Adredge-up is any one of several stages in the evolution of somestars. By definition, during adredge-up, aconvection zone extends all the way from the star's surface down to the layers of material that have undergonefusion. Consequently, the fusion products are mixed into the outer layers of the star's atmosphere, where they can be seen in stellarspectra.

• The first dredge-up

  The first dredge-up occurs when amain-sequence star enters thered-giant branch. As a result of the convective mixing, the outer atmosphere will display the spectral signature ofhydrogen fusion: The¹²C/¹³C andC/N ratios are lowered, and the surface abundances oflithium andberyllium may be reduced. The counter-intuitive existence of lithium-richred giant stars that have gone through first dredge-up may be explained by scenarios such asmass transfer.^[1]

• The second dredge-up

  The second dredge-up occurs in stars with 4–8 solar masses. Whenhelium fusion comes to an end at the core, convection mixes the products of theCNO cycle.^[2] This second dredge-up causes an increase in the surface abundance of⁴He and¹⁴N, whereas the amount of¹²C and¹⁶O decreases.^[3]

• The third dredge-up

  The third dredge-up occurs after a star enters theasymptotic giant branch, after aflash occurs in a helium-burning shell. The third dredge-up brings helium,carbon, and thes-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 acarbon star.^[3]

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 thefirst andthird dredge-ups in their evolution without ever having gone through the second.

• Stellar evolution

• Articles with short description
• Short description is different from Wikidata