PH109 Exploring the Universe, Test #4, Spring 2000

PH109 Exploring the Universe, Test #4, Spring 2000

NAME______

Please indicate the best answers to the following questions on the answer sheet provided. Please answer the essay questions on page where they are printed. Each question is worth 2 points unless noted otherwise.

1. The location of the center of the Galaxy is determined by observations of:

a) Cepheids variables, b) Globular clusters,

c) The spiral arms, d) A visually bright, massive, object around which all objects in the Galaxy move.

2. What two observations of an object allow for a determination of the Milky Way’s mass?

a) Object’s mass and velocity, b) Object’s age and distance from the galactic center

c) Object’s mass and age, d) Object’s velocity and distance from the galactic center

3. What observation of the Galaxy suggests it is much larger than the halo and contains a large amount of matter not in the form of stars?

a) The rotation curve, b) Motions of the globular clusters

c) The shape of the spiral arms, d) Infrared observations of the center of the Galaxy

4. Which of the following is not found in the Galaxy’s spiral arms?

a) young star clusters, b) O and B stars, c) globular clusters, d) emission nebulae

5. The object located at the center of the Galaxy is believed to be a ______.

a) a large cluster of stars, b) an enormous emission nebula

c) a black hole, d) a massive supernova

6. The Magellanic Clouds are known for certain as:

a) Small irregular galaxies that move in orbit around our Milky Way.

b) Regions of active star formation in the outer halo of the Milky Way.

c) Hazy patches of nebulae visible from Earth’s Northern Hemisphere.

d) Dark clouds of dust that are known regions of star formation.

7. Why is the energy source of active galaxies thought to be extremely compact?

a) Their energy is totally nonstellar in origin.

b) Their spectra are like those produced by ordinary stars.

c) They vary on rapid time scales.

d) They can be seen clearly; we can see that the energy source is compact.

8. Why are quasars called “quasi-stellar”?

a) Their spectra strongly resemble the spectra of stars.

b) They are ordinary stars located at vast distances from Earth.

c) They look like stars on photographs.

d) Like the stars visible in our night sky, quasars reside within the boundaries of the Milky Way.

9. What is implied about an active galaxy or quasar that varies its energy output rapidly?

a) There are many separate sources of energy in the object.

b) The source of energy must be numerous supernovae occurring.

c) The source of energy must be relatively small.

d) The energy source must be rapidly rotating.

10. 21 cm radiation is produced by which element?

a) hydrogen, b) helium, c) carbon, d) iron

12. We cannot see the other side of the galaxy primarily because our view is blocked by

a) too many stars, b) glowing interstellar gas, c) interstellar dust, d) planetary nebulae

13. The mass of the galaxy has been found by

a) counting the stars it contains, b) determining its gravitational force on nearby galaxies

c) estimating the number of interstellar clouds, d) applying Kepler’s 3rd law to the Sun’s orbit

14. About how long does it take for the Sun to complete one trip around the galaxy?

a) 25 thousand years, b) 2.5 million years, c) 250 million years, d) 25 billion years

15. The rotation curve of the Milky Way galaxy is non-Keplerian at large distances from the center. This is evidence for

a) rigid body rotation throughout the galaxy

b) large amounts of unseen mass in the outer galaxy or halo

c) gravitational waves in the galaxy

d) spiral arms

16. Which of the following kinds of objects are found in the halo of the galaxy?

a) globular clusters, b) supernovae, c) spiral arms, d) HII regions

17. Which of the following statements about the density wave model for spiral arms is FALSE?

a) material, once it enters a spiral arm, remains there permanently

b) star formation occurs in spiral arms

c) once started, spiral structure lasts a long time

d) spiral arms are NOT winding up as time passes

18. What evidence do we have that there is a massive black hole at the center of the Milky Way?

a) there is rapidly rotating gas very close to the center

b) it is very dark there

c) interstellar dust is being pulled into the central region of the Milky Way

d) the density of stars increases as distance from the center decreases

19. The oldest stars in the galaxy are found

a) in globular clusters in the halo, b) at the center of the galaxy

c) on the front rim of spiral arms, d) in the galactic disk

20. Which type of galaxy contains the least amount of interstellar material?

a) ellipticals, b) spirals, c) barred spirals, d) irregulars

11. A primary difference between spiral and elliptical galaxies is

a) all ellipticals are more massive than spirals

b) ellipticals have more extensive interstellar media than spirals

c) star formation has ceased in ellipticals, but not in spirals

d) spirals have more stars than ellipticals

22. One would expect the HR diagram of an elliptical galaxy to resemble that of a(n)

a) globular star cluster, b) open star cluster, c) star formation region, d) supernova remnant

23. A rotation curve for a galaxy is a plot of

a) orbital speed versus distance from center, b) number of rotations versus time

c) orbital distance versus eccentricity, d) temperature versus luminosity

24. The masses of galaxies are often determined by

a) determining their star formation rates, b) counting their stars

c) measuring their tidal forces on the Milky Way, d) measuring their rotation curves

25. Which of the following would not be useful in determining the distance to another galaxy?

a) parallax, b) Cepheid variables, c) the sizes of HII regions, d) the brightness of a supernova

26. The radio emission from radio galaxies and quasars is due to

a) thermal emission, b) blackbody radiation, c) O and B stars, d) synchrotron radiation

27. How do we know that the quasars are at large distances?

a) they are extremely dim, b) they have large redshifts,

c) they have large parallaxes, d) they are greatly obscured by interstellar dust

28. Quasars were originally discovered as

a) strong radio sources identified with star-like images on photographs

b) variable sources of light

c) bright galaxies, and only later found to be radio sources

d) the only type of radio source within our galaxy

29. What type of galaxy is frequently found at the center of a rich cluster of galaxies?

a) elliptical, b) spiral, c) barred spiral, d) irregular

30. Which of the following statements best describes the distribution of galaxies in the universe?

a) the galaxies lie on sheets and chains surrounding empty regions

b) the galaxies are distributed uniformly in space

c) there are a few large clusters of galaxies with nothing in between

d) there are many clusters near us in space, but nothing beyond

31. Collisions between galaxies:

a) Turn ellipticals into spirals, b) May cause bursts of star formation.

c) Cause large numbers of stars to collide and explode, d) Never occur.

32. Why do the cores of spiral galaxies appear redder than the arms

a) because of reddening due to dust, b) because the cores are bigger

b) because the cores are further away, d) because of black holes in the center

33. Synchrotron radiation is produced by

a) Seyfert galaxies, b) big bang,

c) electrons in magnetic fields, d) star as they get sucked in by black holes

34. What is hardest to explain about spiral arm structure

a) that they ever existed in the first place, b) why galaxies only have two of them

c) why they have sharp boundaries, d) how they maintain themselves

35. Why don’t elliptical galaxies have any spiral arms

a) they have not formed yet, b) they do, we just see them edge on

c) they are old, arms only last a short time, d) collisions tend to destroy structure

36. Irregular galaxies have ill defined shape because

a) they rotate too slow to form a disk, b) active star formation disrupts disk

c) too many black holes distort space time, d) recent collisions caused loss of shape

37. Why do the Magellanic clouds have their name

a) people thought they were magical, b) named after their discoverer

c) they form a large “M” in the sky, d) the name McDonalds was already taken

38. What kind of Galaxy do we live in

a) Globular, b) Spiral, c) Elliptical, d) Irregular

39. What is the name of the nearest large galaxy

a) Milky Way, b) Andromeda, c) Orion, d) Cygnus

40. What astronomer proved that some of the nebulae cataloged by Messier where in fact large external galaxies separate from our galaxy

a) Einstein, b) Copernicus, c) Hawking, d) Hubble

41. (10 points) Compare and contrast a spiral and a elliptical galaxy.

42. (10 points) Explain what an Einstein ring is and how it forms.

41. (10 points) Compare and contrast the disk and halo parts found in spiral galaxies.

42. (10 points) Describe the leading model for the central engine of an active galaxy.

41. (10 points) Explain how we think spiral arms are formed and maintained in spiral galaxies.

42. (10 points) Explain why some astronomers think there is dark matter in our galaxy and describe an experiment that might detect such dark matter.