in a star such as sun what type of energy is transformed into radiant energy help?

Answer 1

The same type of energy that is transformed into radiant energy in all stars, . . thermonuclear energy, the energy coming from the fusion of subatomic particles, i.e. neutrons, protons. Which in the thermonuclear process, gives off gamma rays, x-rays, ultraviolet,visible light, infrared light!
Fortunately for the earth and the life on it, most of the radiant light that actually penetrates through to the surface of the Sun is visible light, ultraviolet and infrared! After a photon's journey time of over 100,000 years to finally reaches the photosphere of the Sun! Or the Sun surface visible to us from earth.
When this radiant infrared, visible and ultraviolet light reaches the surface of the earth, plants can transform these types of radiant light into chemical energy. This photo bio-chemical process in known as photosynthesis.

Answer 2

nuclear fusion.
hydrogen gains electrons due to the immense pressure and heat, to form helium. This process releases a tiny amount of energy millions of times over.