I read this book probably 20 years ago (funny how I keep remembering books I read a long time ago as I try to catalog them all on Goodreads). I can't say I remember the plot in detail, but it was a really fascinating hard SF novel about intelligent life forms who live on the surface of a star. They were very alien but still believable -- Robert L. Forward did a great job of describing their environment and how the incredible heat and gravity affected them. Because of the gravitational effects, time was subjectively "faster" for them, so generations go by in the course of a few hours of "real time." When humans first make contact, the aliens are barely technological, and by the time their ship leaves, the aliens have become so advanced they can actually move stars around. It was a good read, though I don't know well it would stand up twenty years later.