An Etymological Dictionary of Astronomy and Astrophysics
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To grow or increase gradually, by the process of → accretion.

Back-formation from → accretion.

The gas involved in various accretion processes, such as that fed into an → accretion disk, pulled by a compact object, or used in the mass growth of a galaxy.

→ accretion; → gas.

That → accretes, such as → accreting star and → accreting neutron star.

Adjective from → accrete + → -ing.

A → neutron star in a → binary system that accretes matter from the → campion star, either from the → stellar wind or from an → accretion disk that forms if the companion overflows its → Roche lobe. The → gravitational energy from the infalling matter provides at least part of the energy for the observed radiation and the accretion torques dominate the spin evolution. Despite these common properties, accreting → neutron stars display a wide variety of behaviors, depending on the neutron star → magnetic field strength, mass of the companion and properties of → accretion (A. K. Harding, 2013, Front. Phys. 8, 679).

→ accreting; → neutron; → star.

The star which → accretes matter, particularly in its protostellar phase or in a close binary system.

→ accreting; → star.

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.

The time necessary for the → accretion of a definite amount of mass with a fixed → accretion rate.

→ accretion; → time.

An → astronomical object that accretes surrounding material. See also → accretion.

Agent noun, from → accrete + → -or.

To gather or collect, often in gradual degrees; heap up (Dictionary.com).

From ac-, → ad-, + → cumulate; → amass.

A device for storing electricity. An electric current is passed between two plates in a liquid. This causes charges (due to electrolysis) in the plates and the liquid. Same as → secondary cell.

From L. accumulator, from accumulare "to heap up," from → ad- "in addition" + cumulare "heap up," from cumulus "heap."

Anbâré, from anbâr-, anbâštan, → amass; âkumgar, from âkumidan, → accumulate.

1) The state or quality of being → accurate.
2) The degree of nearness of a measured value to the standard or known value of the quantity, not to be confounded with → precision. For example, a refrigerator holds a constant temperature of 5.0 °C. A thermometer is used seven times to read the temperature, with the following results: 6.4, 5.1, 6.3, 4.5, 5.3, 6.1, and 4.1. This distribution does not well match the actual temperature, therefore it lacks accuracy, and shows no tendency toward a particular value; it lacks precision, as well.
If the measured temperatures are 4.8, 5.3, 5.1, 5.0, 4.6, 5.2, and 5.0, the mean value is accurate, because it comes close to the actual temperature, but the distribution shows no clear tendency toward a particular value (lack of precision).
Now suppose that the measured temperatures are 6.2, 6.3, 6.1, 6.0, 6.1, 6.3, and 6.2. In this case every measurement is well off from the actual temperature (low accuracy), but the distribution does show a tendency toward a particular value (high precision).
Finally, if the measured temperatures are 5.0, 5.0, 4.8, 5.1, 5.0, 4.9, and 5.0, the distribution is very near the actual temperature each time (high accuracy), and does show a tendency toward a particular value (high precision).
Accuracy is often given to n → significant digits or n → decimal places. For example e = 2.71828 ... = 2.718 is rounded to two four significant figures or three decimal places. → accurate to n significant figures, → accurate to n decimal places.

From L. accuratus "prepared with care, exact," p.p. of accurare "take care of," from ad- "to" + curare "take care of."

Rašmandi, from rašmand, from raš + adjective forming suffix -mand. Raš, from Av. root raz- "to right, correct, arrange;" compare with Skt. raj "to reign, rule, direct," Gk. oregein "to strech out," L. rego "to direct, lead;" PIE *reg- "to move in a straight line." Similarly, Av. râšta-, rašta- "straight," Skt. rju "straight, right, upright," Gk. orektos "elongated," L. rectus "straight," Ger. recht, E. right. In Mod.Pers. there are several derivatives: râst, râšt (as in afrâšt(an)) "right; true," rasté, rešté, raj, raž, râh, ris, râdé, radé, Lori rezg "row," etc.

1) Conforming exactly to truth or to a standard; free from error.
2) Designating → accuracy.

Accurate, from L. accuratus, → accuracy.

An expression specifying the number of meaningful digits to the right of the → decimal point. For example, e = 2.71828 ... = 2.718 is said to be accurate to three decimal places and 2.72 to two decimal places.

→ accurate; → decimal; → place.

An expression specifying the number of meaningful digits used to express the value of a measured quantity. Same as accurate to n significant figures. For example, e = 2.71828 ... = 2.718 is rounded to four significant digits, and 2.72 to three significant digits. → accurate to n decimal places.

→ accurate; → significant; → digit.

Able to be accused; open to an accusation (of); blameworthy, reprehensible (OxfordDictionaries.com).

→ accuse; → -able.