In scientific writing, Stephen J. Hawking is a luminary. People look up to this well-known scientist and author for his knowledge and insight.
Although luminary can mean an object or celestial body that gives off light, you'll often hear people talk about parties at which there were many luminaries in attendance. In this case, luminary means celebrity or well-known person in sports or politics, science or the arts. Think of them as being bright lights that make a party sparkle.