The energy source of stars is actually nuclear fusion. Deep inside the sun, nuclei of hydrogen come together in a series of reaction whose end product is helium and some excess energy. The sun consumes hydrogen at the rate of 700 million tons per second, and has done so since soon after it formed.
Most other stars generate energy in the same way for most of their lifetimes, only going on to other things when the hydrogen is exhausted. The energy in the sunlight that came in your window today started out in the core of the sun 30.000 years ago, just after the last Neanderthal passed from the scene. It spent most of its lifetime slowly making its way through jostling crowd of atoms in the sun, then had a brief eight-minute sprint through empty space to come to the earth. It is clear then that the energy source of the stars is from nuclear fusion.
A star’s life is a battle between the nuclear fires and gravity. The force of gravity is always pulling the star in on itself. For a moment –as long as the fuel lasts- the star can maintain a precarious equilibrium by using the energy from nuclear reaction to balance the inward pull. The life of every star is a battle between these two competing forces.
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