Although current astronomical techniques have revealed the presence of billions of entire galaxies, stretching to the 'edge'plugin-autotooltip__plain plugin-autotooltip_bigUniverse shape and size

The 'Observable Universe' is limited by the speed of light and the length of time that it has been in existence. It's possible that the 'Observable Universe' is the real size, or, because light has a finite speed, there may be more of the univer… 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 : Science Magazine, 2011

Also see : Sorites paradoxplugin-autotooltip__plain plugin-autotooltip_bigSorites paradox

The Sorites paradox is a philosophical problem dating from the time of Ancient Greece.

It relates to the problem of defining groups - specifically large groups. It has implications for the definitions of 'vagueness' 'fuzziness' 'blurriness' etc etc. Wh… and Galaxy formationplugin-autotooltip__plain plugin-autotooltip_bigGalaxy formation

There are currently two (mutually exclusive) theories regarding the formation of galaxies, which according to current models, all have a supermassive Black Hole at their centre.

1) The galaxy formed around a pre-existing Black Hole which was sufficie…

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