An Etymological Dictionary of Astronomy and Astrophysics
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Same as the → Sedov-Taylor phase.

→ adiabatic; → phase.

In astronomical interferometry, a method using triplets of telescopes in an array to calculate the phase information and get over the effects of atmospheric turbulence. The method, used in high-resolution astronomical observations, both at radio and at optical wavelengths, allows imaging of complex objects in the presence of severe aberrations.

→ closure; → phase.

To make two signals out of phase. For example, to get one signal at its highest peak while the other signal is at its lowest peak; they will be 180 degrees out of phase.

→ de-; → phase.

Same as → out of phase.

Past participle of → dephase.

A fairly long-lived step in the evolution of → low-mass and → intermediate-mass stars when helium burning shifts from the center to a shell around the core. At this phase the stellar luminosity is provided almost entirely by → helium shell burning. The He-shell burning generally adds mass to the growing carbon/oxygen core, resulting in → degenerate matter due to its increasing density.

→ early; → AGB; → phase.

The first phase of → supernova remnant (SNR) evolution in which the surrounding → interstellar medium (ISM) has no influence on the expansion of the → shock wave, and the pressure of the interstellar gas is negligible. The shock wave created by the → supernova explosion moves outward into the interstellar gas at highly → supersonic speed. Assuming that most of the → supernova energy  E_SN is transformed into → kinetic energy of the ejected gas, the ejection velocity v_e can be estimated from E_SN by using E_SN = (1/2) M_ev_e², which leads to v_e = (2E_SN / M_e)^(1/2), where M_e is the ejected mass. The schematic structure of the SNR at this phase can be described as follows: behind the strong → shock front which moves outward into the ISM, compressed interstellar gas accumulates forming a → shell of interstellar gas. This shell of swept-up material in front of shock does not represent a significant increase in the mass of the system. After some time the accumulated mass equals the ejected mass of stellar material, and it will start to affect the expansion of the SNR. By definition, this is the end of the free expansion phase, and the corresponding radius of the SNR, called → sweep-up radius, R_SW, is defined by M_e = (4π/3) R_SW³ρ₀, that is R_SW = (3M_e / 4πρ₀)^(1/3), where ρ₀ is the initial density of the ISM. This radius is reached at the sweep-up time t_SW = R_SW/v_e. The free expansion phase lasts some 100-200 years until the mass of the material swept up by the shock wave exceeds the mass of the ejected material. Then the following → snowplow phase starts.

→ free; → expansion; → phase.

A period in the → pre-main sequence evolution of a low mass star during which the star has negligible nuclear energy production and low internal temperature. Hence energy transport inside the star takes place dominantly through → convection. The star contracts homologously and evolves in the → H-R diagram along the → hayashi track with decreasing → luminosity and nearly constant → effective temperature. The time taken by a star of mass M_* to contract to radius R_* along a Hayashi track is of the order of the → Kelvin-Helmholtz time: t_KH = 10⁷(M_*/M_sun)²/(R_*/R_sun)³ yr.

→ Hayashi track; → phase.

The condition which exists when two waves of the same frequency pass through their maximum and minimum values in a correlated or synchronized way.

→ in; → phase.

Hamfâz, from ham- "together," → com- + fâz→ phase.

The value of the phase corresponding to the origin of time. Same as the → epoch angle.

→ initial; → phase; → angle.

One of the various changes in the apparent shape of the Moon, because as the Moon orbits the Earth different amounts of its illuminated part are facing us. The phases of the Moon include: the → new moon, → waxing crescent, → first quarter, → waxing gibbous, → full moon, → waning gibbous, → last quarter, → waning crescent, and → new moon again.

→ lunar; → phase.

In → photometry of → binary stars or → two-body systems, the number of whole or fractional orbits completed, from the point the photometry begins. The point is conventionally chosen as the position at which the → primary star eclipses the → secondary star, and therefore the → light curve is at a minimum. The phase keeps counting indefinitely, thus the secondary star gets eclipsed at phase 0, 1, 2, 3, and so on. At these phases the primary lies between the secondary and the observer. An orbital phase of 0.5 corresponds to halfway through the binary orbit, 0.75 is three-quarters the way through, and so on.

→ orbital; → phase.

The photometric variability induced by the → orbital motion in a → two-body system.

→ orbital; → phase.

The condition of two oscillators that have the same frequency but different phases. Opposed to → in phase.

→ out; → phase.

1) A particular stage or point in a course, development, or graph varying cyclically; the fractional part of the period through which the time has advanced, measured from some arbitrary origin. Phase is measured like an angle, when a complete cycle is equivalent to a phase of 360° (or 2π radians), or, sometimes, as a number between 0 and 1. Two or more waves of the same frequency are → in phase when their maxima and minima take place at the same moments. Otherwise, they are said to be → out of phase or that they have a → phase difference.
2) A state in which matter can exist, depending on temperature and pressure, e.g. the → solid, → liquid, → gaseous, and → plasma states.
3) A recurring form of the → Moon or a → planet seen in the sky. → lunar phase, → phases of Venus.
4) In a → binary star system, → orbital phase.

Mod.L. phases, plural of phasis, from Gk. phasis "appearance," from stem of phainein "to show, to make appear."

1) Fâz, loanword from Fr., as above.
2) Simâ "face, aspect, resemblance."

1) Physics: Of a → periodic wave, the number of suitable units of angular measure between a point on the wave and a reference point.
2) Astro.: For an object in the solar system, the angle "Sun-object-Earth" that is, the angle between the Sun and the observer as seen from the given object. It is 0° when the object is fully illuminated, 90° when the object is half-illuminated (like the Moon at first quarter and last quarter), and 180° when the object is between Earth and the Sun.
3) More generally, the angle between star light incident onto a related revolving object and the light reflected from the object to the observer (Earth).

→ phase; → angle.

1) Astro.: A curve describing the → brightness of a reflecting → natural satellite as a function of its → phase angle.
2) Math.: A plot of the solution to a set of equations of motion in a phase space as a function of time.

→ phase; → curve.

The ratio of the phase shift of a sinusoidal signal in transmission through a system to the frequency of the signal.

→ phase; → delay.

A graph showing the equilibrium relationships between phases (such as vapor-liquid, liquid-solid) of a chemical compound, mixture of compounds, or solution.

→ phase; → diagram.

The difference of phase (usually expressed as a time or an angle) between two periodic quantities which vary sinusoidally and have the same frequency.

→ phase; → difference.

The condition of temperature and pressure under which different phases (e.g. gas, liquid, and solid) of a substance coexist.

→ phase; → equilibrium.