Luminar Technologies Inc., which produces sensor technology for self-driving cars, started trading publicly Thursday and made its 25-year-old founder and chief executive a billionaire. Austin Russell began Luminar in 2012 at age 17, when he left Stanford University and accepted a fellowship from the foundation of entrepreneur and venture capitalist Peter Thiel to develop the company. Mr. Russell will retain 83% of the voting power in Luminar and serve as its chairman. Luminar "spent a huge amount of time just figuring out what was possible," Mr. Russell said. "In some ways, it's awesome and surreal to be a part of this, going public. In other ways, it just makes a lot of sense."