The layer is the photosphere.
(Granular cells on the Sun are the tops of convection current cells.)
The very center of the core, where nuclear fusion is occurring; millions of degrees.
The Milky Way contains at least 100 billion stars and may have up to 400 billion stars. The exact figure depends on the number of very low-mass, or dwarf stars, which are hard to detect
when does Halley's Comet passes by the earth every