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

In a magnetized wind, the geometric loci of the points where the magnetic pressure equals the flow pressure. See also → Alfven point.

→ Alfven wave; → surface.

The acceleration of gravity at the → event horizon of a → black hole. For a → Schwarzschild back hole it is given by κ = GM/R_Sch² = c⁴/(4GM).

→ black; → hole; → surface; → gravity.

In an → optical system, the → envelope of all the → reflected or → refracted rays (by a → mirror or a → lens respectively) which do not come to a common focal point because of geometrical → aberration. This occurs when parallel rays of light fall on a → concave mirror or when a → convex lens receives parallel light. In the case of → spherical aberration, the caustic surface has an axis of symmetry.

→ caustic; → surface.

An imaginary surface surrounding a body, or group of bodies, over which the gravitational field is of constant strength and, at all points, is directed perpendicular to the surface. For a single star the surface is spherical. In a close binary system the equipotential surface of the components interact to become hourglass-shaped. → Roche lobe; → Lagrangian points.

From → equi-; → potential; → surface.

Geology: The surface of a fracture along which dislocation of adjacent rocks has taken place.

→ falt; → surface.

A surface whose → luminous intensity obeys → Lambert's cosine law. Such a source has a → reflectance that is uniform across its surface and uniformly emits in all directions from all its points. It appears equally bright from all viewing directions. Lambertian surface is a very useful concept for the approximation of radiant power transfer.

→ lambert; → surface.

The set of locations in space corresponding to the → last scattering epoch in the early Universe. It is a spherical surface around the present-day observer from which the → cosmic microwave background radiation appears to emanate.

→ last; → scattering; → surface.

A member of a particularly faint population of galaxies with a central → surface brightness below the brightness of the background sky. The central regions of many of them resemble a → dwarf galaxy, but most of the mass is contained in a large gaseous disk of low density that is observable only with long-exposure optical images or at radio wavelengths. Some are as massive as a large → spiral galaxy, for example Malin 1. The proportion of LSBGs relative to normal galaxies is unknown. They may however represent a significant fraction of mass in the Universe. LSBGs are thought to be primitive systems because they have total masses similar to normal galaxies, but have typically converted less than 10% of their gas into stars. Spiral LSBGs do not obey → Freeman's law.

→ low; → surface; → brightness; → galaxy.

A surface, such as a cylinder or cone, that can be generated by moving a straight line.

Ruled, p.p. of rule; → surface.

Ruyé, → surface; xatt sâxté "built, formed by a line," from xatt→ line; sâxté, p.p. of sâxtan "to build, make, fashion; to adapt, adjust, be fit" (from Mid.Pers. sâxtan, sâz-, Manichean Parthian s'c'dn "to prepare, to form;" Av. sak- "to understand, to mark," sâcaya- (causative) "to teach").

A property of → space-time outside a → rotating black hole, which consists of a surface which geometrically bounds the → ergosphere outward. At the stationary limit a particle would have to move with the local light velocity in order to appear stationary to a distant observer. This is because the space here is being dragged at exactly the speed of light relative to the rest of space. Outside this limit space is still dragged, but at a rate less than the speed of light. Also known as → static limit.

→ stationary; → limit; → surface.

1) The outer face, outside, or exterior boundary of a thing; outermost or uppermost layer or area.
2) Math.: An infinite aggregate of points constituting space of two dimensions; part or all of the boundary of a solid.

→ sur-; → face.

The brightness of an extended object, such as a planet, nebula, galaxy, or the sky background, expressed as magnitudes per unit area (usually square arc second). Surface brightness is calculated by dividing the object's magnitude by its size (→ isophotal radius).

→ surface; → brightness.

A current whose core of maximum velocity is near the surface.

→ surface; → current.

The amount of a quantity distributed over a surface area divided by the area, such as a surface-charge density.

→ surface; → density.

1) The rate at which a small object in free fall near the surface of a body is accelerated by the gravitational force of the body: g = GM / R², where G is the gravitational constant, and M and R are the mass and radius of the object. The surface gravity of Earth is equal to 980 cm s^-2.
2) → black hole surface gravity.

→ surface; → gravity.

Same as → last scattering surface.

→ surface; → last; → scattering.

1) For a star, same as → effective temperature.
2) Meteo.: The air temperature at or near the earth's surface.

→ surface; → temperature.

The inward → attraction of the → molecules at the → surface of a → liquid. The reason is that the molecules at the surface do not have other like molecules on all sides of them and consequently they cohere more strongly to those directly associated with them on the surface. Also called surface energy and capillary forces.

→ surface; → tension.

The double shock structure formed in any two fluids that collide supersonically. A working surface consists of two → shocks, a → bow shock where the ambient material is shocked and accelerated, and a jet shock or → Mach disk, where the → jet material is decelerated. It is common to find multiple working surfaces along the axis of an → Herbig-Haro jet, testifying to recurrent eruptions of the underlying source.

→ work; → -ing; → surface.

In the → restricted three-body problem, a surface which limits the region of space in which a small body can move. In the expression for the → Jacobi integral, the left side value is always positive or nul; hence the particle motion is confined to the region where U ≤ C_J. The surface that limits this region, defined by U = C_J, is called the zero-velocity surface.

→ zero; → velocity; → surface.