SCCC 116 - Honor's Descriptive Astronomy II
Exam 2
Prof. Lacey
6 April 2006

Circle the correct answer to the following multiple choice questions. Each question is worth 2 points.

1. The universe has three possible futures. Which one is correct depends only on the average density of matter in the universe. Why is this?
   1. The density of matter determines the strength of gravity, which decelerates the expansion over time.
   2. The density of matter determines the rate of formation of black holes which will eventually collapse the universe.
   3. The density of matter tells astronomers whether new matter is constantly forming, thereby producing a steady-state.
   4. If the density is sufficiently high, the geometry of space may be curved.
   5. The density of visible matter must exactly equal the dark force energy.
   Answer: A Page Ref: 26.4R

2. To which of these phenomena are X-ray busters most similar
   1. nova
   2. planetary nebulae
   3. type I supernovae
   4. type II supernovae
   5. hypernovae
   Answer: A Page Ref: 22.3N

3. What explanation does general relativity provide for gravity?
   1. Gravity is the weakest of the four universal forces.
   2. Gravity is a result of curved spacetime.
   3. Gravity is directly proportional to mass.
   4. Gravity is inversely proportional to radius.
   5. All of the above are correct.
   Answer: B Page Ref: 22.6R

4. Which of the following are attracted by gravity?
   1. antimatter
   2. any object with mass
   3. electromagnetic radiation
   4. neutrinos
   5. all of the above
   Answer: E Page Ref: 22.7R

5. What would happen if mass is added to a 1.4 solar mass white dwarf?
   1. The star would explode as a nova.
   2. The star's radius would increase
   3. The star would immediately collapse into a black hole
   4. The star would erupt as a carbon detonation (type I) supernova.
   5. The core would collapse as a type II supernova.
   Answer: D Page Ref: 22.1R

6. Which statement about Population I is incorrect?
   1. The bright blue stars that dominate the night sky belong to it.
   2. The Sun is one of its older members.
   3. Their orbits lie close to the plane of the Galaxy.
   4. Its stars surround the Galaxy in the extended halo.
   5. Its stars are richer in heavier elements made in supernovae.
   Answer: D Page Ref: 23.3R

7. Which statement about Population II is incorrect?
   1. These older stars are richer in heavier elements they have made in their cores.
   2. Its most notable groupings are the globular star clusters.
   3. Its brightest stars are evolved red giants.
   4. They lie anywhere from the galactic nucleus out to deep into the halo.
   5. Its oldest members may be over ten billion years old.
   Answer: A Page Ref: 23.3

8. Star formation ceased first in the:
   1. nucleus.
   2. spiral arms.
   3. bulge.
   4. halo.
   5. galactic disk.
   Answer: D Page Ref: 23.4N

9. The cosmic microwave background is important mostly because:
   1. its detection lead to the discovery of dark energy.
   2. its detection opened a new form of radiation to observation.
   3. it confirmed a major prediction made by the Big Bang theory.
   4. it showed that the universe is open.
   5. it showed that the universe is closed.
   Answer: C Page Ref: 26.7R

10. Of what use are the Doppler shifts of the 21-cm radio signals?
    1. They let astronomers map the large-scale motion and distribution of gas in the Milky Way's spiral arms.
    2. The can more easily pick up the heat of brown dwarfs and dark matter.
    3. They are used to study distant stars in the Galaxy's halo and corona.
    4. They let astronomers "see" into black holes.
    5. They let astronomers learn about the process of star formation.
    Answer: A Page Ref: 23.5R

11. Which of the following is not typical of the Galaxy's spiral arms?
    1. giant molecular clouds
    2. young star clusters
    3. OB associations
    4. globular clusters
    5. emission nebulae
    Answer: D Page Ref: 23.5R

12. The leading explanation for the existence of spiral arms is:
    1. shock waves from cyclic supernovae explosions.
    2. shock wave from the quasar-like eruption of the galactic core.
    3. passages of spiral density waves through the interstellar medium.
    4. passages of massive globular clusters through the galactic plane.
    5. tidal streams of small galaxies being torn up by our monster Galaxy.
    Answer: C Page Ref: 23.5N

13. What two observations of an object allow for a determination of the Milky Way's mass?
    1. object's age and chemical composition
    2. object's mass and velocity
    3. object's age and distance from Galactic Center
    4. object's mass and age
    5. object's velocity and distance from the galactic center
    Answer: E Page Ref: 23.6R

14. What property is common to all spiral galaxies?
    1. a very flat bluish disk, compared to the dimensions of the halo
    2. ongoing star formation, chiefly in the spiral arms
    3. abundant interstellar gas, chiefly in the spiral arms
    4. richer in metals than the spectra of ellipticals
    5. All of the above are correct.
    Answer: E Page Ref: 24.1R

15. If the density of the universe is less than critical, then:
    1. the universe will "survive" to expand forever.
    2. the universe on average must be less than 2.73K in temperature.
    3. the universal expansion must stop within about twenty billion years.
    4. the universe must be static, and the red shifts are not cosmological.
    5. the universe will begin contracting immediately into the "Big Crunch."
    Answer: A Page Ref: 26.3R

16. Which relation matches the rotation of a spiral galaxy to its luminosity?
    1. Hertzsprung-Russell diagram
    2. Hubble's Law
    3. Hubble's Tuning Fork Diagram
    4. the Tully-Fisher Relation
    5. the Chandrasekhar Limit
    Answer: D Page Ref: 24.2N

17. Why does the Cepheid "standard candle" have limited usefulness beyond 20 Mpc?
    1. Distant galaxies are seen in the past, when Cepheid variables behaved differently than they do today.
    2. Distant galaxies are so young they do not contain Cepheids.
    3. Cepheids are too faint to be seen beyond that distance, even with HST.
    4. The light variability of Cepheids diminishes with distance so they do not appear to vary there.
    5. Older distant Cepheids are of population II, where the period-luminosity relation no longer works.
    Answer: C Page Ref: 24.2R

18. What is the most likely source of energy for active galaxies and quasars?
    1. collisions of large spiral galaxies
    2. large clusters of very massive, luminous stars
    3. a single supermassive, superluminous star
    4. accretion onto a supermassive black hole
    5. numerous supernovae from rapid star formation in young galaxies
    Answer: D Page Ref: 24.5R

19. A galaxy is at a distance of one billion light years. Which of the following is true?
    1. We see the galaxy the way it will be in one billion years.
    2. We see the galaxy the way it was one billion years ago.
    3. We see the galaxy the way it was when the universe was one billion years old.
    4. We see what our galaxy will be like in one billion years.
    5. We have no knowledge of anything at that distance.
    Answer: B Page Ref: 25.5R

20. Because almost all of the galaxies are redshifted, we know that:
    1. there was no beginning to time.
    2. we are at or close to the center of the universe.
    3. the universe is expanding.
    4. the sky is dark at night.
    5. All of the above are correct.
    Answer: C Page Ref: 26.2R

21. Homogeneity and isotropy, taken as assumptions regarding the structure and evolution of the universe, are known as:
    1. Obler's Paradox.
    2. Hubble's Law.
    3. Wien's Law.
    4. the Cosmological Principle.
    5. the Grand Unified Theory.
    Answer: D Page Ref: 26.1R

22. The type of radiation coming from active nuclei due to electrons spiraling in very strong magnetic fields at relativistic speeds is:
    1. positron radiation.
    2. thermal energy.
    3. a blackbody curve.
    4. synchrotron radiation.
    5. nuclear energy.
    Answer: D Page Ref: 24.5N

23. What is implied about an active galaxy or quasar that varies its energy output rapidly?
    1. The source of energy must be relatively small.
    2. The source is due to a collision between matter and antimatter.
    3. The energy source must be rapidly rotating.
    4. The source of energy must be a chain reaction of supernovae occurring.
    5. There are many separate tiny sources of energy in the object.
    Answer: A Page Ref: 25.4R

    
    
    Answer the remaining problems on separate sheets of paper. Remember to use only significant figures in the answers and show all work! Remember to include units for all numerical answers.
    
    
    

24. (5 pts) Why do we not see all neutron stars as pulsars?

    We can only see a neutron star as a pulsar when the beamed emission of the neutron star is pointed in the direction of the Earth.

25. (5 pts) What is time dilation?

    Time dilation is an effect of special relativity that causes clocks in frames that are moving at speeds near the speed of light to run more slowly than clocks in frames at rest.

26. Finding distances is one of the most fundamental problems in astronomy.
    1. (4 pts) List all the rungs to the distance ladder in order from closest to farthest.
       
       1. Closest: radar ranging
       2. Parallax
       3. Spectroscopic parallax
       4. Variable stars (Cepheids, RR Lyraes)
       5. Tully-Fisher Relationship
       6. Type Ia Supernovae
       7. Farthest: Hubble's Law

    2. (4 pts) Describe one of the methods that can be used to find the distance to a nearby galaxy.

       The distance to a nearby galaxy can only be determined by Variable Stars, Tully-Fisher, Type Ia Supernovae. Spectroscopic parallax can only be used in our own Milky Way as the individual stars in a cluster need to be resolved so that we can place each star on the HR diagram. We can only do this within our own glalaxy. A nearby galaxy has local motions that make Hubble's Law inapplicable.

       Variable Stars: Measure the period of the luminosity change. Then use the relationship between period and absolute luminosity to get the absolute magnitude. We observe the apparent magnitude. Use $D=10pc \times 10^{\frac{m-M}{5}}$.

       Tully Fisher: Measure the rotational velocity of a galaxy. Tully Fisher found a correlation between the rotational velocity of a galaxy and its absolute luminosity, thus absolute magnitude. We observe the apparent magnitude. Use $D=10pc \times 10^{\frac{m-M}{5}}$.

       Type Ia SNe: White dwarfs that siphon mass from a binary companion. When the mass of the white dwarf reaches 1.4 M$_{\odot}$, the star explodes in a supernova. These explosions all involve the same type of star at the same mass, so every Type Ia SN has the same explosion, and the same absolute luminosity. Use the absolute luminosity to get the absolute magnitude. We observe the apparent magnitude. Use $D=10pc \times 10^{\frac{m-M}{5}}$.

27. (3 Pt's) Name 3 parts that make up our Milky Way Galaxy.
    

    Disk, Bulge, Halo
    

28. (3 pts) What is the Schwarzschild radius of a billion solar mass black hole?
    

    $R_{Schwarzschild}~(km)=M_\odot\times 3~km$
    

    For 1 billion $M_\odot$:
    $R_{Schwarzschild}~(km)=1\times 10^9~M_\odot \times 3~km =3\times 10^9~km$
    

29. (4 pts) What evidence exists for dark matter in our Milky Way Galaxy?

    Rotational curves of galaxies point to missing mass in the outer halos of galaxies (including our own). Also examining the motions of galaxies in galaxy clusters lead to the conclusion that unseen mass must be present to explain the motions of the galaxies due to gravity.

30. Hubble's Law is used to calculate distances to far away galaxies.
    1. (4 pts) Calculate the recessional velocity of a galaxy with a redshift of z=0.1. $z_{line1}=0.10$,      $z=v/c$ for low velocities
       

       $v_{r}=zc=0.10~c, v_{r}=zc=0.10~c=0.10 \times 3\times 10^5 km/s=3\times10^4 km/s$
       

    2. (4 pts) Use Hubble's Law to determine the distance to the above galaxy? Assume H$_0$=70 km/s/Mpc.

       Hubble Law: $v_{recessional}=H_0 d$
       

       $d= \frac{V_r}{ H_0}=\frac{3\times10^4 km/s}{70 km/s/Mpc}= 430 Mpc$