What is supernova?
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A star begins its life as a large and comparatively cool mass of gas. The contraction of this gas and the subsequent rise of temperature continue until the interior temperature of the star reaches a value of about 1,000,000° C (1,800,000° F). At this point nuclear reactions take place, the net result of which is that the nuclei of hydrogen atoms combine with deuterons (nuclei of so-called heavy hydrogen atoms) to form nuclei of helium. The latter reaction liberates large amounts of nuclear energy, and the further contraction of the star is halted.

When the release of energy from the deuteron-hydrogen nucleus reaction ends, contraction begins anew, and the temperature of the star increases again until it reaches a point at which a nuclear reaction can occur between hydrogen and lithium and other light metals present in the body of the star. Again energy is released and contraction stops.When the lihium and other light materials are consumed, contraction resum and the star enters the final stage of development. This thermonuclear reaction is characteristic of the main sequence of stars. The star gradually swells and becomes a red giant. It attains its greatest size when all its central hydrogen has been converted into helium. If it is to continue shining, the temperature at the centre must rise high enough to cause fusion of the helium nuclei. Durinthis process the star probably becomes much smaller and denser. When it has exhausted all possible sources of nuclear energy, it may contract further and become a white dwarf. This final stage may be marked by the stellar explosions known as novae. When a star sheds its outer envelope explosively as a nova or supernova, it returns to the interstellar medium elements heavier than hydrogen that it has synthesized in its interior. Future generations of stars formed from this material will therefore start life with a richer supply of heavier elements than the earlier generations of stars. Stars that shed their outer layers in a non-explosive fashion become planetary nebulae, old stars surrounded by spheres of gas radiating across a range of wavelength.

A schematic diagram showing the main sequence and areas occupied by stars at different stages of evolution is Hertzsprung Russell diagram. Absolute magnitude is plotted against color index in this example. The dashed lines link stars of different luminosities with the same spectral types but different color indices.

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Colour, temperature or some other comparable quantity may be substitude for spectral type as the quantity plotted along the horizontal axis. Temperature conventionally decreasses towards the right. Eighter magnitude or luminosity relative to the Sun are frequently used for the verticl scale, The resulting graph may also be called a colour magnitude diagram or colour luminosity diagram according to the actual quantities used.

 As a result of gravitational forces acting against the nuclear structure of the core of a fuel depleted star, tremendous shock waves are generated which cause the outside layers of the star to be blown away from the core. A catastrophic stellar explosion in which so much energy is released that the supernova alone can outshine an entire galaxy of billions stars. In addition to the radiant energy produced, ten times as much energy goes into the kinetic energy of the material blown out by the explosion, and a hundred times as much is curried off by neutrions. This can happen in one of two ways depending on the type of supernova.

Type I Supernovae. These type of supernovae involve two stars, one of them being a white dwarf whose gravitational attraction is so intense that it is capable of siphoning off material from its companion. Unfortunately for the star (but fortunately for us at a long distance!), the white dwarf exceeds its Chandrasekhar limit of stability causing it to go into thermonuclear instability and produces one of the largest explosions known in the Universe, the Type I SN. There are currently three types of Type I SNe accepted by the astronomical community in general. The subclass types (Ia, Ib, and Ic) are basically determined by the state of the white dwarf's companion star, though to qualify as a Type I SN the companion should have expelled its hydrogen layer. Mike Richmond's SN Taxonomy table gives a good schematic idea about the (more or less) current thinking on the topic.

 Type II (Core Collapse) Supernovae. Gravitational forces condensing hydrogen gas raises the temperature at the center of the star to the point where nuclear fusion is initiated. According to the Onion Skin Model (illustrated above), the following sequence occurs. Hydrogen is fused into helium and energy is given off in the process. As more helium accumulates at the center, the temperature rises due to compression until another nuclear fusion is initiated. This time helium is converted to carbon and oxygen and additional energy is given off during the nuclear fusion. A similar process continues with carbon and oxygen fusing to neon, magnesium, and oxygen. These elements then undergo another fusion process as the temperature and pressure increase to produce silicon and sulfur. The latter two elements then fuse into iron. During each nuclear fusion, energy is given off. This takes two orders of magnitude less time to happen than on the previous fusion. However, nuclear fusion stops at iron because energy is no longer produced by fusion. The irocore collapses very quickly (within hours or less). Since the iron core can collapse only so far and can no longer undergo fusion, it becomes extremely hot and now begins to expand rapidly. This occurs while the star's outer shells are rushing in to fill the void left by the collapsed iron core. The expanding iron and the collapsing outer gases collide with each other producing tremendous shock waves which blow the outer layers away from the core, thus causing the supernova's gigantic explosion.

Remember that these are only models attempting to explain these massive explosions. They can change at any time! It is fun attending a conference on the topic of SNe. The research astronomers exchange videos with each other to show how their current model actually accomplishes the explosive event. The onion skin model (for type II SN and possibly one component of a type Ib SN) serves to illustrate the layered structure of massive stars with lighter gases on the outside becoming heavier gases on the inside through nuclear fusion.

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What happens after the explosion depends on the type and mass of the progenitor stars. Mostly they produce a gas cloud called a supernova remnant which initially expands at a rate of about 10,000 km/s. Gradually the expansion rate slows down while dissipating into the interstellar medium, seeding the neighborhood with heavy elements and providing the necessary shock waves for new stellar formation. The Crab Nebula, M1 (image), is a remnant of the supernova of 1054 (which occurred within our Milky Way Galaxy).

Edited by Boris Andreic and Vibor Jelic

 

Literature:                                                                                                                                    Encarta 97 Enciclopedia                                                                         http://cssa.stanford.edu/~marcos/sne.html                                http://www.chapman.edu/oca/benet/mrgalaxy.htm

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