1. The one component of the material of the Milky Way Galaxy that prevents us from seeing and photographing the galactic center atoptical wavelengths is
A very cold hydrogengas.
B interstellardust.
C the glare of light fromnearby stars.
D hot hydrogen gas.

2. Radio waves of 21-cm wavelength originate from which componentof the interstellar medium?
A Cool, carbonmonoxide, CO.
B Cold, molecular hydrogen,H2.
C Hot, ionized atomichydrogen.
D Cool, neutral atomichydrogen.

3. The dimensions of the disk of our Milky Way Galaxyare
A diameter 10,000light-years; thickness, 28,000 light-years.
B diameter 2000light-years; thickness, 100,000 light-years.
C diameter 28,000light-years; thickness 2000 light-years.
D diameter 150,000light-years; thickness, 2000 light-years.

4. Cepheid variable stars are invaluable in astronomy because of the close relationship between
A the peak wavelengthof their spectra and their surface temperatures.
B the red shift of theirspectrum and their distance from the Sun.
C their apparent magnitudeand their pulsation period.
D their luminosity, orabsolute magnitude, and their pulsation period.

5. The event that settled the Shapley-Curtis debate about "spiralnebulae" was
A Edwin Hubblemeasuring the distance to the Andromeda galaxy.
B Edwin Hubble showing thatthe universe was expanding.
C Albert Einstein showingthat gravity can bend the path of light.
D Arno Penzias and RobertWilson detecting the cosmic microwave background radiation.

6. Which of the following could explain the rotation curve of the Milky Way?
A A large amount ofdark matter.
B A large amount of darkenergy.
C A small modification toNewton's law of gravity.
D A and B.
E A and C.