An Etymological Dictionary of Astronomy and Astrophysics
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The first step carried out in a → fuzzy logic system during which a → crisp set of → input data are gathered and converted to a → fuzzy set using fuzzy → linguistic variables, fuzzy linguistic terms, and → membership functions.

Verbal noun of → fuzzify; → -tion.

A region of the Galaxy whose boundaries are set by its calm and safe environment and access to the chemical materials necessary for building terrestrial planets similar to the Earth. → circumstellar habitable zone; → habitable zone.

→ galactic, → habitable; → zone.

In the → Galactic coordinate system, the angle between the → Galactic Center and the projection of the object on the → Galactic plane. Galactic longitude, usually represented by the symbol l^II, ranges from 0 degrees to 360 degrees.

→ galactic; → longitude.

The revolving of the gaseous and stellar content of a galaxy around its central nucleus. The rotation is not uniform, but differential. One revolution of the Sun within our own Galaxy takes about 220 million years, or one cosmic year.

→ galactic; → rotation.

The discrepancy between observed galaxy → rotation curves and the theoretical prediction, assuming a centrally dominated mass associated with the observed luminous material.

→ galactic; → rotation; → problem.

The study dealing with the processes that gave rise to galaxies in a remarkably → early Universe. See also → structure formation, → protogalaxy

→ galaxy; → formation.

Same as → Galilean satellites.

→ Galilean; → moon.

The method of relating a measurement in one → reference frame to another moving with a constant velocity with respect to the first within the → Newtonian mechanics. The Galilean transformation between the coordinate systems (x,y,z,t) and (x',y',z',t') is expressed by the relations: x' = x - vt, y' = y, z' = z. Galilean transformations break down at high velocities and for electromagnetic phenomena and is superseded by the → Lorentz transformations.

→ Galilean; → transformation.

The coating of steel or iron with → zinc, either by immersion in a bath of molten zinc or by electrolytic deposition from a solution of zinc sulfate, to give protection against corrosion.

Verbal noun of → galvanize.

The study of → gamma rays from → extraterrestrial → sources, especially → gamma-ray bursts.

→ gamma ray; → astronomy.

The constraint on the → baryon number density at T ~ 10⁹ K in the early → expanding Universe. Gamow recognized that a key to the element buildup is the reaction n + p ↔ d + γ. Deuterium needs to be produced in sufficient abundance for higher elements to form, but if all → neutrons are immediately locked up into → deuterium, no higher elements can form either. The Gamow condition is expressed by n_b<σv>t ~ 1, where n_b is the baryon number density, σ is the cross section for the reaction at relative → velocity v, and t the expansion time-scale for the → Universe. This means that the time-scale for the above reaction is comparable to the expansion time. From this condition the baryon number density at the start of element buildup is found to be n_b ~ (σvt)^-1 ~ 10¹⁸ cm^-3 at T = 10⁹ K (P. J. E. Peebles, 2013, Discovery of the Hot Big Bang: What happened in 1948, arXiv.1310.2146).

→ Gamow barrier; → condition.

For a given quantity of an → ideal gas, the product of its → pressure and the → volume divided by the → absolute temperature (R = PV/T).

→ gas; → constant.

An equation that links the pressure and volume of a quantity of gas with the absolute temperature. For a gram-molecule of a perfect gas, PV = RT, where P = pressure, V = volume, T = absolute temperature, and R = the gas constant.

→ gas; → equation.

An → isotope separation process using the different diffusion speeds of → atoms or → molecules for separation. This process is used to divide → uranium hexafluoride (UF₆) into two separate streams of U-235 and U-238. Before processing by gaseous diffusion, uranium is first converted from → uranium oxide (U₃O₈) to UF₆. The UF₆ is heated and converted from a solid to a gas. The gas is then forced through a series of compressors and converters that contain porous barriers. Because uranium-235 has a slightly lighter isotopic mass than uranium-238, UF₆ molecules made with uranium-235 diffuse through the barriers at a slightly higher rate than the molecules containing uranium-238. At the end of the process, there are two UF₆ streams, with one stream having a higher concentration of uranium-235 than the other (EVS, a Division of Argonne National Laboratory).

→ gaseous; → diffusion.

A class of elementary particles that includes the gluon, photon, W⁺, W^-, and Z⁰ particles, each having an integral spin.

→ gauge; → boson.

A change of the fields of a gauge theory that does not change the value of any measurable quantity.

→ gauge; → transformation.

A theoretical frequency distribution for a set of variable data, usually represented by a bell-shaped curve with a mean at the center of the curve and tail widths proportional to the standard deviation of the data about the mean.

→ Gaussian; → distribution.

A method of solving a matrix equation of the form A x = b, where A is a matrix and x and b are vectors. The process consists of two steps, first reducing the elements below the diagonal to 0 and second, back substituting to find the solutions.

→ Gaussian; → elimination.

The function e^-x2, whose integral in the interval -∞ to +∞ gives the → square root of the → number pi: ∫e^-x2dx = √π. It is the function that describes the → normal distribution.

→ Gaussian; → function.

The constant, denoted k, defining the astronomical system of units of length (→ astronomical unit), mass (→ solar mass), and time (→ day), by means of → Kepler's third law. The dimensions of k² are those of Newton's constant of gravitation: L ³M ^-1T ^-2. Its value is: k = 0.01720209895.

→ Gaussian; → gravitational; → constant.