There's this really nice, very educational website called Astronomy Picture of the Day, which has a gigantic archive of astronomical images with descriptions written by astronomers. They have a search feature, and I used it to search for M5. I noticed that they only have one color image of M5 (here it is), and it's nothing special. So I thought as a side project, we'd make a color image of M5 and submit it to APOD.
So, to make a color image we gathered our data for M5 in three filters: the R filter for red, the V filter for green, and the B filter for blue. A full-color image can be made by combining just red, green, and blue. Here are the R, V, and B images we used:
When we looked very closely at the images, we noticed that they don't exactly line up: if you "blink" back-and-forth between them, the stars move by a pixel or so from one image to another. So the first thing we did was use the IRAF task imalign to line the images up (this is called "registering" the images). Then we used the R,V,B images to drive the red, green, and blue channels of a PNG image to produce our color image (click on it to see it bigger):
Since this globular cluster is old (how old? that's part of the project!) there are more reddish stars than blueish ones. Note that almost all the bright stars are reddish --- that's because the red giant branch is brighter than the main sequence. We talked a little about how there could be bright blue stars still in the cluster, stars called blue stragglers that don't fit in to our nice story of cluster evolution in the CMD.