Astronomy midterm question: deep in the thick of things

Astronomy 210 Midterm 2, Spring Semester 2009
Cuesta College, San Luis Obispo, CA

[20 points.] Consider the following observation by Henry Freudenreich:

"The central reason astronomers have been slow to understand the Milky Way is simply because we are deep in the thick of things: The other stars, the gas and especially all the dust in the disk prevent us from seeing the full extent of the galaxy's structure."
--Henry Freudenreich, Am. Sci. vol. 87 no. 5 p. 418 (1999)

Explain how it is still possible to map the spiral arm structure of the Milky Way.

Solution and grading rubric:

• p = 20/20:
  Correct. Massive stars, due to their extremely short lifetimes, are born and die only in the spiral arms; and are bright enough to be seen through some of the obscuring gas and dust. Similarly for HII (emission) nebulae. Radio waves emitted from cold hydrogen gas are not obscured by gas and dust. Observing the locations of all three result in a spiral arm map of the Milky Way.
  
• r = 16/20:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors.
  
• t = 12/20:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. Describes direct or indirect evidence other Milky Way properties, such as its thin disk structure, size/mass, and/or location of the center of the Milky Way.
  
• v = 8/20:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. May use other spiral galaxies to infer that the Milky Way must also have spiral arms, or some other observational evidence that does not directly reveal the structure of the Milky Way.
  
• x = 4/20:
  Implementation/application of ideas, but credit given for effort rather than merit.
  
• y = 2/20:
  Irrelevant discussion/effectively blank.
  
• z = 0/20:
  Blank.
  

Grading distribution:
Section 30676
p: 11 students
r: 4 students
t: 18 students
v: 15 students
x: 9 students
y: 0 students
z: 0 students

A sample "p" response (from student 0008):

Another sample "p" response (from student 0321):

A sample "p" response (from student 9891, who is scared of the implications of this response):

A sample "x" response (from student 2626), going out on a limb: