1. Briefly describe how the spiral density wave theory accounts for the existence of spiral arms in some spiral galaxies. 2. How is it possible that galaxies in our Local Group still remain to be...

1. Briefly describe how the spiral density wave theory accounts for the existence of spiral arms in some spiral galaxies.

2. How is it possible that galaxies in our Local Group still remain to be discovered? In what part of the sky are these galaxies located? What sorts of observations might reveal these galaxies?