NAEP Science Grade 12

Category - Earth Science

Astronomers have speculated about what creates different types of galaxies. The leading hypothesis is that the:
  1. Collision of disk-like, relatively small galaxies produces larger spiral and elliptical galaxies.
  2. Presence or absence of a black hole determines whether a galaxy will develop into an elliptical or spiral type of galaxy.
  3. Gravitational attraction of the matter in galaxies causes them to change type as they age.
  4. Abundance or scarcity of heavy elements determines whether elliptical or spiral galaxies will form from preexisting gas clouds.
Explanation
Correct Response: A. There is observational evidence that mergers and collisions of galaxies played a major role in the formation of different types of galaxies in the early universe and that these types of gravitational interactions have continued, especially in galaxy clusters. The composition and structure of different types of galaxies indicate that the structure of large spiral galaxies, elliptical galaxies, and some irregular galaxies results from gravitational interactions during mergers and collisions. Black holes (B) at the center of galaxies are not believed to determine whether a galaxy has a spiral or elliptical structure. Gravitational attraction of the matter in a galaxy is central to that galaxy's evolution (C), but does not explain the origin of different types of galaxies. The relative amount of heavy elements in a galaxy (D) relates to the evolution of the stars in the galaxy, but is not responsible for the evolution of its large-scale structure.
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