Experiments have shown that light of a given energy (frequency) is not something that can be broken up indefinitely. Rather for a given frequency it comes in discrete bundles with energy hf where h is Planck's constant and f is the frequency. These discrete bundles of light are known as photons. It is often useful to think of light as a bunch of particle photons. Other times it is useful to think of light as a wave. Astronomers do both as needed.
The idea of the photon was first put forward by Albert Einstein in his theory of the photoelectric effect.