For stars with masses approximately equal to that of the Earth's sun, the period of expansion of the shell and core shrinkage will continue, but the gravitational contraction will not release enough energy to reach the 600 million K needed to start nuclear fusion of carbon. Densities in the core continue to increase until about 10,000,000,000 kg per cubic meter, when the carbon ash in the core attains an electron degenerate state and can be compressed no further. At this stage temperatures stop rising, since there is no internal nuclear fusion and gravitation contraction has stopped, so no more potential energy is released. Thus, when the helium fuel is all burned, the fires go out.
When all the helium is used from the core and just a dense carbon ash remains, the core of the star is essentially dead. The inner core continues to accumulate the spent carbon while outer core layers form shells that burn helium and hydrogen at an intense pace and the outermost part of the star continues to expand. The intense burning in the helium shell is very unstable in this stage, and stars typically experience a series of helium shell flashes from the fluctuating temperatures at high pressures. These helium flashes send intense radiation waves to the star's surface, causing it to violently pulsate. Expansion of the outer layers of the star causes the temperature of the outer layers to drop, which has the effect of allowing electrons that were dissociated by the earlier higher temperatures to recombine with atoms, releasing additional photons in the process. The energy from these photons causes the outer envelope of the star to move progressively further from the core of the star, causing the radius to oscillate in progressively larger variations in distance from the core. This process causes the outer layers of the star to be ejected to space at tens of miles (km) per second.
The wild oscillations of the star and ejection of material from the outer layers forms a new highly unusual looking stellar structure called a planetary nebula. This nebula has a small, dense core made of the spent carbon ash with a shell of material still burning helium into carbon. A span of relatively empty space about the size of Earth's solar system surrounds the core, succeeded outward by a glowing ring consisting of the ejected outer layers of the former giant star. The Milky Way Galaxy contains more than 1,000 known planetary nebulae, which last only for a very brief period of stellar evolution, typically a few tens of thousands of years as compared to the billions of years of the star's previous life history. The planetary nebula stage marks the death, or the final active stage of the star, so the thousand or so planetary nebula known from the Milky Way represent the stars currently (when the light was emitted) going through their death throes. When stars die, however, they do not go away, they simply disappear from sight. As the dead stars fade from view they go through two more stages, the white dwarf and the black dwarf stages.
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