Our home galaxy, the Milky Way, doesn't have a lot of companions. It's a member of a small cluster known as the Local Group -- a collection of a few dozen mostly small galaxies. The Milky Way is one of only two big galaxies in the entire group.
But many galaxies have lots of companions. An example is Abell 2151, also known as the Hercules Cluster. It's about a half-billion light-years away, spread across the constellation Hercules, which is high in the west at nightfall.
The cluster contains hundreds of galaxies. Many of them are big spirals like the Milky Way. And many of those galaxies take companionship to the extreme: they're merging to form even-bigger galaxies.
A galactic merger is a messy process. The galaxies begin by sweeping past each other. The gravity of each galaxy tugs at the other, pulling out streamers of stars that can stretch across hundreds of thousands of light-years. Many of the stars will leave their parent galaxies behind and fly off into intergalactic space.
The gravitational pull also causes big clouds of gas to ram together, triggering the birth of thousands or millions of new stars.
Over time, the galaxies loop around each other, moving closer on each pass. Eventually, they slam together, triggering the birth of more new stars. They soon use up their gas, though, so starbirth comes to a halt. The merged galaxies lose their spiral shapes, and instead look like a fuzzy football -- the final step for close galactic companions.
Script by Damond Benningfield, Copyright 2010