Galactic Engines

摘要

Galaxies are phenomenal objects in every way. They are the basic organization of visible matter in the universe, which contains as many as a hundred billion of them. They each commonly pack hundreds of billions of stars. Many are found solo in space; others are found in gravita-tionally linked pairs, familial groups, and clusters. They can be spiral, elliptical, or irregular in shape. Most are photogenic.