What is a Galaxy, Anyway? | Hacker News Reader

What is a Galaxy, Anyway?

astrobasics.blogspot.com

24 pointssaundby6 comments

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?

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.

Perhaps that's because the article isn't about the formation of galaxies but about the perception of what a galaxy is?

Besides, it's a fairly long post without even getting into the nuts and bolts of astrophysics. ;)

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.

Visible mass is sufficient to explain the formation of stars and galaxies. What is a problem is the "rotation curve" of galaxies - that is orbital velocity around galactic centre as a function of distance from galactic centre. It's observed to be linear - ie stars far out are orbiting at the same velocity as stars closer in. In fact velocity should decrease with distance - see http://en.wikipedia.org/wiki/Galaxy_rotation_curve Dark matter has been postulated as an explanation - there must be other material that we can't see in the galaxy that results in the observed linear rotation curve.

Of course this assumes that the 1/r^2 holds for all distances. This is by no means certain, and it may well be that the linear curve is hint of new physics.

Edit: For other tantalising evidence that all is not well with 1/r^2, check out the problem with the trajectories of Pioneers 10 & 11:

http://moriond.in2p3.fr/J07/trans/wednesday/reynaud.pdf

Super-summary: a bunch of stars.

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