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.
HINT:
đ Dredge-up in the context of Carbon burning process
The carbon-burning process or carbon fusion is a set of nuclear fusion reactions that take place in the cores of massive stars (at least 4Â Mâ at birth) that combines carbon into other elements. It requires high temperatures (>5Ă10Â K or 50Â keV) and densities (>3Ă10Â kg/m).
These figures for temperature and density are only a guide. More massive stars burn their nuclear fuel more quickly, since they have to offset greater gravitational forces to stay in (approximate) hydrostatic equilibrium. That generally means higher temperatures, although lower densities, than for less massive stars. To get the right figures for a particular mass, and a particular stage of evolution, it is necessary to use a numerical stellar model computed with computer algorithms. Such models are continually being refined based on nuclear physics experiments (which measure nuclear reaction rates) and astronomical observations (which include direct observation of mass loss, detection of nuclear products from spectrum observations after convection zones develop from the surface to fusion-burning regions â known as dredge-up events â and so bring nuclear products to the surface, and many other observations relevant to models).
In this Dossier
- â Core Definition: Dredge-up
- đ Dredge-up in the context of Carbon burning process
- Dredge-up in the context of NGC 5315
Dredge-up in the context of NGC 5315
NGC 5315 is a planetary nebula in the southern constellation Circinus. Of apparent magnitude 9.8 around a central star of magnitude 14.2, it is located 5.2 degrees west-southwest of Alpha Circini. It is only visible as a disc at magnifications over 200-fold. The nebula was discovered by astronomer Ralph Copeland in 1883. The central star has a stellar class of WC4 and is hydrogen deficient with an effective temperature of 76-79Â kK. The distance to this nebula is not known accurately, but is estimated to be around 6.5 kilolight-years.
This planetary nebular has a slightly elliptical form, a complex structure, and a ring that is somewhat broken. It shows a typical abundance of carbon and a slightly enhanced nitrogen abundance. Radial velocity studies indicate that the star may be a member of a binary system. The nebula does not show enrichment of s-process elements. This suggests that the star's asymptotic giant branch stage may have been truncated by interaction with the companion. Alternatively, the star may be low in mass and did not undergo third dredge-up, or the star's s-process elements were heavily diluted by the envelope during the AGB phase.
View the full Wikipedia page for NGC 5315