The shape of the Milky Way...

Cecil's Columns/Staff Reports
1

So, as to the question on The Shape of the Milky Way - didn’t I read recently that the Milky Way is in the process of digesting a dwarf galaxy in the constellation Sagittarius ?

Unless I’m mistaken, that would give us a distinctly un-spiral lump on that side…

2

Well, it’s probably the galactic “collisions” (not an accurate term since galaxies basically pass through each other) that set up the density waves that gave us spiral arms. But I think that the dwarf galaxy that we’re “digesting” is currently out of us but being pulled back in again. It might pass though another time or two before it is fully incorporated. Here’s an APOD link that says more:
http://antwrp.gsfc.nasa.gov/apod/ap980216.html

BTW, I especially like the fact that you plug “Astronomy Picture of the Day”, which is where I’ve learned 90% of what I know about astronomy.

Here’s an APOD that gives a little more on how they know there’s a central bar in the Milky way.
http://antwrp.gsfc.nasa.gov/apod/ap960201.html

Here’s some on the use of infrared to determine our shape:
http://antwrp.gsfc.nasa.gov/apod/ap000130.html
http://antwrp.gsfc.nasa.gov/apod/ap000518.html

3

So we’re gonna collide with the Andromeda Galaxy in three billion years?

… before or after lunch?

4

I’m betting it’ll happen right around dessert.

5

Yes, but it’s not very big, and the dwarf galaxy is not easily detectable. (not at all, with the naked eye). The bulge to which I was referring in the report is an integral part of our own Galaxy.

And the collision with M31 is by no means certain. It’s easy to measure the radial velocity, using Doppler shift, and we’re quite certain that the two galaxies are getting closer together. On the other hand, it’s very difficult to measure transverse velocity: About the only way is to take very high resolution images, separated in time, and see if the position on the sky is changing. It’s still quite possible, from the limits of our measurement, that the Andromeda Galaxy will miss us by quite a bit.

And Dex, I don’t know about you, but I’m not going to wait three billion years to have lunch. I’m going to say “after”.