When a new star forms, it derives its energy from hydrogen in its core, with hydrogen atoms undergoing a process of nuclear fusion to produce helium. During this phase of its existence, the star belongs to the 'main sequence' (with less massive examples typically being described as 'dwarfstars'). A star can continue to generate energy in this way for billions of years; less massivestars tend to have longer lifespans on the main sequence, while more massivestars consume their hydrogen reserves more quickly, and may only remain on the main sequence for a matter of tens of millions of years.
Regardless of the star'smass, it will eventually reach a point where the original reserves of hydrogen in its core have been consumed. At that point, the star will undergo radical transformations in its chemistry and structure, and will move away from the main sequence. At this point, as the star enters a new phase of its existence, it is described as an evolvedstar. The nature of the next phase of the star's evolution will depend on its initial mass.
Arcturus, or Alpha Boötis, a relatively nearby star that belongs to the evolved redgiant class, and has a diameter some twenty times that of the Sun. In approximately five billion years, the Sun itself will begin the process of expansion that will see it evolve into a giantstar of this kind. Imagery provided by Aladin sky atlas
A star on the main sequence maintains its structure because the energy produced by its core counterbalances the pressure of gravity, but as it evolves away from the main sequence, this balance breaks down. With no further hydrogen fusion to maintain it, the core will begin to contract until, in all but the least massivestars, it reaches a point where helium fusion can begin. Meanwhile the star's outer shell expands, and hydrogren fusion can continue in this shell, ultimately producing a swollen redgiantstar, which will then slowly cool to form a white dwarf and expel its outer shell to form a planetary nebula. This is the typical fate of less massivedwarfstars like the Sun.
More massivestars - those with masses of more than about fifteen times that of the Sun - evolve in more complex ways, because the mass of the star is sufficient to support different phases of nuclear fusion. These stars evolve to form supergiants, composed of multiple concentric shells, each fusing progressively heavier atoms, from carbon to iron. As a supergiant evolves, physical forces will eventually causes its core to collapse, with dramatic results. This core collapse is cataclysmic, and depending on its mass, will form either a neutron star or a black hole. Meanwhile the outer shells of the massivestar explode outward in an event known as a supernova.
