dredge-up

The term dredge-up, in stellar evolution, refers to the movement of fused material in/near the stellar core to the outer portion or surface of the star through convection. This occurs during some phases of stellar evolution, the events referred to as dredge-ups. The dredged-up material and the reduction of displaced material can sometimes be discerned in the spectrum of the star. Such dredge-up events occur in stars more massive than red dwarfs, that enter the red-giant branch (RGB). (A red dwarf has convection from its core throughout its main sequence rather than as a few distinct events). Current stellar models presume (up to) three occurrences in the sufficiently-massive stars:

• First dredge-up - entering the RGB.
• Second dredge-up - when helium fusion ceases (end of RGB), for stars in the 4-8 solar mass range.
• Third dredge-up - during a helium flash while on the asymptotic giant branch (AGB).