An Etymological Dictionary of Astronomy and Astrophysics
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Any equation governing the behavior of a → perturbation.

→ perturbation; → equation.

Approximate method of solving a difficult problem if the equations to be solved depart only slightly from those of a problem already solved.

→ perturbation; → method.

The variation in brightness of a target as the phase angle (the angle between Sun and observer as seen from the target) varies between 0° and 180°. The directional distribution of reflected (or scattered) radiation. The phase angle is the supplement of the scattering angle (the angle between the incident ray and the emerging ray); in other words, the sum of the phase angle and the scattering angle is always 180° (Ellis et al., 2007, Planetary Ring Systems, Springer).

→ phase; → function.

Modulation in which the phase angle of a sine-wave carrier is caused to depart from the carrier angle by an amount proportional to the instantaneous magnitude of the modulating wave.

→ phase; → modulation.

A measure of the relative phase in the image as function of frequency. It is the phase component of the → optical transfer function. A relative phase change of 180°, for example, results in an image with the black and white areas reversed.

→ phase; → transfer; → function.

The changing of a substance from one phase to another, by → freezing, → melting, → boiling, → condensation, or → sublimation. Also known as phase transformation. A well known phase transition is the transition from → water to → ice. Phase transitions are often associated with → symmetry breaking. In water there is a complete symmetry under rotations with no preferred direction. Ice has a crystal structure, in which certain orientations in space are preferred. Therefore, in transition from water to ice the continuous rotational symmetry is lost.

→ phase; → transition.

A correlation between the peak brightness of → Type Ia supernovae and the decline rate of their → light curve (15 days after the maximum). The decline rate is also correlated to the width of the peak brightness of the supernova. The brightest events are the broadest in time and brighter SNe Ia decline more slowly than dimmer ones. Applying the Phillips relation reduces the dispersion in the light curves of Type Ia SNe thus making them precise distance indicators which can be observed over large distances.

Named after Mark M. Phillips (1951-), American astronomer (Phillips et al. 1993, ApJ 413, L105); → relation.

A situation in which all of the energy of a photon is transferred to an atom, molecule, or nucleus.

→ photo- + → absorption.

Th desorption of surface substances by ultraviolet radiation.

→ photo-; → desorption.

The process by which atomic nuclei are broken apart into their constituent protons and neutrons by the impact of high energy gamma photons. Photodisintegration takes place during the core collapse phase of a → Type II supernova explosion.

→ photo- + → disintegration.

The → dissociation of a → chemical compound by → radiation  → energy.

Verbal noun of → photodissociate; → -tion.

A neutral region at the boundary of a → molecular cloud created by the penetration of → far ultraviolet (FUV) radiation from associated stars. The FUV radiation (6 eV ≤ hν ≤ 13.6 eV) dissociates the molecules and heats the gas and dust. A warm, atomic → H I region is thus created and the chemistry and thermal balance of the region are determined by the penetrating FUV photons. The progressive absorption of FUV photons leads to the occurrence of transitions between atomic and molecular phases, such as H I/H₂ and C II/C I/CO transitions. By extension, any neutral region where the physics is controlled by FUV photons can be called a PDR, as it is the case for → diffuse interstellar clouds or the edge of → circumstellar disks. The PDR concept was first studied by A. G. G. M. Tielens and D. Hollenbach (1985, ApJ 291, 722).

→ photodissociation + → region.

A process going on in a molecular cloud surface whereby the surface material ionized by ultraviolet photons of neighboring stars is dissipated.

→ photo- + → evaporation.

The mechanism of raising an electron to higher energies by photon absorption, when the energy of the photon is too low to cause photoionization.

→ photo- + → excitation.

The physical process in which an incident high-energy photon ejects one or more electrons from an atom, ion, or molecule.

→ photo- + → ionization.

To cause, or to undergo → photoionization.

→ photo-; → ionize.

Subject to, or produced by → photoionization.

→ photo-; → ionized.

A calibration which converts the measured relative magnitudes into an absolute photometry.

→ photometric + → calibration.

Same as → physisorption.

→ physical; → adsorption.

The state of a → physical system regarding its temperature, density, pressure, etc. at a given time.

→ physical; → condition.