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Dredge ups1/20/2024 ![]() ![]() This third dredge–up transports large amounts of freshly synthesized carbon to the star’s surface, and the star’s spectrum thus exhibits prominent absorption bands of carbon–rich molecules like C 2, CH, and CN. Still later, during the AGB stage, a third dredge–up can occur if the star has a mass greater than about 2 M ⊙. A second dredge–up occurs after core helium fusion ceases, further altering the abundances of carbon, nitrogen, and oxygen. Convection dips so deeply into the star that material processed by the CNO cycle of hydrogen fusion (see Section 16-1) is carried up to the star’s surface, changing the relative abundances of carbon, nitrogen, and oxygen. The first dredge-up takes place after core hydrogen fusion stops, when the star becomes a red giant for the first time. At these times, convection can “dredge up” the heavy elements produced in and around the core by nuclear fusion, transporting them all the way to the star’s surface. ![]() During the final stages of a star’s life, however, the convective zone can become so broad that it extends down to the star’s core. ![]() In the Sun, convection dominates only the outer layers, from around 0.71 solar radius (measured from the center of the Sun) up to the photosphere (recall Figure 16-4). ![]()
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