An Etymological Dictionary of Astronomy and Astrophysics
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فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory

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Number of Results: 134 Search : form
specific star formation rate (sSFR)
  نرخ ِ آبیزه‌ی ِ دیسش ِ ستارگان   
nerx-e âbize-ye diseš-e setâregân

Fr.: taux de formation d'étoiles spécifique   

Star formation rate per unit → mass. More specifically, the → star formation rate in a galaxy divided by the → stellar mass of the galaxy. Observations of galaxies over a wide range of → redshifts suggest that the slope of the SFR-M* relation is about unity, which implies that their sSFR does not depend strongly on stellar mass. Specific star formation rates increase out to z ~ 2 and are constant, or perhaps slowly increasing, from z = 2 out to z = 6, though with a large scatter, sSFR ~ 2-10 Gyr-1 (Lehnert et al., 2015, A&A 577, A112, and references therein).

specific; → star; → formation; → rate.

star formation
  دیسش ِ ستاره   
diseš-e setâré

Fr.: formation d'étoiles   

The process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. As a branch of astronomy, star formation includes the study of the interstellar medium and molecular clouds as precursors to the star formation process as well as the study of young stellar objects.

star; → formation.

star formation efficiency (SFE)
  کارایی ِ دیسش ِ ستاره   
kârâyi-ye diseš-e setâré

Fr.: efficacité de formation d'étoiles   

The degree to which stars form in a system, such as a → molecular cloud or a → galaxy. It is given by the ratio of the total mass of stars to the initial gas mass: εSFE = Mstars / (Mstars + Mgas).

star formation; → efficiency.

star formation history
  تاریخ ِ دیسش ِ ستاره   
târix-e diseš-e setâré

Fr.: histoire de formation d'étoiles   

The → star formation rate as a function of time.

star; → formation; → history.

star formation quenching
  اسرش ِ دیسش ِ ستارگان   
osereš-e diseš-e setâregân

Fr.: assèchement de formation d'étoiles   

The premature termination of star formation process in some galaxies. The ultimate quenching of star formation is caused by stripping of the gas reservoir which will finally turn into stars. A wide variety of mechanisms have been proposed to provide quenching. For example, → major mergers can transform spiral galaxies into ellipticals, and may also quench future star formation by ejecting the → interstellar medium from the galaxy via starburst, → active galactic nucleus, or shock-driven winds. In rich clusters, where merging is less efficient because of the large relative velocities of galaxies, rapid encounters or fly-bys may cause the formation of a bar and growth of a spheroidal component instead of larger scale star formation. Also, cold gas can be stripped out of the galaxy both by tidal forces and ram pressure in the intracluster medium. Similarly, the hot halo that provides future fuel for cooling and star formation may be efficiently stripped in dense environments, thus quenching further star formation (see, e.g., Kimm et al., 2009, MNRAS 394, 1131, arXiv:0810.2794).

star; → formation; → quench.

star formation rate
  نرخ ِ دیسش ِ ستاره   
nerx-e diseš-e setâré

Fr.: taux de formation d'étoiles   

The rate at which a molecular cloud or a galaxy is currently converting gas into stars. It is given by the ratio of the number of stars to the star formation time-scale.

star formation; → rate.

star formation region
  ناحیه‌ی ِ دیسش ِ ستاره   
nâhiye-ye diseš-e setâré

Fr.: région de formation d'étoiles   

A region in the → interstellar medium where processes of → star formation are going on or have occurred in the past.

star; → formation; → region.

star formation time scale
  مرپل ِ زمانی ِ دیسش ِ ستاره   
marpel-e zamâni-ye diseš-e setâre

Fr.: échelle de temps de formation d'étoiles   

The time necessary for a star to form. It depends inversely on the stellar mass.

star formation; → time scale.

star-forming galaxy
  کهکشان ِ دیسنده‌ی ِ ستاره   
kahkešân-e disnade-ye setâré

Fr.: galaxie de formation d'étoiles   

A galaxy that is located on the → galaxy main sequence in the plane relating → star formation rates to total stellar masses.

star; → formation; → galaxy.

star-forming region
  ناحیه‌ی ِ دیسش ِ ستاره   
nâhiye-ye diseš-e setâré

Fr.: région de formation d'étoiles   

A region in which → star formation is going on.

star; → formation; → region.

stimulated star formation
  دیسش ِ گوالیده‌ی ِ ستاره   
diseš-e gavâlide-ye setâré

Fr.: formation stimulée d'étoiles   

A process in which a star is not formed spontaneously but is provoked by the action of external forces, such as pressure and shock on a molecular cloud by close-by → massive stars, → supernova explosions, etc. See also → sequential star formation.

Stimulated, p.p. of → stimulate; → star formation.

stochastic self-propagating star formation
  دیسش ِ ستارگان با خود-توچش ِ کاتورگین   
diseš-e setâregân bâ xod-tuceš-e kâturgin

Fr.: formation d'étoiles par auto-propagation stochastique   

A mechanism that could be responsible for global → spiral structure in galaxies either by itself or in conjunction with spiral → density waves. In this mechanism, star formation is caused by → supernova-induced → shocks which compress the → interstellar medium. The → massive stars thus formed may, when they explode, induce further → star formation. If conditions are right, the process becomes self-propagating, resulting in agglomerations of young stars and hot gas which are stretched into spiral shaped features by → differential rotation. Merging of small agglomerations into larger ones may then produce large-scale spiral structure over the entire galaxy. The SSPSF model, first suggested by Mueller & Arnett (1976) was developed by Gerola & Seiden (1978). While the → density wave theory postulates that spiral structure is due to a global property of the galaxy, the SSPSF model examines the alternative viewpoint, namely that spiral structure may be induced by more local processes. The two mechanisms are not necessarily mutually exclusive, but they involve very different approaches to the modeling of galaxy evolution. The SSPSF gives a better fit than the density wave theory to the patchy spiral arms found in many spiral galaxies. However, it cannot explain → galactic bars.

stochastic; → self; → propagate; → star; → formation.

structure formation
  دیسش ِ ساختار   
diseš-e sâxtâr

Fr.: formation des structures   

The study of the processes that gave rise to the apparition of matter concentrations, such as → superclusters of galaxies, → galaxy clusters, and galaxies, in a homogeneous → expanding Universe. Cosmic structures are believed to result from → density fluctuations that existed in the → early Universe before radiation and matter decoupled (→ decoupling era or → recombination era). Initial → quantum fluctuations in the → inflaton field were expanded by → inflation. Inflation amplified them up to scales that correspond to those of galaxy clusters and beyond. Generally, a model of structure formation includes three main ingredients: 1) background cosmology, 2) model for fluctuation generation, and 3) types of → dark matter.
See also:
bottom-up structure formation, → hierarchical structure formation, → Silk damping, → top-down structure formation.

structure; → formation.

terraform
  زمین‌دیسیدن   
ziman-disidan

Fr.: terraformer   

To alter the environment of a planet or moon in a → terraforming process in order to make it habitable for life forms.

Probably taken from noun terraform, from L. terra "earth," → terrestrial; → form.

From zamin, → earth, + disidan "to → form."

terraforming
  زمین‌دیسش   
zamin-diseš

Fr.: terraformation   

The hypothetical process of altering the environment (atmosphere, temperature, surface topography, or ecology) of another planet or moon to improve the chances of survival of an indigenous biology or to allow habitation by terrestrial life forms. See also → ecopoiesis.

Verbal noun of → terraform. The term first appeared in a science fiction novel, Seetee Shock (1949) by Jack Williamson, an American science fiction writer; but the actual concept pre-dates this work.

top-down structure formation
  دیسش ِ ساختار از بالا به پایین   
diseš-e sâxtâr az bâlâ bé pâyin

Fr.: formation des structures du haut vers le bas   

A cosmological model of → structure formation in which larger structures, such as galaxy → superclusters or perhaps even the vast → filaments and → voids, form earlier and then they fragment into smaller structures such as individual galaxies. Opposite of → bottom-up structure formation.

top; → down; → structure; → formation.

transform
  ۱) ترادیس؛ ۲) ترادیسیدن   
1) tarâdis (#); 2) tarâdisidan (#)

Fr.: 1) transformée, transformation; 2) transformer   

1) Math.: A mathematical quantity obtained from a given quantity by an algebraic, geometric, or functional transformation.
The transformation itself.
2) To change in form, appearance, or structure; to change in condition, nature, or character; convert.
Physics: To change into another form of energy.
To increase or decrease (the voltage and current characteristics of an alternating-current circuit), as by means of a transformer.
Math.: To change the form of (a figure, expression, etc.) without in general changing the value.

trans- + → form.

transformation
  ترادیسش، ترادیس   
tarâdiseš (#), tarâdis (#)

Fr.: transformation   

1) The act or process of transforming. The state of being transformed.
2) The relationship between description of a physical phenomenon in one → reference frame to another. → Galilean transformation; → Lorentz transformation.
3) Math.: The act, process, or result of transforming or mapping.

Verbal noun of → transform.

transformer
  ترادیسگر، ترادیسنده   
tarâdisgar (#), tarâdisandé (#)

Fr.: transformateur   

A device that converts low voltages to higher voltages, or vice versa. A transformer consists of a primary coil and a secondary coil, both traversed by the same magnetic flux.

transform + → -or.

triggered star formation
  دیسش ِ ماشه‌ای ِ ستاره   
diseš-e mâše-yi-ye setâré

Fr.: formation d'étoiles déclanchée   

The formation of second-generation stars in a → molecular cloud, as set off by the action of → massive stars. → sequential star formation; → radiation-driven implosion.

trigger; → star formation.

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