Dredge-up
Encyclopedia
Dredge-up refers to a period in the evolution of a star
where a surface convection zone
extends down to the layers where material has undergone nuclear fusion
. As a result, the fusion products are mixed into the outer layers of the stellar atmosphere and these nuclide
s can appear in the spectrum
of the star.
The first dredge-up occurs when a main sequence
star enters the red giant branch (RGB). 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 second dredge-up occurs in stars with 4 – 8 solar mass
es of material. When helium
fusion comes to an end at the core, convection mixes the products of the CNO cycle
. This second dredge up results in an increase in the surface abundance of 4He and 14N, while the amount of 12C and 16O decreases.
The third dredge-up occurs after a massive star enters the asymptotic giant branch
(AGB) and a flash
occurs along a helium burning shell. This dredge-up causes helium, carbon
and the s-process
products to be brought to the surface. The result is an increase in the abundance of carbon relative to oxygen, which can create a carbon star
.
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 experienced by the star. As a result, lower-mass stars experience the first and third dredge-ups in their evolution but not the second.
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
where a surface convection zone
Convection zone
The convection zone of a star is the range of radii in which energy is transported primarily by convection. In the radiation zone, energy is transported by radiation...
extends down to the layers where material has undergone nuclear fusion
Stellar nucleosynthesis
Stellar nucleosynthesis is the collective term for the nuclear reactions taking place in stars to build the nuclei of the elements heavier than hydrogen. Some small quantity of these reactions also occur on the stellar surface under various circumstances...
. As a result, the fusion products are mixed into the outer layers of the stellar atmosphere and these nuclide
Nuclide
A nuclide is an atomic species characterized by the specific constitution of its nucleus, i.e., by its number of protons Z, its number of neutrons N, and its nuclear energy state....
s can appear in the spectrum
Astronomical spectroscopy
Astronomical spectroscopy is the technique of spectroscopy used in astronomy. The object of study is the spectrum of electromagnetic radiation, including visible light, which radiates from stars and other celestial objects...
of the star.
The first dredge-up occurs when a main sequence
Main sequence
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 Hertzsprung and Henry Norris Russell...
star enters the red giant branch (RGB). As a result of the convective mixing, the outer atmosphere will display the spectral signature of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
fusion: the 12C
Carbon-12
Carbon-12 is the more abundant of the two stable isotopes of the element carbon, accounting for 98.89% of carbon; it contains 6 protons, 6 neutrons, and 6 electrons....
/13C
Carbon-13
Carbon-13 is a natural, stable isotope of carbon and one of the environmental isotopes. It makes up about 1.1% of all natural carbon on Earth.- Detection by mass spectrometry :...
and C
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
/N
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
ratios are lowered, and the surface abundances of lithium
Lithium
Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...
and beryllium
Beryllium
Beryllium is the chemical element with the symbol Be and atomic number 4. It is a divalent element which occurs naturally only in combination with other elements in minerals. Notable gemstones which contain beryllium include beryl and chrysoberyl...
may be reduced.
The second dredge-up occurs in stars with 4 – 8 solar mass
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
es of material. When helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
fusion comes to an end at the core, convection mixes the products of the CNO cycle
CNO cycle
The CNO cycle is one of two sets of fusion reactions by which stars convert hydrogen to helium, the other being the proton–proton chain. Unlike the proton–proton chain reaction, the CNO cycle is a catalytic cycle. Theoretical models show that the CNO cycle is the dominant source of energy in stars...
. This second dredge up results in an increase in the surface abundance of 4He and 14N, while the amount of 12C and 16O decreases.
The third dredge-up occurs after a massive star enters the asymptotic giant branch
Asymptotic Giant Branch
The asymptotic giant branch is the region of the Hertzsprung-Russell diagram populated by evolving low to medium-mass stars. This is a period of stellar evolution undertaken by all low to intermediate mass stars late in their lives....
(AGB) and a flash
Helium flash
A helium flash is the runaway fusion of helium in the core of low mass stars of less than about 2.25 solar masses and greater than about 0.5 solar mass, or on the surface of an accreting white dwarf star. They may also occur in the outer layers of larger stars in shell flashes...
occurs along a helium burning shell. This dredge-up causes helium, carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
and the s-process
S-process
The S-process or slow-neutron-capture-process is a nucleosynthesis process that occurs at relatively low neutron density and intermediate temperature conditions in stars. Under these conditions the rate of neutron capture by atomic nuclei is slow relative to the rate of radioactive beta-minus decay...
products to be brought to the surface. The result is an increase in the abundance of carbon relative to oxygen, which can create a carbon star
Carbon star
A carbon star is a late-type star similar to a 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 all the oxygen in the atmosphere, leaving carbon atoms free to form other carbon compounds,...
.
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 experienced by the star. As a result, lower-mass stars experience the first and third dredge-ups in their evolution but not the second.