

What is a Galaxy, Anyway? - saundby
http://astrobasics.blogspot.com/2010/08/what-is-galaxy-anyway.html

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jacoblyles
I love learning about the development of the knowledge that we take for
granted today. Only a few short years ago, we didn't even know what a galaxy
was. Who knows what we will discover in the future?

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CytokineStorm
Did anyone else think it was strange that this article didn't even mention the
importance of dark matter to the formation of galaxies? Without the enormous
quantity of dark matter that gravitationally binds our galaxy together the
centrifugal force would cause the galaxy to fly apart.

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frossie
_Without the enormous quantity of dark matter that gravitationally binds our
galaxy together the centrifugal force would cause the galaxy to fly apart._

I think you have confused yourself. Dark matter was postulated to explain why
the velocity of the outer stars in our galaxy is _higher_ than expected.
Otherwise you would expect things to be slower the further out you get, just
like in the solar system where the orbital speed of say, Neptune, is much
slower than that of the Earth.

Dark matter is not necessary to explain galactic formation _per se_. Dark
matter can be used to explain _observed_ galactic formation, but in fact many
suggested forms of dark matter make galaxy formation harder, not easier.

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Groxx
Super-summary: a bunch of stars.

