Stars 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... 

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). 

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. 

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. 

Hayashi track The line on a Hertzsprung-Russell diagram during which a star s luminosity stays relatively constant, while its temperature continues to decrease, helium burning A series of nuclear reactions in which helium nuclei fuse to make larger atomic nuclei, helium flash A period of very rapid helium burning that occurs in the core of a star with low mass. 

Following the core helium flash, the star quickly ( 106 years) moves to the Horizontal Branch (HB), where it burns 4He in a convective core, and hydrogen in a shell (that provides most of the luminosity). This corresponds to points 10-13 in Figure 3. The coulomb repulsion is larger for He than for H,... 

Fig. 4. (Top panel) radiated (dashed line) and He-burning (solid line) luminosity during the core helium flash for the 1M , Z = 0.004 model. (Bottom panel) mass of the temperature maximum as a function of time. At the flash peak, the maximum temperature is 0.2M0 from the centre of the star. The evolution of a 1M , Z = 0.02 star would be very similar although somewhat less extreme owing to the higher initial metallicity...

During the next 100000 years the situation is as follows in the non degenerate core, helium burns to carbon, the luminosity of the star is now 1/100 as it was at the time of the helium flash. The star shrinks, its surface temperature increases. In the H-R diagram (Fig. 9.10) the stars move to the left of the red giant branch. In this horizontal branch, stars burn helium in the core to carbon and hydrogen to He in a shell. Differences in their chemical composition affect where these stars fall on the horizontal branch. 

Several scenarios for creating such stars have been proposed. Lambert (1986) discussed the possibility of an explosion at the helium core flash ejecting the... 

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