Glossary of terms used in PHY320

α-β-γ theory
is the theory of primordial nucleosynthesis put forward by Alpher and Gamow (Gamow added Bethe's name as a joke) in the late 1940s. The theory attempted to explain nuclear abundances by a single process, namely successive neutron capture in the early Universe. The theory breaks down beyond 4He, owing to the lack of any stable nuclides with atomic masses 5 and 8.
alpha-process element
is an element whose main isotope can be made up of an integer number of helium-4 nuclei (alpha particles) – i.e. one with Z = 2n and A = 2Z, where n is a positive integer. The alpha-process elements are C, O, Ne, Mg, Si, S, Ar, Ca and Ti. Alpha-process elements are synthesised in the silicon-burning stage immediately preceding the explosion of core-collapse supernovae, and also during the explosion itself: they are the principal product of SNe II (in contrast to SNe Ia, which make mostly iron-peak elements).
AGB (asymptotic giant branch) star
is a star in the evolutionary stage beyond helium core burning. AGB stars initially derive their energy from helium fusion around the carbon/oxygen core, but subsequently alternate between fairly long periods of hydrogen shell fusion and short bursts of helium shell fusion (when enough helium builds up). The helium burning episodes (thermal pulses) start abruptly with a helium shell flash, which induces strong convective mixing in the star (dredge-up). AGB stars are believed to be the main site of the s-process, and the elements produced by this are transported to the star's outer envelope by dredge-up. Note that although AGB stars are red and are giants, they are not technically red giants – this name refers to an earlier stage of stellar evolution.
Atmophile
is an element which occurs primarily in gaseous form: H, C, N and the noble gases. Atmophiles are depleted in material produced at high temperatures, such as the Earth. Note that oxygen is not an atmophile, despite the fact that O2, CO2 and H2O are all volatiles: most oxygen is locked up in solid silicates, making oxygen a lithophile.
Big Bang Nucleosynthesis
see Primordial Nucleosynthesis.
By-passed Nuclei
are proton rich nuclei which cannot be produced by the neutron capture process. They may have been produced by proton capture, neutron emission (due to the absorption of a gamma ray by the nucleus, for example) or an inverse beta process caused by the absorption of a positron in the nucleus.
Cerenkov Effect (also spelled Cherenkov)
is the emission of light when a charged particle passes through a medium at a velocity which is faster than the velocity of light in that medium. It is used in the detection of cosmic rays, contributing to the measurement of their charge and hence to the identification of their chemical type.
Chalcophile
is an element which prefers to combine with sulphur rather than oxygen. These are the "poor metals" and semi-metals in groups 11-16 of the periodic table (but not elements with atomic number Z < 16, which are lithophiles, or gold, which is a siderophile). Chalcophiles are depleted in the crust relative to the mantle because sulphides are denser than silicates, and the less metallic chalcophiles (e.g. selenium) are depleted on the Earth as a whole because they can form volatile hydrides, and so are lost from the inner solar nebula.
Chandrasekhar mass
is the mass (1.4 solar masses) at which gravity overcomes electron degeneracy, causing the collapse of the body in question. It is important in understanding supernovae, both core-collapse (caused by the collapse of the iron core of an evolved massive star) and Type Ia (caused by the collapse of a white dwarf which has reached the Chandrasekhar mass through accretion or merger).
Chondrites
are stony meteorites which, amongst meteoric evidence, are thought to give the best guide to the original abundances of the elements in the solar system. They are so-called because they generally contain small spherical bodies called chondrules. The classification of chondrites is extremely complicated, but it appears that the most primitive and unaltered composition is probably that of the CI carbonaceous chondrites, ironically a subtype which does not in fact contain chondrules.
Chondrules
are small spherical grains of silicate minerals, found in chondritic meteorites. They are believed to form in free space, which explains their spherical shape, and then subsequently become incorporated into a stony matrix. Their diameters range from 0.02 mm (in CH chondrites) to 10 mm (in CBa chondrites).
CNO cycle
is a set of reactions which converts hydrogen into helium-4 using carbon-12 as a catalyst. Starting with p + 12C → 13N + γ, protons are successively added to the seed nucleus until a final alpha-decay releases the completed 4He along with the original 12C (along the way, 13N and 15O undergo β+-decay to convert two protons into neutrons). Proton capture on 14N is much the slowest reaction in the CNO cycle, so in stars using this reaction set (main-sequence stars heavier than the Sun, and red giants) there is a net conversion of carbon-12 to nitrogen-14.
(Primary) Cosmic Rays
are the particles which impinge on the top of the Earth's atmosphere. They are one of the sources of information about the relative abundances of elements in the Universe. Although they are predominantly protons, their elemental composition does exhibit differences with respect to the so-called "universal" abundances, especially for light elements such as Lithium and Boron. The showers of particles produced when primary cosmic rays interact with the atmosphere are called secondary cosmic rays: these include muons, which are the principal source of cosmic ray background in particle physics experiments.
Curve of Growth
is the theoretical relationship between the relative spectral absorption line intensity and the number of absorbing atoms. It plays an important role in the interpretation of absorption spectra in terms of chemical abundances. It includes the effects of line broadening and saturation. Such calculated curves have been verified both from laboratory experiments and stellar observations.
Doppler Broadening
is the broadening of spectral lines due to the motion of emitting or absorbing atoms. It is important that this effect is taken into account when using the spectra of stars to estimate their chemical composition. Doppler broadening causes a Gaussian line profile and is usually the dominant effect at small and moderate optical depth.
Electromagnetic Shower
is the cascade production of electrons, positrons and photons which is a part of the sequence of secondary effects which occurs when a cosmic ray interacts with an atom in the Earth's atmosphere. Measurements on the shower can be used to determine the energy of the original primary particle.
Equivalent Width
is a measure of the intensity of an absorption line. It is expressed in Angstrom units (or nanometres) and signifies the removal of an amount of radiation equivalent to that contained in the neighbouring continuous spectrum over such a wavelength interval. It is used when extracting chemical abundances from stellar spectra.
Fermi Mechanism
is a suggested way in which cosmic rays are accelerated to high energies by successive collisions with magnetised dust clouds. It is reasonably successful in predicting the observed primary cosmic ray energy spectrum.
Forbidden Line
is a spectral line corresponding to a transition that is not allowed by the selection rules of quantum mechanics (i.e. it's not an electric dipole transition). Forbidden lines are actually rather common in astrophysics, as they occur when the density of the material is low enough to suppress the usual collisional de-excitation mechanism. They are useful in setting upper limits to the density of their source.
Helium Abundance
is the amount of helium in the Universe taken as a fraction of the whole or in relation to the amount of hydrogen. In astrophysics textbooks the helium fraction by mass is usually denoted by Y (hydrogen fraction is X and heavy element content Z). Helium is mostly attributed to primordial nucleosynthesis and its measured abundance serves as a check on the standard model of that process.
Helium flash
is the runaway onset of helium fusion in degenerate material. A helium flash occurs for low to medium mass stars at the tip of the red giant branch, when hydrogen fusion in a shell around the degenerate helium core finally raises the temperature of the helium to the ignition point for the triple-alpha process. Helium shell flashes occur during the later evolution of AGB stars.
Interstellar Medium (ISM)
is the gas and dust within a galaxy. This is a mixture of primordial material which has never formed stars and material enriched by stellar nuclear reactions, dispersed into the ISM through supernova explosions, planetary nebulae and mass ejections from evolved stars. The gradual enrichment of the ISM is described by models of galactic chemical evolution.
Iron Peak
is the name given to the relatively high abundance of the elements around iron (from about chromium, Z = 24, to zinc, Z = 30), which produces a prominent peak in the measured elemental abundances. These nuclei have the highest nuclear binding energies, of about 8.8 MeV/nucleon, and are therefore the most likely to be formed in conditions of thermodynamic equilibrium, such as are believed to occur in the central regions of supernovae.
Lithophile
is an element which readily forms solid oxides or silicates. Lithophiles include the metals in groups 1-6 of the periodic table (except molybdenum, which is a siderophile), boron, aluminium, silicon, phosphorus, oxygen and the halogens, as well as the lanthanides (rare earths) and the actinides (uranium and thorium). The Earth's crust and mantle are largely composed of lithophiles, particularly oxygen and silicon (in the form of silicate), magnesium, calcium and aluminium.
Local Approximation
is the assumption that relative abundances produced by the s-process are inversely proportional to the neutron capture cross-sections. This works well in the regions away from closed shell (so-called magic number) nuclei such that the product of these two quantities is reasonably constant in those regions.
Magic Numbers
are particular numbers of protons and neutrons which produce exceptionally stable nuclei. They are the numbers required to close a nucleon shell – the nuclear equivalent to the atomic shell closures associated with the chemically inert noble gases. They have a marked effect on the relative abundances of elements synthesised by the r-process of neutron capture.
Metallicity
is the heavy element content of a star (the astrophysicist's periodic table goes "hydrogen, helium, metals"). It is generally denoted by Z. "High" metallicity stars – like the Sun – typically contain 1-4% heavy elements by mass; low-metallicity stars can have ≪ 1%.
Natural Broadening
is the broadening of spectral lines due to the finite lifetime of the excited state (ΔE Δt > ℏ). Because it causes a Lorentzian line profile, with very broad wings, it dominates in lines of very large optical thickness, even if it is completely negligible compared to Doppler broadening for optically thin lines.
Neutron Absorption
see Neutron Capture.
Neutron Capture
is the process whereby elements heavier than iron are synthesised. It is linked to the conversion of neutrons into protons (via beta decay) in this element-producing sequence. Slow or rapid neutron capture (s-process and r-process) get their names from the rate of neutron capture relative to the beta decay lifetime.
Nuclear Fission
is the breakup of heavy nuclei into two medium mass nuclei with an accompanying release of energy. This form of nuclear instability ultimately limits the extent to which heavy nuclei can be synthesised. It places an upper limits on the atomic number (A) of nuclei produced by the r-process.
Nuclear Fusion
see Thermonuclear Fusion.
Photodisintegration
is the breakup of nuclei by the absorption of energetic photons. This can occur in the core of a massive star which has proceeded to the end of the fusion sequence. With no further energy-producing fusion reactions taking place, the iron core shrinks and heats up until there are thermal photons with sufficient energy to break up the 56Fe nuclei.
Populations I, II and III
are the names given respectively to high-metallicity stars formed from material significantly enriched by prior generations of stellar nucleosynthesis, old, low-metallicity stars formed from minimally enriched material, and a hypothetical initial population formed from completely unenriched material consisting only of hydrogen and helium. Population I is found in the discs of spiral galaxies and in irregular galaxies, Population II in the bulges and haloes of spiral galaxies and in elliptical galaxies (though it should be noted that not all Population II regions are metal-poor: the defining characteristic is really the lack of recent star formation). Nobody has yet discovered an extant Population III star, but there are theoretical reasons to believe that zero-metallicity stars would be very massive, and would have exploded as supernovae after a very short time.
Primordial Nucleosynthesis
is the production of chemical elements in the first few minutes after the "Big Bang". It produces only the light nuclides 2H, 3He, 4He and 7Li. Measurements of the abundances of these nuclear species provides a valuable crosscheck on the standard model of the early evolution of the Universe.
Red Giants
are stars which have just left the main sequence after completing the consumption of hydrogen in their cores. They are powered by hydrogen fusion in a shell outside the inert helium core. Red giant stars make most of the 14N observed in the Universe: it is produced as a by-product of the CNO cycle.
r-process
is the creation of elements by neutron capture at a rate which is rapid compared to the beta decay lifetime of the nuclei produced. This process initially produces super-neutron-rich, extremely unstable nuclei which subsequently undergo repeated beta decays, eventually leading to stable nuclei which tend to be on the neutron-rich side of the line of maximum binding energy. The r-process must happen – nuclides which cannot possibly be produced by the s-process are quite abundant in the Galaxy – but its exact site is still very uncertain.
Siderophile
is an element which dissolves readily in molten iron, and therefore tends to be carried into the Earth's core along with the iron during gravitational differentiation. The siderophiles are transition metals: Mn, Fe, Co, Ni, Mo, Ru, Rh, Pd, Re, Os, Ir, Pt and Au. As a consequence of differentiation, the Earth's crust and mantle are heavily depleted of siderophiles: for example, the Earth as a whole is about 32% iron by mass, but the crust is only 5% and the mantle 6%.
Solar Neutrinos
are neutrinos resulting from the energy-producing reactions within the Sun. For a long time, measured neutrino fluxes at the Earth did not agree with predictions from solar models, but it is now known that this was a consequence of neutrino oscillation (the neutrinos are produced as electron-type neutrinos, but on their way out of the Sun can be transformed into other types, with a probability that depends on the neutrino's energy but which is quite high for the neutrinos that are most easily observed, i.e. the higher energy ones).
Spallation
is the production of light nuclei in collisions between either an energetic heavy nucleus and a low energy proton of the interstellar medium or an energetic proton and a low energy heavy nucleus of the interstellar medium. This process accounts for the relatively high abundance of Be, Li and B in cosmic rays.
s-process
is the creation of elements by neutron capture at a rate which is slow compared to the beta decay lifetime of the nuclei produced. This process produces nuclei which are close to the line of beta stability (maximum binding energy) on the chart of nuclides. The most likely site of the s-process is thermal-pulsing AGB stars, though stars fusing helium in their cores may also contribute.
Supernovae
represent the explosive end of a star's life. They are usually caused by the collapse of a core which has reached the Chandrasekhar mass – either the iron core of an evolved massive star, or a white dwarf which has gained mass by accretion or merger. Supernovae are responsible for the production of iron-peak and alpha-process elements, and may also contribute to the r-process.
Thermonuclear Fusion
is the fusing together of light nuclei to produce medium mass nuclei. The process is exothermic and is responsible for the production of elements up to iron. it is the process which powers stars as light sources – for example the production of helium from hydrogen is the process which maintains stars while they are on the main sequence.
Triple-alpha process
is the process by which three helium-4 nuclei fuse to form carbon-12. This goes in two steps: first two 4He nuclei fuse to form highly unstable 8Be, and then this 8Be nucleus is hit (during its extremely fleeting 0.07 fs existence) by another 4He nucleus, forming an excited state of carbon-12. The existence of this excited state, which makes the production of 12C resonant, is the reason that carbon is a common element in the universe, and was first deduced by Fred Hoyle because of this.
Waiting Point
is the point on the chart of nuclides where a nucleus will not undergo further neutron capture until beta decay has occurred. This is a feature of the r-process of element synthesis and leads to enhanced production of certain nuclei.
Wolf-Rayet star
is an evolved massive star (> 20 solar masses) which is losing mass in a strong stellar wind, diagnosed by strong emission lines in the spectrum. Wolf-Rayet stars may be the precursors of some Type Ibc supernovae (showing no hydrogen lines because the hydrogen envelope has been lost) – especially certain highly luminous SNe Ic – and perhaps also some Type IIn.