Hidden chaos in cosmic order

摘要

In popular literature and undergraduate textbooks, galaxies are often called the building blocks of the Universe. That implies that galaxies are complete, indivisible, monolithic blocks that make up all other cosmic structures. Indeed, for a long time that view was shared by many astronomers. After all, most of the galaxies that we see in books and on the Internet - and even in Chicago's O'Hare International Airport, which celebrated the 2009 International Year of Astronomy by decorating its passageways with astronomical images -are perfect in the symmetries of their shapes. This respect for the strict beauty of galactic shapes is, to a large degree, due to Edwin Hubble. His classification of galaxies' on the basis of their shapes and structural properties is still in wide use today. But the perfection is never complete, and Hubble had to dump into a class of 'irregular' galaxies all the exceptions that did not fit into his two main classes of perfect ellipsoids or spiral disks. These irregular galaxies are often in the process of collision, or 'merger', with other galaxies. Galaxies are large and complex systems, and when they collide, everything becomes a mess.