Photosphere

The photosphere is the surface of a star. The photosphere is located below the corona and chromosphere layers of a star's atmosphere.

In 2153, Vissians were able to travel well into the photosphere of G-type stars due to the trinesium hulls on their starships. At the time, flying into stars was not within the capacity of Starfleet ships. 

In 2368, Kurn took the Hegh'ta into a star's corona and approached dangerously close to the photosphere before turning and going to warp in order to escape two other Klingon Birds-of-Prey chasing them. 

After being towed into the interior of a Dyson sphere in 2369, auxiliary power needed to be diverted to the thrusters so that the USS Enterprise (NCC-1701-D) could achieve a stable orbit outside of the photosphere of the star located inside the sphere. 

The photosphere of a star could be masked out by the viewscreen of starships such as the Enterprise. In 2364, the crew of the Enterprise masked out the photosphere of a star in the Delos system in order to better study solar flares and magnetic activity occurring below.