Stellar evolution looks at the life history of stars. By what we understand now, stars shine with the energy they produce through nuclear fusion. The particulars of the fusion process in a given star may change during the course of its life, and produce spectacular changes in the appearance and makeup of the star. The types of changes that stars undergo, depending on their size, is the topic of stellar evolution.
Stars make the universe interesting. Basically every chemical element that we know of, except hydrogen and helium, comes from the variety of nuclear reactions that occur in the late phases of a massive star's life. Further, it is primarily by the explosions of massive, dying stars that these newly created elements are injected into the interstellar medium. From there, they can be incorporated into new stars and planets.
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