A stellar physicist was someone who studied stellar physics.

The Ferengi Doctor Reyga was a stellar physicist. (TNG: "Suspicions")

The USS Enterprise-D had a team of stellar physicists on board. In 2368, they and the planetary evolution team both wished to use the thermal imaging array first. When Data informed Geordi La Forge of this, he told the android to tell them to flip a coin, adding that everyone needed to cooperate. (TNG: "The Game")