An Etymological Dictionary of Astronomy and Astrophysics
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A quantity that measures the extent to which the magnetic field lines wrap and coil around each other. It is closely related to field line topology. Magnetic helicity is defined by: H_M = ∫ A . B dV, where A is the vector potential of the magnetic field and the integration is over a volume V. → helicity; → kinetic helicity

→ magnetic; → helicity.

A correlation between the → stellar mass (or → luminosity) and the → gas metallicity of → star-forming galaxies (Lequeux et al. 1979) according to which massive galaxies have higher gas metallicities. Several large galaxy surveys, such as the → Sloan Digital Sky Survey (SDSS), have confirmed that galaxies at all → redshifts with higher stellar masses retain more metals than galaxies with lower stellar masses. Besides the dependence on stellar mass, other studies have found further dependences of gas metallicity on other physical properties at a given mass, such as → specific star formation rate, → star formation rate, and stellar age. These higher dimensional relations could provide additional constraints to the processes that regulate the metal enrichment in galaxies. In addition to gas metallicity, also the → stellar metallicity of galaxies is found to correlate with the stellar mass, suggesting the mass-metallicity relation already existed at early epochs of galaxy evolution (Lian et al., 2017, MNRAS 474, 1143, and references therein).

→ mass; → metallicity; → relation.

The amount of information which can be retained in a memory, usually expressed as the number of words which can be retained. For comparison of different memories this number is expressed in bits.

→ memory; → capacity.

In a star, nebula, or galaxy, the proportion of the material that is made up of → metals, that is elements heavier than → helium. It is generally denoted by Z. The term "metallicity" is a misnomer used in astrophysics.
1) In practice, the metallicity of stars is usually expressed by the number ratio of → iron atoms to → hydrogen atoms per unit volume, with respect to the solar values: [Fe/H] = log₁₀(N_Fe/N_H)_star - log₁₀(N_Fe/N_H)_Sun, where N_Fe and N_H are the numbers of iron and hydrogen atoms per unit volume. In fact it is taken to be equal to the iron → abundance with respect to the solar value. The solar logarithmic iron abundance is 7.50 ± 0.04 (Asplund et al. 2009, ARAA 47, 481), with respect to that of hydrogen which, by convention, is 12.00. Stellar metallicity is often expressed in mass fraction. See also → solar metallicity.
2) Nebular metallicity is often defined as the relative abundance of → oxygen: (N_O/N_H)_neb/(N_O/N_H)_Sun, where N_O and N_H represent the numbers of oxygen and hydrogen atoms per unit volume.

From metallic, from → metal + → -ity.

A plot representing the number of stars (or systems) per metallicity interval, usually expressed in [Fe/H] (abundance of → iron relative to → hydrogen).

→ metallicity; → distribution; → function.

The decrease in the → abundances of → heavy elements in a → disk galaxy as a function of distance from the center. Radial metallicity gradients are observed in many galaxies, including the → Milky Way and other galaxies of the → Local Group. In the case of the Milky Way, several objects can be used to determine the gradients: → H II regions, → B stars, → Cepheids, → open clusters, and → planetary nebulae. The main diagnostic elements are oxygen, sulphur, neon, and argon in photoionized nebulae, and iron and other elements in Cepheids, open clusters, and stars. Cepheids are probably the most accurate indicators of abundance gradients in the Milky Way. They are bright enough to be observed at large distances, so that accurate distances and spectroscopic abundances of several elements can be obtained. Average abundance gradients are generally between -0.03 → dex/kpc and -0.10 dex/kpc, with a a flattening out of the gradients at large galactocentric distances (≥ 10 kpc). The existence of these gradients offers the opportunity to test models of → chemical evolution of galaxies and stellar → nucleosynthesis.

→ metallicity; → gradient.

The → heat capacity of one → mole of substance: C_μ = μ C, where μ is the → molecular weight and C the → specific heat capacity. The molar heat capacity of water is practically 18 cal/mole.C°.

Molar, adj. of → mole; → heat; → capacity.

The sum of → absorption coefficient (κ_ν) and → scattering coefficient (σ_ν) at a given frequency: k_ν = κ_ν + σ_ν. See also the → Rosseland mean opacity.

→ monochromatic; → opacity.

1) The state of being multiple, made of several components.
2) In atomic and nuclear physics, the number of → levels into which the energy of an atom, molecule, or nucleus splits as a result of → Russell-Saunders coupling between → orbital angular momentum and → spin angular momentum. It is given by 2S+1, where S is the total electron → spin quantum number. The multiplicity of an energy level is indicated by a left superscript to the value of L, where L is the resultant electron orbital angular momentum of the individual electron orbital angular momenta.
3) In → statistical mechanics, the number of → microstates corresponding to a given → macrostate.

→ multiple; → -ity.

1) General: The state or quality of being opaque.
2) A measure of the absorption of photons on their way from the stellar center to the surface. Opacity depends upon the frequency of the radiation, the density, the chemical composition, and the thermodynamic state of the gas. For a given density, the hotter the gas the lower the opacity, since the gas absorbs less readily, as described by → Kramers' law. Conversely, the cooler the gas the higher the opacity. See also → stellar pulsation, → kappa mechanism, → valve mechanism, → partial ionization zone.

From Fr. opacité, from L. opacitatem (nom. opacitas) "shade, shadiness," from opacus "shaded, dark, opaque."

Kederi, from keder "opaque," from Ar. kader + -i suffix forming nouns from adjectives.

The velocity of an object in a given orbit around a gravitating mass. For a perfect circular orbit, the velocity is described by the formula V =√[G(M + m)/r], where G is the gravitational constant, M the mass of the primary gravitating body, m the mass of the orbiting object, and r the radius of the orbit.

→ orbital; → velocity.

The speed necessary to form a parabolic orbit around a gravitational center. It is also the minimum speed necessary to escape from the gravitational pull of a body.

→ parabolic; → velocity.

1) Velocity with respect to the Local Standard of Rest.
2) Any velocity a galaxy has with respect to us that is not a Hubble law velocity due to the expansion of space.

→ peculiar; → velocity.

A state or condition characterized by regular repetition in time or space.

→ periodic + → -ity.

The speed at which any fixed phase (individual wave) in a → wave packet travels. It is expressed as v_ph = ω/k, where ω is the → angular frequency and k the → wave number. See also the → group velocity.

→ phase; → velocity.

The property which enables a material to be → deformed permanently without → rupture during the application of a → force. An → elastic material becomes plastic above its → yield point. See also → elasticity, → ductility.

→ plastic; → -ity.

The → angular velocity of a star deduced from the → rotational broadening of its → spectral lines. It is expressed as v sini, where i is the → inclination of the rotational axis with respect to the normal to the → plane of the sky. The real equatorial rotational velocity can be determined only if the inclination of the rotational axis is known.

Projected, p.p. of → project; → rotational; → velocity.

1) Extensive mention in the news media or by word of mouth or other means of communication.
2) The state of being public, or open to general observation or knowledge (Dictionary.com).

→ public; → -ity.

The component of a three-dimensional velocity vector of an object directed along the line of sight. It is measured by examining the Doppler shift of lines in the spectrum of astronomical objects.

→ radial; → velocity.

A curve describing the variation of the radial velocity of a star, due to the Doppler effect, under the gravitational effect of a secondary body (companion or exoplanet). The amplitude of these variations depends upon the mass of the secondary and its distance from the star.

→ radial velocity; → curve.