An Etymological Dictionary of Astronomy and Astrophysics
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In the → Goldschmidt classification, any → chemical element that has an → affinity to combine with → iron rather than some other element. These elements are concentrated in the → Earth's core. The group includes → iron (Fe), → nickle (Ni), → cobalt (Co), → platinum (Pt), → gold (Au), → tin (Zn), and → tantalum (Ta). The siderophile elements include → highly siderophile element (HSE).

→ siderophile; → element.

In models of star formation, an isothermal sphere in which the density distribution in the static or nearly static outer envelope obeys an r^-2 power law. In the limit of infinite central concentration, the unstable equilibrium approaches the singular isothermal sphere which has the density and mass distributions ρ(r) = (a²/2πG)r^-2 and M(r) = (2a²/G)r, where a is the isothermal → sound speed inside the cloud, G is the → gravitational constant, and r the distance from the center (F. H. Shu, 1977, ApJ 214, 488).

→ singular; → isothermal; → sphere.

A ∩-shaped tube with unequal arms that is used to move a liquid from one level to a lower level via a third level higher than either. Once the short arm is filled, for example, by suction, the liquid flows down in the long arm under the action of gravity due to mass excess in it.

From Fr. siphon, from L. sipho (genitive siphonis), from Gk. siphon "pipe, tube," of unknown origin.

The number of → edges.

→ size; → graph.

A type of spectrograph that uses a slit to provide resolution.

→ slit; → spectrograp.

A numerical method for modeling → compressible hydrodynamic flows, which uses particles to simulate a continuous fluid flow. Because the system of hydrodynamical basic equations can be analytically solved only for few exceptional cases, the SPH method provides a numerical algorithm to solve systems of coupled → partial differential equations for continuous field quantities. The main advantage of the method is that it does not require a computational grid to calculate spatial → derivatives and that it is a Lagrangian method, which automatically focuses attention on fluid elements. The equations of motion and continuity are expressed in terms of ordinary differential equations where the body forces become classical forces between particles. This method was first independently developed by Lucy (1977, AJ 82, 1013) and Gingold & Monaghan (1977, MNRAS 181, 375).

Smoothed Particle Hydrodynamics, first used by Gingold & Monaghan (1977); → smooth; → particle; → hydrodynamics.

The third phase in the evolution of a → supernova remnant (SNR) occurring after the → Sedov-Taylor phase when the mass of the swept-up material becomes much larger than the amount of the ejected material. The SNR is surrounded by a cool → shell of accumulated material that is being pushed from behind, similar to what occurs for a snowplow. During this phase, → radiative cooling becomes important and the total energy is no longer conserved. Also called the → radiative phase.

→ snowplow; → phase.

The abundance of a → chemical element as determined from the observation of solar → spectral lines. The solar chemical composition is an important ingredient in our understanding of the formation, structure and evolution of both the Sun and our solar system. Furthermore, it is an essential reference standard against which the elemental contents of other astronomical objects are compared (Asplund et al. 2009, arXiv:0909.0948). The photospheric abundances relative to hydrogen are not representative of the → protosun, or global → solar system abundances. This is because heavy-element fractionation in the Sun has altered photospheric abundances (Lodders 2003, ApJ 591, 1220).

→ solar; → photospheric; → abundance.

The branch of astrophysics concerned with the study of the physical properties of the Sun based on the most detailed observations which can be obtained for a star.

→ solar; → physics.

Any of the various phenomena observable on the Earth that are caused by the influence of the Sun, such as aurora borealis.

→ solar; → terrestrial; → phenomenon.

The branch of condensed matter physics concerned with the study of rigid matter or solids in terms of their constituent particles (electrons and nuclei). The bulk of solid-state physics theory and research is focused on the electromagnetic, thermodynamic, and structural properties of crystalline solids.

→ solid state; → physics.

The half of the → Earth or another → north pole between the → south pole and the → equator.

→ southern; → hemisphere.

The → extreme adaptive optics system and → coronagraphic facility at the → European Southern Observatory (ESO) → Very Large Telescope (VLT) (UT3) available from May 2014. Its primary science goal is imaging, low-resolution spectroscopic, and polarimetric characterization of → exoplanetary system at → visible and → near-infrared wavelengths (0.5-2.32 μm). SPHERE is capable of obtaining → diffraction-limited images at 0''.02 to 0''.08 resolution depending on the wavelength. Its → spectral resolution is 30 to 350, depending on the mode.

→ spectro-; → polarimetric; → high; → contrast; → exoplanet.

An instrument that disperses the light into spectral lines and records them.

→ spectro-; → -graph.

An instrument for recording monochromatic images of the Sun.

→ spectro-; → heliograph.

An instrument designed to measure the intensity of a particular spectral line or a series of spectral lines.

→ spectro-; → photometer.

Of or relating to → spectrophotometry.

→ spectrum; → photometry.

In astronomy, measurement of the absolute fluxes of the components of different frequencies in the spectrum of a light source.

→ spectrum; → photometry.

A solid geometric figure generated by the revolution of a semicircle about its diameter; equation: x² + y² + z² = r².

M.E. spere, from O.Fr. espere, from L. sphæra "globe, ball, celestial sphere," from Gk. sphaira "globe, ball," of unknown origin.

Koré, loan from Ar. kurat.
Sepehr "sphere, celestial globe, heavens, sky;" Mid.Pers. spihr "sphere, sky, firmament, fate;" Av. spiti- in compounds: spiti-dôiθra- "with clear eyes;" Proto-Iranian *spiθra- (in proper name); cf. Skt. śvitrá- "white, whitish."

The region of space around one of the bodies in a system of two celestial bodies where a third body of much smaller mass is influenced by the gravitational field of that body. The sphere of influence of a planet with respect to the Sun has a radius given by: R = R_P(M_P/M_S)^2/3, where R_P is the radius of the planet's orbit around the Sun, M_P is the mass of the planet, and M_S is the solar mass. The sphere of influence of the Earth has a radius of about 927,000 km or slightly under 150 Earth radii. Beyond this limit, a space probe will come under the influence of the Sun.

→ sphere; → influence.