of 100 MSun having the luminosity of 106LSun.


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