Why Haven't the Arms of Spiral Galaxies Wound Up After All This Time? (forbes.com)

StartsWithABang writes: When you take a look at a spiral galaxy in the night sky, it seems obvious that the stars on the inner parts of the galaxy are going to orbit in less time than the stars in the outer part. This turns out to be true, something we've figured out even though the timescales for galaxies to complete a full revolution are far longer than we've ever been able to observe. But one thing that doesn't happen is that the arms don't "wind up," meaning that the galaxies don't see the spiral patterns intensify as they age. Even though we first observed spiral structure in galaxies back in the mid-1800s, we didn't understand what the cause of this effect was for over 100 years. Yet now, not only do we understand it, but we can explain why galaxies will never wind up over time, and how this effect is true with or without dark matter.

Comments

Actual Link

[cfalcon]

Actual Link (warning: still Forbes): http://www.forbes.com/sites/st...

Spiral compression waves

[MichaelSmith]

Spiral arms are not solid structures which can wind up. They are zones of star formation created by waves which propagate through the galaxy.

Better source than Forbes - PBS (Phil Plait)

[donkeyb]

Phil Plait's excellent series of PBS shorts explains all this in a better fashion (I think), and he doesn't spam himself all over slashdot, so is more deserving of our time :) Galaxies part 1 is here: https://www.youtube.com/watch?... Part 2 is linked from there. :)