Why is the sun a star?
A star is any self-luminous body which produces energy within its core through nuclear fusion. In the case of the Sun or any other main sequence star, the core is mainly made up of hydrogen, and this is gradually being converted into helium. This process (known as the PP chain if you care to look that up) involves the the fusion of two hydrogen ions (protons) to form a helium nucleus. The complete process involves the emission of a neutrino and gamma ray photon, the latter of which forms the origin of sunlight. By estimating how much hydrogen there is in the core of the Sun, we can estimate that the Sun will last in its present state for about another 4.5ish billion years.
