A star like the Sun is called a main sequence star. It is considered as young age star. Sun is a medium sized star and its mass determines how long it can continue in its present form.
Its heat and light are a result of the thermonuclear fusion reactions that occur in its core. The proton cycle and carbon cycle are responsible for generating the immense energy which is released in the form of heat and light. The amount of nuclear fuel ( hydrogen helium) present in the sun indicate that it can continue glowing at this state for another 5 or 5.5 billion years more. Sun formed abut 4.6 or 5 billion years ago, as shown from the calculations.
