A dredge-up is any one of several stages in the evolution of some stars. By definition, during a ''dredge-up'', a

convection zone A convection zone, convective zone or convective region of a star is a layer which is unstable due to convection. Energy is primarily or partially transported by convection in such a region. In a radiation zone, energy is transported by radiatio ...

extends all the way from the star's surface down to the layers of material that have undergone

fusion Fusion, or synthesis, is the process of combining two or more distinct entities into a new whole. Fusion may also refer to: Science and technology Physics *Nuclear fusion, multiple atomic nuclei combining to form one or more different atomic nucl ...

. 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

*;''The first dredge-up'': The first dredge-up occurs when a

main-sequence In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar He ...

star enters the

red-giant branch The red-giant branch (RGB), sometimes called the first giant branch, is the portion of the giant branch before helium ignition occurs in the course of stellar evolution. It is a stage that follows the main sequence for low- to intermediate-mass sta ...

. As a result of the convective mixing, the outer atmosphere will display the spectral signature of

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

fusion: The C/ C and C/ N ratios are lowered, and the surface abundances of

lithium Lithium (from el, λίθος, lithos, lit=stone) is a chemical element with the symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard conditions, it is the least dense metal and the least dense solid ...

and

beryllium Beryllium is a chemical element with the symbol Be and atomic number 4. It is a steel-gray, strong, lightweight and brittle alkaline earth metal. It is a divalent element that occurs naturally only in combination with other elements to form m ...

may be reduced. *;''The second dredge-up'': The second dredge-up occurs in stars with 4–8 solar masses. When

helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...

fusion comes to an end at the core, convection mixes the products of the

CNO cycle The CNO cycle (for carbon–nitrogen–oxygen; sometimes called Bethe–Weizsäcker cycle after Hans Albrecht Bethe and Carl Friedrich von Weizsäcker) is one of the two known sets of fusion reactions by which stars convert hydrogen to helium, ...

. This second dredge-up causes an increase in the surface abundance of He and N, whereas the amount of C and O decreases. *;''The third dredge-up'': The third dredge-up occurs after a star enters the asymptotic giant branch, after a

flash Flash, flashes, or FLASH may refer to: Arts, entertainment, and media Fictional aliases * Flash (DC Comics character), several DC Comics superheroes with super speed: ** Flash (Barry Allen) ** Flash (Jay Garrick) ** Wally West, the first Kid F ...

occurs in a helium-burning shell. The third dredge-up brings helium,

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon mak ...

, 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: 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

{{star Stellar evolution