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

A combining form of self with a range of related meanings.

From M.E., from O.E. self, seolf, sylf "one's own person, same;" cf. O.Fris. self, Du. zelf, O.H.G. selb, Ger. selbst.

Xod-, from xod; Mid.Pers. xwad "self; indeed;" Av. hva- "self, own."

The decrease in the radiation from a material caused by the absorption of a part of the radiation by the material itself.

→ self-; → absorption.

The → state or → condition of being aware of one's own → personality or → individuality.

→ self-; → awareness.

The characteristic of a system of masses, such as a star, kept together by mutual gravity.

→ self; → gravitate.

The → gravitational attraction of a system of masses, such of a planet, that allows the system to be held together by their mutual gravity. Self-gravity between atoms allows a → star to hold together, despite tremendous temperature and pressure. Similarly, to be considered a → planet, a body must have enough mass so that its self-gravity pulls it into a near-spherical shape.

→ self-; → gravity.

The inductance associated with an isolated electric circuit that is characteristic of the circuit's physical design.

→ self-; → inductance.

The generation of a voltage in a circuit due to self-inductance, the polarity of which tends to oppose the changing current in the circuit.

→ self-; → induction.

A model of → accretion disk around a → pre-main sequence star or a → protostar in which the outer parts of the disk are geometrically flat, in contrast to a → flared disk. Inward of a certain radius (0.5-1 AU from the star) the dust in the disk evaporates. Because the dust is the main source of opacity and the gas in the disk is usually optically thin, the irradiation burns a hole in the disk. Moreover, the inner rim puffs up, similarly to the case of flared disks. The difference lies in the outer parts. The inner rim casts its shadow over the disk all the way out. Since the disk thickness is almost constant, no photons can reach the surface of the disk and the outer parts of the disk remain shadowed by the inner rim and the midplane temperatures decrease accordingly. This model explains the observed → spectral energy distribution of some pre-main sequence stars, such as HD 101412. It also accounts for the observed weak → far infrared→ excess, weak or no → PAH emission, and weak or no [O I] emission.

→ self-; → shadow; → disk.

Gerdé, → disk; xod-, → self-; pardé, → screen.

The phenomenon whereby the → photodissociation transitions of a molecule in interstellar clouds become → optically thick, so that the molecule in question is "shielded" by other molecules against dissociating stellar → far-ultraviolet (FUV) photons. In the case of → molecular hydrogen (H₂), when the → column density exceeds 10¹⁴ cm^-2, the UV absorption bands become optically thick, and H₂ undergoes self-shielding. More specifically, all of the photons that could lead to UV photodissociation are absorbed by H₂ in the outer layers of the cloud, hence protecting the H₂ within the cloud. Self-shielding occurs in → diffuse interstellar clouds exposed to the interstellar → radiation field or in → molecular clouds in proximity to sources of UV photons. Dust can also absorb UV photons, further limiting the photodissociation, but it dominates only when the local UV radiation field is unusually intense relative to the density of the cloud.

→ self-; → shield; → -ing.

1) Of a geometric figure, having a structure analogous or identical to its overall structure. → fractal.
2) The quality of a variable entity in which the shape does not change with time, such as a → self-similar process.

→ self-; → similar.

A process that is invariant in distribution under scaling of time. Schematically, images taken of such a process at different time scales will look similar.

→ self-; → similar; → process.

The property of being → self-similar.

→ self-; → similarity.

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.