Not much. The prevailing idea was put forward by William Herschel in 1795. The Sun was a dark body surrounded by a luminous atmosphere. Sunspots let you see through the atmosphere to the black body inside which some said was populated by beings of some kind. It wasn't until 1815 that Fraunhofer designed a spectroscope to view hundreds of 'spectral lines' and 1859 that Kirchoff showed these lines came from incandescent hot gas. By the 1820's even the Corona was only recently discovered and believed to be dust particles given off by the sun. The origin of the Sun's energy was unknown and thought to be a chemical process of some kind.
All answers are provided by Dr. Sten Odenwald (Raytheon STX)
NASA IMAGE/POETRY Education and Public Outreach program.