Supernova

From Aetilc

Supernova, a massive star in the latter stages of stellar evolution that suddenly contracts and then explodes, increasing its energy output as much as a billionfold. Supernovas are the principal distributors of heavy elements throughout the universe; all elements heavier than iron are produced in supernovas. Supernovas also are the principal heat source for interstellar matter and may be a source of cosmic rays. Recent discoveries have confirmed an underlying connection between supernovas and gamma-ray bursts (GRBs). Both are associated with the deaths of massive stars and they often happen nearly simultaneously. There is no generally agreed upon model for how a massive star explodes. However, the association with gamma rays has renewed interest in the role played by stellar rotation and magnetic fields.

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[edit] Formation

Astronomers did not know what causes a star to explode in a super nova until the 1939, when an astrophysicist pieced together the sequence of events leading up to a supernova. He also calculated a figure for the mass of a star (known as Chandrasekhar's limit) that would determine if it would end up as a neutron star or a black hole.

Various theories have been proposed to explain the reasons a star explodes outward while collapsing inward. One theory is that the explosion is caused by a final burst of uncontrolled nuclear fusion. A more recent theory is that the explosion is due to the ejection of a wave of high-energy subatomic particles called neutrinos (electrically neutral particles in the lepton family). Just before the supernova came into view, a surge of neutrinos was detected in laboratories around the world. This supernova, called Supernova 1987A, was the first visible to the naked eye since 1604.

[edit] Distribution of Supernovas

At peak intensity, a supernova can shine as brightly as the entire galaxy in which it occurs. Novas are less spectacular and more common; they increase in brightness only by a few thousand times, and several occur in our galaxy every year. Supernovas can occur in that small percentage of stars having a mass greater than 8 to 10 times the mass of the sun and perhaps in certain binary stars.

[edit] Theoretical Models of Supernovas

[edit] Type I Supernovas

In a star about to become a Type I supernova, the star's hydrogen is exhausted, and the star's gravity pulling inward overcomes the forces of its thermonuclear fires pushing the material outward. As the core begins to contract, the remaining hydrogen ignites in a shell, swelling the star into a giant and beginning the process of helium burning. Eventually the star is left with a still contracting core of carbon and oxygen. If the star, now a white dwarf, has a nearby stellar companion, it will begin to pull matter from the companion. In many stars the excess matter is blown off periodically as a nova; if it is not, the star continues to get more and more massive until the matter in the core begins to contract again. When the star gets so massive that it passes Chandrasekhar's limit (1.44 times the sun's mass), it collapses very quickly and all of its matter explodes.

[edit] Type II Supernovas

Type II supernovas involve massive stars that burn their gases out within a few million years. If the star is massive enough, it will continue to undergo nucleosynthesis after the core has turned to helium and then to carbon. Heavier elements such as phosphorus, aluminum, and sulfur are created in shorter and shorter periods of time until silicon results. It takes less than a day for the silicon to fuse into iron; the iron core gets hotter and hotter and in less than a second the core collapses. Electrons are forced into the nuclei of their atoms, forming neutrons and neutrinos, and the star explodes, throwing as much as 90% of its material into space at speeds exceeding 18,630 mi (30,000 km) per sec. After the supernova explosion, there remains a small, hot neutron star, possibly visible as a pulsar, surrounded by an expanding cloud.

[edit] Words to Know

  • Black hole: Remains of a massive star that has burned out its nuclear fuel and collapsed under tremendous gravitational force into a single point of infinite mass and gravity.
  • Chandrasekhar's limit: Theory that determines whether an exploding supernova will become either a neutron star or a black hole depending on its original mass.
  • Neutrino: High-energy subatomic particle with no electrical charge and no mass, or such a small mass as to be undetectable.
  • Neutron star: Extremely dense, neutron-filled remains of a star following a supernova.
  • Nuclear fusion: Merging of two hydrogen nuclei into one helium nucleus, with a tremendous amount of energy released in the process.
  • Pulsar: Rapidly spinning, blinking neutron star.
  • Radio waves: Electromagnetic radiation, or energy emitted in the form of waves or particles.
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