“What exactly is a galaxy? Surprising as it may sound, astronomers don't have an answer to this fundamental question. There's no agreement on when a collection of stars stops being a cluster and starts being something more.
You might think a galaxy is simply a large group of stars, but just how many stars does it take? Astronomers tend to call five or so stars a 'group' and a hundred or more a 'cluster'. At some point, a cluster becomes a galaxy—the Oxford English Dictionary suggests 'millions or billions' of stars is enough—but there has never been an official threshold.”
source: Science Magazine, 2011
Also see :