|
A helium flash is a very brief thermal runaway nuclear fusion of large quantities of helium into carbon through the triple-alpha process in the core of low mass stars (between 0.8 solar masses () and 2.0 〔(Chapter 9: Post-main sequence evolution through helium burning )〕) during their red giant phase (the Sun is predicted to experience a flash 1.2 billion years after it leaves the main sequence). A much rarer runaway helium fusion process can also occur on the surface of accreting white dwarf stars. Due to the inability of these low mass stars to counter the pressure of gravity by fusing helium when the hydrogen in the core is exhausted, eventually a flash of very intense helium fusion (or burning) occurs because the accumulation of helium which has become degenerate matter has reached a certain percentage of the total helium in the core where it is supported against gravitational collapse by quantum mechanical pressure rather than thermal pressure. This process causes an increase in the temperature and density of the core which then finally does undergo helium burning when the core reaches 100 million kelvin but at a tremendous rate because this burning does not act to expand the matter against the pressure of gravity and hence cool it (as does happen in main sequence stars which burn too much hydrogen) because it is degenerate, and a fundamental quality of degenerate matter is that changes in temperature will not affect the volume of the mass, and thus there is no regulation of the rate of fusion through hydrostatic equilibrium. The very high density speeds up the fusion rate until there is a runaway nuclear reaction which lasts a few minutes and which briefly emits energy at a rate comparable to the whole Milky Way. In the case of normal low mass stars, the energy is absorbed by the star and thus is undetected by observation and is solely described by astrophysical models. The process ends when the material is heated to the point where thermal pressure again becomes dominant, and the material then expands and cools. It is estimated that the electron-degenerate helium core weighs about 40% of the star mass and that 6% of the core is converted into carbon.〔(The End Of The Sun )〕 ==Red giants== During the red giant phase of stellar evolution in stars with less than 2.0 the nuclear fusion of hydrogen ceases in the core as it is depleted, leaving a helium-rich core. While fusion of hydrogen continues in the star’s shell causing a continuation of the accumulation of helium ash in the core, making the core denser, the temperature still is unable to reach the level required for helium fusion, as happens in more massive stars. Thus the thermal pressure from fusion is no longer sufficient to counter the gravitational collapse and create the hydrostatic equilibrium found in most stars. This causes the star to start contracting and increasing in temperature until it eventually becomes compressed enough for the helium core to become degenerate matter. This degeneracy pressure is finally sufficient to stop further collapse of the most central material but the rest of the core continues to contract and the temperature continues to rise until it reaches a point () at which the helium can ignite and start to fuse. The explosive nature of the helium flash arises from its taking place in degenerate matter. Once the temperature reaches 100 million–200 million kelvin and helium fusion begins using the triple-alpha process, the temperature rapidly increases, further raising the helium fusion rate and, because degenerate matter is a good conductor of heat, widening the reaction region. However, since degeneracy pressure (which is purely a function of density) is dominating thermal pressure (proportional to the product of density and temperature), the total pressure is only weakly dependent on temperature. Thus, the dramatic increase in temperature only causes a slight increase in pressure, so there is no stabilizing cooling expansion of the core. This runaway reaction quickly climbs to about 100 billion times the star's normal energy production (for a few seconds) until the temperature increases to the point that thermal pressure again becomes dominant, eliminating the degeneracy. The core can then expand and cool down and a stable burning of helium will continue.〔 〕 A star with mass greater than about 2.25 starts to burn helium without its core becoming degenerate, and so does not exhibit this type of helium flash. In a very low-mass star (less than about 0.5 ), the core is never hot enough to ignite helium. The degenerate helium core will keep on contracting, and finally becomes a helium white dwarf. The helium flash is not directly observable on the surface by electromagnetic radiation. The flash occurs in the core deep inside the star, and the net effect will be that all released energy is absorbed by the entire core, leaving the degenerate state to become nondegenerate. Earlier computations indicated that a nondisruptive mass loss would be possible in some cases,〔(Two- and three-dimensional numerical simulations of the core helium flash ) by Deupree, R. G.〕 but later star modeling taking neutrino energy loss into account indicates no such mass loss.〔(A Reexamination of the Core Helium Flash ) by Deupree, R. G.〕〔(Multidimensional hydrodynamic simulations of the core helium flash in low-mass stars ) by Mocák, M.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「helium flash」の詳細全文を読む スポンサード リンク
|