Dredge-up refers to a period in the evolution of a star where a surface convection zone extends down to the layers where material has undergone nuclear fusion. As a result, the fusion products are mixed into the outer layers of the stellar atmosphere and these nuclides can appear in the spectrum of the star.
The first dredge-up occurs when a main sequence star enters the red giant branch (RGB). 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 second dredge-up occurs in stars with 4 – 8 solar masses of material. When helium fusion comes to an end at the core, convection mixes the products of the CNO cycle. This second dredge up results in an increase in the surface abundance of 4He and 14N, while the amount of 12C and 16O decreases.
The third dredge-up occurs after a massive star enters the asymptotic giant branch (AGB) and a flash occurs along a helium burning shell. This dredge-up causes helium, carbon and the s-process products to be brought to the surface. The result is an increase in the abundance of carbon relative to oxygen, which can create a carbon star.
The names of the dredge-ups are set by the evolutionary and structural state of the star in which each occurs, not by the sequence experienced by the star. As a result, lower-mass stars experience the first and third dredge-ups in their evolution but not the second.
- ^ Lambert, D. L. (1992). "Observational Effects of Nucleosynthesis in Evolved Stars". In Mike G. Edmunds and Roberto J. Terlevich. Elements and the Cosmos. University of Cambridge. pp. 92–109. ISBN 052141475X.
- ^ a b Kwok, Sun (2000). The origin and evolution of planetary nebulae. Cambridge University Press. p. 199. ISBN 0521623138.
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