====== Galaxies (definition of) ======
{{tag>Undefined}}

Although current astronomical techniques have revealed the presence of billions of entire galaxies, stretching to the [[content:physics:cosmology:universe_shape|'edge']] of the known universe, a strict definition of what a galaxy actually //is// has not been generally agreed.

>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 :// [[http://www.sciencemag.org/news/2011/01/what-galaxy|Science Magazine, 2011]]

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Also see : [[content:philosophy:sorites_paradox]] and [[content:physics:cosmology:galaxy_formation]]