Early in the 20th century, our galaxy was estimated to be:

Early in the 20th century, our galaxy was estimated to be:
 
  A) about 100,000 ly in diameter.
  B) about 35,000 ly (10 kpc) in diameter.
  C) about 10 million ly in diameter.
  D) not much larger than our solar system.
  E) about 2.5 million ly in diameter.

Question 2

Other than its atmosphere, name two properties of Venus that are unique among the terrestrial planets.
 
  What will be an ideal response?

Question 3

A cool, thin gas produces absorption lines.
 
  Indicate whether the statement is true or false

Question 4

The Huygens probe took numerous pictures as it descended to Titan's surface in 2005. What did the pictures show?
 
  A) primitive life forms
  B) a densely cratered surface
  C) lava flows of molten basalt
  D) features or erosion, including what appeared to be dry river valleys and lakebeds

Question 5

In the process of pair production, which of the following is a possible product pair?
 
  A) electron-neutron
  B) proton-neutron
  C) electron-positron
  D) proton-positron
  E) electron-proton