Hello and welcome to Petco Pet Education Center, formerly Petcoach. It is impossible to tell from the picture how this happened, or how deep it is, but what can be determined is that it is a severely infected wound. Possible causes are penetrating traumas (sticks, bites, etc.), some parasites/worms can cause wounds like this, and some tumors can present in abnormal ways, however that is less likely.
I strongly recommend that you have Star seen by a vet as soon as possible, as the wound appears deep and is definitely infected. It needs to be cleaned up, explored, and medications provided.
Best of luck with Star, I hope she feels better soon!