Supernova is an exploding star that can become billions of times as bright as the sun before gradually fading from view. At its brightest, a supernova may outshine an entire galaxy. The explosion throws a large cloud of gas into space at speeds of up to 10 percent of the speed of light, which is 186,282 miles (299,792 kilometers) per second. The mass of the expelled material may exceed 10 times the sun’s mass (see Sun (Mass and density)). Most supernovae reach their peak brightness in one to three weeks and shine intensely for several months. Astronomers often classify a supernova according to the type of explosion involved. The two main types are: (1) thermonuclear supernovae and (2) core-collapse supernovae.
Thermonuclear supernovae
are thought to occur in certain binary stars. A binary star is actually a pair of stars that lie close together and orbit each other. Some such pairs include a white dwarf—a small, dense star made up of mostly carbon and oxygen. If the two stars lie close enough to each other, the gravitational pull of the white dwarf draws mass from the larger companion. When the white dwarf reaches a mass about 1.4 times that of the sun, atomic nuclei begin to fuse (join together) in its interior. This reaction, a type of thermonuclear fusion, releases a tremendous amount of energy. The rapid release of energy produces a thermonuclear explosion that destroys the white dwarf.
Thermonuclear supernovae serve an important role in cosmology, the study of the universe’s structure and development. Because all thermonuclear supernovae explode at about the same mass, they reach almost exactly the same peak brightness. Astronomers can therefore determine such a supernova’s distance by measuring how bright it appears from Earth. Studies of the distances of supernovae have revealed information about the universe’s size, shape, and development. Astronomers sometimes refer to thermonuclear supernovae as Type Ia supernovae, using a classification system based on the spectrum (range of light) a supernova emits.
Core-collapse supernovae
occur when single stars that are at least roughly 8 to 11 times as massive as the sun run out of fuel to sustain fusion. When this happens, the core of iron built up by the fusion process suddenly collapses. The collapse releases a huge amount of energy in the form of neutrinos (electrically neutral subatomic particles) and electromagnetic radiation (electric and magnetic energy). This energy causes the star to explode as a supernova.
Results of supernovae.
Supernovae leave behind various types of objects. Some leave a small, extremely dense object called a neutron star (see Neutron star). After other supernovae, an invisible object called a black hole may remain. A black hole has such a powerful gravitational pull that not even light can escape it (see Black hole). Some supernovae leave behind no object at all.
In 1054, Chinese astronomers recorded a supernova so bright that it was visible during the day. The explosion left behind a pulsar (rapidly spinning neutron star) and a huge cloud of gas and dust now known as the Crab Nebula. In 1987, an exploding star in the Large Magellanic Cloud, the galaxy closest to our own Milky Way, became the first supernova to be visible to the unaided eye in almost 400 years. In 2007, astronomers reported a stellar explosion that released more than 100 times the energy of a typical supernova. They estimated the star’s mass at more than 100 times that of the sun.
Scientists think that supernovae created all the heavier elements, such as iron, gold, and uranium, that are found on Earth and throughout the universe. Many cosmic rays appear to originate in supernovae. Cosmic rays are electrically charged, high-energy particles that travel through space (see Cosmic rays). Observations suggest that some core-collapse supernovae may produce gamma-ray bursts. A gamma-ray burst is a brief, extremely powerful flash of gamma rays—the most energetic form of electromagnetic radiation.
