A dredge-up is any one of several stages in the evolution of some
star
A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...
s. 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.
Multiple stages
*;''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
C/
C and
C/
N ratios are lowered, and the surface abundances of
lithium and
beryllium may be reduced.
*;''The second dredge-up'': The second dredge-up occurs in stars with 4–8
solar mass
The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass ...
es. 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 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
The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (about 0.5 to 8 solar masses) lat ...
, 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
A carbon star (C-type star) is typically an asymptotic giant branch star, a luminous red giant, whose atmosphere contains more carbon than oxygen. The two elements combine in the upper layers of the star, forming carbon monoxide, which consumes mos ...
''.[
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
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Stellar evolution