An Etymological Dictionary of Astronomy and Astrophysics
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The angle between the light entering and exiting the prism when the light passing through the prism is parallel to the prism's base. Angle of minimum deviation (D) is used to measure the → index of refraction (n) of the prism glass, because: n = sin [(A + D)/2]/sin (A/2), where A is the → prism angle.

→ angle; → minimum; → deviation.

→ prism angle.

→ angle; → prism.

The angle between the reflected ray and the normal to the reflecting surface.

→ angle; → reflection.

The angle between the direction in which a ray is refracted and the normal to the refracting surface.

→ angle; → refraction.

Unit of length used to describe wavelengths and interatomic distances. 1 Å = 10^-10 m.

Named after Anders Jonas Ångström, Swedish physicist and astronomer who founded the science of spectroscopy and discovered by studying the solar spectrum that there is hydrogen in the Sun's atmosphere; → unit.

Having, forming, or consisting of an → angle or angles.

From L. angularis "having corners or angles," from angulus→ angle.

Zâviye-yi, adj. from zâviyé, → angle.

The rate of change of → angular velocity. It is equal to the → first derivative of the → angular velocity: α = dω/dt =d²θ/dt² = a_t/r, where θ is the angle rotated, a_t is the linear tangential acceleration, and r is the radius of circular path.

→ angular; → acceleration.

The apparent diameter of an object in angular measure.

→ angular; → diameter.

1) The ratio of an object's → linear size (l) to its → angular size (δθ, in → radians), that is D_A = l/δθ. It is used to convert observed angular separations into proper separations at the source.

2) In cosmology, a distance defined as the ratio of an object's physical transverse size (l) to its angular size (δθ). It is used to convert angular separations in telescope images into proper separations at the source. The angular diameter distance is defined by: D_A = l / δθ. Consider a light source of size l at r = r₁ and t = t₁ subtending an angle δθ at the origin (r = 0, t = t₀). The proper distance between the two ends of the object is related to δθ by: δθ = l / [a(t₁). r₁], where a(t₁) is the → scale factor at the present epoch. Therefore, D_A = r₁ / (1 + z). The angular diameter distance has the particularity that it does not increase infinitely with z→ ∞. It gets its maximum value at a → redshift of ~ 1 and then decreases for higher z. Therefore, more distant objects appear larger in angular size. This is explained by considering the size of the Universe when the light of the object was emitted. At that time the Universe was smaller and therefore the object occupied a larger fraction of the size of the Universe. In other words, objects appear larger because the entire Universe acts as a → gravitational lense.

→ angular; → diameter; → distance.

A high-contrast imaging technique that reduces minute temporal and spatial → seeing fluctuations and facilitates the detection of faint point sources, in close separation from their stars. It consists of the acquisition of a sequence of images with an → altazimuth mounting telescope while the instrument field derotator is switched off. This keeps the instrument and telescope optics aligned and allows the field of view to rotate with respect to the instrument. For each image, a reference → point spread function (PSF) is constructed from other appropriately selected images of the same sequence and subtracted to remove quasistatic PSF structure (Marois et al. 2006, ApJ 641, 556).

→ angular; → differential; → imaging.

The rate of change of the angles of emergence θ of various wavelengths from a dispersing prism: dθ/dλ.

→ angular; → dispersion.

Between two points A and B, the angle → subtended by lines drawn from an observing point O to A and B. Same as → angular separation.

→ angular; → distance.

The number of complete rotations per unit time, expressed in radian per second: ω = 2πν, where ν is the frequency (cycles per second).

→ angular; → frequency.

Same as → rotational energy.

→ angular; → kinetic; → energy.

The product of → moment of inertia and → angular velocity; synonymous with moment of momentum about an axis. Angular momentum is a vector quantity; it is conserved in an isolated system.

→ angular; → momentum.

A problem encountered by the → cold dark matter model of galaxy formation. The model predicts too small systems lacking → angular momentum, in contrast to real, observed galaxies. → cusp problem; → missing dwarfs.

→ angular; → momentum; → catastrophe

The ratio J/M, where J is the → angular momentum of a → rotating black hole and M the mass of the black hole.

→ angular; → momentum; → parameter.

1) The fact that the Sun, which contains 99.9% of the mass of the → solar system, accounts for about 2% of the total → angular momentum of the solar system. The problem of outward → angular momentum transfer has been a main topic of interest for models attempting to explain the origin of the solar system.
2) More generally, in star formation studies, the question of the origin of the angular momentum of a star and the evolution of its distribution during the early history of a star. Consider a filamentary molecular cloud with a length of 10 pc and a radius of 0.2 pc, rotating about its long axis with a typical → angular velocity of Ω = 10^-15 s^-1. At a matter density of 20 cm^-3, the cloud is about 1 → solar mass. The cloud collapses to form a star with radius of 6 x 10¹⁰ cm. The conservation of angular momentum (∝ ΩR²) requires that as the radius decreases from 0.2 pc to the stellar value, a factor of 10⁷, the value of Ω must increase by 14 orders of magnitude to 10^-1 s^-1. The star's rotational velocity will be 20% the speed of light and the ratio of → centrifugal force to gravity at the equator will be about 10⁴. Observational data, however, indicate that the youngest stars are in fact rotating quite slowly, with rotational velocities of 10% of the → break-up velocity. The angular momentum problem was first studied in the context of single stars forming in isolation (L. Mestel, 1965, Quart. J. R. Astron. Soc. 6, 161). For more information see, e.g., P. Bodenheimer, 1995, ARAA 33, 199; H. Zinnecker, 2004, RevMexAA 22, 77; R. B. Larson, 2010, Rep. Prog. Phys. 73, 014901, and references therein.

→ angular; → momentum; → problem.

A process whereby in a rotating, non-solid system matter is displaced toward (→ accretion) or away from (→ mass loss) the rotation center. See also → magnetorotational instability.

→ angular; → momentum; → transfer.

Same as → angular momentum transfer.

→ angular; → momentum; → transport.