An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

The ability of a substance to flow; reciprocal of → viscosity.

→ fluid + → -ity.

Flux of radiation that falls on a detector per unit surface area of the detector per unit bandwidth of the radiation per unit time.

→ flux; → density.

1) Condition or quality of being formal; accordance with required or traditional rules, procedures, etc.
2) Strict adherence to established rules and procedures (Dictionary.com).

→ formal; → -ity.

Optics: If the intensity in an interference fringe pattern has the maximum and minimum values I_max and I_min, the visibility is defined by the relation ν = (I_max - I_min) / (I_max + I_min), where 0 ≤ ν ≤ 1. In terms of the intensities of the two interfering waves: ν = 2(I₁ . I₂)^1/2 / (I₁ + I₂).

→ fringe; → visibility

The division of galaxies into a "red sequence" and a "blue sequence" in the → color-magnitude diagrams of galaxies involving large statistical surveys. In both sequences, redder galaxies tend to be brighter. The blue sequence is truncated at the red magnitude ~ -22, while the red sequence extends to brighter magnitudes. The division between the two classes of galaxies is associated with a critical stellar mass ~ 3 × 10¹⁰ Msun. Galaxies below the critical mass are typically blue, star forming spirals and reside in the field. Galaxies above the critical mass are dominated by red spheroids of old stars and live in dense environments (Kauffmann et al, 2003, MNRAS 341, 33 & 54).

→ galaxy; → bimodality.

The principle according to which the fundamental laws of physics are the same in all frames of reference moving with constant velocity with respect to one another (→ inertial reference frames). Same as → Galilean invariance and → Newtonian relativity.
See also: → Galilean transformation, → Einsteinian relativity.

→ Galilean; → relativity.

The metallicity derived from observations of the gas component of a galaxy. It is mainly measured from optical → emission lines using primarily oxygen abundances. The gas → metallicity is one of the most important tools to investigate the evolutionary history of galaxies. The reason is that the gas metallicity of galaxies is basically determined by their star-formation history. Recent observational studies has allowed the investigation of the gas metallicity even in → high redshift beyond z = 1, such as → Lyman break galaxies, submillimeter-selected high-z galaxies, and so on. Such observational insights on the metallicity evolution of galaxies provide constraints on the theoretical understandings of the formation and the evolution of galaxies.

→ gas; → metallicity.

The total electric flux ψ out of an arbitrary closed surface in free space is equal to the net charge within the surface divided by the → permittivity. In differential form: ∇ . E = ρ/ε₀, where ρ is the → charge density and ε₀ the permittivity. The integral form of the law: ∫E . dS = Q/ε₀ (closed surface integral). This is one of the four → Maxwell's equations.

→ gauss; → law; → electricity.

Math.: The condition of having → Gaussian distribution. The extent to which something is Gaussian.

→ Gaussian + → -ity.

The theory of → gravitation developed by Albert Einstein (1916) that describes the gravitation as the → space-time curvature caused by the presence of matter or energy. Mass creates a → gravitational field which distorts the space and changes the flow of time. In other words, mass causes a deviation of the → metric of space-time continuum from that of the "flat" space-time structure described by the → Euclidean geometry and treated in → special relativity. General relativity developed from the → principle of equivalence between gravitational and inertial forces. According to general relativity, photons follow a curved path in a gravitational field. This prediction was confirmed by the measurements of star positions near the solar limb during the total eclipse of 1919. The same effect is seen in the delay of radio signals coming from distant space probes when grazing the Sun's surface. Moreover, the space curvature caused by the Sun makes the → perihelion of Mercury's orbit advance by 43'' per century more than that predicted by Newton's theory of gravitation. The → perihelion advance can reach several degrees per year for → binary pulsar orbits. Another effect predicted by general relativity is the → gravitational reddening. This effect is verified in the → redshift of spectral lines in the solar spectrum and, even more obviously, in → white dwarfs. Other predictions of the theory include → gravitational lensing, → gravitational waves, and the invariance of Newton's → gravitational constant.

→ general; → relativity.

The natural variations in the → geomagnetic field due to interactions of the Earth's field and → magnetosphere with energetic particles from the Sun.

→ geomagnetic; → activity.

The process by which fluctuations in an infinite medium of size greater than a certain length scale (the Jeans length) grow by self-gravitation.

→ gravitational; → instability.

1) The apparent force of → gravitation on an object at or near the surface of a star, planet, satellite, etc.
2) Same as → gravitation and → gravitational interaction.

From L. gravitatem (nom. gravitas) "weight, heaviness," from gravis "heavy," from PIE base *g^wrə- "heavy" (cf. Mod.Pers. gerân "heavy;" Av. gouru- "heavy;" Skt. guru- "heavy, weighty, venerable;" Gk. baros "weight," barys "heavy;" Goth. kaurus "heavy").

Gerâni, noun of gerân "heavy, ponderous, valuable," from Mid.Pers. garân "heavy, hard, difficult;" Av. gouru- "heavy" (in compounds), from Proto-Iranian *garu-; cognate with gravity, as above.

An important astronautical technique whereby a → spacecraft takes up a tiny fraction of the → orbital energy of a planet it is flying by, allowing it to change → trajectory and → speed. Since the planet is not at rest but gravitating around the Sun, the spacecraft uses both the orbital energy and the gravitational pull of the planet. Also known as the slingshot effect or → gravitational slingshot. More specifically, as the spacecraft approaches the planet, it is accelerated by the planet's gravity. If the spacecraft's velocity is too low, or if it is heading too close to the planet, then the planet's → gravitational force will pull it down to the planet. But if its speed is large enough, and its orbit does not bring it too close to the planet, then the gravitational attraction will just bend the spacecraft's trajectory around, and the accelerated spacecraft will pass rapidly by the planet and start to move away. In the absence of other gravitational forces, the planet's gravity would start to slow down the spacecraft as it moves away. If the planet were stationary, the slow-down effect would be equal to the initial acceleration, so there would be no net gain in speed. But the planets are themselves moving through space at high speeds, and this is what gives the "slingshot" effect. Provided the spacecraft is traveling through space in the same direction as the planet, the spacecraft will emerge from the gravity assist maneuver moving faster than before.

→ gravity; assist, from M.Fr. assister "to stand by, help, assist," from L. assistere "assist, stand by," from → ad- "to" + sistere "to cause to stand," from PIE *siste-, from *sta- "to stand" (cognate with Pers. istâdan "to stand").

Yâri "assistance, help; friendship," from yâr "assistant, helper, friend," from Mid.Pers. hayyâr "helper," hayyârêh "help, aid, assistance," Proto-Iranian *adyāva-bara-, cf. Av. aidū- "helpful, useful."

→ gravity darkening.

→ gravity; → brightening.

The darkening, or brightening, of a region on a star due to localized decrease, or increase, in the → effective gravity. Gravity darkening is explained by the → von Zeipel theorem, whereby on stellar surface the → radiative flux is proportional to the effective gravity. This means that in → rotating stars regions close to the pole are brighter (and have higher temperature) than regions close to the equator. Gravity darkening occurs also in corotating → binary systems, where the → tidal force leads to both gravity darkening and gravity brightening. The effects are often seen in binary star → light curves. See also → gravity darkening exponent. Recent theoretical work (Espinosa Lara & Rieutord, 2011, A&A 533, A43) has shown that gravity darkening is not well represented by the von Zeipel theorem. This is supported by new interferometric observations of some rapidly rotating stars indicating that the von Zeipel theorem seems to overestimate the temperature difference between the poles and equator.

→ gravity; → darkening

According to the → von Zeipel theorem, the emergent flux, F, of total radiation at any point over the surface of a rotationally or tidally distorted star in → hydrostatic equilibrium varies proportionally to the local gravity acceleration: F ∝ g_eff^α, where g_eff is the → effective gravity and α is the gravity darkening coefficient. See also the → gravity darkening exponent.

→ gravity; → darkening; → coefficient.

The exponent appearing in the power law that describes the → effective temperature of a → rotating star as a function of the → effective gravity, as deduced from the → von Zeipel theorem or law. Generalizing this law, the effective temperature is usually expressed as T_eff∝ g_eff^β, where β is the gravity darkening exponent with a value of 0.25. It has, however, been shown that the relation between the effective temperature and gravity is not exactly a power law. Moreover, the value of β = 0.25 is appropriate only in the limit of slow rotators and is smaller for fast rotating stars (Espinosa Lara & Rieutord, 2011, A&A 533, A43).

→ gravity; → darkening; → exponent.

Same as → g mode

→ gravity; → mode.

Transient → streamers which form when → clumps of particles begin to collapse under their own → self-gravity but are sheared out by → differential rotation. This phenomenon is believed to be the source of → azimuthal asymmetry in → Saturn's → A ring (Ellis et al., 2007, Planetary Ring Systems, Springer).

→ gravity; → wake.