Is the sun on fire?
No, the sun is not on fire. The sun work by a process called Thermonuclear Fusion.
The process works only due to the sun mass. The sun is massive and as a result, the sun severely distorts space and time. The result of this distortion is the phenomenon of gravity.
The sun works by squeezing Hydrogen atoms together so that they fuse and form helium releasing energy in the form of heat and light.
Daily Physics 