The most massive stars in the universe end their lives in cataclysmic explosions called supernovae. Of the four supernovae types, the two that occur most commonly are Type Ia and Type II supernovae. Type Ia supernovae occur in what's called a binary system, in which a pair of small stars orbit one another. By contrast, Type II supernova events involve very large individual stars called supergiants.
Any medium-mass or larger star, from one about 40 percent the size of our Sun to one several times greater, relies on energy produced in its core to offset the enormous force of gravity pulling matter toward its center. Nuclear fusion produces this sustaining energy. When hydrogen nuclei -- a star's primary fuel supply -- fuse with one another, helium nuclei are created. Although helium is heavier than hydrogen, the mass of the single helium nucleus is less than the total mass of the two hydrogen nuclei from which it formed. The difference is released as energy. A star will blaze away as long as there is enough hydrogen to fuel the reactions. Depending on its mass, this can range from a few million years for the most massive stars to several billion years for less massive stars, including the Sun.
When a star at least four times more massive than our Sun runs out of hydrogen, gravity's influence becomes predominant, and the star's solid core begins to contract. As it contracts, it heats up. If it reaches a high enough temperature, a different type of fusion begins. At this point, the helium nuclei act as fuel and begin to fuse with one another. As the star swells to become a supergiant, the gravitational force is once again opposed by the outward pressure of energy from the material undergoing fusion. After the supply of helium is exhausted, the star repeats this process using the carbon and oxygen nuclei produced by helium fusion as its next source of fuel.
The cycle of producing and using heavier and heavier elements continues until all possible fuel sources are exhausted. When this happens, the core instantaneously collapses, and an enormous shock wave ripples outward through the star. This massive explosion, or supernova, jettisons solar gases and heavy elements into space. Depending on the size of the star, the remains of the collapsed core form either an incredibly dense, spinning neutron star or a black hole. Unlike Type Ia supernovae, which fade in a characteristic manner -- rapidly at first, then more slowly -- Type II supernovae fade differently from one event to another.
To learn more about another kind of supernova, check out Birth of a Supernova, Type Ia.
To learn more about the remnants of a type II supernova, check out Pulsars: Little Green Men.
To learn about how certain elements are created in a star, check out The Elements: Forged in Stars.