Helium flashover

nuclear fusion
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Helium flash occurred in the low-mass star core with 0.8 solar mass (M ⊙) to 2.0 M ⊙ Red Giant The stage is a very short runaway heat nuclear fusion A large amount of helium becomes carbon through two ionization processes (it is predicted that the sun will occur 1.2 billion years after leaving the main sequence belt). [1] Many rare runaway helium fusion processes can also be White dwarf accretion On the surface. Because these low-mass stars cannot perform helium fusion reaction to resist the effect of gravity when the hydrogen in the core is exhausted, it will eventually be because helium is quantum mechanical Degeneracy State pressure is in balance with gravity, not blocked by thermal pressure Gravitational collapse When this helium accumulates to a certain proportion in the core, it will undergo very intense helium fusion (combustion). This extrusion process leads to the increase of core temperature and density. Finally, when the core temperature reaches 100 million K, it will expand and resist gravity at an amazing rate, and make the temperature drop (because there is too much hydrogen in the main sequence zone, it will not happen).
However, the basic characteristic of degenerate matter is that the temperature change will not affect the volume, so it is not limited by the rule of hydrostatic balance passing fusion rate. The very high density accelerates the fusion rate, leading to runaway nuclear reaction, which releases the energy equivalent to the entire galaxy in a few minutes. This is purely described by the model of astrophysics, because the energy of normal low mass stars will be absorbed by the outer atmosphere and cannot be found and observed. At the end of this process, the material is heated until thermal pressure becomes dominant again, so the material will expand and cool. It is estimated that about 40% of the core mass is electron degenerate helium, and 6% of the core is converted to carbon. [2]
Chinese name
Helium flashover
Foreign name
helium flash
Interpretation
Degenerate matter Natural explosion

brief introduction

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Helium flash is in the core of a medium mass star, or White dwarf Surface deposited helium Sudden nuclear fusion
It is Degenerate matter Natural explosion. When Degeneracy pressure (purely a function of density) When the thermal pressure (proportional to density and temperature) is exceeded, the correlation between total pressure and temperature is weak.

process

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Helium flash
Once the temperature reaches 100 million to 200 million K nuclear fusion , the temperature will increase rapidly, which will further increase Helium fusion The rate and reaction area, but will not increase the pressure, so the core will not be stable expansion (and cooling). The runaway thermal reaction makes the energy released by the star exceed the normal value very quickly (only a few seconds) fixed star 1 million times, until the increased temperature makes the thermal pressure grasp the advantage again, which can be ignored Degeneracy pressure
For a medium mass star, gravity Collapse The core density of the star is very high, so helium flash will occur when the hydrogen in the core is exhausted. During contraction, the core temperature becomes higher and higher until the outer shell expands outward Red Giant Phase of. When the star continues to shrink due to gravity, it finally becomes Degenerate matter Degeneracy Makes the star's temperature rise, and Helium combustion It began to approach the end of the explosion.
When hydrogen comes from companion Accumulate to White dwarf After that, hydrogen usually fuses into helium. These helium forms a helium shell on the surface, and when the amount of helium is enough, helium flash may occur, which is under the condition of thermal runaway fusion Supernova people say that Type I supernova It is the result of helium flash.
Shell helium flash It is similar to helium combustion, although it does not need to rely on Degenerate matter , but it will occur periodically in the shell outside the core of the asymptotic giant star branch star.
Two four He nucleated eight Be Nucleus Extremely unstable, if we are lucky to have another four He fusion can form twelve C。 This process is also called 3 α reaction. Due to the combustion process Hydrogen combustion Very short, Helium combustion The process is called helium flash.

Red Giant

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For stars with mass less than 2.0M ⊙, in the red giant stage of star evolution, hydrogen in the core has been exhausted, leaving a helium rich core. While the hydrogen in the shell continues to fuse, so that the helium ash in the core continues to accumulate, increasing the density of the core, but the temperature still does not meet the requirements for helium fusion in stars with higher mass. Therefore, the thermal pressure generated from nuclear fusion is not enough to create Hydrostatic balance And resist gravitational collapse. This causes the star to increase the heat content per unit volume, causing the temperature to rise until enough helium is compressed in the core to become degenerate matter. This degenerate pressure is finally enough to prevent the core from further collapsing, but the rest of the core will continue to shrink and make the temperature continue to rise until it reaches this point (≈ 1 × 10K), so that helium can ignite and start to fuse. [3-5]
The naturally occurring helium flash originates from degenerate matter. Once the temperature reaches 100 million to 200 million K, the helium core will carry out the 3 helium process, and the temperature will rise rapidly, further improving the helium fusion rate, and because the degenerate material is a good conductor of heat, the reaction area will be expanded.
However, because the degenerate pressure (purely a function of density) exceeds the thermal pressure (proportional to density and temperature), the correlation between total pressure and temperature is weak. Therefore, the dramatic warming only slightly increases the pressure, without expansion cooling of the stable core.
This runaway reaction quickly (for a few seconds) causes the star to generate hundreds of billions of times the energy of the normal star, until the temperature rises again to the thermal pressure becomes the dominant force again, eliminating the degenerate state. The core can then expand and continue to steadily burn the remaining helium. [6]
For stars with a mass of more than 2.25M ⊙, the core starts to burn helium in the core before entering the degenerate state, so this type of helium flash does not occur. For stars with very low mass (less than 0.5M ⊙), the core will never be reset enough to ignite helium. The degenerate core will continue to be maintained and eventually become a helium white dwarf star.
Helium flash is not directly observed by the electromagnetic waves radiated from the surface. The flash occurs deep in the core, and the net effect is that the whole core absorbs the released energy and leaves the degenerate state to become non degenerate matter. Earlier calculations showed that in some cases there would be a non splitting mass loss, [7] However, the energy loss of neutrinos was later added to the calculation, which showed that there was no such direct loss. [8-9]

White dwarf conjoint

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When hydrogen is accreted from the companion star of a white dwarf, the absorption rate range of hydrogen to helium is very narrow, but the hydrogen layer of most systems is developed inside the degenerate white dwarf. These hydrogen can form hydrogen shells near the surface of stars. When the mass of hydrogen is large enough, the runaway fusion causes the nova. In some conjoined star systems, hydrogen fusion on the surface can make a large amount of helium establish an unstable helium flash. In some conjoint star systems, the companion star may have lost a lot of hydrogen and donated helium rich materials to the compact star. Note that there may be neutron stars similar to lightning.

Shell helium flash

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Shell helium flash is a similar phenomenon, but it is not so intense, there is no runaway helium ignition, and it does not occur in the degenerate state. They will appear periodically in the outer layer of the core of the asymptotic giant star branch star. This is in the late life of the giant star stage. The star has exhausted most of the available helium fuel in its core. Its core is now a helium core composed of carbon and oxygen. Helium continues to burn in the outer shell of the core, but this thin layer will stop with the depletion of helium. This allows the hydrogen fusion on the upper layer of the helium layer to continue. After enough helium is accumulated, the helium fusion is ignited again, leading to temporarily brightened and expanded pulsars (this change will be delayed for several years, because it will take years to ignite the helium fusion again and transmit energy to the surface). This pulse may last for hundreds of years, and the period of occurrence may be 10000 to 100000 years. After the flash, the helium fusion continues to decay exponentially, accounting for about 40% of the cycle period, and the shell is consumed. The thermal pulse may cause the outflow of dust and gas, forming the arcuate shell. [10]