An Etymological Dictionary of Astronomy and Astrophysics
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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 proportion of the solar matter made up of → chemical elements heavier than → helium. It is denoted by Z, which represents the sum of all elements heavier than → helium, in mass fraction. The most recent determination of the solar Z gives a value of 0.0134 (Asplund et al. 2009, ARAA 47, 481), corresponding to the present-day photospheric composition.

→ solar; → metallicity.

The metallicity derived from observations of stars in galaxies. It is mainly based on spectral → absorption lines in → ultraviolet (UV) and optical ranges. Stellar metallicity is a direct measure of the amount of metals in a galaxy, since large part of heavy elements lies in its stars.

→ stellar; → metallicity.

A very → metal-rich star whose iron → metallicity, [Fe/H], exceeds 0.20 → dex. Examples include HD 32147, HD 121370, and HD 145675 (Feltzing & Gonzalez, 2001, A&A 367, 253).

→ super-; → metal; → rich; → star.