Most of my galaxy compilation posters address broad traits, such as the Brightest Galaxies or Superthin Galaxies.  For this poster, I am focusing on a specific galaxy detail that I find to be interesting.

Star formation is seen in galaxies as bright blue regions, usually in the inner half of the disk especially near the bar or core.  Scientists believe that a primary star forming mechanism is bar-driven inflows of gas towards the center of the galaxy.  My Starburst Galaxy Collection shows many galaxies with these beautiful bright star-forming regions in the inner half of the galaxy disk.

On occasion, I have been surprised to see these star-forming regions not in the inner half of the galaxy disk, but in the outer half.  This poster shows 20 galaxies which have star-forming occuring in the outer half of the galaxy disk.  Because of the color and detail of these star-forming regions, the galaxies of the poster are good imaging targets.

Sometimes, as in the 1st row of the poster, the star-forming region is only seen in a short outer portion of an arm.  Other times,  as in the 2nd poster row, star-forming can be seen around the entire galaxy rim.  The 3rd row shows cases where star-forming shows up throughout the outer disk. 

I do not know how such star-forming activity can occur so remote from the galaxy core and bar.  In many cases, I believe that the star-forming has been initiated by gravitational interaction with a companion galaxy.  This is certainly the case with the Collisional Ring Galaxies of the 4th row of the poster.  Although companions are not seen clearly in most of these galaxies, they may have already passed by or been absorbed.

Each of the object images on this poster has been posted to my Astrobin account.  The links to each individual image are below in case you wanted to see more detail and a description of each object:

NGC 2196
NGC 2750
NGC 2713
UGC 1775
IC 226
UGC 5055
NGC 2935
NGC 1954
UGC 5644
NGC 1358
NGC 922
NGC 7610
NGC 2805
NGC 578
UGC 312
PGC 2248
PGC 25211
PGC 41524
IC 298
NGC 2793

All of the above objects are contained in my Astrobin Star-forming Outer Disk Collection.

If you enjoyed this compilation poster, please see my collection of 80 such posters here and my Deep Sky Compendium.