The Nature of Light, Science Question on thermodynamics

According to the first law of thermodynamics, energy is neither created nor destroyed. What does this say about a substance or object that is emitting light?

Matter converts to energy (and vice versa).

Two molecules of hydrogen (matter) are fused together into another molecule of helium (matter). That process releases a particle of energy (a photon). It's called nuclear fusion and occurs every second in every star in the universe.