Published in ScienceNOW, 27 Jan 2011
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. Now, in an echo of the recent wrangling over Pluto’s status as a planet, a pair of astrophysicists from Australia and Germany want to start a debate on the issue—and they have even set up a Web site for people to cast their votes.
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. [...]
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