Dredge-up
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A dredge-up is any one of several stages in the evolution of some stars . 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 . Consequently, the fusion products are mixed into the outer layers of the star's atmosphere, where they can be seen in stellar spectra .
Multiple stages
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- The first dredge-up
- The first dredge-up occurs when a main-sequence star enters the red-giant branch . As a result of the convective mixing, the outer atmosphere will display the spectral signature of hydrogen fusion: The 12 C / 13 C and C / N ratios are lowered, and the surface abundances of lithium and beryllium may be reduced.
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- The second dredge-up
- The second dredge-up occurs in stars with 4–8 solar masses . When helium fusion comes to an end at the core, convection mixes the products of the CNO cycle . [1] This second dredge-up causes an increase in the surface abundance of 4 He and 14 N, whereas the amount of 12 C and 16 O decreases. [2]
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- The third dredge-up
- The third dredge-up occurs after a star enters the asymptotic giant branch , after a flash occurs in a helium-burning shell. The third dredge-up brings helium, carbon , and the s -process products to the surface, increasing the abundance of carbon relative to oxygen; in some larger stars this is the process that turns the star into a carbon star . [2]
Note:
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 in which they occur in any one star. Some lower-mass stars experience the
first
and
third
dredge-ups in their evolution without ever having gone through the second.
References
- ↑ Lambert, D.L. (1992). "Observational effects of nucleosynthesis in evolved stars". In Edmunds, Mike G.; Terlevich, Roberto J. (eds.). Elements and the Cosmos . University of Cambridge . pp. 92–109. ISBN 0-521-41475-X .
- 1 2 Kwok, Sun (2000). The origin and evolution of planetary nebulae . Cambridge University Press . p. 199. ISBN 0-521-62313-8 .
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