Glossary

Introduction

This glossary collects the technical terms, acronyms, and abbreviations used most often in the book. The definitions are operational rather than encyclopedic: they explain how each term functions inside stellar nucleosynthesis, where it appears in the physical argument, and why it matters for interpreting abundances. For formal nuclear-data terminology and evaluated nuclear definitions, consult the IAEA Nuclear Data Section [International Atomic Energy Agency] .

A

$\alpha$ element. Element whose synthesis is strongly connected to alpha-particle capture or alpha-like assembly in hydrostatic or explosive burning. The usual astrophysical group includes O, Ne, Mg, Si, S, Ar, and Ca. These elements are mainly produced by massive stars and core-collapse supernovae, so enhanced $[\alpha/\mathrm{Fe}]$ in old stars indicates enrichment before Type Ia supernovae had time to add much iron.

$\alpha$-rich freeze-out. Explosive-burning regime in which material reaches nuclear statistical equilibrium at temperatures above several $10^{9}$ K and then expands too rapidly for all alpha particles to be assembled into iron-group nuclei. The surviving alpha particles modify the final composition, enhancing nuclei such as $^{44}\mathrm{Ti}$ and affecting the production of $^{56}\mathrm{Ni}$, the main power source of many supernova light curves.

AGB (Asymptotic Giant Branch). Late evolutionary phase of low- and intermediate-mass stars. An AGB star has a degenerate carbon-oxygen core, a helium-burning shell, a hydrogen-burning shell, and a large convective envelope. During the thermally pulsing AGB phase, third dredge-up brings carbon and s-process material to the surface, and strong winds return that material to the interstellar medium.

AME (Atomic Mass Evaluation). Reference evaluation of nuclear masses, mass excesses, separation energies, and related quantities. AME data determine reaction Q-values, drip-line locations, and many thresholds used in nuclear reaction networks. AME2020 is the modern benchmark cited in this book.

APOGEE (Apache Point Observatory Galactic Evolution Experiment). Infrared spectroscopic survey of Milky Way stars. APOGEE is important for Galactic chemical evolution because it provides large homogeneous abundance samples across the disk, bulge, and halo, including stars obscured by dust in optical surveys.

B

B²FH. Short name for the 1957 paper by Margaret Burbidge, Geoffrey Burbidge, William Fowler, and Fred Hoyle, Synthesis of the Elements in Stars. It organized the field into named processes such as the s process, r process, p process, and equilibrium burning, creating much of the language still used in stellar nucleosynthesis.

BBN (Big Bang Nucleosynthesis). Production of light nuclei during the first minutes of cosmic history. Standard BBN makes most primordial $^{4}\mathrm{He}$, deuterium, $^{3}\mathrm{He}$, and traces of $^{7}\mathrm{Li}$, while producing essentially no heavy elements because of the mass gaps at $A = 5$ and $A = 8$.

Beryllium transport. Cameron-Fowler mechanism for lithium production. $^{7}\mathrm{Be}$ is synthesized in a hot layer through $^{3}\mathrm{He}(\alpha,\gamma)^{7}\mathrm{Be}$, carried quickly to cooler material, and then decays by electron capture into $^{7}\mathrm{Li}$. The mechanism operates in intermediate-mass AGB stars and in classical novae.

BNS (Binary Neutron Star). A compact binary containing two neutron stars. BNS mergers eject neutron-rich matter and are confirmed r-process sites through GW170817 and its kilonova AT2017gfo.

Branching point. Unstable nuclide on a neutron-capture path where neutron capture and beta decay occur on comparable timescales. Branching points turn abundance ratios into diagnostics of neutron density, temperature, and exposure time. Examples include $^{85}\mathrm{Kr}$, $^{151}\mathrm{Sm}$, and thermally coupled $^{176}\mathrm{Lu}$.

C

CEMP star (Carbon-Enhanced Metal-Poor star). Metal-poor star with strong carbon enhancement. The subclasses encode different enrichment histories: CEMP-s stars usually involve AGB mass transfer, CEMP-no stars are linked to early massive-star enrichment, and CEMP-r or CEMP-r/s stars trace neutron-capture events.

Chart of the nuclides. Two-dimensional map of nuclei in the $(N,Z)$ plane. Isotopes of the same element have the same $Z$, isotones have the same $N$, and isobars have the same $A$. Nucleosynthesis paths, beta decays, neutron captures, proton captures, and drip lines are all most clearly described on this chart.

Chemical tagging. Method for identifying stars that formed together by comparing detailed multi-element abundance patterns. In principle, stars born in the same cluster preserve a shared chemical fingerprint even after the cluster disperses. In practice, chemical tagging requires high-dimensional abundance data and careful control of observational uncertainties.

CNO cycle. Hydrogen-burning cycle in which carbon, nitrogen, and oxygen nuclei catalyze the conversion of four protons into one alpha particle. It dominates main-sequence energy production in stars more massive than about the Sun and controls many nitrogen and oxygen isotope signatures in evolved stars.

Collapsar. Collapse of a rapidly rotating massive star to a black hole plus accretion disk, often associated with long gamma-ray bursts. Some collapsar models produce neutron-rich disk outflows and have been proposed as r-process contributors, especially in the early Galaxy.

Compactness parameter. Dimensionless measure of a massive star’s presupernova core structure, commonly evaluated around an enclosed mass such as $2.5\,M_\odot$. High compactness usually means a harder-to-explode core, stronger fallback, and a greater chance of black-hole formation.

Cosmochronometer. Radioactive abundance ratio used as a clock for stellar populations or nucleosynthetic events. Examples include Th/Eu, U/Th, and Re/Os. The method depends on knowing both the present abundance and the production ratio at the source.

D

DD scenario (Double Degenerate). Type Ia supernova channel in which two white dwarfs merge after losing orbital energy to gravitational radiation. DD systems are a major candidate channel for normal Type Ia events and for some subtypes.

DDT (Delayed Detonation Transition). Type Ia supernova model in which an initially subsonic deflagration transitions to a supersonic detonation. The transition changes how much material burns at high density and therefore affects the yield of $^{56}\mathrm{Ni}$ and neutron-rich iron-group isotopes.

Drip line. Boundary beyond which adding a neutron or proton no longer produces a bound nucleus. The neutron drip line limits the most neutron-rich nuclei relevant to the r process. The proton drip line limits the proton-rich path followed by the rp process.

DTD (Delay-Time Distribution). Distribution of times between the formation of a progenitor system and its event. DTDs are central for Type Ia supernovae, neutron-star mergers, and chemical-evolution models because delayed enrichment changes abundance trends.

E

ECSN (Electron-Capture Supernova). Explosion triggered when electron captures on nuclei such as Mg and Ne remove pressure support from an oxygen-neon-magnesium core. ECSNe are expected near the boundary between super-AGB evolution and ordinary massive-star core collapse.

EMP star (Extremely Metal-Poor star). Star with $[\mathrm{Fe/H}] < -3$. EMP stars preserve chemical information from the early Galaxy, when only a small number of enrichment events had contributed to the gas from which each star formed.

EOS (Equation of State). Relation among pressure, density, temperature, and composition. In neutron-star physics, the EOS determines radii, maximum masses, tidal deformabilities, and merger outcomes. In stellar evolution, the EOS closes the equations of structure.

Eu/Fe. Europium-to-iron abundance ratio. Since Eu is mostly r-process in the solar abundance pattern, high $[\mathrm{Eu/Fe}]$ identifies r-process enhanced stars. The usual r-I and r-II classifications use this ratio together with barium constraints.

F

FAIR (Facility for Antiproton and Ion Research). Radioactive-beam and nuclear-physics facility at GSI Darmstadt. It is relevant to nucleosynthesis because unstable nuclei far from stability control r-process and rp-process flows.

Fission cycling. Regime of a strong r process in which superheavy nuclei fission and feed fragments back into lower mass ranges. Repeated cycling can reduce memory of the initial seed distribution and help explain robust heavy r-process abundance patterns.

FRIB (Facility for Rare Isotope Beams). Radioactive-ion beam facility at Michigan State University. FRIB measures masses, beta-decay lifetimes, and reaction constraints for unstable nuclei that cannot be studied with stable beams.

FRUITY (FRANEC Repository of Updated Isotopic Tables and Yields). Public database of AGB stellar yields computed with FRANEC models. It is used to compare AGB nucleosynthesis predictions with stellar abundances and presolar grain data.

G

GCE (Galactic Chemical Evolution). Modeling of how gas and stellar abundances evolve in a galaxy through star formation, stellar yields, gas inflow, outflow, mixing, and delay times. GCE translates single-source yields into observed abundance trends.

GCR (Galactic Cosmic Rays). High-energy particles, mostly protons and alpha particles, accelerated in the Galaxy. Their spallation reactions on C, N, and O nuclei are the main source of beryllium and boron and a significant source of lithium.

GW170817. Binary neutron-star merger observed on August 17, 2017, through gravitational waves and electromagnetic emission. Its kilonova provided direct evidence that neutron-star mergers synthesize r-process elements.

H

HBB (Hot Bottom Burning). Proton-capture burning at the base of the convective envelope in intermediate-mass AGB stars. HBB converts dredged-up $^{12}\mathrm{C}$ into $^{14}\mathrm{N}$, changes CNO isotopic ratios, activates NeNa and MgAl chains, and can temporarily produce lithium.

HR diagram. Hertzsprung-Russell diagram, usually luminosity versus effective temperature. Stellar evolutionary tracks on this diagram organize the phases in which different burning processes occur.

Hypernova. Energetic core-collapse supernova with kinetic energy significantly above ordinary events, often around $10^{52}$ erg. Hypernovae are associated with massive progenitors, strong asymmetry, broad-lined Type Ic spectra, and in some cases long gamma-ray bursts.

I

i process (Intermediate neutron-capture process). Neutron-capture regime with neutron densities between the s and r processes. It may occur during proton-ingestion events or convective-reactive burning and can produce abundance patterns not explained by a pure s or r process.

IMF (Initial Mass Function). Birth distribution of stellar masses. Because yields depend strongly on initial mass, the IMF weights how much each mass range contributes to the chemical enrichment of a stellar population.

ISM (Interstellar Medium). Gas and dust between stars. It is enriched by stellar winds, AGB ejecta, novae, supernovae, and compact-object mergers, and it is the reservoir from which later generations of stars form.

J

JINA REACLIB. Public library of thermonuclear reaction rates in a standard parameterized format used by many nuclear reaction networks and stellar-evolution codes.

JUNA (Jinping Underground Nuclear Astrophysics). Underground accelerator facility in China designed for low-background measurements of nuclear reactions at astrophysical energies.

JWST (James Webb Space Telescope). Infrared space telescope used for high-redshift galaxies, dust, stellar populations, and transients. In nucleosynthesis studies it is especially relevant for early chemical enrichment, dust formation, and kilonova follow-up.

K

KADoNiS (Karlsruhe Astrophysical Database of Nucleosynthesis in Stars). Database of Maxwellian-averaged neutron-capture cross sections, especially important for s-process calculations around thermal energies such as $kT = 30$ keV.

Kilonova. Optical-infrared transient powered by radioactive decay of freshly synthesized r-process nuclei in neutron-star merger ejecta. The color depends strongly on opacity, which is high for lanthanide-rich material.

Knee. Bend in an abundance trend such as $[\alpha/\mathrm{Fe}]$ versus $[\mathrm{Fe/H}]$. It usually marks the metallicity at which delayed Type Ia supernova iron becomes important relative to prompt core-collapse enrichment.

L

LBV (Luminous Blue Variable). Massive, unstable star near the Eddington limit, capable of strong and sometimes eruptive mass loss. LBV-like episodes can alter the final mass and envelope structure of supernova progenitors.

LECR (Low-Energy Cosmic Rays). Sub-GeV cosmic-ray component. LECRs are important for LiBeB spallation and gamma-ray line excitation, but they are difficult to infer near Earth because solar modulation reshapes the local spectrum.

Ledoux criterion. Convective stability criterion including the composition gradient. It is stricter than the Schwarzschild criterion when a stabilizing molecular-weight gradient is present and is important for semiconvection and boundary mixing.

LMXB (Low-Mass X-ray Binary). Binary system in which a neutron star or black hole accretes from a low-mass companion. Accreting neutron-star LMXBs are the standard hosts of Type I X-ray bursts.

LUNA (Laboratory for Underground Nuclear Astrophysics). Underground accelerator at Gran Sasso. The underground environment suppresses cosmic-ray background and enables measurements of very low cross sections near stellar energies.

M

MACS (Maxwellian-Averaged Cross Section). Neutron-capture cross section averaged over a Maxwell-Boltzmann distribution. MACS values are basic inputs for s-process calculations and are tabulated in databases such as KADoNiS.

Mass cut. Boundary in a core-collapse supernova between material ejected and material that remains in or falls back onto the compact remnant. The mass cut strongly affects iron-group yields and is a major uncertainty in one-dimensional models.

MESA (Modules for Experiments in Stellar Astrophysics). Open-source one-dimensional stellar-evolution code used for stellar structure, evolution, and some nucleosynthesis applications.

Metallicity. Abundance of elements heavier than helium. It can mean total heavy-element mass fraction $Z$ or an observational proxy such as $[\mathrm{Fe/H}]$. Metallicity controls opacity, mass loss, seed abundances, and many yields.

Mixing-length theory. Phenomenological description of convection in one-dimensional stellar models. It represents convective transport through a characteristic length proportional to the pressure scale height and must be calibrated.

N

NACRE (Nuclear Astrophysics Compilation of REaction rates). Compilation of thermonuclear reaction rates used in stellar and explosive nucleosynthesis. NACRE-II is the updated version cited in this book.

NanoSIMS. High-spatial-resolution secondary ion mass spectrometer used to map isotopic ratios in sub-micron presolar grains. It is one of the main tools that turned presolar grains into a quantitative nucleosynthesis laboratory.

NICER (Neutron star Interior Composition Explorer). X-ray instrument on the International Space Station used to constrain neutron-star masses and radii through pulse-profile modeling.

NLTE (Non-Local Thermodynamic Equilibrium). Radiative-transfer regime in which atomic level populations are not set by local thermodynamic equilibrium. NLTE corrections are essential for precision photospheric abundances.

NNDC (National Nuclear Data Center). Nuclear-data center at Brookhaven National Laboratory. NNDC tools provide evaluated half-lives, decay modes, nuclear levels, gamma rays, and other nuclear structure data.

NSE (Nuclear Statistical Equilibrium). High-temperature regime in which strong and electromagnetic reactions are fast enough to equilibrate the nuclear composition. The result depends mainly on temperature, density, and electron fraction $Y_e$.

NSM (Neutron Star Merger). Coalescence of two neutron stars. NSMs eject neutron-rich matter, power kilonovae, and are confirmed r-process sites.

$\nu$ process. Nucleosynthesis driven by neutrino interactions, mainly in core-collapse supernovae. Neutrino spallation and charged-current reactions contribute to selected nuclei such as $^{7}\mathrm{Li}$, $^{11}\mathrm{B}$, $^{138}\mathrm{La}$, and $^{180}\mathrm{Ta}$.

$\nu p$ process. Proton-rich nucleosynthesis enabled by antineutrino captures in neutrino-driven winds. Neutrons created by antineutrino interactions allow flow past waiting points and may contribute to light p nuclei such as Mo and Ru isotopes.

O

Overshoot. Convective penetration beyond the formal stability boundary. Overshoot affects core sizes, thermal pulses, third dredge-up, $^{13}\mathrm{C}$ pocket formation, and the final yields of many stellar models.

P

p nuclei. Stable proton-rich isotopes that are shielded from production by the s and r processes. They are usually attributed to the gamma process plus secondary channels such as the $\nu$ process, $\nu p$ process, rp process, and Type Ia supernova contributions.

PISN (Pair-Instability Supernova). Complete disruption of a very massive star triggered by electron-positron pair production in the oxygen core. PISNe produce distinctive odd-even abundance patterns and large metal yields, but observational evidence remains limited.

PNS (Proto-Neutron Star). Hot, lepton-rich neutron star formed after core bounce in a core-collapse supernova. It cools by intense neutrino emission and drives neutrino-heated outflows.

$^{13}\mathrm{C}$ pocket. Thin AGB intershell region enriched in $^{13}\mathrm{C}$ by partial proton mixing after third dredge-up. It supplies neutrons for the main s process through $^{13}\mathrm{C}(\alpha,n)^{16}\mathrm{O}$.

Population III. First generation of stars, formed from metal-free primordial gas. Their masses, explosion channels, and chemical signatures are still inferred indirectly from simulations and extremely metal-poor stars.

pp chain. Proton-proton chain converting hydrogen into helium in low-mass main-sequence stars. Its branches, pp-I, pp-II, and pp-III, determine the solar neutrino spectrum and the Sun’s main energy production.

Q

QSE (Quasi-Statistical Equilibrium). Regime in which subsets of nuclear reactions are internally equilibrated while the full network is not. QSE clusters appear during oxygen burning, silicon burning, and some explosive photodisintegration flows.

R

r process (Rapid neutron-capture process). Neutron-capture process in which captures occur faster than beta decays. It requires very neutron-rich conditions and produces many of the heaviest nuclei, including much of Eu, Pt, Au, Th, and U.

RIB (Radioactive Ion Beam). Beam of unstable nuclei used to study masses, decays, and reaction properties far from stability. RIB facilities are essential because many r-process and rp-process nuclei cannot be reached with stable beams.

RIMS (Resonance Ionization Mass Spectrometry). Laser-based mass spectrometry technique that selectively ionizes chosen elements. It is used to measure trace heavy-element isotopes in individual presolar grains.

rp process (Rapid proton-capture process). Explosive proton-capture sequence on the proton-rich side of stability. It operates in Type I X-ray bursts and can run up to the SnSbTe region under favorable conditions.

S

s process (Slow neutron-capture process). Neutron-capture process in which beta decay usually occurs before the next neutron capture. It proceeds near the valley of stability and has weak, main, and strong components associated with massive stars and AGB stars.

SAGB (Super-AGB). Massive AGB-like star that ignites carbon but does not proceed through the full massive-star burning sequence. It may leave an ONeMg white dwarf or collapse through electron captures.

SASI (Standing Accretion Shock Instability). Hydrodynamic instability of the stalled shock in a core-collapse supernova. It can help create large-scale motions that support shock revival.

Schwarzschild criterion. Convective stability criterion based on the comparison of radiative and adiabatic temperature gradients. Unlike the Ledoux criterion, it does not include the composition gradient.

SD scenario (Single Degenerate). Type Ia supernova channel in which a carbon-oxygen white dwarf accretes material from a non-degenerate companion and approaches explosive conditions.

SFR (Star Formation Rate). Rate at which gas is converted into stars. It controls how quickly massive-star enrichment occurs and how soon delayed sources such as Type Ia supernovae appear in abundance trends.

Spite plateau. Nearly constant lithium abundance observed in warm metal-poor halo stars. Its value is below the standard Big Bang nucleosynthesis prediction, forming the cosmological lithium problem.

SSM (Standard Solar Model). Calibrated model of the Sun’s structure and evolution over $4.567$ Gyr. It is constrained by luminosity, radius, surface composition, helioseismology, and solar neutrino measurements.

Superwind. Final high mass-loss stage of an AGB star. Pulsation and dust-driven winds remove most of the envelope, determine the number of remaining thermal pulses, and therefore strongly affect final yields.

T

TDU (Third Dredge-Up). Convective mixing episode after an AGB thermal pulse. TDU brings carbon and s-process material from the intershell into the envelope and is the mechanism that creates many carbon stars.

TP-AGB (Thermally Pulsing AGB). Late AGB phase in which the helium shell becomes thermally unstable and produces repeated pulses separated by quiescent interpulse phases.

U

UMP star (Ultra Metal-Poor star). Star with $[\mathrm{Fe/H}] < -4$. Such stars are rare probes of the earliest enrichment events in the Galaxy.

Universality of the r-process pattern. Observation that many r-process enhanced metal-poor stars share a nearly identical heavy-element pattern over selected atomic-number ranges after scaling. It suggests robust nuclear or astrophysical conditions, though deviations occur for lighter neutron-capture elements and actinides.

V

Valley of beta stability. Region of the chart of the nuclides occupied by stable or very long-lived nuclei. It lies near $N \approx Z$ for light nuclei and bends toward neutron excess for heavy nuclei.

W

Waiting point. Nuclide where reaction flow stalls because the next capture is inhibited and beta decay must occur first. Waiting points shape r-process abundance peaks and control the duration and light curve of rp-process X-ray bursts.

Weak r process. Neutron-capture component that mainly produces first-peak r-process material rather than the full heavy r-process pattern. It may occur in moderately neutron-rich outflows from mergers or supernova-related environments.

WR star (Wolf-Rayet star). Massive evolved star stripped of its hydrogen-rich envelope, with strong winds and emission-line spectra. WR winds contribute to helium, carbon, oxygen, and $^{26}\mathrm{Al}$ enrichment.

X

XRB (X-Ray Burst). Type I thermonuclear burst on the surface of an accreting neutron star in a low-mass X-ray binary. XRBs are powered by unstable hydrogen and helium burning and host the rp process.

Y

$Y_e$ (Electron or proton fraction). Number of protons per baryon in electrically neutral matter. It controls neutron richness and is one of the decisive variables for r-process nucleosynthesis in supernovae and neutron-star mergers.

Yield. Mass of a given isotope or element produced and ejected by a star or event. Yields from AGB stars, novae, supernovae, and neutron-star mergers are the main inputs of Galactic chemical-evolution models.

Z

$Z$. Atomic number when referring to nuclei; total heavy-element mass fraction when referring to stellar composition. The meaning is determined by context.

$Z_\odot$. Solar metallicity. Modern abundance compilations give a heavy-element mass fraction close to $0.014$, although individual element abundances and model-dependent values differ slightly.