Dredge-up

Types

• 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 ¹²C/¹³C and C/N ratios are lowered, and the surface abundances of lithium and beryllium may be reduced. The counter-intuitive existence of lithium-rich red giant stars that have gone through first dredge-up may be explained by scenarios such as mass transfer.

• 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. This second dredge-up causes an increase in the surface abundance of ⁴He and ¹⁴N, whereas the amount of ¹²C and ¹⁶O decreases.

• 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.

Note that not all stars will experience all three dedge-up types. For example, some lower-mass stars experience the first and third dredge-ups in their evolution without ever having gone through the second. A period of convection does occur on these stars at the start of the asymptotic giant branch, but does not reach as deep into the star as the first dredge-up; despite this, this period of convection is still sometimes referred to as the second dredge-up for comparison with the evolution of higher-mass stars.