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

A range of frequencies that can pass through a filter such as one in an electrical circuit.

From → band + pass, from O.Fr. passer, from V.L. *passare "to step, walk, pass," from L. passus "step, pace;" cf. Pers. pâ "foot," pey "step."

Gozar-bând, from gozar "passage, transit, passing," gozaštan "to pass, cross, transit," from Mid.Pers. vitârtan + bând, → band.

An electric filter that transmits a known band of frequencies but suppresses unwanted frequencies above and below this band.

→ bandpass; → filter.

The portion of the electromagnetic spectrum that is permitted to pass through an electronic device, such as a radio telescope detector. The term refers to either a wavelength interval or a frequency interval.

Bandwidth, from → band + → width.

Bândpahnâ, from bând, → band, + pahnâ, → width, from pahn "wide," → broad.

The brightest star in the constellation → Andromeda with an average → apparent visual magnitude of 2.05. It is a red (B - V = +1.57), → giant star of → spectral type M0 III. Beta And lies at a distance of 197 ± 7 → light-years (61 ± 2 → parsecs). It has a mass of 3-4 Msun (→ solar mass), a → luminosity of ~ 2,000 Lsun (→ solar luminosity), and a radius of 100 Rsun (→ solar radius). Its other designations include: Mirach, Merach, Mirac, Mizar, 43 Andromedae, BD+34°198, HD 6860, HIP 5447, HR 337, LTT 10420, and SAO 54471. Beta And happens to lie nearly along the → line of sight to the galaxy → NGC 404. This galaxy, known as → Mirach's Ghost, is visible seven arc-minutes away.

Beta, a Greek letter of alphabet used in the → Bayer designation; → Andromeda

An object that seems likely to be a → black hole, but waits for more observational confirmations.

→ black; → hole; → candidate.

A mechanism for the extraction of energy from a rotating → Kerr black hole. It relies on the assumption that the material → accreted by a → black hole would probably be → magnetized and increasingly so as the material gets closer to the → event horizon. Since all black holes of current astrophysical interest are probably accreting from magnetized disks, this has led to suggestions that the Blandford-Znajek process plays a vital role in → active galactic nuclei (AGN) and other accreting black hole systems. The power, P, generated is given by: P = (4π/μ₀) B²R_S²c, where B is the → magnetic field of the → accretion disk, and R_S is the → Schwarzschild radius of the black hole. As an example, for a 10⁸ solar mass black hole with a 1 T magnetic field, the power generated is approximately 2.7 × 10³⁸ W. In perspective, the annual energy consumption of the world is estimated around 5 × 10²⁰ J. The example case presented produces more energy in a single second than the entire globe consumes in a year. While this is a bold claim to make, it is only an example case where not all the energy produced is extractable as useable energy. However, at that point, even a system which is less that < 10-15 % efficient would be sufficient to supply enough energy to power the world for a full year. Of course, the system itself is limited in its lifetime due to the extraction of energy by slowing down the rotation of the black hole. Hence, the system can only exist as long as the black hole has angular momentum, continuing to rotate. At some point, the rotation will cease and the energy source will be unusable (D. Nagasawa, PH240, Stanford University, Fall 2011).

Blandford, R. D., & Znajek, R. L., 1977, MNRAS 179, 433; → process.

Photometric measurements carried out through filters with a band-width (about one-tenth the central wavelength) in the range 30-100 nm. Typical examples are Johnson photometry, Krons-Cousins RI photometry, and the six-color system.

→ broad; → band; → photometry.

The → SI unit of → luminous intensity in a given direction; symbol cd. It is defined by taking the fixed numerical value of the → luminous efficacy of → monochromatic radiation of → frequency 540 × 10¹² Hz, K_cd, to be 683 when expressed in the unit lm W^-1, which is equal to cd sr W^-1, or cd sr kg^-1 m^-2 s³, where the kilogram, meter and second are defined in terms of → Planck's constant (h), → velocity of light (c), and Δν_Cs.

From L. candela, → candle.

1) An applicant or suitable person for a position.
2) An astronomical entity which is being considered for belonging to a special class of entities; e.g. → black hole candidate, → dark matter candidate, → supernova candidate.

From L. candidatus "clothed in white" (reference to the white togas worn by those seeking office), from candidus "shining white," from candere "to shine," cf. Skt. cand- "to shine," candra "bright; the Moon;" PIE base *kand- "to glow, to shine."

Nâmzad, literally "nominated," from nâm, → name, + zad, p.p. of zadan "to strike" (Mid.Pers. zatan, žatan, O.Pers./Av. jan-, gan- "to strike, hit, smite, kill," Skt. han- "to strike, beat," Gk. theinein "to strike," L. fendere "to strike, push," Gmc *gundjo "war, battle;" PIE *g^when- "to strike, kill").

1) A cylinder or block of wax, tallow, or other fatty substance with a central wick, which is burned to produce light.
2) A unit of luminous intensity, superseded by the → candela.

M.E., O.E. candel, from L. candela "a light, torch," from candere "to shine," candidus "shining white" (E. candidate); cf. Skt. cand- "to shine, to glow," candati "shines," candra- "shining, glowing, the Moon;" Gk. kandaros "coal;" PIE base *kand- "to glow, to shine."

Šam', loan from Ar.

Small-scale variations in the position of the Earth's geographical poles within an irregular circle of 3 to 15 metres in diameter. It seems to result from two nearly circular components, a seasonal variation in the mass distribution on the Earth (ice, snow, atmosphere) and movements of matter within the Earth.

Named after Seth Carlo Chandler (1846-1913), the American astronomer who discovered the phenomenon; → wobble.

An astronomy satellite launched by NASA in 1999 July, specially designed to detect X-ray emission from very hot regions of the Universe such as exploded stars, clusters of galaxies, and matter around black holes. Chandra carries a high resolution mirror (aperture 1.2 m, focal length 10 m), two imaging detectors (HRC and ACIS), and two sets of transmission grating spectrometer (LETG and HETG). Important Chandra features are: an order of magnitude improvement in spatial resolution, good sensitivity from 0.1 to 10 keV, and the capability for high spectral resolution observations over most of this range. Chandra was initially given an expected lifetime of 5 years, but on 4 September 2001 NASA extended its lifetime to 10 years "based on the observatory's outstanding results." Among the results obtained using Chandra one can mention the spectacular image of the → supernova remnant Cassiopeia A. See also → X-ray astronomy.

Initially called Advanced X-ray Astrophysics Facility (AXAF), the satellite was renamed the Chandra X-ray Observatory in honor of Subrahmanyan Chandrasekhar, the 1983 Nobel Prize in Physics, → Chandrasekhar limit. Moreover, Chandra, or candra- means "moon" or "shining" in Skt., from cand- "to give light, shine;" cf. Gk. kandaros "coal;" L. candela "a light, torch," from candere "to shine;" → X-ray; → Observatory.

A limiting mass of about 1.44 Solar masses that the theory predicts a non-rotating → white dwarf can attain without collapsing to become a → neutron star or a → black hole. Over this → critical mass, the degeneracy pressure will be unable to bear the load of the bulk mass.

Named after Subrahmayan Chandrasekhar (1910-1995), Indian-born American astrophysicist who, with William A. Fowler, won the 1983 Nobel Prize for Physics for his research on white dwarfs; → limit.

A band in the → absorption spectrum of → ozone (O₃) extending in the → visible from 400 nm to 700 nm. → Hartley band, → Huggins band.

J. Chappuis, Acad. Sci., Paris, C. R. 91, 985 (1880).

A → sequential star formation model involving → massive stars and → H II regions. The energetic ultraviolet photons from a massive star born in a → molecular cloud drive a spherical → ionization front radially outward from the star at a velocity much higher than the → sound speed in the cold neutral gas. The supersonic expansion of the H II region through the surrounding neutral gas creates a → shock front, sweeping up an increasingly massive and dense shell of cool neutral gas. This is the collect phase of the process in which the H II region simply acts like a snowplough. If the expansion of the H II region continues for long enough, the surface density of the shell increases to the point where the shell becomes self-gravitating. The shell is then expected to collapse and fragment. Individual fragments may then enter a non-linear collapse phase, possibly forming massive stars. This model was first proposed by Elmegreen & Lada (1977, ApJ 214, 725), who used a one-dimensional analysis. Whitworth et al. (1994, MNRAS, 268, 291) developed an analytical model for the collect and collapse process which predicts the fragmentation time, the size, number, and mass of the fragments (see also Elmegreen 1998, in ASP Conf. Ser. 148, Origins, eds. Woodward et al., p. 150 and references therein). → stimulated star formation, → triggered star formation.

→ collect; → collapse; → model.

In the energy spectrum of a solid, a range of energies in which electrons can move freely under the influence of an electrical field. Metals have many electrons in this range, insulators have none. In semiconductors the conduction band contains few electrons provided by impurity atoms or ejected from the valence bands by thermal energy or photon absorption.

→ conduction; → bande.

A principle, resulting from Einstein's theory of → special relativity whereby in any → closed system the sum of mass and energy remains → constant.

→ conservation; → mass; → energy.

An → absorption band of molecular origin characterizing the spectra of → late-type stars (G0 and later, e.g. → S star). → Cyanogen absorption is an important → luminosity criterion for → low-mass stars, and is more pronounced in → giant stars than in → dwarf stars of the same → spectral type.

→ cyanogen; → band.

A hypothetical physical entity capable of accounting for the corresponding observed phenomena involving → dark matter. → Non-baryonic dark matter candidates include → WIMPs, → neutralinos, → axions, → gravitinos. Among → baryonic dark matter candidates can be noted ordinary and heavy → neutrinos, clouds of → neutral hydrogen gas, and compact objects.

→ dark; → matter; → candidate.

The two branches of mathematics that make up the → calculus. → differential calculus; → integral calculus.

→ differential; → integral; → calculus.