Well, I guess one could quibble as to what the "most" common name of the cluster might be. As I was looking for some images of the M13 bacteriophage, I was sort of hoping that it would vaguely resemble a globular cluster. But alas, it's filamentary in appearance. So no other similarity besides the designation.

Assuming my answer is correct. And yes, I'm well aware of the common advice about making assumptions. Here's my question:

Let's say you were observing Jupiter through a telescope. You spot shadows cast on the planet's surface by 3 of the 4 Galilean moons. Even if you cannot identify the moons (or their shadows) individually, you can know the identity of 1 of the moons with complete certainty. Which one is it, and why?