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

1) A series of very closely spaced absorption lines in stellar spectra resulting from the absorption of light by molecules. Bands caused by titanium oxide (→ TiO bands) and carbon compounds occur in the spectra of low temperature M and C stars.
2) A range of wavelengths, usually in electromagnetic radiation, that are absorbed by a given substance. Absorption bands are characteristic of molecules and correspond to changes of electron orbits in the molecules. See also → anomalous dispersion.

→ absorption; → band.

The fraction of normally incident light that is absorbed per unit path length or by a unit mass of absorbing medium.

→ absorption; → coefficient.

A graphic representation of the amount of radiant energy absorbed by a material as a function of the wavelength.

→ absorption; → curve.

A dip in a spectrum indicating that light has been absorbed on the path from the emitting source to the observer. It may be a line, corresponding to a well-defined atomic or molecular transition, or a more complex feature whose physical origin is not necessarily understood.

→ absorption; → feature.

A dark line in the spectrum of a source produced by a lower temperature gas cloud lying between the source and the observer.

→ absorption; → line.

A dark cloud of dust and gas that absorbs light from and impedes the view of background stars; dark nebula.

→ absorption; → nebula.

A spectrum of absorption lines or bands, produced when light from a hot source, itself producing a continuous spectrum, passes through a cooler gas.

→ absorption; → spectrum.

1) The mental process in which an element or quality is separated from a total object. Also the result of this process.
2) Chemistry: → abstraction reaction.

Verbal noun from → abstract; → -tion.

Chemistry: A bimolecular chemical reaction that involves removal of an atom or ion from a molecule. For example, hydrogen abstraction from methane: CH₄ + Cl → CH₃ + HCl.

→ abstraction; → reaction.

The motion of an object subject to → acceleration. Opposite to → uniform motion.

Accelerated, from → accelerate; → motion.

Jonbeš, → motion; šetâbdâr "accelerated," from šetâb→ accelerate + dâr "having, possessor" (from dâštan "to have, to possess," Mid.Pers. dâštan, O.Pers./Av. root dar- "to hold, keep back, maitain, keep in mind;" cf. Skt. dhr-, dharma- "law;" Gk. thronos "elevated seat, throne;" L. firmus "firm, stable;" Lith. daryti "to make;" PIE *dher- "to hold, support").

→ accelerating Universe.

→ accelerating; → expansion; → Universe.

The rate at which the velocity of an object changes with time.

Acceleration, from accelerate, from L. accelerare "quicken," from → ad- "to" + celerare "hasten," from celer "swift" (cf. Skt. car, carati "to move, go, drive," Gk. keles "fast horse, horse race," Av. kar- "to walk, move, go around," Mod.Pers. cal, calidan "to move, to go, to walk" (jald? "quick, active, brisk"), Gilaki/Hamadâni jal "quick, fast,"Lori žil "motion, impulse"); PIE *kel- "to drive, set in swift motion."

Šetâb "quickness, haste, speed," Mid.Pers. ôštâp "hurry, haste," ôštâftan "to hurry, hasten," from *abi.stap-, from abi- " to; in addition to; against" + *stap- "to oppress," Arm. (loanword) štap "haste, trouble."

The acceleration that an object experiences because of gravity when it falls freely close to the surface of a massive body, such as a planet. Same as → gravitational acceleration.

→ acceleration; → gravity.

A measure of the departure from a constant rate of the acceleration of the Universe, expressed by: q(t) = R(t)R ^..(t)/R ^.2(t), where R(t) represents the size of the Universe at time t. Traditionally, a negative sign is inserted in the above equation for the → deceleration parameter.

→ acceleration; → parameter.

1) The process by which an object increases its mass under the influence of its → gravitational attraction. Accretion plays a key role in a wide range of astrophysical phenomena. In particular stars result from the accretion of material by a → protostar from a surrounding → molecular cloud. The accumulation of mass on the protostar involves the formation of an → accretion disk. Theoretical and observational investigations of protostars and newborn stars indicate the important role of → magnetic fields in this process. They favor the magnetospheric accretion model for mass transfer from the circumstellar disk onto the newborn star. In this model, the stellar magnetosphere → truncates the disk at a few stellar radii. Gas from the disk accretes onto the star along the magnetic field lines and hits the stellar surface at approximately the → free fall velocity, causing a strong accretion shock. Various → emission lines, such as the hydrogen → Balmer series, He I 5876 Å, Brγ 2.17 μm, and so forth are formed in the infalling magnetospheric flow. Moreover, optical/ultraviolet excess continuum emission is produced in the → accretion shocks. The accretion is accompanied by mass ejection through collimated → bipolar jets.
2) Accumulation of dust and gas onto larger bodies by → coalescence under the influence of their mutual → gravitational attraction or as a result of chance collisions.
See also:
→ accretion column, → accretion disk, → accretion flow, → accretion rate, → accretion shock, → accretion time.

L. accretionem (nom. accretio, gen. accretionis) "a growing larger," from stem of accrescere, from ad- "to" + crescere "to grow".

Farbâl from prefix → far- which conveys "increase, abundance" + bâl, from bâlidan "to grow, to wax great," Mid.Pers. vâlitan, Av. varəd-, varədait- "to increase, augment, strengthen, cause to prosper," Skt. vrdh-, vardhati.

The channel through which matter is accreted onto a body such as a → protostar, → white dwarf, → neutron star, or → black hole. The accreting body possesses magnetic fields strong enough to disrupt the → accretion flow and carry the material through column-shaped channels directly on to a small fraction of the stellar surface near the magnetic poles.

→ accretion; → column.

A rotating disk of gas and dust formed around a center of strong gravity that pulls material off a surrounding or near-by gaseous object. Accretion disks are associated with several astrophysical objects such as → binary stars, → protostars, → white dwarfs, → neutron stars, and → black holes. Accretion disk forms because the infalling gas does not directly crash the accreting object due to its too high → angular momentum. The individual particles go into a circular orbit around the accretor because the circular orbit has the lowest energy for a given angular momentum. A spread in angular momentum values will give a population of particles moving on different orbits, so that a rotating disk of matter forms around the object. The matter in the disk becomes very hot due to internal friction and → viscosity as well as the tug of the accreting object. Since this hot gas is being accelerated it radiates energy and loses angular momentum and falls onto the accretor. Theoretical and observational pieces of evidence point to the importance of → magnetic fields in the accretion process. According to current models, the stellar magnetosphere → truncates the disk at a few stellar radii. Gas from the disk accretes onto the star along the magnetic field lines and hits the stellar surface at approximately the → free fall velocity, causing a strong accretion shock. See also → flared disk, → self-shadowed disk, → protoplanetary disk, → alpha disk model.

→ accretion; → disk.

1) Flow of matter during an accretion process.
2) In a → binary system, flow of matter from the losing-mass → companion toward the compact one. The flow can be from a → stellar wind or through the → inner Lagrangian point.
3) → cold accretion flow, → hot accretion flow.

→ accretion; → flow.

The amount of mass → accreted during unit time. The accretion rate for the → collapse of a singular → isothermal sphere is expressed by: dM/dt = 0.975 c_s³/G, where c_s is the isothermal → sound speed (Shu 1977, ApJ 214, 488). This relation can be written as: dM/dt = 4.36 x 10^-6 (T / 20 K)^3/2 in units of solar masses per year, where T is the temperature. Observed temperatures of 10-20 K in regions of → low-mass star formation imply accretion rates of about 10^-6 to 10^-5 solar masses per year. Accretion rates for → massive stars amount to values of 10^-4 to 10^-3 solar masses per year.

→ accretion; → rate.

A → shock wave occurring at the surface of a compact object or dense region that is accreting matter with a → supersonic velocity from its environment. In the case of → young stellar objects the process is believed to take place by funneled streams in the form of → accretion columns that originate in the surrounding → accretion disk and flow along the → field lines of the → protostar → magnetosphere. The gas falls supersonically onto the surface of the central body and its impact produces strong shocks of a few million → kelvin, a phenomenon that is observable in → X-rays.

→ accretion; → shock.