An Etymological Dictionary of Astronomy and Astrophysics
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A galaxy whose → metallicity is smaller than that of the → Milky Way galaxy.

Adj. from → metal deficiency; → galaxy.

Describing an → astronomical object in which the → chemical abundances of → heavy elements are lower than that of a reference value (usually solar or of Milky Way).

→ metal; → poor.

Same as → metal-deficient galaxy.

→ metal; → poor; → galaxy.

An environment (→ galaxy, → nebula) whose → metallicity is larger than that of the → Milky Way galaxy.

→ metal; → rich; → environment.

A star whose → metal content is higher than the → solar metallicity. The stars that harbor → extrasolar planets tend to be considerably more metal-rich than the average → Population I star in the Galactic neighborhood. See also → super-metal-rich star.

→ metal; → rich; → star.

Any language that is used to describe a language. See also → object language.

→ meta-; → language.

Of, relating to, or consisting of metal.

→ metal; → -ic.

A kind of → degenerate matter resulting from hydrogen gas when it is sufficiently compressed to undergo a phase change to liquid or solid state. Metallic hydrogen is thought to be present in compressed astronomical objects, such as the interiors of the solar system planets Jupiter and Saturn. Above the core of these planets (at a temperature of 10,000 degrees and a pressure of 3 million bars) the electrons are squeezed out of the hydrogen atoms and the fluid starts to conduct like a metal.

→ metallic; → hydrogen.

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.

An excited state in an atom, which is at the origin of the spectral lines called → forbidden lines. The time duration of the excited state being relatively long, under laboratory conditions the atom cannot pass directly to the ground state by emitting radiation. In the extremely rarefied interstellar medium, however, such highly improbable transitions do occur.

→ meta-; → stable; → state.

A streak of light caused when a → meteoroid enters Earth's → atmosphere and becomes incandescent, mostly from → friction with the air at high speed. Meteors are also referred to as shooting stars. Very bright meteors are called → fireball or → bolide. Most of visible meteors arise from particles ranging in size from about that of a small pebble down to a grain of sand, and generally weigh less than 1-2 grams. The brilliant flash of light from a meteor is mainly caused by the → meteoroid's high level of → kinetic energy as it collides with the atmosphere at high speeds (11-72 km/s). The increase in the number of meteors visible toward the end of the night results from the fact that the Earth rotates about its axis in the same direction as it orbits the Sun. This means that the leading edge (morning side) of the Earth encounters more meteoroids than the trailing edge (evening side). In general, 2 to 3 times as many meteors can be seen in the hour or so just before morning twilight, than can be seen in the early evening. Moreover, the numbers of random, or sporadic, meteors vary from season to season, due to the tilt of the Earth on its axis and other factors. See also → meteor shower.

From M.Fr. meteore, from M.L. meteorum (nom. meteora), from Gk. ta meteora "the celestial phenomena," pl. of meteoron, literally "thing high up," neuter of meteoros (adj.) "high up," from → meta- "over, beyond" + -aoros "lifted, hovering in air," related to aeirein "to raise."

Šahâb, from Ar. Šihâb.

A → meteorite impact crater located about 55 km east of Flagstaff, near Winslow in the northern Arizona desert of the United States. Meteor Crater is about 1,200 m in diameter and some 170 m deep. It is thought to have formed between 20,000 to 50,000 years ago, by the impact of a small → asteroid about 25 m in diameter. Same as → Barringer Crater.

→ meteor; → crater.

The reflection of → radio waves from transmitters located on the ground by a → meteor or by the corresponding trail left behind. When a meteor enters the Earth's upper atmosphere it excites the air molecules, producing a streak of light and leaving a trail of ionization behind it tens of kilometers long. This ionized trail occurs typically at a height of about 85 to 105 km, and may persist for less than 1 second up to several minutes.

→ meteor; → echo.

An increased number of → meteors all appearing to → diverge from the direction of a single point, called → radiant. Meteor showers occur → annually on the same dates, when the Earth crosses through a → meteoroid stream. Meteor showers are named after the → constellation in which the radiant is located. For example, the → Perseids's radiant lies near the top of the constellation Perseus. Most meteor showers are caused by → comets. As a comet orbits the Sun it sheds an icy, dusty → debris stream along its orbit. When the Earth's orbit intersects the dust trail, more meteors are seen as the cometary debris encounters our planet's → atmosphere. In the case of the → Geminids and → Quadrantids, those meteor showers come from the debris scattered by orbiting → asteroids. Typical meteor showers show 15 to 100 meteors per hour at their peak. On very rare occasions, during a → meteor storm, thousands of meteors fall per hour. Prominent meteor showers are: → Quadrantids, → Lyrids, → Eta Aquariids, → Delta Aquariids, → Perseids, → Orionids, → Taurids, → Leonids, → Geminids, → Ursids, → Alpha Capricornids.

→ meteor; → shower.

An extremely intense → meteor shower, in which hundreds or even many thousands of → meteors per hour may be observed. During the great → Leonids meteor storm of 1833 an estimated number of about 150,000 meteors fell per hour.

→ meteor; → storm.

The ionization of air molecules by the heat generated when a meteorite enters the atmosphere.

→ meteorite, → ionization.

An object of → extraterrestrial origin that survives entry through the atmosphere to reach the Earth's surface. → Meteors become meteorites if they reach the ground. See also → stony meteorite, → iron meteorite, → stony-iron meteorite, → chondrite, → micrometeorite , → achondrite, → CAI meteorite, → differentiated meteorite, → undifferentiated meteorite, → Hoba meteorite, → Jilin meteorite, → Martian meteorite, → meteorite flux.

From → meteor + -ite a suffix of nouns.

Šaxâné "metor," may be from šaxudan, šaxânidan "to scratch, to thrust, to assail," as the meteor light scratches the dark night.
Šahâbsang, from šahâb, → meteor, + sang, → stone.
Âsmânsang, from âsmân, → sky, + sang, → stone.

The total mass of extraterrestrial objects that land on Earth during a given time period. The meteorite flux is currently estimated to be about 10⁷ to 10⁹ kg yr^-1. Much of this material is dust-sized objects called → micrometeorites.

→ meteorite; → flux.