You may remember the story of Pluto. A few years back, as astronomers found more and more objects which blurred the line between planet, asteroid and other, it became apparent that the word “Planet” lacked a proper definition. So, after two weeks debate at the IAU General Assembly in Prague, a definition was voted in by over 400 delegates which happened to exclude the ex-planet Pluto. Now, as astronomers find more and more small galaxy-like objects, we find a similar problem with the word ‘Galaxy’.
Duncan Forbes and Pavel Kroupa, in their recent paper, present some possible components of a formal definition of a galaxy. While any new definition is going to cause controversy, it’s unlikely to bother the general public as much as the Pluto debacle, since very few people could name any galaxies at all. Even those which may be familiar (Milky Way galaxy, Great Galaxy of Andromeda, for example) are not at risk of losing their status. Still, the authors would prefer not to impose their own definitions on the astronomical community. Instead, they’ve chosed to call to the Wisdom of the Crowds, asking their readers to vote on their favourite criteria. You can take part by clicking through to their online poll. I’d recommend skimming over the paper first, though, to understand all the options!