Resource: Birth of a Supernova, Type Ia

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. The more powerful Type Ia supernovae occur in what's called a binary system, in which a pair of 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 the 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.

A star between 0.5 and 1.4 times the size of our Sun does not explode when it has used up its fuel supply. Instead, it evolves into a smaller, denser, and cooler body called a white dwarf. After one of the stars in a binary system has become a white dwarf and the second star exhausts its fuel, the white dwarf's gravity field draws in the gases escaping from its dying companion. The white dwarf increases in mass until its core can no longer support itself, and it collapses in a huge explosion.

Supernovae are rare -- so rare that only three or four can be expected to occur in our galaxy each century. One was last observed in the Milky Way in 1604. However, thanks to continual improvements in telescope and instrument technology, astronomers can more easily detect and monitor supernovae in other galaxies. And although they cannot predict when a star will explode, observing the event and its aftermath can provide important clues to the workings of the universe. All Type Ia supernovae fade in a characteristic manner: rapidly at first, then more slowly. By analyzing the light they emit, scientists can measure distances to faraway galaxies and the speed at which the universe is expanding. They can also identify the elements from which the stars were made. And as the explosion fades to obscurity in a year or two, a gas remnant continues to mark where it occurred. From recent Hubble research, we know that Type Ia supernovae are not all of the same brightness, as was previously postulated.

To learn more about how scientists analyze light spectra to determine the elements that comprise a star, check out The Origin of the Elements.

To learn about how certain elements are created in a star, check out The Elements: Forged in Stars.

To learn more about another kind of supernova, check out Birth of a Supernova, Type II.

• What is a red giant? What is a white dwarf?
• What happens to the core of the red giant? Why?
• What are the different courses of events in the birth of Type Ia and Type II supernovae? What are the similarities between the two processes? (for both Type Ia and II resources)