|M4 in Scorpius. Click for larger view.|
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
Finally, M4 has the distinction of being the first globular cluster in which a millisecond pulsar was discovered.