22.4 Further Evolution of Stars
3 min read•june 12, 2024
Stars evolve through various stages, fusing heavier elements as they age. From hydrogen to helium, then carbon and beyond, each step requires higher temperatures. This process, called , creates the elements that make up our universe.
Low-mass stars end their lives as , while massive stars go supernova. These explosive events distribute newly formed elements into space. Through these processes, stars are responsible for creating and spreading the building blocks of life throughout the cosmos.
Late Stellar Evolution and Nucleosynthesis
Core processes after hydrogen depletion
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- Helium begins when the core temperature reaches 100 million K, higher than hydrogen fusion due to the increased Coulomb barrier
- fuses three helium-4 nuclei to form carbon-12 (carbon, oxygen)
- Helium burning shell surrounds an inert carbon core in the star's interior
- Carbon fusion commences at even higher temperatures of 600 million K
- Produces heavier elements such as oxygen, neon, and magnesium (silicon, sulfur)
- Neon, oxygen, and silicon burning occur at successively higher temperatures in massive stars
- Lead to the formation of elements up to iron-56, the most stable nucleus (nickel, cobalt)
- These processes are examples of stellar nucleosynthesis, the creation of chemical elements within stars
Formation of planetary nebulae
- Low to intermediate mass stars (0.8 to 8 solar masses) form planetary nebulae at the end of their lives
- Red giant phase involves the ejection of outer layers through strong stellar winds (, pulsations)
- Exposed hot core () ionizes the previously ejected gas, causing it to glow
- Planetary nebulae exhibit a variety of shapes, including spherical, elliptical, bipolar, or irregular
- Emission nebulae due to the presence of ionized gas (hydrogen, helium, heavier elements)
- Short-lived, lasting tens of thousands of years compared to the millions or billions of years in a star's lifetime
- Enriched in heavy elements produced by the star during its evolution (carbon, nitrogen, oxygen)
Synthesis of new elements
- Fusion of heavier elements beyond helium occurs in the late stages of stellar evolution
- Carbon, neon, oxygen, and silicon burning in massive stars
- Each stage requires progressively higher temperatures and densities
- Produces elements up to iron-56, the most stable nucleus
- (slow ) occurs in (AGB) stars
- Neutrons are captured by atomic nuclei, followed by beta decay (electron emission, proton conversion)
- Creates elements heavier than iron, such as strontium and barium (rubidium, yttrium)
- (rapid neutron capture) takes place during explosive events
- Supernova explosions and neutron star mergers provide high neutron flux
- Allows for rapid neutron capture, creating the heaviest elements (gold, platinum, uranium)
- is responsible for the production of elements beyond iron
- Explosive nucleosynthesis occurs during core-collapse supernovae (Type II, Ib, Ic)
- High temperatures and densities enable the formation of heavy elements (zinc, selenium, krypton)
- Supernovae disperse these newly synthesized elements into the interstellar medium (enrichment, chemical evolution)
Late-stage stellar processes
- Stellar winds play a crucial role in mass loss and shaping the surrounding environment
- occurs in massive stars when fusion can no longer support the star's weight
- supports white dwarfs against further gravitational collapse
Key Terms to Review (21)
Asymptotic Giant Branch: The Asymptotic Giant Branch (AGB) is a region in the Hertzsprung-Russell diagram that represents the late evolutionary stage of low-to-intermediate mass stars. It is characterized by the star's expansion into a red giant and the onset of thermal pulsing, which drives the star's further evolution towards the end of its life cycle.
Core Collapse: Core collapse refers to the final stage of a massive star's evolution, where the core of the star implodes under its own gravity, leading to a catastrophic explosion known as a supernova. This process is a critical component in the life cycle of stars and the formation of various celestial objects.
Electron Degeneracy Pressure: Electron degeneracy pressure is a type of quantum mechanical pressure that arises in extremely dense stellar matter, such as in the cores of white dwarf stars or the interiors of neutron stars. It is a fundamental force that counteracts the gravitational forces that would otherwise cause the star to collapse under its own weight.
Fusion: Fusion is the process where two light atomic nuclei combine to form a heavier nucleus, releasing an enormous amount of energy. This process powers stars, including our Sun, and is fundamental to understanding stellar evolution and the universe's energy dynamics.
Mass loss: Mass loss is the process by which a star loses its mass over time, primarily through stellar winds and other ejection mechanisms. This phenomenon significantly influences the star's evolution and eventual end state.
Neutron Capture: Neutron capture is a nuclear process in which a nucleus absorbs a neutron, forming a heavier isotope of the same element. This process is particularly important in the context of stellar evolution, as it plays a crucial role in the further evolution of stars.
Planetary Nebulae: Planetary nebulae are shells of ionized gas expelled from a star, typically a red giant, during the final stages of its life. These colorful, glowing clouds of gas are a crucial part of the life cycle of cosmic material and the evolution of stars.
R-process: The r-process, or rapid neutron capture process, is a series of nuclear reactions that occur in extremely hot and dense environments, such as supernovae and neutron star mergers. This process is responsible for the formation of heavy elements, including many of the naturally occurring elements heavier than iron, through the rapid absorption of neutrons by atomic nuclei.
S-process: The s-process, or slow neutron capture process, is a nucleosynthetic process that occurs in stars and is responsible for the creation of about half of the stable, heavy elements heavier than iron in the periodic table. It involves the slow absorption of neutrons by atomic nuclei, allowing them to build up to higher atomic weights through a series of nuclear reactions.
Stellar Nucleosynthesis: Stellar nucleosynthesis is the process by which new atomic nuclei are created inside stars through nuclear fusion reactions. This process is responsible for the creation and distribution of the elements that make up the universe, from the lightest elements like hydrogen and helium to the heavier elements like carbon, oxygen, and iron.
Stellar wind: Stellar wind is a stream of charged particles released from the upper atmosphere of a star. It can significantly influence the surrounding interstellar medium and contribute to star formation processes.
Stellar Wind: Stellar wind is a stream of charged particles and radiation that flows outward from the surface of a star, driven by the intense heat and pressure within the star's atmosphere. This continuous outflow of material plays a crucial role in the evolution of stars and their surrounding environments.
Supernova Nucleosynthesis: Supernova nucleosynthesis is the process by which heavy elements are created during the explosive death of a massive star. This occurs when a star's core collapses, triggering a powerful supernova explosion that generates the extreme temperatures and pressures necessary for the fusion of lighter elements into heavier ones.
Triple-alpha process: The triple-alpha process is a set of nuclear fusion reactions by which three helium-4 nuclei (alpha particles) are transformed into carbon-12. It occurs in the cores of stars during their red giant phase when temperatures and pressures are extremely high.
Triple-Alpha Process: The triple-alpha process is a series of nuclear fusion reactions that occur in the cores of massive stars, particularly during the late stages of their life cycles. This process is responsible for the production of carbon and other heavier elements essential for the formation of planets and life.
Type Ib Supernova: A Type Ib supernova is a specific type of stellar explosion that occurs when a massive star, typically a Wolf-Rayet star, runs out of nuclear fuel and undergoes a catastrophic collapse. This event is characterized by the absence of hydrogen lines in the supernova's spectrum, distinguishing it from the more common Type II supernovae.
Type Ic Supernova: A Type Ic supernova is a specific type of stellar explosion that occurs when a massive star, typically 8-15 times the mass of the Sun, exhausts its nuclear fuel and collapses under its own gravity, resulting in a catastrophic explosion. These supernovae are characterized by the lack of hydrogen and helium lines in their spectra, indicating that the progenitor star has lost its outer hydrogen and helium layers prior to the explosion.
Type II supernova: A type II supernova is a powerful explosion that occurs when a massive star exhausts its nuclear fuel and its core collapses. This leads to the ejection of the star's outer layers into space.
Type II Supernova: A Type II supernova is a catastrophic explosion of a massive star at the end of its life cycle, marking the violent death of a star that has exhausted its nuclear fuel and collapsed under its own gravity.
White dwarf: A white dwarf is the remnant of a low to medium mass star that has exhausted its nuclear fuel and shed its outer layers. It is incredibly dense, with a mass comparable to the Sun but a volume similar to Earth.
White Dwarf: A white dwarf is the dense, compact remnant of a low-mass star that has exhausted its nuclear fuel and shed its outer layers, leaving behind a core composed primarily of degenerate matter. This stellar endpoint is a crucial component in understanding the evolution of stars and the structure of the universe.