ASTR 103 - Quiz

Supplement 20 - Our Galaxy

True-False

1. Proper motion is the angular change in position of a star on the celestial sphere per year.
2. The point on the sky toward which the Sun appears to be moving relative to its neighbors is called the apex of the solar motion.
3. The spheroidal population stars are those stars that make up the disk portion of the Galaxy.
4. The nucleus is the outer spherical portion of the Galaxy surrounding the disk.
5. The galactic orbits of most of the stars in the Sun's neighborhood are approximately circular.
6. A star's space velocity is its velocity measured relative to the Sun.
7. The disk population stars are the older stars found in the Galactic halo and nucleus.
8. The spiral density wave theory is the concept of expanding compressional waves spreading out from the Galactic center, and where interstellar matter encounters these waves it is compressed and forms the Galaxy's spiral arms.
9. The center of our Galaxy lies well beyond the stars of the constellation Sagittarius.
10. Large portions of the spiral arms in our Galaxy have been traced from observations of the 21-cm line of atomic hydrogen.
11. The distribution of stars shows a strong increase in concentration toward the plane of the Earth's equator.
12. A firm realization that our Galaxy was only one of many galaxies in the Universe came from the pioneering work of William Herschel.
13. Different rates of rotation for the Galaxy have been found associated with distinctive parts of the Galaxy as defined by the ages and characteristics of its component stars.
14. The Sun is located about 30,000 ly from the center of the Galaxy and close to the mid-plane of the disk.
15. Most, some 55 percent, of the stars in our Galaxy are white dwarfs.
16. Of the 400 hundred billion stars in our Galaxy, only about 10,000 are O stars.
17. About 40 percent of the mass of our Galaxy is in the form of M-type main-sequence stars.
18. About 40 percent of the luminosity of the Galaxy is provided by supergiants of all spectral types, red giants, and main- sequence O stars.
19. Evidence, although somewhat sketchy, exists to show that the halo is much larger and more massive than the visible component that we readily detect.
20. The nucleus is one of the more active regions in the Galaxy for star formation.

Multiple Choice

Our Galaxy is broken up into several definable regions. They are the:

a. nucleus,

b. spiral arms,

c. disk,

d. halo.

Use these regions as the answers to the following questions. An answer may be used more than once.

1. The youngest of the disk population stars are located in the ___ of the Galaxy.
2. The oldest of the spheroidal population stars are located in the ___ of the Galaxy.
3. Where are most of the globular clusters located?
4. Where is the Sun located in the Galaxy?
5. The oldest of the disk population stars are located in the ___ of the Galaxy.
6. Where does most of the Galaxy's interstellar gas and dust reside?
7. Where are most of the open clusters located?
8. In what region of the Galaxy would one look to find the O and B supergiants?
9. Where are the stars located in the Galaxy whose chemical compositions contain the smallest fraction of elements heavier than hydrogen and helium?
10. Where are the stars located whose orbits about the nucleus of the Galaxy are approximately circular?

Essay

1. Discuss the motions of stars in the solar neighborhood. How is the solar neighborhood, along with the Sun, moving in the Galaxy? Is the rotation the same for all parts of the Galaxy?
2. If the density of nearby stars given in the caption of Figure 11.9 were typical of the entire Galaxy, then for the volume of the disk given in Table 13.1, how many stars would the disk contain? For the volume of the halo, how many stars would it contain? What do these numbers tell you about the density of stars throughout the Galaxy?
3. In Table 13.3, examples of H-R diagrams are given for each of the population types in the table. Figures 12.10, 12.11, 15.7, and 15.8 show these representative H-R diagrams. Describe their differences as you see them. What can you learn from this comparison?
4. Of what significance is the Galactic year to astronomers? Is it a fundamental measurement of time in the Universe? Do you find it an instructive measure of the Galaxy and its history?
5. Do all the tracers of the spiral arms in our Galaxy mark the same feature? If not, do astronomers have an explanation for the differences? Can you propose a reason for any differences? If the same, can you propose a reason why they should be the same features?
6. Describe astronomers' current views on the chemical evolution of the Galaxy. How is the chemical composition of the Galaxy likely to change in the future? Is there an end to the chemical changes the Galaxy can undergo?

Problems

1. The relation for the visual absolute magnitude M(v) is
   
   M(v) = m(v) + 5 - 5{log[r(pc)]} - A(v)
   M(v) = m(v) + 7.57 - 5{log[r(ly)]} - A(v)
   where m(v) is the visual apparent magnitude, r(pc) is the distance in parsecs, r(ly) is the distance in light years, and A(v) is the visual absorption by interstellar matter in magnitudes. Interstellar absorption has been found empirically to be given by
   
   A(v) = 3.3.E(B-V),
   where E(B-V) is the excess of the measured value of color index over the expected intrinsic color. If the color excess for Alnilan (Orionis) is +0.08, what should its visual absolute magnitude corrected for interstellar absorption be?
2. If the emission of X-rays by the Galaxy was as great as the emission of visible light is, how many X-ray sources would the Galaxy contain assuming they possess the same X-ray brightness as the actual ones do, or L(x) = 10⁴[L(sun)]?