Supernova

I

Introduction

Supernova, violent explosion that occurs when gravitation causes a star to collapse onto itself. Supernovas, also called supernovae, can be brighter than all the stars in an entire galaxy combined and can shine for weeks or months. The extreme conditions in supernova explosions forge atomic particles into chemical elements—all the atoms in the universe that are heavier than iron were formed in supernovas. The explosions also spread gas and dust into space where the chemically enriched interstellar material can form new stars and planets—almost every substance on Earth is made in part or in whole from the ashes of supernovas.

Supernovas can leave behind some of the strangest objects in the universe—neutron stars and black holes—and may give off bursts of gamma rays and cause ripples in space-time called gravitational waves. A supernova also unleashes intense electromagnetic radiation that could destroy all life on Earth if one exploded too near to our solar system. The extreme brightness of some supernovas allows astronomers to measure the distances and motions of galaxies far back into time, indicating how fast the universe is expanding.

Supernovas are very rare events and the vast majority of stars, including our Sun, do not have enough mass to reach a supernova stage. Because the universe is filled with billions of galaxies, astronomers are able to observe a few hundred distant supernovas a year. Throughout the observable universe, in fact, a supernova goes off about every second. However, most of these events are not detected because astronomers cannot scan the entire universe at once. Over the past 1,000 years only about a half-dozen supernovas have exploded in our own Milky Way Galaxy.

Supernovas are distinct from novas (from the Latin term nova stella meaning “new star”). Novas occur in double-star systems that have a normal star and a white dwarf star orbiting close together around a common center of gravity. Gas drawn off the normal star explodes on the surface of the white dwarf star in a nuclear reaction that brightens then fades. The process can happen repeatedly. A supernova is a one-time event that affects the core of the star, not only its surface, and is billions of times brighter than a nova.

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II

Types of Supernovas

How a supernova explodes and what kind of object is left by the explosion depends on basic features of the star itself.

A star forms from a cloud of gas and dust that contracts in on itself from the force of gravity. If the object at the center of the collapsing cloud has at least 8 percent the mass of the Sun, gravity can compress the gas in the core of the object to a point where the mutual repulsion of atomic nuclei is overcome and nuclear fusion of hydrogen into helium begins. In the process a small amount of matter is turned into large amounts of energy as expressed in Einstein’s famous equation E = mc² (energy equals mass times the speed of light squared). The energy released at the star’s core heats and pushes back the outer layers of gas, and the star begins to shine.

A star’s existence is a constant battle between gravity pulling gases in toward the center and energy pushing back. Over a star’s life, lighter elements are fused together into heavier elements. The star’s final fate is tied to how long it can sustain nuclear fusion. As a basic rule, the more massive the star, the hotter and faster it will burn up its nuclear fuel, and the more cycles of burning heavier elements it can reach. An extremely massive star may last only for a few million years, compared to the 10-billion-year life span of our Sun. See also Life Cycles and Ages of Stars: Star (astronomy).

In very large stars known as supergiants, the fusion processes can continue to create and burn nuclei of heavier elements including carbon, oxygen, nitrogen, neon, and silicon. Fusion of different elements occurs at different layers in the stars, giving them an onion-like structure. If these stars are less than 10 times the mass of the Sun, they generally will shed enough of their outer layers to end up as white dwarf stars. Supergiant stars that have at least 8 to 10 times the mass of the Sun can explode as supernovas.

A

Classification of Supernovas by Light Spectrum

The spectrum of the light from supernovas is an important source of information that can be used to help classify supernova explosions. Spectral lines indicate what chemical elements are present and what temperatures occur in the explosion. These spectral lines vary as the brightness of a supernova fades.

Astronomers use the term Type I for supernovas that do not show spectral lines of hydrogen and Type II to describe supernovas that do. It is thought that Type I supernovas explode after the stars have lost their outer layers of hydrogen gas. Type II supernovas explode at a stage when the stars still have hydrogen gas in their atmospheres.

Scientists recognize a number of subtypes for Type I supernovas, known as Type Ia, Type Ib, and Type Ic. Like Type Ia supernovas, Types Ib and Ic lack hydrogen lines in their spectra. However, they result from a different explosion mechanism from Type Ia—core collapse of a giant star rather than from the thermonuclear explosion of a white dwarf star. Different types of supernovas also have distinct light curves or patterns of brightness over time. Type Ia supernovas are typically brighter than other supernovas, while Type Ib and Type Ic are generally dimmer.

The expanding debris from Type I supernovas gets its energy for shining from the radioactive decay of isotopes created in the explosion. Type II supernovas are thought to shine as the envelope of material that surrounded the giant star before it exploded is heated by the shock wave created in the core collapse event.

B

Classification of Supernovas by Explosion Mechanism

Another way to classify supernovas is by how they explode. Two basic mechanisms involving gravitation are thought to cause a star to collapse onto itself, generating a supernova explosion: (1) core collapse and (2) excess matter added to a white dwarf, causing a thermonuclear explosion.

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