Research on physics expertise has predominantly focused on cognitive differences between physics experts and novices where the novices are high school or introductory college students and the experts are university physics professors or graduate doctoral students. Most physics expertise studies declare the experts to be the physics faculty without justifying this decision. To establish more clearly the characteristics of physics experts, we conducted a qualitative interview pilot study of three university physics professors. The professors each had an hour-long interview where they were asked about their experiences of becoming a physics expert. We present the analysis of the question, 'What makes a physics expert?' Analysis of the data resulted in the construction of a model of physics expertise, which indicates that a physics expert is a specific physics expert first, acquires general physics expert characteristics and then becomes an expert in physics or a boundary crosser.