![]() ![]() When helium fusion comes to an end at the core, convection mixes the products of the CNO cycle. The second dredge-up The second dredge-up occurs in stars with 4–8 solar masses.As a result of the convective mixing, the outer atmosphere will display the spectral signature of hydrogen fusion: The 12C/ 13C and C/ N ratios are lowered, and the surface abundances of lithium and beryllium may be reduced. The first dredge-up The first dredge-up occurs when a main-sequence star enters the red-giant branch.Consequently, the fusion products are mixed into the outer layers of the star's atmosphere, where they can be seen in stellar spectra. By definition, during a dredge-up, a convection zone extends all the way from the star's surface down to the layers of material that have undergone fusion. ![]() A dredge-up is any one of several stages in the evolution of some stars.
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