(via APOD: 2012 September 30 - A Galaxy Collision in NGC 6745)
Image Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration
Acknowledgment: Roger Lynds (KPNO/NOAO) et al.
So, it would seem that Hubble snagged a shot of galactic collision aftermath. Here we see NGC 6745, who has just interacted with a smaller galaxy off the image to the lower right. In fact, you can see a tail of dust being pulled off in that direction.
It is likely that NGC 6745 used to be a regular spiral galaxy, but has been warped by this encounter.
Interestingly, stars are generally far enough apart that even when a whole galaxy collides with another, stars very rarely run into each other. However… the gas and dust in the interstellar medium, the magnetic fields being generated all over the place, even the dark matter, all of these things do interact, and sometimes “collide” in interesting ways.
Generally when dust and gas clouds collide, they end up compressing each other, and the gravitational tidal effects will create some interesting pressure patters too. So, you end up with a burst of star formation in various places. You can see the bright spots of sudden and intense star formation in the “tail” of gas and dust being pulled along in the lower right.