Globular Cluster Photo Series (Part 1): M4

Today I thought I'd start off a little series of some of the objects I've been spending my time imaging up on Mauna Kea while volunteering. I've imaged a lot of things, mostly on a "let's see what's out tonight, pick something that sounds interesting, and image it" basis, but last fall I had an idea to try and photograph as many of the Milky Way's globular clusters as I could. Having cracked the secret to getting pictures from my data, I start off by presenting to you an image of the globular cluster Messier 4, in Scorpius.

M4 in Scorpius. Click for larger view.

It looks like there's another small cluster (possibly of the open variety) in the bottom right of the frame, but I don't know what it's called.

Messier 4 is a very rich globular cluster, and was the first globular cluster in which individual stars were resolved. It also happens to one of the closest globular clusters to Earth, at a mere 7,200 light years. Because it's so close, it appears fairly large on the sky: depending on where you want to draw the cutoff point, it appears nearly the same size as the full moon does, 36.0 arcminutes across (the moon varies between 29.3 to 34.1 arcminutes depending on its position in orbit). For ease of comparison, I've decided not to crop the images from the size they come out of the camera so you can easily see the difference in apparent sizes between objects. (This might come back to bite me when I'm looking at really small clusters, especially since M4 is one of the larger ones on the sky, but we'll see how it goes.) Keep in mind that apparent size differences are not absolute size differences, which is why I will post the actual size when possible; M4 is about 35 70 light years across. (35 light years is its radius.)

Finally, M4 has the distinction of being the first globular cluster in which a millisecond pulsar was discovered.