The Milky Way was demoted to 2 spirals instead of 4 at the recent AAS meeting

http://www.space.com/scienceastronomy/0806...piral-arms.html

That's two major arms. Apparently, other arms are considered minor. Having two major arms is consistent with having a bar.

Indeed:

I wrote this up here, if anyone's interested...

http://journals.aol.com/stuartatk/Cumbrian...../3712

Fascinating. As far as I know, it's always been a given that barred spiral galaxies are the result of galactic collision and coalescence. More recently, it's been suggested that every spiral galaxy of whatever form is the result of close encounters between galaxies, if not downright collisions.

So -- it becomes obvious that our own Milky Way is the result of at least one major collision between two galaxies. When you look at it that way, the Magellanic clouds become very understandable -- they're the outlying detritus of a collision (perhaps more than one), left gravitationally attached to our galaxy but flung out far enough to appear separate.

Next thing is to see if we can model the Milky Way backwards in time and get a rough idea of how many billions of years ago our most recent galactic collision occurred. (The next one, of course, is in about 2.5 billion years, when M31 collides with us.)

If the last collision occurred, say, six billion years ago or more, it's only of academic interest to us. But what if it occurred about 4.5 billion years ago? Or 3.5 billion years ago?

Is it possible that the LHB was the result of a gravitational shake-up of our solar system caused by the general upheaval of the collision? Or is it possible that our very own star and planets were formed when the last collision occurred? We do observe vast fields of star formation where other galaxies' gas and dust clouds collide with each other.

Just some cosmological images flitting through my fever-oppressed brain, here (been home sick the last few days with a flu bug that just doesn't seem to want to go away...)

-the other Doug