Helium flash

After the flash, the HCS eventually merges with the convective envelope and the surface composition is enhanced in CNO elements (the star being now a carbon star, as X(C)/X(0) = 4.8). When the model evolves to the next helium flash, the temperature at the former OVHECS (between 0.5058 M0 and... 

Eventually the He burning converts the core to carbon. Core thermonuclear shutdown occurs but fusion continues in the shell around the core, producing He followed by periods of helium flashes, causing wild variations in the luminosity. The Sun will then develop a super wind by convection that will blow off the overshell of the star, leaving a hot core behind. The expelled material forms a shell... 

Helium flash A rapid burst of nuclear reactions in the hydrogen-shell burning phase of stellar evolution. 

In this physical state, the onset of nuclear reactions can have explosive consequences. It is believed that nuclear combustion in a degenerate medium is responsible for the so-called helium flash that shakes small, ageing stars, but also for type la supernovas (to be discussed shortly). There are two types of star in which quantum pressure counterbalances the force of gravity, viz. white dwarfs and neutron stars. In the first case, the pressure is exerted by electrons, in the second, by neutrons. 

All stellar evolution can be summed up by a simple rule the star tries to make itself as small as possible. Its life story is one of contraction, but in a discontinuous manner, with sometimes long pauses during which it maintains its size. There are phases when the outer layers are driven off by radiation pressure (stellar winds, ejection of the envelope) and brief periods when the star violently readjusts itself, but without breaking apart (helium flash, thermal pulses). 

The evolutionary paths of the central density and temperature are plotted by the solid lines in Figure 1 for cases A-C. The dashed lines show the paths for the same accretion rates as for cases A-C but with the NCO reaction switched off. The dotted lines denote the ignition curves for the 3a and NCO reactions. Note that the NCO reaction dominates over the 3a reaction to heat up the core. The inclusion of the NCO reaction leads to the ignition of the helium flash at considerably lower density. 

In case A, once pc reaches pth, produced 4C quickly burns to 180. The core halts further contraction. The helium flash is initiated by the NCO reaction. In cases B and C, the condition Pc > P th s realized. Then 4N is completely converted into 4C in the central region. However, 4C has not yet been processed into 80, because Tc is too low for the a-capture to occur. The central density continues to increase until the helium flash is triggered by C burning. 

Referring to a specified sequence of a 0.7 M star presented by Sweigart and Gross (1978), we calculate the red giant sequence from the subgiant branch to the onset of the helium flash. The initial composition is X( He) = 0.999 and X( N) = 0.001. 

Sir, you mean that this helium flash occurs within a few hours after helium fusion begins ... 

Mr. Plex leans closer. Sir, the helium flash—is it bright ... 

It is bright deep inside the star. The peak luminosity during the helium flash can be 1014 solar luminosities or about 100 times the entire energy output of our Galaxy. However, this huge amount of energy does not destroy the star. 

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