![]() The red-giant stage of the star's evolution is finally completed by the ejection of the outer mass and the formation of a planetary nebula.įor a solar mass star, the red-giant phase normally lasts just around a billion years overall, with practically all of that time being spent on the red-giant branch. Instead, the star will expel its outer layers near the conclusion of the asymptotic-giant-branch phase, leaving the star's core exposed and generating a planetary nebula before eventually becoming a white dwarf. However, fusion in a star's degenerate carbon-oxygen core will never begin in a star with a mass of fewer than eight times that of our Sun. This process of fusing helium tends to result in the formation of a carbon-oxygen core. Once the core has degenerated, it will continue to heat up until it reaches a temperature of about 10 8 K, which is hot enough to start the triple-alpha process of fusing helium to carbon, so-called because it involves three helium-4 isotopes or alpha particles.Īs a result, the star enters a second red-giant phase, the asymptotic giant branch. When the core of the Sun and other stars less massive than about two times our Sun becomes sufficiently dense, electron degeneracy pressure will stop them from further collapsing. The star's mass determines the course of its evolution as it progresses down the red-giant branch. ![]() Once the star cools sufficiently it stops expanding, its luminosity begins to rise, and it starts to ascend the red-giant branch of the Hertzsprung-Russell (H-R) diagram. ![]() The stage where the star is in the process of cooling and expanding is often called the subgiant stage. Because the star's surface has expanded, the energy at the surface is dissipated, and the star's surface cools. The extra heat from this causes the outer layers of the star to expand greatly.īurning hydrogen in the shell results in what has been called the mirror principle when the core within the shell contracts, the layers of the star outside the shell must expand. ![]() Nuclear processes can no longer take place in the star's core as it runs out of hydrogen fuel, which causes the core to start contracting under the star's gravity.Īs the core collapses, the shell of plasma surrounding the core becomes hot enough to begin fusing hydrogen. ![]()
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