Nova (or a recoccuring nova): cataclysmic nuclear explosion on a white dwarf which causes a brightening of the star. Novae are thought to occur on the surface of a white dwarf in a binary system. If the two stars of the system are sufficiently near to one another, material can be pulled from the companion star's surface onto the white dwarf. A nova is caused by the accretion of hydrogen onto the surface of the star, commencing a runaway fusion reaction.
Supernova (Type I and Type II): explosion of a massive supergiant star. May shine w brightness of 10 billion suns. The total energy output may be 1044 joules, as much as the total output of the sun during its 10 billion year lifetime. The likely scenario is thatfusion proceeds to build up a core of iron. The "iron group" of elements around mass number A=60 are the most tightly boundnuclei, so no more energy can be gotten from nuclear fusion.
Can be classified as Type I or Type II depending upon the shape of their light curves and the nature of their spectra.
Supernovae are classified as Type I if their light curves exhibit sharp maxima and then die away gradually. The maxima may be about 10 billion solar luminosities. Type II supernovae have less sharp peaks at maxima and peak at about 1 billion solar luminosities. They die away more sharply than the Type I. Type II supernovae are not observed to occur in elliptical galaxies, and are thought to occur in Population I type stars in the spiral arms of galaxies. Type I supernovae occur typically in elliptical galaxies, so they are probably Population II stars.
Photodisintegration: Occurs when a high-energy photon is absorbed by an atomic nucleus. The nucleus splits to form lighter elements, releasing a neutron, proton or alpha particle in the process. Photodisintegration in a core-collapse supernova undoes hundreds of thousands of years of nuclear fusion by splitting the iron nuclei into helium nuclei and neutrons.
Neutronization: A process by which, during the collapse of the core of a star, protons and electrons are forced together. The extremely high density of the core causes electrons to combine with protons, forms neutrons and neutrinos.
Chandrasekhar mass: Astrophysicist, Subrahmanyan Chandrasekhar, showed that it is impossible for a white dwarf star, which is supported solely by a degenerate gas of electrons, to be stable if its mass is greater than 1.44 times the mass of the Sun. If such a star does not completely exhaust its thermonuclear fuel, then this limiting mass may be slightly larger. All direct mass determinations of actual white dwarf stars have resulted in masses less than the Chandrasekhar limit. A star that ends its nuclear-burning lifetime with a mass greater than the Chandrasekhar limit must become either a neutron star or a black hole.
Stellar nucleosynthesis: process by which the natural abundances of the chemical elements within stars vary due to nuclear fusion reactions in the cores and overlying mantles of stars. Core fusion increases the atomic weight of its gaseous elements, causing pressure loss and contraction accompanied by increase of temperature. Structural changes of the star (evolution) become necessary to stabilize it. The energy released from nuclear fusion reactions accounted for the longevity of the Sun as a source of heat and light. The fusion of nuclei in a star, starting from its initial hydrogen and helium abundance, provides that energy and synthesizes new nuclei as a byproduct of that fusion process.
Neutron capture: nuclear reaction in which an atomic nucleus and one or more neutrons collide and merge to form a heavier nucleus. Neutron capture plays an important role in the cosmic nucleosynthesis of heavy elements. In stars it can proceed in two ways: as a rapid (r-process) or a slow process (s-process). Nuclei of masses greater than 56 cannot be formed by thermonuclear reactions (i.e. by nuclear fusion), but can be formed by neutron capture/
Neutron activation analysis: can be used to remotely detect the chemical composition of materials. This is because different elements release different characteristic radiation when they absorb neutrons. This makes it useful in many fields related to mineral exploration and security.
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