4 ideas
In this galaxy, no species reaches sapience alone. An older 'patron' race genetically and culturally raises a promising animal species to intelligence. In return, the new 'client' species serves its patron for a period of indenture measured in millennia, so status and debt pass down chains of patron lineage.
Humanity has no known patron and claims to have evolved intelligence on its own, which Galactic orthodoxy treats as impossible. Humans have also uplifted chimpanzees and dolphins, which gives them patron standing. The missing origin story therefore becomes a political weapon: whoever can 'prove' a hidden patron gains a claim on humanity and its clients.
From a base on Mercury, a joint human and alien expedition dives shielded ships into the Sun's chromosphere to study toroidal, magnetically structured 'Solarian' life-forms. When Jeff, an uplifted chimpanzee scientist, dies after his solo ship is destroyed, suspicion falls on the Solarians and on human error.
A ship cannot simply insulate itself inside a star, because heat keeps flowing in until it is removed. The Sundiver ships stay cool by using a refrigeration laser to concentrate their interior heat into a beam hotter than the surrounding plasma and fire it back out. In the same way, the Sun's life is imagined as self-sustaining magnetic plasma structures rather than chemistry-based organisms.