The energy is produced by nuclear reactions in the core of the Sun. Apparent Magnitude Apparent Magnitude tells how bright an object is as seen from the Earth. It was originally a scale set up for measuring the brightness of the stars. The brightest stars were called First Magnitude, the next brightest were called Second Magnitude, etc. The brighter a star, the smaller its magnitude.
Many stars are brighter than first magnitude.
Some stars are so bright they have negative magnitudes. The faintest stars visible to the naked eye are sixth magnitude. On this scale the Sun is so bright its magnitude is the high negative value of -27. This is the temperature of the visible solar surface. There are many methods of measuring this temperature.