An Etymological Dictionary of Astronomy and Astrophysics

One of → Saturn’s rings, lying beyond the → C ring and before the → A ring, extending from 92,000 to 117,300 km (width 25,300 km) from the center of Saturn. The B ring is bounded by the → Huygens Division.

See also: → ring.

A star of → spectral type B, whose spectrum is marked by absorption lines of hydrogen; also known as B-type star. B-type optical spectra are characterized by the presence of neutral He
lines (mainly He I 4471 Å) at about type B9. He I strengthens up to about B2, then
decreases. Ionized helium (mainly He II 4541 Å) first appears at about B0. Most metallic lines are absent or weak, except some absorption lines for the higher ionization states of → silicon, → oxygen, → carbon, and → magnesium. These are hot stars with → effective temperatures ranging from
about 10,000 K at B9 to nearly 30,000 K at B0. They are between 3 and 20 → solar masses. Some famous examples include: → Rigel, → Achernar, → Hadar.

See also: → star.

A → polarization component in the → cosmic microwave background radiation that depends only on → curl, is independent of → gradient, and has → handedness that distinguishes left from right. The B-mode is due to only → vector perturbations or → tensor perturbations. It has two types, the first type, which constitutes the majority of the B-mode polarization,
results from → lensing by galaxies that twist the → E-mode polarized light on its journey from the other side of the → observable Universe.
The second type can be produced only by → gravitational waves, not by density perturbations.
This type of B-mode is incredibly faint, producing temperature variations of about 0.4 microK and accounting for just one part in 10 million in the CMB temperature distribution. It is expected to be generated during cosmic → inflation shortly after the → Big Bang. The → BICEP2 team announced in March 2014 that they had detected the second type of B-modes, consistent with inflation and gravitational waves in the → early Universe. The detected degree scale B-mode polarization has a tensor-to-scalar ratio, r = 0.2 (+0.07, -0.05), which is a measure of the amplitude of the primordial gravitational waves.

See also: B, indicating magnetic-field like; → mode; → polarization.

A division of → C-type asteroids whose members have relatively low albedos (0.04 to 0.08) and the → ultraviolet absorption below 0.5 μm is small or absent. Examples include → 2 Pallas, 379 Huenna (diameter 62 km), and → 101955 Bennu.

See also: → type; → asteroid.

Same as → B star.

See also: B, letter of alphabet used in the → Harvard classification; → type; → star.

A → Be star with → forbidden lines in emission in its spectrum. B[e] stars show large → infrared excess due to → circumstellar dust emission. See also → supergiant B[e] star, → pre-main sequence B[e] star, → compact planetary nebula B[e] star, → symbiotic B[e] star, and → unclassified B[e] star.

See also: B, referring to the spectral type; e for emission lines, brackets for distinction from Be; → star.

A method used to determine the size of certain types of pulsating stars, including Cepheids, from their magnitude variations (photometry) and the corresponding radial velocities (spectroscopy).

Etymology (EN): Baade, from Walter (Wilhelm Heinrich) Baade (1893-1960), German/American astronomer, who made important contributions to the research on variable stars; Wesselink, from Adriaan J. Wesselink (1909-1995), Dutch/American astronomer,
the originator of the method. → method.

An area of the sky with relatively low amounts of  → interstellar dust along the → line of sight, occurring toward the  → constellation of  → Sagittarius, near the → globular cluster NGC 6522. The window, through which stars in the → Galactic bulge are visible, lies 3.9 degrees south of the → Galactic center, corresponding to a line of sight that passes within 1,800 → light-years of the → Milky Way’s core. It is named after Walter Baade, who used it to observe → RR Lyrae stars  in the → Galactic bulge region. 

See also: → Baade-Wesselink method; → window.

A crystal device made of two → quartz  → prisms
with equal acute angles assembled in a rhombus that is used in analyzing → polarized light. The wedge-shaped prisms are placed against each other and can be displaced along their plane of contact by a → micrometer. Thus they form a parallel plate of variable thickness allowing the optical retardation to be adjusted.

See also: Jacques Babinet (1794-1872), French physicist; → compensator.

One of several points on the sky where the degree of → linear polarization is zero in skylight. See also → neutral point; → Arago point, → Brewster point.

See also: → Babinet compensator; → point.

The → diffraction pattern for an → aperture is the same as the diffraction pattern for its → complementary aperture.

See also: → Babinet compensator; → principle.

1. General: Related to or located at the back “the side or surface opposite the front or face.”
   

2. The side of the → planispheric astrolabe
   opposite the hollow of the → mater, → tympanum, and → rete. At its center is hinged the → alidade, whose rim slides on a set of graduated circular scales engraved on the mater. The number of scales varies from one instrument to another. On this face the user can perform observations and read the data needed to set the front side of the instrument in the proper position (online museo galileo, VirtualMuseum).

Etymology (EN): Back, from M.E., from O.E. bæc; akin to O.H.G. bah “back.”

Etymology (PE): Pas-, from pas “behind” (e.g.: pas-e pardé “behind the curtain”); Mid.Pers. pas “behind, before, after;” O.Pers. pasā “after;” Av. pasca “behind (of space); then, afterwards (of time);” cf.
Skt. paścā “behind, after, later;” L. post “behind, in the rear; after, afterwards;” O.C.S. po “behind, after;” Lith. pas “at, by;”
PIE *pos-, *posko-.
Pošt-, from pošt, variant of pas.

In a radiotelescope, the unit forming the end part of the reception chain. It generally consists of a spectrometer and performs frequency analysis of the signals. → front-end.

Etymology (EN): Back-end, from → back + end, from
M.E., O.E. ende (cf. Du. einde, O.H.G. enti “top, forehead, end,” Ger. ende, Goth. andeis “end”), originally “the opposite side,” from PIE *antjo “end, boundary,” from base *anta-/*anti- “opposite, in front of, before.”

Etymology (PE): Pas-tah, from pas, → back, + tah “end,” Mid.Pers. tah “bottom.” The origin of this term is not clear. It may be related to Gk. tenagos “bottom, swamp,” Latvian tigas«i>*tingas < *tenegos “depth,” PIE *tenegos “water bottom.”

General: That part of a view or scene that serves as a setting for the main objects, persons, etc.
Astro.: The part of an observed field which is not physically related to the objects of interest and lies behind them.

Etymology (EN): Background, from → back + ground, from M.E., from O.E. grund; akin to O.H.G. grunt “ground.”

Etymology (PE): Paszaminé, from pas-, → back, + zaminé “ground,” from zamin “ground,” → earth.

See also: → foreground.

An unwanted signal in a system which is producing or recording a signal.
For instance, a randomly fluctuating signal superimposed on the signal from a cosmic radio source.

See also: → background; → noise.

The isotropic residual microwave radiation in space left from the primordial → Big Bang. Same as
→ cosmic microwave background (CMB) radiation.

See also: → background; → radiation.

1. (v.tr.) To deflect photons or particles in a direction opposite to their initial path.
   
2. Same as → backscattering.
   
3. The radiation or particles so deflected.

See also: From → back + → scattering.

The light that has undergone → backscattering.

See also: → backscatter; → backscattering.

Scattering of radiation or particles through angles greater than 90° with respect to the original direction of motion.

See also: → scattering.

A copy of computer files that is stored separately from the original in order to protect against loss of data.

Etymology (EN): Backup “substitute, support,” from → back + up.

Etymology (PE): Poštvân “prop, support, help” from pošt,
→ back, + -vân suffix denting protection, variant of -bân.

An auxiliary observing program to be carried out at telescope in case the atmospheric conditions make the main program unfeasible.

Etymology (EN): Backup, from → back + up;
→ program.

Etymology (PE): Barnâmé, → program; yadaki “reserve, substitute,” from yadak “a led horse.”

1. Toward the back or rear.
   

2. Directed toward the back or past.

See also: → back; → -ward.

A sort of → greenhouse effect in → stellar atmospheres where the deeper layers heat up due to overlying → opacity. The presence of numerous → bound-bound opacities of → metals amplifies the → scattering of → photons, in particular their → backscattering, forcing the → temperature to increase in order to conserve the radiation flux and the transport of energy from the interior to the outer parts of the atmosphere.

See also: → back; → warming; → effect.

Column of a → CCD detector that does not correctly read out charge.

Etymology (EN): Bad, from M.E. badde, but the origin of the word is not clear; → column.

Etymology (PE): Sotun, → column; bad, from Mid.Pers. wad, maybe from Old Iranian *vata- “small;” cf. Scythian bata- “small, bad,” Sogdian wtγy “suffer, sorrow.”

A phenomenon that occurs during a total eclipse of the Sun. Just prior to and after totality, sunlight shines through the lunar valleys on the Moon’s limb, causing the dark face of the Moon to appear to be surrounded by a shining “necklace of pearls”.

Etymology (EN): Baily, from Francis Baily (1774-1844), English amateur astronomer, who discovered the phenomenon during the solar eclipse of 1836. Beads “a necklace of beads or pearls; a rosary,” from bead “a small, often round piece of material, such as glass, plastic, or wood, that is pierced for stringing or threading,” from M.E. bede “rosary bead,” from O.E. bed, bedu, gebed “prayer;” PIE *gwhedh- “to ask, pray”.

Etymology (PE): Mohrehâ “beads,” from mohré “a kind of small shell resembling pearls; glass or coral beads,” cf. Khotanese mrâhe, may be related to morvârid, → pearl,

• -hâ suffix of plurals.

Something edible placed on a hook or in a trap
to attract fish or other animals as prey.

Etymology (EN): M.E., from O.Norse beita “food,” O.E. bat “food,” literally “to cause to bite.”

Etymology (PE): Cašté “bait,” related to câšni “taste,” cašidan, caš- “to taste,” câšt “breakfast;” Mid.Pers. câšt “meal,” câšnig “taste;” cf. Skt. cas- “to eat;” Proto-Ir. caš- “to eat, to drink; to drip.”

1a) A weighing device.

1b) The constellation → Libra.

2. A state of equilibrium.

Etymology (EN): M.E balaunce, from O.Fr. balance “balance, scales for weighing,” from M.L. bilancia, from L.L. bilanx, from L. (libra) bilanx “(scale) having two pans,” possibly from L. bis “twice” + lanx “dish, plate, scale of a balance.”

Etymology (PE): Tarâzu, → Libra.

Having little or no hair on the scalp. → bald patch.

Etymology (EN): Bald from M.E. ball(e)d; Celtic bal “white patch, blaze;” Gk. phalios “having a white spot;” L. fulica “coot;” → patch.

Etymology (PE): Kal “bald,” Mid.Pers. gar “bald;”
Av. kaurva- “bald;” cf. Skt. kulva- “bald, thin-haired;” L. calvus “naked, bald.”
Kacal may be related to kal.
Tâs, variant taz “bald;” Tabari tisâ, Saraxsi, Lâsgardi, Sangesari tusâ “empty;” related to tohi “empty;” Mid.Pers. tuhig; Av. taoš- “to become empty,” pres. tusa-, caus. taošaya-, tusən “they lose their posture;” cf. Skt. tuccha-, tucchya- “empty;” L. tesqua, tesca “deserted place;” Russ. tošcij “hollow;” PIE base *teus- “to empty.”

The location on the surface of the → Sun where → coronal  → magnetic field lines become tangent to the → photosphere. Bald patches play an important role in solar → magnetohydrodynamics.

See also: → bald, such called because of visual reference to a haircut (Titov et al. 1993, A&A 276, 564); → patch.

A set of nebular → emission line diagrams used to distinguish the ionization mechanism of → nebular gas. The most famous version consists of [N II]λ6584/Hα versus [OIII] λ5007/Hβ. The next two more commonly used BPT diagnostics are [S II] λλ6717,6731/Hα versus [O III] λ5007/Hβ and [O I] λ6300/Hα versus [O III]λ5007/Hβ. These diagrams use strong, optical lines of close proximity in the ratios to limit → reddening and → spectrophotometric effects. They are able to clearly distinguish different classes of → ionization, for example → LINERs from normal → H II regions and → active galactic nuclei.

See also: Baldwin, J. A., Phillips, M. M., Terlevich, R., 1981 PASP 93, 5; → diagram.

A spherical or approximately spherical body, either solid or hollow.

Etymology (EN): From M.E. bal, balle, from O.Fr.; cf. O.H.G. ballo, Ger. Ball; PIE root *bhel- “to blow, swell.”

Etymology (PE): Tup “ball,” initially “clmup, aggregation, parcel, group” (tup tup “many”); Tabari tupa “compressed, assembled,” tuppi “round;” Kurd. top “ball,” topâl “round;”
guy, → globe.

A rare form of lightning occurring as a bright red globe observed floating or moving through the atmosphere close to the ground. It usually is seen shortly before or after, or during, a → thunderstorm. Its duration varies from a few seconds to a few minutes. See also → Saint Elmo’s fire.

See also: → ball; → lightning.

Of or relating to → ballistics.

A missile that after being launched and guided in the early part of its flight, travels unpowered in a ballistic trajectory.

See also: → ballistic; → missile.

Transfer of microbes and biochemical compounds from a planet to another due to meteoric impacts. Debris being knocked off a planet like Mars can reach escape velocity and enter the atmosphere of another planet with passenger micro-organisms intact.

See also: → ballistic; → panspermia.

A curved path followed by an unpowered object that is being acted upon only by gravitational forces and the friction of the medium through which it moves.

See also: → ballistic; → trajectory.

Audible disturbance or wave caused by the compression of air ahead of a projectile in flight.

See also: → ballistic; → wave.

The science of the motion and behavior of → projectiles. The study of the functioning of firearms.

Etymology (EN): From L. ballista “ancient military machine for hurling stones,” from Gk. ballistes, from ballein “to throw,” from PIE *g^welH₁- “to throw;” cf. Pers. garzin “arrow;” Av. niγr- “to throw down;” Khotanese (+ *abi-, *ui-) bīr- “to throw, sow;” Proto-Iranian *garH- “to throw.”

Etymology (PE): Partâbik, from partâb “a throw, an arrow that flies far,” partâbidan “to throw,” + -ik, → -ics;
partâbšenâsi, from partâb + -šenâsi,
→ -logy.

A branch of modern astronomy in which balloons are used to carry telescopes and instruments to high altitudes (up to 50 km) for observation.

Etymology (EN): Balloon, from Fr. ballon, from It. dialectal ballone, augmentative of balla, ball, from P.Gmc. *ball-, from PIE *bhel- “to blow, swell”. → astronomy.

Etymology (PE): Axtaršenâsi, → astronomy; bâlon, from Fr. ballon.

A remotely guided or automatic telescope carried to high altitudes by a balloon.

Etymology (EN): → balloon astronomy; borne “a past participle of bear," from O.E. beran “bear, bring, wear,” from P.Gmc. *beranan (O.H.G. beran, Goth. bairan “to carry”), from PIE root *bher-; “to carry;” compare with Av./O.Pers. bar- “to bear, carry,” bareθre “to bear (infinitive),” bareθri “a female that bears (children), a mother,” Mod.Pers. bordan “to carry,” Skt. bharati “he carries,” Gk. pherein, L. fero “to carry.” → telescope.

Etymology (PE): → balloon astronomy. Bord in bâlon-bord “borne, carried,” from Mod.Pers. bordan “to bear, carry,” as explained above. Durbin, → telescope.

From Johann Jakob Balmer (1825-1898),
Swiss mathematician and physicist, who explained the visible spectral lines of the hydrogen spectrum in 1885.

A continuous range of wavelengths in the Balmer spectrum of hydrogen corresponding to transitions between the energy levels n = 2 and n = ∞.

See also: → Balmer; → continuum.

The intensity ratio among the couple of relatively adjacent → Balmer lines, for example Hα/Hβ and Hβ/Hγ, which have well-known theoretical values. They are used to determine the → interstellar extinction.

See also: → Balmer; → decrement.

An abrupt decrease in the intensity of the continuum at the limit of the → Balmer series of hydrogen (at about 3650 Å), caused by the energy absorbed when electrons originally in the second → energy level are ionized. Same as → Balmer jump.

See also: → Balmer; → discontinuity.

A special solution of the mathematical formula which represents the wavelengths of the various spectral series of hydrogen in which the lower energy level is n = 2.

See also: → Balmer; → formula.

Same as → Balmer discontinuity.

See also: → Balmer; → jump.

The wavelength in the blue end of the → Balmer series, at 3646 Å, near which the separation between successive lines decreases and approaches a → continuum.

See also: → Balmer; → limit.

The → spectral lines making up the → Balmer series.

See also: → Balmer; → line.

A series of hydrogen → spectral lines (Hα, Hβ, Hγ, and others) that lies in the visible portion of the spectrum and results when electrons from upper → energy levels (n > 2) undergo → transition to n = 2.
Historically, Balmer emission lines (mainly Hα) from ionized nebulae were first observed by O. Struve and C. T. Elvey (1938, ApJ 88, 364). This was an important indication of the existence of hydrogen, in the ionized state, in the → interstellar medium.

See also: → Balmer; → series.

General:1) A strip serving to encircle and bind one object or to hold a number of objects together. 2) A strip or stripe that contrasts with something else in color, texture, or material.
Physics: 1) A specific range of wavelengths or frequencies of electromagnetic radiation.
2) Closely packed spectral lines that appear to form a continuous group. → absorption band;
→ emission band.

Etymology (EN): From M.E. bende, O.E. bend, from O.Fr. bande, bende, P.Gmc. *bindan, from PIE *bendh- “to bind” (cf. Goth bandi “that which binds;” Av./O.Pers. band- “to bind, fetter,” banda- “band, tie,” Skt. bandh- “to bind, tie, fasten,” bandhah “a tying, bandage.”

Etymology (PE): Bând, adoption from E. band, which is cognate and synonymous with Pers. band, present tense stem of bastan “to bind, shut,” Mid.Pers. bastan, band, Av./O.Pers. band-, as explained above. See also → strip.

A location on the spectrogram of a molecule at which the lines of a band stack.

Etymology (EN): Band head, from → band + head, from O.E. heafod “top of the body,” also “upper end of a slope,” also “chief person, leader,” from P.Gmc. *khaubuthan, from PIE *kauput- “head” (cf. Skt. kaput-, L. caput “head,” Lori kapu “head,” kapulek “skull, middle of the head”).

Etymology (PE): Bândsar, from → bând + sar “head,” soru, sorun “horn,” karnâ “a trumpet-like wind instrument” (originally made from animal horns), variant sornâ “a wind instrument;” Mid.Pers. sar “head,” sru “horn;” Av. sarah- “head,” srū- “horn, nail;” cf. Skt. śiras- “head, chief;” Gk. kara “head,” karena “head, top,” keras “horn;”
L. cornu “horn,” cerebrum “brain;” P.Gmc. *khurnaz (E. horn; Ger. Horn, Du. horen), from PIE *ker- “head, horn.”

A spectrum which consists of a number of bands each having one sharp edge. Each band is composed of a large number of closely spaced emission or absorption lines. Band spectra are typical of molecules. Bands produced by titanium oxide, zirconium oxide, and carbon compounds are characteristic of low temperature stars.

Etymology (EN): Band spectrum, from → band + → spectrum.

Etymology (PE): Binâb-e bândi, from binâb, → spectrum + bândi, relating to bând, → band.

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

Etymology (EN): 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.”

Etymology (PE): 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.

See also: → 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.

Etymology (EN): Bandwidth, from → band + → width.

Etymology (PE): Bândpahnâ, from bând, → band, + pahnâ, → width, from pahn “wide,”
→ broad.

1. A unit of → pressure, not belonging to the → International System (SI), equivalent to 10⁶→ dynes per cm² and 0.987 → atmospheres.
   

2. → stellar bar, → galactic bar, → barred spiral galaxy.

Etymology (EN): 1) From Gk. baros “weight,” cf. Skt guru, L. gravis; PIE *gwere- “heavy;” cf. Pers. bâr “weight,” gerân “heavy,” L. brutus “heavy, dull, stupid, brutish,” Skt. bhara- “burden, load,” bharati “he carries;” PIE *bher- “carry, give birth.”

2. From O.Fr. barre, from V.L. *barra “bar, barrier,” or perhaps from Gaulish *barro “summit.”

Etymology (PE): 1) Loan from Fr., as above.

2. Mile, from Ar. mil “any long and narrow piece of metal,” + -é noun suffix.

A whitish, malleable, metallic → chemical element; symbol Ba. → Atomic number 56; → atomic weight 137.33; → melting point 725°C; → boiling point 1,640°C; → specific gravity 3.5 at 20°C. Barium was discovered by the Swedish pharmacist and chemist Carl Wilhelm Scheele in 1774. It was first isolated by the British chemist Humphry Davy in 1808.

See also: From Mod.L., from Gk. barys “heavy,” from the mineral barytes “heavy spar” (BaSO₄),
in which the element was discovered; cognate with Pers. bâr “weight,” → bar.

A type of star, usually G or K → giants, whose spectra show unusually strong absorption lines of → barium, → strontium, and other → s-process elements.

See also: → barium; → star.

The external covering on the trunks, boughs, and branches of trees.

Etymology (EN): M.E., from O.Norse börkr “bark.”

Etymology (PE): Kâlun, from Mâzandarâni kâlun “bark,” variants (Dâmqân) kul “bark,” (Tâti) lo “bark,” (Yazd, Mâzandarân) kol “bark,” (Nâin) kuluz “egg shell,” (Aftar) cokola “egg shell, pistaschio shell,” pukel, → shell, keler, → scalp, probably related to (Khotan Sacca) karastra- “fur garment,” (Waxi) kurust “bark of tree,” from PIE root *(s)ker- “to cut off,” from which are derived L. cortex “bark,” corium “thick skin,” scortum “hide,” and Persian carm “leather.”

A → negative lens placed in a telescope between the → objective and the → ocular. Its diverging action reduces the convergence of the light cone, forming a larger image at a slightly greater distance.

See also: Peter Barlow (1776-1862), English physicist; → lens.

In nuclear physics, unit of area for measuring the cross-sections of nuclei. 1 barn equals 10^-24 sq. cm.

Etymology (EN): Barn, from O.E. bereærn “barn,” lit. “barley house,” from bere “barley” + aern “house.” The use of barn in nuclear physics comes from the fact that the term denotes also “an unexpectedly large quantity of something.” It seems that when physicists were first studying nuclear interactions, they found out that the interaction probabilities, or cross-sections, were far more larger than expected; the nuclei were `as big as a barn'.

From Edward Emerson Barnard (1857-1923) American astronomer who made several obserational discoveries.

A very faint nebular shell of huge size enveloping the central portion of Orion.

Etymology (EN): Named after → Barnard, who discovered the loop in 1895; → loop.

A → red dwarf in the constellation → Ophiuchus discovered in 1916 by E.E. Barnard, that until 1968 had the largest → proper motion of any star. It moves on the sky 10.3 arcseconds per year, which means that it travels the equivalent of a lunar diameter every 180 years. It is the second nearest star system to the Sun.

Etymology (EN): In honor of → Barnard; → star.

A prefix meaning → pressure used in the formation of compound words, such as → baroclinic, → barometer, → barotropic.

Etymology (EN): Baro- combining form of Gk. baros “weight;” cognate with Pers. bâr “weight,” gerân “heavy;” cf. Skt. guru, L. gravis; PIE *gwere- “heavy;”
L. brutus “heavy, dull, stupid, brutish;” Skt. bhara- “burden, load,” bharati “he carries;” PIE *bher- “carry, give birth.”

Etymology (PE): Fešâr-, → pressure.

Of, pertaining to, or characterized by → baroclinicity. Sometimes called → barocline.

See also: → baro-; → -cline; → -ic.

1. A type of instability occurring within a rapidly → rotating star where non-axisymmetric motions can separate surfaces of constant pressure from → equipotential surfaces.
   

2. A hydrodynamic instability associated with a baroclinic layer of the atmosphere. It arises from temperature variation along the pressure surfaces. Baroclinic instability is associated with the vertical → shear of the mean flow, which is related to the horizontal temperature gradient by the thermal wind equation. Instabilities in a baroclinic region grow by converting potential energy associated with the mean horizontal temperature gradient into kinetic energy through ascending warm air and descending cold air (Rasmussen & Turner (eds.), Polar Lows, Cambridge Univ. Press, 2003).

See also: → baroclinic; → instability.

The state of stratification in a fluid in which surfaces of constant pressure do not coincide with those of constant density, but intersect. Where baroclinicity is zero, the fluid is → barotropic. Same as baroclinity.

See also: → baroclinic; → -ity.

Instrument for measuring the atmospheric pressure. It is used in determining height above sea level and predicting changes in weather.

See also: → baro- + → -meter.

A law which describes the vertical pressure distribution in the lower parts of Earth’s atmosphere. The atmospheric pressure decreases exponentially from any reference surface as the altitude increases.

See also: → barometer; → law.

In a fluid, conditions where surfaces of constant pressure
are parallel to surfaces of constant temperature. This state is equivalent to zero
→ baroclinicity.

See also: → baro-; → -tropic.

A gas whose density is a function solely of pressure.

See also: → barotropic; → gas.

A hydrodynamical instability that arises when the horizontal → shear gradient becomes very large. Barotropic instabilities grow by extracting kinetic energy from the mean flow field.

See also: → barotropic; → instability.

A state of a fluid in which the surfaces of constant density coincide with surfaces of constant pressure (isobaric).

See also: → barotropic gas.

Having a bar like structure.

See also: → bar.

A transitional class of object between the classic spiral galaxies and true irregular systems. The → Large Magellanic Cloud, the nearest and best studied example of the class, is, contrary to popular opinion, not an irregular galaxy. The LMC and other members of the SBm class have definite structural signatures. They are generally dominated by a pronounced asymmetric bar – one that is offset from the optical center of the galaxy – with a nascent spiral arm emanating from one end. As is the case with irregular galaxies, the optical centers of SBm type systems are not particularly special places. Disk systems later than Sc characteristically lack a central stellar concentration in addition to having weak spiral structure; this is true of SBm-type galaxies. SBm galaxies are typically very active in their star formation activity, often containing a large star-forming complex situated at one end of the bar. Beyond these general trends there is a tremendous amount of dispersion in physical properties within the SBm class, particularly in the strength of the spiral structure. At one extreme are the “one-armed” spirals such as NGC 3664 and NGC 4027 which are dominated by single, looping spiral arm. On the other hand NGC 4861 shows little evidence of spiral structure and it is dominated by a large star-forming complex at one end of its bar. The class smoothly leads to the Barred Magellanic irregulars (IBm) which show no indication of spiral structure

(Wilcots et al. 1996, AJ 111, 1575).

See also: → Magellanic; → spiral; → galaxy.

A → spiral galaxy that exhibits a bar-shaped structure in its nucleus. → galactic bar.

See also: → barred; → spiral; → galaxy.

A defect in an optical system in which magnification decreases with distance from the optical axis,
whereby the image of a square appears barrel-shaped. Opposite of → pincushion distortion.

Etymology (EN): Barrel, M.E. barel, from O.Fr. baril; → distortion.

Etymology (PE): Cowlegi, → distortion; celiki, relating to celik “barrel”.

General: Anything that prevents passage or blocks.
Physics: 1) A region where the potential energy is greater than the total energy of a particle, whereby the particle cannot go through.
2) The depletion layer of a P-N junction in a diode.

Etymology (EN): O.F. barrière “obstacle,” from V.L. *barraria, from *barra “bar, barrier.”

Etymology (PE): Varqé, from varq “a mound, a dam” + -é nuance suffix. Varq is
probably related to Av. vâra- “barrage,” vara- “enclosure,” var- “castle,” Mid.Pers. var “enclosure,” from Av. root var- “to cover, to conceal;” variants: barq (Torbat Heydariyei), valgâ (štiyâni), var (Qomi); cf. Skt. vatra- “a dike, a dam,"varana- “rampart, wall,” from vr- “to obstruct, close, cover, hide; to choose.”

Same as → Meteor Crater.

See also: Names after Daniel Barringer (1860-1929), American geologist, who bought the Crater in 1903, convinced that it was made by a huge → meteorite; → crater.

The center of mass of a system of bodies.

Etymology (EN): From Gk. barus “heavy,” → bar, +
→ center.

Etymology (PE): Gerânigâh, from gerâni “weight;” cognate with Gk. barus, → bar, + gâh “place.”

A → coordinate time having its spatial origin at the solar system barycenter. It is intended to be used as the independent variable of time for all calculations pertaining to orbits of planets, asteroids, comets, and interplanetary spacecraft in the solar system. → Barycentric Dynamical Time (IDB).

Etymology (EN): → barycenter; → coordinate; → time.

A time scale previously used in calculations of the orbits of solar system objects (planets, asteroids, comets, and interplanetary spacecrafts). It was based on the Terrestrial Dynamical Time, but took the relativistic effect of time dilation into account to move the origin to the solar system barycenter. It is now superseded by → Barycentric Coordinate Time (TCB).

See also: → barycenter; → dynamical; → time.

The → Julian Date referenced to the → barycenter of the → solar system. The BJD is more precise than the → Heliocentric Julian Day because the Sun is not stationary. It moves due to the → gravitational attraction of Jupiter and the other planets.

See also: → barycentric; → Julian Date.

The hypothetical mechanism of creating the → baryon asymmetry in the → Universe. Universe. Explaining the observed matter asymmetry is an important open question in physical cosmology. → Sakharov conditions.

See also: From baryo-, from → baryon + → -genesis.

Any of the class of the heaviest → subatomic particles that includes → protons, → neutrons, as well as a number of short-lived particles whose decay products include protons. Baryons obey the → Fermi-Dirac statistics. They form a subclass of the → hadrons and
are further subdivided into → nucleons and → hyperons.

See also: Gk. barys “heavy” + → -on, from “fermion.”

In cosmology, one of a series of peaks and troughs that are present in the power spectrum of matter fluctuations after the → recombination era, and on large scales. At the time of the Big Bang, and for about 380,000 years afterwards, Universe was ionized and photons and baryons were tightly coupled. Acoustic oscillations arose from perturbations in the primordial plasma due to the competition between gravitational attraction and gas+photons pressure. After the epoch of recombination, these oscillations froze and imprinted their signatures in both the → CMB and matter distribution. In the case of the photons, the acoustic mode history is manifested as the high-contrast Doppler peaks in the temperature anisotropies. As for baryons, they were in a similar state, and when mixed with the non-oscillating → cold dark matter perturbations, they left a small residual imprint in the clustering of matter on very large scales, ~100 h^-1Mpc (h being the → Hubble constant in units of 100 km s^-1 Mpc^-1). The phenomenon of BAOs, recently discovered using the Sloan Digital Sky Survey data, is a confirmation of the current model of cosmology. Like → Type Ia supernovae, BAOs provide a → standard candle for determining cosmic distances. The measurement of BAOs is therefore a powerful new technique for probing how → dark energy has affected the expansion of the Universe (see, e.g., Eisenstein 2005, New Astronomy Reviews 49, 360; Percival et al. 2010, MNRAS 401, 2148).

See also: → baryon; → acoustic; → oscillation.

The observation that in the present → Universe there is → matter but not much → antimatter. Observations do not show the presence of galaxies made of antimatter, nor gamma rays are observed that would be produced if large entities of antimatter would undergo → annihilation with matter. However, the → early Universe could have been baryon symmetric, and for some reason the matter excess has been generated, through some process called → baryogenesis. → Sakharov conditions.

See also: → baryon; → asymmetry.

1. The difference between the total number of → baryons and
   the total number of → antibaryons in a system of → subatomic particles. It is a measure of → baryon asymmetry and is defined by the quantity η = (n_b - n_b^-)/n_γ, called the → baryon-photon ratio, where n_b is the → comoving number density of baryons, n_b^- is the number of antibaryons, and n_γ is that of photons. The value of η for the → cosmic microwave background radiation (CMBR) has been very well determined by the → WMAP satellite to be η = (6.14 ± 0.25) x 10^-10. The baryon number is assumed to be constant. The photons created in stars amount to only a small fraction, less than 1%, of those in the CMBR.
   

2. A property of an → elementary particle represented by a
   → quantum number. It is
   equal to +1 for a baryon and -1 for an antibaryon.
   → Bosons, → leptons, and → mesons have a baryon number B = 0. → Quarks and → antiquarks have baryon numbers of B = +1/3 and -1/3, respectively. The baryon number is → conserved in all observed types of particle-particle interaction.

See also: → baryon; → number.

The → baryon number compared with the number of photons in the → Universe. The baryon-photon ratio can be estimated in a simple way. The → energy density associated with → blackbody radiation of → temperature  T is aT⁴, and the mean energy per photon is ~kT. Therefore, the number density of blackbody photons for T = 2.7 K is: n_γ = aT⁴/kT = 3.7 x 10² photons cm^-3, where a = 7.6 x 10^-15 erg cm^-3 K^-4 (→ radiation density constant)
and k = 1.38 x 10^-16 erg K^-1 (→ Boltzmann’s constant). The number density of baryons can be expressed by ρ_m/m_p, where ρ_m is the mass density of the Universe and m_p is the mass of the → proton (1.66 x 10^-24 g). → CMB measurements show that the baryonic mean density is ρ_m = 4.2 x 10^-31 g cm^-3 (roughly 5% of the → critical density). This leads to the value of ~ 2 x 10^-7 for the number density of baryons. Thus, the baryon/photon ratio is approximately equal to η = n_b/n_γ = 2 x 10^-7/3.7 x 10² ~ 5 x 10^-10. In other words, for each baryon in the Universe there is 10¹⁰ photons. This estimate is in agreement with the precise value of the baryon-photon ratio 6.14 x 10^-10 derived with the → WMAP. Since the photon number and the baryon number are conserved, the baryon-photon ratio stays constant as the Universe expands.

See also: → baryon; → photon; → ratio.

→ Dark matter made up of → baryons that are not luminous enough to produce any detectable radiation. It is generally believed that most dark matter is → non-baryonic.
The baryonic dark matter could reside in a number of forms, including cold gas and compact objects.

See also: → baryonic; → dark; → matter.

Ordinary matter composed of → baryons, i.e. → protons and → neutrons, as distinct from → non-baryonic, exotic forms.

See also: Adj. of → baryon; → matter.

A dark fine-grained → igneous rock typically composed of → plagioclase with → pyroxene and → olivine and often displaying a columnar structure.

See also: From L.L. basaltes, misspelling of L. basanites “very hard stone,” from Gk. basanites, from basanos “touchstone,” from Egyptian baban “a stone used by the Egyptians as a touchstone of gold.”

1. The bottom support of anything; a fundamental principle or groundwork.
   

2. Geometry: The line or surface on which a figure is assumed to stand.
   

3. Arithmetic: The number which, raised to various powers, forms the main counting units of a system. Thus 10 is the base of the decimal system.
   

4. Logarithm: The number a in the equation N = a^x. The base of common logarithm is 10.
   

5. A centre of operations or supply, such as a → database.
   

6. Chemistry: A substance that reacts with an acid to yield a salt and water.

Etymology (EN): M.E., from O.Fr. bas, from L. basis “foundation,” from Gk. basis “step, pedestal,” from bainein “to step.”

Etymology (PE): Pâyé “base,” from pâ, pây “foot,” from Mid.Pers. pâd, pây;
Av. pad-, cf. Skt. pat: Gk. pos, genitive podos; L. pes; PIE *pod-/*ped-.
Pâygâh, from pâyé + gâh “place” (O.Pers. gāθu-; Av. gātav-, gātu- “place, throne, spot;” cf. Skt. gâtu- “going, motion; free space for moving; place of abode;” PIE *gwem- “to go, come.”
Bâz, loan from Fr., as above.

1. In radio interferometry, the separation between the electrical, or phase centers of two interferometer elements.
   

2. In spectroscopy, the contribution to a spectrum from phenomena
   that are not of astronomical interest, such as frequency dependent properties of the receivers, or by astronomical sources other than
   those under study.

See also: → base; → line.

Chemistry: Of or denoting or of the nature of or containing a → base. Same as → alkaline.

See also: → base; → -ic.

A large impact crater on a planet or moon, typically several hundred kilometers across, flooded with basaltic lava and surrounded by concentric rings of faulted cliffs.

Etymology (EN): From O.Fr. bacin, from V.L. *baccinum, from L. bacca “water vessel,” perhaps originally Gaulish.

Etymology (PE): Howzé, from howz “pond, a large reservoir of water” (from Ar. hauz) + -é noun suffix.

A combination of → cells connected together so as to produce useful electrical energy.

Etymology (EN): M.Fr. batterie “a grouping of artillery pieces for tactical purposes,” from O.Fr. baterie “beatng, thrashing, assault,” from battre “to beat,” from L. battuere “to beat.”

Etymology (PE): Bâtri, loanword from Fr., as above.

A body of water forming an indentation of the shoreline, larger than a cove but smaller than a → gulf (Dictionary.com).

Etymology (EN): M.E. baye, from M.Fr. baie, from L.L. bâia, perhaps ultimately from Iberian bahia.

Etymology (PE): Bâhé, loan from Sp. bahia.

A stellar designation system in which a specific star is identified by a Greek letter, followed by the genitive form of its hosting
→ constellation’s Latin name. For example, Alpha Eridani, Delta Cephei, Lambda Bootis. The Greek alphabet has only 24 letters. In case a single constellation contained a larger number of stars, Bayer amended with Latin letters: upper case A, followed by lower case b through z (omitting j and v), for a total of another 24 letters. Bayer did not go beyond z, but later astronomers added more designations using both upper and lower case Latin letters, the upper case letters following the lower case ones in general. Examples include, for Vela: a Vel (Velorum), z Vel, A Vel, Q Vel; for Scorpius: d Sco (Scorpii), A Sco; for Leo: b Leo (Leonis), o Leo, A Leo, → c Orionis.

Compare with the → Flamsteed designation.

See also: First introduced by Johann Bayer (1572-1625) in his atlas
Uranometria, published in 1603 at Augsburg, Germany; → designation.

A theorem in probability theory concerned with determining the → conditional probability of an event when another event has occurred.
Bayes’ theorem allows revision of the original probability with new information. Its simplest form is: P(A|B) = P(B|A) P(A)/P(B), where P(A): independent probability of A, also called prior probability; P(B): independent probability of B; P(B|A): conditional probability of B given A has occurred; P(A|B): conditional probability of A given B has occurred, also called posterior probability. Same as Bayes’ rule.

See also: Named after its proponent, the British mathematician Reverend Thomas Bayes (1702-1761). However, Bayes did not publish the theorem during his lifetime; instead, it was presented two years after his death to the Royal Society of London.

Being, relating to, or denoting statistical methods based on → Bayes’ theorem.

See also: Referring to → Bayes’ theorem.

An approach to → statistical analysis in which → unknowns to be estimated have a prior → probability distribution which combined with the information from data produces a posterior probability distribution for the target quantities.

See also: → Bayesian; → inference.

A mathematical framework described by the prior distribution of a random parameter and by the likelihood of the observations. In this framework, all information on the random parameter based on the observations is included in the posterior distribution
which can be obtained using → Bayes’ theorem (see, e.g., Andrieu et al., 2001, “An Introduction to Monte Carlo Methods for Bayesian Data Analysis,” in Nonlinear Dynamics and Statistics, ed. A. I. Mees, Boston: Birkhäuser).

See also: Bayesian; → model.

An approach to model selection in which one bases inference on an average of all possible models instead of a single best model. The BMA is largely used in various branches of knowledge to properly account for model uncertainty in performing predictions.

See also: → Bayesian; → model; → average.

The episodic occurrence of abrupt → mass loss in → Be stars resulting in → Balmer lines in emission and → infrared excess. The Be phenomenon results from a combination of a long-term secular effect and short-term instabilities, such as pulsation. The secular evolution brings the star close enough to the critical → break-up velocity, so that the additional velocity field due to the instability may allow some mass ejection (Maeder 2011).

See also: → Be star; → phenomenon.

A hot star of → spectral type B showing → Balmer lines in emission. Be stars are fast rotators (spinning at about 200 km/sec) and have strong → stellar winds with important → mass loss.

See also: B, referring to the spectral type; e for emission lines;
→ star.

1. A collection of nearly parallel → light  → rays or a concentrated stream of → particles. See also → beam of light.
   

2. The area of the sky being observed at any one time by a → radio telescope.
   

3. The size of the → diffraction pattern; synonymous with point spread function.

Etymology (EN): M.E. beem, from O.E. beam “tree;” akin to O.H.G boum “tree,” Ger. Baum.

Etymology (PE): Tâbé, from tâb; tâbidan “light; to shine”

• -é noun suffix.

A parameter indicating the quality of an antenna as a direction measuring device. It is given by the ratio of the total
received power contained in the main beam of an antenna
to the total power (including the sidelobes); the same as main beam efficiency. See also → beamwidth.

See also: → beam; → efficiency.

A relatively large bundle of → rays of light. See also → pencil of light.

See also: → beam; → light.

A narrow unidirectional flow of particles

See also: → beam; → particle.

A partially reflecting mirror which permits a part of the light beam to pass through and reflects the rest.

Etymology (EN): → beam; splitter, from to split, from M.Du. splitten, from P.Gmc. spl(e)it-, PIE (s)plei- “to split, splice.”

Etymology (PE): Fâqgar, from fâq “split, breach, division” + tâbé→ beam.

In single dish radio astronomy, any technique which forms the difference of signals received from two (or more) pointings on the sky without physically moving the main reflector of the antenna. By rapidly forming differences between sky positions that do and do not contain astronomical sources, beam switching can minimize the corruption of spectral baselines by non-idealities in the instrumental frequency response, or of continuum observations by atmospheric fluctuations.

See also: → beam; → switching.

The angle between the two directions in the main beam at which the power response has fallen to half its maximum value. → beam efficiency.

Etymology (EN): From → beam + → width.

Etymology (PE): Tâbe-pahnâ, from tâbé, → beam, + pahnâ “width,” → broad.

The periodic and alternatively strengthening and weakening of two waves of similar frequencies when they interfere with one another. In particular, the soft and loud sounds created by the
interference of two sound waves of similar frequencies.

Etymology (EN): M.E. beten, from O.E. beaten, from P.Gmc. *bautan; IER *bhau- “to strike.”

Etymology (PE): Zaneš, noun from zan- present tense stem of zadan “to beat, strike” + -š verbal noun suffix. Zadan from Mid.Pers. zatan, žatan; O.Pers./Av. jan-, gan- “to strike, hit, smite, kill” (jantar- “smiter”); cf.
Skt. han- “to strike, beat” (hantar- “smiter, killer”); Gk. theinein “to strike;” L. fendere “to strike, push;” Gmc. *gundjo “war, battle;” PIE *g^when- “to strike, kill.”

A Cepheid variable in which two or more almost identical periods of variability pass into and out of phase with each other, producing periodic amplitude fluctuations in their light curves. Beat periods are typically about 2 hours.

See also: → beat; → Cepheid.

One of the frequencies that results from the combination of two waves of slightly different frquencies. A beat frequency is equal to the absolute value of the difference between the two frequencies. An unknown frequency can be determined by beating it with a reference frequency. More specifically, when the two frequencies are superimposed, the phase difference will change with time and wave interference alternate between constructive and destructive. The alterations of intensity brings about a beat frequency.

See also: → beat; → frequency.

A system for estimating and reporting wind speeds which has 13 standardized categories and associated descriptions. The Beaufort scale ranges from 0 for complete calm to 12 for a cyclone. In this scale, the wind speed (in km/h) equals 3B^1.5, where B is the Beaufort number of the wind. The scale was originally devised for use at sea but has subsequently been modified for use over land.

See also: Named after Admiral Sir Francis Beaufort (1774-1857), who introduced the first version of the system in 1805; → scale.

A combination of qualities that pleases the mind.

Etymology (EN): M.E. be(a)ute, from O.Fr. beautez, beltet “beauty, seductiveness, beautiful person” from V.L. bellitatem (nominative bellitas) “state of being handsome,” from L. bellus “pretty, charming.”

Etymology (PE): Zibâyi, from zibâ “beautiful,” related to zib “beauty, elegance, ornament,” zibidan “to suit, to adorn.”

A large, amphibious rodent of the genus Castor, having sharp incisors, webbed hind feet, and a flattened tail, noted for its ability to dam streams with trees, branches, etc. (Dictionary.com).

Etymology (EN): M.E. bever, O.E. beofor, befor; cognate with Ger. Biber; Av. bawra-, bawri- “beaver” (Mid.Pers. bawrak “beaver”);
Skt. babhrav- “reddish-brown,” babhrus- “mangoose,” L. fiber “beaver” (Fr. bièvre “River of Beavers”); O.H.G. biorr; Lith. bêbrus; Rus. bobr “beaver.”

Etymology (PE): Bidastar “beaver,” of unknown origin.

For the reason that; on account of.

Etymology (EN): M.E. bi cause “by cause,” from O.E. bi “by” (akin to Du. bij, O.H.G. bi, Ger. bei, Goth. bi) + → cause.

Etymology (PE): Zirâ, variants azirâ, zirâk, azirâk, from Mid.Pers. êt rây cê or ê(t) râd cê “because,” literally “this reason why;” from êd “this”
(O.Pers. aita; Av. aēta “this;” cf. Skt. etá); rây, → reason; cê “what” (O.Pers/Av. ci- “what, any,” collateral stem to ka- “who?, what?;” cf. Skt. ka-; Gk. po-; L. quo-; E. what, who; PIE *qwos/*qwes).

A compact infrared source in the Orion molecular cloud (OMC-1). It is thought to be a very dusty compact H II region surrounding a young B0 or B1 star.

See also: After Eric Becklin (1940-), and Gerry Neugebauer (1932-) who discovered the object in 1967; → object.

To come, change, or grow to be.

Etymology (EN): M.E. becumen; O.E. becuman “happen, come about,” also “meet with, arrive;” akin to Du. bekomen, O.H.G. biqueman “obtain,” Ger. bekommen, Goth. biquiman; from be- a prefix denoting several meanings, and → come.

Etymology (PE): Šodan “to become, to go, to pass, to change,” from Mid.Pers. šudan, šaw- “to go;” Av. š(ii)auu-, šiyav- “to move, go,” šiyavati “goes,” šyaoθna- “activity; action; doing, working;” O.Pers. šiyav- “to go forth, set,” ašiyavam “I set forth;” cf. Skt. cyu- “to move to and fro, shake about; to stir,” cyávate “stirs himself, goes;” Gk. kinein “to move;” Goth. haitan “call, be called;” O.E. hatan “command, call;” PIE base *kei- “to move to and fro.”

The → SI unit of → radioactivity. One becquerel corresponds to the → disintegration of one atom per second. Abbreviation: Bq. → curie.

See also: Named for the French physicist Henri Becquerel (1852-1908), who discovered radioactivity in 1896.

A → dwarf spheroidal galaxy belonging to the → Local Group that was discovered serendipitously in 2019. Bedin I was detected using extremely deep → HST images (V ~ 30 mag) obtained for the purpose of investigating the → white dwarf cooling track of the Galactic → globular cluster NGC 6752.

Bedin I is too faint and too close to the core of NGC 6752 for detection in earlier surveys. The discovery paper derives a → distance modulus of (m - M)₀ = 29.70 ± 0.13 mag from the observed → red giant branch, i.e. ~ 8.7 → megaparsecs (~ 30 million → light-years), and a size of ~ 840 × 340 pc, about one-fifth the size of the → Large Magellanic Cloud. This object is most likely a relatively isolated → satellite galaxy of the nearby great → spiral galaxy NGC 6744. The study suggests the presence of an old (~ 13 Gyr) and → metal-poor ([Fe/H] ~ -1.3) population in Bedin I.

See also: Named after the first author of the discovery paper, L. R. Bedin et al., 2019, MNRAS 484, L54.

Solid → rock present beneath any → soil, → sediment, or other surface cover. In some locations it may be exposed at earth’s surface.

Etymology (EN): Bedrock, from bed (O.E. bed, from P.Gmc. *badjam “sleeping place dug in the ground;” PIE *bhedh- “to dig, pierce”) + → rock.

Etymology (PE): Sangbastar, from sang “stone, rock,”
→ stone, + bastar “bed” (Mid.Pers. vistarak, cognate with Mod.Pers. gostar “a bed; spreading; scatterer,” Av. star-, starəta- “spread,” from star- “to spread,” Skt. strnâti).

Any tree of the genus Fagus, especially Fagus sylvatica of Europe, having smooth grayish bark. In Iran Fagus orientalis populates Caspian highland forests from Âstârâ to Gorgân.

Etymology (EN): M.E. beche, from O.E. becew; cf. Dutch beuk, Ger. Buche, from PIE root *bhagos “beech tree;” also cf.
Gk. phegos “oak,” L. fagus “beech,” Rus. buzina “elder.”

Etymology (PE): Râš, as named in Gilân, Râmsar, Šahsavâr, Kalârdašt. Its other names: (Mâzandarân) mers, (Ammaârlu, Manjil) râj, (Tevâleš, Mt. Dorfak) aluš, alâš, âlâš, (Nur) celer, celar (Iranica).

→ Praesepe.

Manner of behaving or acting.

Etymology (EN): M.E. behavour, from behaven “to behave,” altered by model havour, from O.Fr. avoir “to have.”

Etymology (PE): Raftâr “behavior, walking, going,” from raft past tense stem of raftan “to go, walk, proceed” (present tense stem row-, Mid.Pers. raftan, raw-, Proto-Iranian *rab/f- “to go; to attack”)

• -âr prefix forming action noun.

The mathematical expression giving the → entropy, S, of a → black hole as a function of the area of its → event horizon, A: S = (kc³A)/(4Għ), where k is → Boltzmann’s constant, ħ is the → reduced Planck’s constant, and G the → gravitational constant. It can also be expressed by S = (kA)/(4l_P²), where l_P is the → Planck length. The existence of this entropy led to the prediction of the → Hawking radiation, because an entropy is associated with a temperature
and a temperature to a → thermal radiation. The entropy of a black hole increases continuously because the fall of material into it increases its area.

See also: For Jacob D. Bekenstein (1947-), an Israeli theoretical physicist, who contributed to the foundation of black hole thermodynamics; → formula.

A dimensionless unit to measure sound. One bel designates a ratio 10:1 between two quantities, P₁ and P₀, which have the dimension of a power: n [B] = log (P₁/P₀), in Bel units, with → natural logarithm. If one sound is 2 bels louder than another, this means the louder sound is 100 times more intense than the fainter one. It is also common to use this definition for quantities that are proportional to a power, such as energy, work, intensity, or voltage. The bel was too large for everyday use, so the → decibel (dB), equal to 0.1 bel, is more commonly used.

See also: This unit was put forward by engineers of the Bell telephone network in 1923 and named in honor of the inventor of the telephone, Alexander Graham Bell (1847-1922), who also invented techniques for teaching speech to the deaf.

One of the small satellites of → Uranus discovered from the Voyager 2 photographs taken during its encounter with the planet in 1986.

Etymology (EN): Named after the heroine in Alexander Pope’s The Rape of the Lock.

Any of a large number of inequality relations developed to study the → hidden variable hypothesis suggested in the → EPR paradox. Using Bell’s inequalities, the → Aspect experiment showed that no local hidden variable theory can make predictions in agreement with those of quantum mechanics. If, in a measurement, the inequality is violated, the measurement is in agreement with the predictions of the quantum theory. If the equality is satisfied, it suggests that a classical, causal, and local model is adequate to explain the outcome of the measurements.
See also → quantum entanglement.

See also: John Stewart Bell (1928-1990); → inequality.

A bright, blue → giant star ( → spectral type B2 III), one of the main stars
of the constellation → Orion. With a visual magnitude of 1.64,
it is about 1000 times more luminous than the Sun, and lies at a distance of 243 → light-years.

Etymology (EN): From L. bellatrix “a female warrior,” fem. of bellator, from bellum “war.”

Etymology (PE): Merzam, Ar. name of the star; its other name is Nâjed.

1. (with preposition to) To be the property of.
   

2. (with preposition to) To be a part or adjunct of.

Etymology (EN): M.E. belongen, from be- intensive prefix, + longen “to go,” from O.E. langian “pertain to, to go along with;” akin to Du. belangen, Ger. belangen; of unknown origin.

Etymology (PE): Pargetidan, literally “to surround, to relate with” (on the model of L. pertinere “pertain,” Skt. parigraha- “surrounding; relation to”),
from parget “to hold, seize, take around,” from par- “around,” → peri-, + get “to take, sieze,” as in Tâleši gate “to take,” Târi gata, Sorxeyi, Lâsgardi, Semnâni, Šâhmerzâdi -git- “take, seize,” variants of gereftan “take, hold,” → concept.

If x is an → element of a → set S, then x belongs to S and this is written x ∈ S.

See also: Third person present verb of → belong.

A strip of leather or cloth worn around the waist.
Something that resembles this type of band, e.g. → Gould’s Belt, → Belt of Venus.

Etymology (EN): O.E. belt, from P.Gmc. *baltjaz, from L. balteus “girdle;” → Orion.

Etymology (PE): Kamarband “belt,” from kamar “waist” (Mid.Pers. kamar “waist; belt, girdle,” Av. kamarâ- “belt”) + band “a band, tie, belt.”

→ Orion’s Belt.

See also: → belt; → Orion.

A pink to brownish border above the horizon separating the Earth’s dark shadow on the sky from the sky above it. The Belt of Venus appears during a cloudless twilight just before sunrise or after sunset. It is due to scattered red sunlight in the atmosphere. Also called anti-twilight arc.

See also: → belt, → Venus.

A → European Space Agency (ESA) mission aimed at studying → Mercury, the least explored planet in the inner → Solar System. It was launched on 20 October 2018.

Among several goals, BepiColombo will make a complete map of Mercury at different wavelengths. It will chart the planet’s → mineralogy and elemental → composition, determine whether the interior of the planet is molten or not, and investigate the extent and origin of Mercury’s → magnetic field, the properties of its → magnetosphere, and history of the planet.

The trajectory will also be modified by eight planetary flybys: of Earth in April 2020, Venus in 2020 and 2021, and then six times of Mercury itself between 2021 and 2025. BepiColombo will enter Mercury orbit in December 2025.

BepiColombo is a joint mission between ESA and the Japanese Aerospace Exploration Agency (JAXA), executed under ESA leadership.

See also: Named after Giuseppe (Bepi) Colombo (1920-1984), a scientist who studied Mercury’s orbital motion in detail as well as orbits and interplanetary travel in general.

→ Coma Berenices.

The equation expressing → Bernoulli’s theorem: P + (1/2)ρV² + ρgz = constant, where P is the fluid → pressure, V is → velocity, ρ is → density,
g is the acceleration due to → gravity, and z is the vertical reference → level. The theree terms are called → static pressure, → dynamic pressure, and
→ hydrostatic pressure, respectively. The Bernoulli equation states that the total pressure along a → streamline is → constant.

See also: → Bernoulli’s theorem; → equation.

A statement of the → conservation of energy in the → steady flow of an → incompressible, → inviscid fluid. Accordingly, the quantity (P/ρ) + gz + (V²/2) is → constant along any → streamline, where P is the fluid → pressure, V is the fluid → velocity, ρ is the mass → density of the fluid, g is the acceleration due to → gravity, and z is the vertical → height. This equation affirms that if the internal velocity of the flow goes up, the internal pressure must drop. Therefore, the flow becomes more constricted if the velocity field within it increases. Same as the → Bernoulli equation.

See also: After Daniel Bernoulli (1700-1782), the Swiss physicist and mathematician who put forward the theorem in his book Hydrodynamica in 1738; → theorem.

1. A small, round fruit on particular plants and trees.
   

   2. Botany: Any of various small edible fruits such as the raspberry, blackberry, and strawberry.

Etymology (EN): M.E. berye, from O.E. berie “berry, grape,” cf. M.Du. bere, Ger. Beere, O.Sax. winberi, Gothic weinabasi “grape,” Norwegian and Danish bær, of unknown origin.

Etymology (PE): Pelâr, from Hamadâni, Malâyeri pellâr, pellâra “berry, grape berry;” cf. Laki, Xonsâri palâra “raisin grape,” Aligudarzi pellâr “part of a raison grape,” of unknown origin (related to berry, as above?).

A grey, very hard metallic chemical element; symbol Be. → Atomic number 4; → atomic weight 9.01218;
→ melting point about 1,278°C; → boiling point 2,970°C (estimated); → specific gravity 1.85 at 20°C; → valence +2. Beryllium occurs as beryl, from which it is obtained by electrolysis. Used for light alloys which are corrosion resistant. Beryllium was discovered by Louis Nicolas Vauquelin (1763-1829) in 1798. First isolated by Friedrich Wöhler (1800-1882) in 1828.

See also: From L. beryll(us), from beryl, a mineral, beryllium aluminum silicate, Be₃Al₂Si₆O₁₈, M.E. beril, from O.Fr., from L. berillus, from Gk. beryllos, + → -ium.

From Friedrich Wilhelm Bessel (1784-1846), German astronomer and mathematicians, who made fundamental contributions to positional and spherical astronomy.

A linear second-order differential equation, the solutions to which are called Bessel functions.

Etymology (EN): From → Bessel; → equation

Etymology (PE): Hamugeš, → equation.

Same as → 61 Cygni, the first star whose distance was measured, by Friedrich Bessel in 1838.

See also: → Bessel; → star.

Of or pertaining to Friedrich Wilhelm Bessel (1784-1846) or to his discoveries. → Besselian day number → → Bessel equation → Besselian star constant → → Besselian year.

See also: → Bessel.

Any of the five quantities denoted by A, B, C, D, and E used in conjunction with → Besselian star constants for the reduction of a star’s → mean catalog place to its → apparent place.

See also: → Besselian;
→ day; → number.

Any of the eight quantities denoted by a, b, c, d (for → right ascension) and a’, b’, c’, d’ (for → declination) used in conjunction with → Besselian day numbers for the reduction of star’s → mean catalog place.

See also: → Besselian;
→ star; → constant.

The period taken for the right ascension of the mean Sun to increase by 24 hours. The starting point is when the mean Sun’s longitude is 280°, corresponding roughly to January 1. It is virtually the same as the tropical year.

See also: → Besselian; → year.

In a scatter plot, a mathematical line or curve that passes as near to as many of the data points as possible.

Etymology (EN): Best, M.E., from O.E. betst, akin toi O.E. bot “remedy.” Fit, from M.E. fitten “to marchal troops,” from or akin to M.Dutch vitten “to be suitable.”

Etymology (PE): Behtarin supperlative of beh “good, fine” (Mid.Pers. veh “better, good,” O.Pers. vahav-, vahu-, Av. vah-, vohu- “good,” cf. Skt. vasu- “good,” Hittite wasu-, Gaulish vesus “good”) + saz, from sazidan “to be fit, proper,” from Mid.Pers. saz, sazistan “to be fitting, proper.”

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.

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

The second brightest star in the constellation → Cepheus and the prototype of → Beta Cephei variables. It is a variable B2 type → giant star with a visual magnitude of 3.23 varying with a period of 4.57 hours. Its mass is a dozen times that of the Sun. Beta Cephei is a → triple system lying at a distance of about 600 → light-years. The inner → spectroscopic companion, → spectral type A, is only about 45 AU away, and takes around 90 years to orbit. The third visual companion is at least 2400 AU away, with an orbital period of at least 30,000 years.

See also: Beta (β), the second letter of the Gk. alphabet; → Cepheus.

A variable star, of early B or late O types, undergoing radial pulsations with short periods (< 1 day). Beta Cephei stars are confined within a narrow band of the → H-R diagram above the upper → main sequence. They are believed to be near the end of core hydrogen-burning stars of approximately 10 to 20 solar masses. The famous bright stars → Spica and → Mirzam belong to this family.

See also: → Beta Cephei; → variable.

The transformation of a → radioactive nuclide in which a → beta particle is emitted. In beta minus decay, a → neutron changes into a → proton, → antineutrino, and → electron: n → p + e + ν^-. Beta plus decay involves the conversion of a proton to a neutron, → positron, and → neutrino: p → n + e⁺ + ν.

See also: Beta (β), from → beta particle; → decay.

→ beta particle.

See also: → beta particle; → minus; → decay.

An → electron or a → positron emitted from an unstable nucleus during a → radioactive process known as → beta decay.

See also: The term “beta particle” relates to the early history of the → radioactivity studies when the nature of the emergent particles was not elucidated; → particle.

The second brightest star, with an apparent magnitude of 3.86, in the southern constellation → Pictor. Beta Pic is a young star of spectral type A lying 63 → light-years away. It has a luminosity 8.6 times that of the Sun and its surface temperature is 8250 K. Beta Pic is surrounded by a dust and gas disk stretching 400 A.U. away from the star in each direction, 10 times the average distance of Pluto from the Sun. The disk is not symmetric, one side is brighter than the other. Moreover, it has an inner clear zone about the size of our solar system (some 30 A.U.). Recently a probable giant → exoplanet lying in the disk has been imaged.

See also: Beta (β), the second letter of the Gk. alphabet; → Pictor.

→ beta particle.

See also: → beta particle; → plus; → decay.

A solid object with a mass about 10^-18-10^-15 kg in → interplanetary space that moves in hyperbolic orbit as a result of the solar → radiation pressure.

See also: → meteoroid.

The → red supergiant that is the second brightest star in the constellation → Orion. Betelgeuse is one of the biggest stars known with a size of almost 1,000 times larger than the Sun, corresponding to an angular diameter of 43.76 ± 0.12 milli-arcseconds (Perrin et al. 2004, A&A 418, 675). It is a → semiregular variable whose → apparent visual magnitude varies between 0.2 and 1.2 shining very rarely more brightly than its neighbor → Rigel. The energy released by Betelgeuse is estimated to be only 13% in the form of visible light, with most of its radiation being at → infrared wavelengths. The distance of Betelgeuse is 643±146 → light-years (Harper et al. 2008, AJ 135, 1430), while its luminosity
is about 140,000 times that of the Sun (→ solar luminosity). Its → spectral type is M2 Iab, its → surface temperature about 3,600 K, and its → initial mass 10 to 20 → solar masses (M_sun).
Neilson & Lester (2011, arXiv:1109.4562) recently proposed a mass of 11.6 (+5.0, -3.9) M_sun
for Betelgeuse, while Dolan et al. (2008, BAPS 53, APR.S8.6) obtained about 21 M_sun. Its → rotation period is estimated to be about 17 years (Uittenbroek et al. 1998, AJ 116, 2501). Recent observations with the → Very Large Telescope resolve not only the apparent surface of Betelgeuse, but also reveal a large and previously unknown plume of gas extending into space from the surface of the star (Kervella et al. 2009, A&A 504, 115).
The plume extends to at least six times the diameter of the star, corresponding to the distance between the Sun and Neptune. This detection suggests that the whole outer shell of Betelgeuse is not shedding matter evenly in all directions. More recently, an image of the surface of the star was obtained using long → baseline → interferometry at infrared wavelengths (Haubois et al. 2009, A&A 508, 923). It shows the presence of an irregular flux distribution possibly caused by enormous → convective cells. A very large dusty envelope has also been observed at larger distances from the star (Kervella et al. 2011, A&A 531, A117).

Etymology (EN): Betelgeuse, from Ar. Ibt al-Jauza’ (ابط‌الجوزاء)
“the armpit of Jauza’,” from ibt “armpit” + Jauza’ “Orion.”

Etymology (PE): Ebtoljowzâ, from Ar. Ibt al-Jauza’.

A unit of energy equal to 10⁴⁴ → joules or 10⁵¹ → ergs, corresponding to the amount of energy liberated in a typical → supernova explosion. It is used by some theoreticians.

See also: In honor of Hans Bethe (1906-2005), Nobel Prize in Physics (1967), for his work on the theory of stellar nucleosynthesis.
The unit name was proposed by Steven Weinberg (1933-) in 2006 for Bethe’s contributions to the supernova research aftre 1980.

An → ansatz initially used to deal with → antiferromagnetism
in a quantum system. It has been generalized to various quantum → n-body problems.

See also: First introduced by Hans Bethe (1906-2005), → bethe; → ansatz.

Same as → blue horizontal branch star.

See also: → blue; → horizontal; → branch; → star.

Prefix denoting “two, both, twice.”

Etymology (EN): From L. bi-, cognate of Gk. di-, O.E. twi-; cf. Av. bi- “two,” biš “twice,” Mod.Pers. do “two,” PIE *dwo- “two.”

Etymology (PE): Do, Mid.Pers. do, Av. dva-, Skt. dvi-, Gk. duo, L. duo, E. two, Ger. zwei, Fr. deux.

A cosmological model based on the theory of → general relativity, which is homogeneous but → anisotropic. There are actually ten dinstinct Bianchi types, classified according to the particular kinds of symmetry they posses.

See also: Luigi Bianchi (1856-1928), Italian mathematician; → cosmological; → model.

1. General: Leaning of the mind toward or away from something; especially: a personal and sometimes unreasoned judgment.
   Statistics: A → systematic error introduced by selecting items from a wrong population, favoring some of the elements of a population, or poorly phrasing questions.
   

2. Electronics: A → voltage applied to a device to establish a reference level for operation. Same as initial voltage, initial tension (Ger. die Vorspannung).
   In a → CCD detector, electronic → offset which prevents negative signal.
   

3a) General: To give a settled and often prejudiced outlook to.

3b) Electronics: To apply a small voltage to a device to control its operation. See also: → bias error, → bias frame, → bias offset, → bias voltage, → biased, → biased estimate, → biased galaxy formation, → biased sample, → biased statistics, → debias, → kinematic bias, → Lutz-Kelker bias, → Malmquist bias, → observational bias, → sampling bias, → unbiased, → unbiased estimator.

Etymology (EN): From M.Fr. biais “slant,” perhaps ultimately from Gk. epikarsios “slanting, oblique,” from epi- “upon” + karsios “oblique.”

Etymology (PE): Varak, from var “side, direction”

• -ak suffix denoting relation, affinity, similarity (as in dastak, poštak, pašmak, xarak, nâxonak, mušak, eynak). Note also var in yekvar “slanting, inclined”.
  
  Pištaneš, only in the electronic sense, from piš, → pre- + taneš, → tension.
  Varak dâdan, from varak + dâdan “to give,” → datum.

A measurement error that remains constant in magnitude for all observations; for example an incorrectly set zero adjustment.

See also: → bias; → error.

CCD frame with exposure time set to zero and giving the bias level.

Etymology (EN): → bias; → frame.

Etymology (PE): Nurdâd, → exposure; varak→ bias

In → CCD detectors, same as → bias and → offset.

See also: → bias; → offset.

A voltage applied or developed between two electrodes as a bias.

See also: → bias; → voltage.

Marked by or exhibiting bias; characterized by settled and often prejudiced outlook.

Etymology (EN): Biased, from → bias + adj. suffix -ed.

Etymology (PE): Varakdâr, from varak, → bias + -dâr “possessing, having,” from dâštan “to possess, to have.”

Of a population parameter, if the mean or expectation of the statistics is not equal to the parameter.

Etymology (EN): → biased; → estimate.

Etymology (PE): barâvard, → estimate; varakdâr→ biased.

The theory that bright galaxies form preferentially from anomalously overdense perturbations in the → early Universe.

See also: → biased; → galaxy; → formation.

A sample that is not a true representative of a → statistical population to which generalizations are to be made. A sample which is not → randomly constituted.

See also: → biased; → sample.

A statistics based on a → biased sample.

See also: → biased; → statistics.

A birefrigent crystal, such as mica, that is characterized by having two optical axes along which light is propagated with equal velocities.

See also: Biaxial, from → bi- “two” + axial, from → axe; → crystal.

A → microwave → polarimeter designed specifically to target the → B-mode signature of → inflation in the → cosmic microwave background polarization. BICEP2 observed from the South Pole for three seasons from 2010 to 2012. This 26 cm aperture → telescope comprised an all-cold refracting optical system equipped with a → bolometer array of 512 → detectors (256 pixels) operating at 150 GHz.

See also: BICEP2, the upgraded version of the first BICEP, short for Background Imaging of Cosmic Extragalactic Polarization.

Describing a lens with two concave faces.

See also: → bi-; → concave; → lens.

A → spherical lens with two convex faces. The radii of curvature for the two surfaces may or may not be the same.

See also: → bi-; → convex; → lens.

A comet having a short period of 6.62 years discovered by Biela.
It broke up on its 1846 return and subsequently gave rise to a spectacular meteor shower.

Etymology (EN): In honor of Wilhelm von Biela (1782-1856), Austrian military officer and amateur astronomer, who re-discovered the comet Biela in 1826, although it had been seen first in 1772. → comet.

A → meteor shower, originating from → Biela’s comet, whose → radiant lies in the constellation → Andromeda; also called → Andromidids.

Etymology (EN): Bielids, From Biel(a) + → -ids suffix denoting “descendant of, belonging to the family of.”

1. To divide into two branches.
   

2. Divided into two branches.

Etymology (EN): M.L. bifurcatus, from L. → bi- “two,”

• furca “pitchfork; fork used in cooking,” of uncertain origin.

Etymology (PE): Dogalidan, from Gilaki dogal “fork, two-branched,” cf. Tabari dekal, dokkal, doqâla, from do, → two, + gal, kal “branch, part,” cf. Kurd. (Kurmanji) kar “part, piece,” cognate with Pers. kârd “knife,” (+ *niš-) nišgarda “cobbler’s knife;” Mid.Pers. kârt “knife,” karēnītan, karītan “to cut,”
(+ *fra-) fragard “chapter, section;” Av. karət- “to cut;” Proto-Iranian kart- “to cut;” cf. Skt. kart- “to cut;”
Gk. karpos “fruit;” L. carpere “to cut, divide, pluck;” PIE base (s)ker- “to cut;” + -idan infinitive suffix. See also → shear.

1. A branching or division into two parts; a splitting apart.
   

2. A sudden qualitative change in the behavior of a → dynamical system.
   The equation describing the evolution of the system finds several solutions for certain values of a parameter. Systems undergoing a bifurcation do not necessarily return to their original state, even if the parameter returns to its nominal value. See also → chaos.

See also: Verbal noun of → bifurcate.

The point or moment in the evolution of a → dynamical system that occurs if a parameter passes through a critical point. At this point the system branches into any number of qualitatively new types of behavior.

See also: → bifurcation; → point.

1. A theory which studies how, in certain nonlinear systems, there may be paths and shifts in behavior dependent on small changes in circumstances or the current position of the system.
   

2. Math.: The study of the behavior of a solution of a nonlinear problem in the neighborhood of a known solution, particularly as a parameter varies.

See also: → bifurcation; → theory.

Of considerable size, number, quantity, large.

Etymology (EN): M.E., northern England dialect, of unknown origin.

Etymology (PE): Bozorg, → large.

Meh “great, large;” Mid.Pers. meh, mas; Av. maz-, masan-, mazant- “great, important,” mazan- “greatness, majesty,” mazišta- “greatest;” cf. Skt. mah-, mahant-; Gk. megas; L. magnus; PIE *meg- “great.”

A theory which states that the → Universe came into existence in an “instantaneous” event some 14 billion years ago. Matter
was created in that initial event and as time has gone by the Universe has expanded and the contents evolved into the galaxies and stars and of today. The Big Bang is sometimes described as an “explosion.” However, matter and energy did not erupt into a pre-existing space, since they came into being simultaneously with space and time.

Etymology (EN): → big;
bang “a sudden loud noise, as of an explosion” (probably of Scandinavian origin; akin to Icelandic banga “to hammer”). The term was coined by Fred Hoyle in 1950 in the course of discussions entitled “the Nature of the Universe” broadcasted by BBC. Hoyle’s intention was a pejorative term in order to ridicule the theory which his own → steady state theory contested.

Etymology (PE): Meh Bâng, from meh “great, large,” → big, + bâng “voice, sound, clamour,” (Mid.Pers. vâng, Av. vaocanghê “to declare (by means of speech”), vacah- “word,” from vac- “to speak, say,” cf. Mod.Pers. vâžé “word,” âvâz “voice, sound, song,” Skt. vakti “speaks, says,” vacas- “word;”
Gk. epos “word,” L. vox “voice;” PIE base *wek- “to speak”).

→ Big Bang; → model.

The production of → light elements, roughly three minutes after the → Big Bang when the temperature of the → Universe dropped from 10³² K to approximately 10⁹ K. In a short time interval → protons and → neutrons collided to produce → deuterium. Most of the deuterium then fused with other protons and neutrons to produce → helium and a small amount of → tritium. The element
→ lithium 7 could also arise form the coalescence of one tritium and two deuterium nuclei. According to the Big Bang nucleosynthesis theory, roughly 25% of the mass of the Universe consists of helium. It also predicts about 0.01% deuterium, and even smaller quantities of lithium. These predictions depend critically on the → baryon-photon ratio. Same as → primordial nucleosynthesis.

See also: → Big Bang; → nucleosynthesis.

A hypothetical state of → infinite energy density representing an infinite → gravitational field and infinite → space-time curvature.

The singularity arises from using Einstein’s theory of → general relativity concerning gravity. We know, however, that when the density and heat become extremely large, quantum physics of gravity becomes important. Yet Einstein’s equations ignore quantum effects. In other words, in certain extreme conditions, Einstein’s equations do not apply.

See also: → Big Bang; → singularity.

→ Big Bang; → theory.

The broad continuum feature dominating the optical-ultraviolet spectra of AGNs. Most current models attribute the big blue bump to thermal emission from an optically thick accretion disk.

See also: → big; → blue; bump, → bump Cepheid.

The state of extremely high density and temperature into which a closed → Universe would → collapse in the distant future. If the Universe has a mass density exceeding the critical threshold, then gravity will eventually halt the expansion and cause the Big Crunch.

Etymology (EN): → big; crunch “to crush, grind, or tread noisily; the act or sound of crunching,” alteration of craunch, possibly of imitative origin.

Etymology (PE): Meh “large, big,” → big; romb, → collapse, from rombidan “to collapse;” rombeš-e farjâmin “final collapase,” from rombeš verbal noun of rombidan; farjâmin, → late.

A group of seven stars, an → asterism, lying inside the Northern constellation → Ursa Major. They are:
→ Dubhe, → Merak, → Phad, → Megrez, → Alioth, → Mizar, and → Alkaid. The group is also known as the Plough in Great Britain.

Etymology (EN): → big; dipper a popular U.S. name for the asterism known in Britain as The Plough or Charles’ Wain, from dip O.E. dyppan “immerse,” from P.Gmc. *dupjanan.

Etymology (PE): Haft barâdarân “the seven brothers,” from haft “seven” (Mid.Pers. haft, Av. hapta, cf. Skt. sapta, Gk. hepta, L. septem, P.Gmc. *sebun, Du. zeven, O.H.G. sibun, Ger. sieben, E. seven; PIE *septm)

• barâdarân, plural of barâdar “brother” (Mid.Pers. brad, bardar, O.Pers./Av. brātar-, cf. Skt. bhrátar-, Gk. phrater, L. frater, P.Gmc. *brothar; PIE base *bhrater- “brother”).
  Haftowrang, Mid.Pers. haptôiring, from Av. haptôiringa- “with seven marks,” from hapto- “seven,“as above, + iringa- “mark,” cf. Skt. linga- “mark, token, sign.”
  Camcé “a spoon, ladle; a wooden bowl or cup;”
  bozorg “big, large.”

A type of → interstellar dust grains with a size ranging from 150 to 1000 Å. Big grains consist of graphite and silicates. They
are in → thermal equilibrium with the radiation field and their emission can be described by a modified → blackbody radiation following from → Kirchhoff’s law.

See also: → big; → grain.

A cosmological hypothesis regarding the ultimate fate of the → Universe whereby in a far future galaxies and stellar systems would be torn apart due to the → accelerating expansion of the Universe depending on the kind of the → dark energy content of the Universe. According to this hypothesis, after the disruption of galaxies, stars, and planets even atoms might not be able to withstand the internal force of the expansion imposed by the dark energy.

Etymology (EN): → big; M.E. rippen, origin obscure, cf. Frisian rippe “to tear, rip,” M.Du. reppen, rippen “to pull, jerk,” Swed. reppa, Dan. rippe “to tear, rip.”

Etymology (PE): Meh “large, big,” see under → big; gosast stem of gosastan “to tear, cut, break,” from Mid.Pers. wisistan “to break, split,” Av. saed-, sid- “to split, break,” asista- “unsplit, unharmed,” Skt. chid- “to split, break, cut off,” PIE base *skei- “to cut, split,” cf. Gk. skhizein “to split,” L. scindere “to split,” Goth. skaidan, O.E. sceadan “to divide, separate.”

A → mapping  f from a → set  A onto a set B which is both an → injection and a → surjection. More explicitly, for every element b of B there is a unique element a of A for which f(a) = b. Also known as → bijective mapping.

See also: From bi- + → injection.

Of or pertaining to a → bijection.

See also: → bi-; → injection.

Same as → bijection.

See also: → bijective; → mapping.

Same as → isomorphism.

See also: → bijective; → morphism.

Having or providing two modes, methods, systems, etc., in particular having or occurring with two statistical modes.

See also: → bi-; → modal.

A concept of → star formation in which → high-mass stars and → low-mass stars form in different physical conditions involving different → molecular clouds.

Following the pioneering suggestion of Herbig (1962), successive investigations have generally supported the idea that star formation proceeds bimodally with respect to stellar mass. The star formation rate appears to differ both spatially and temporally for low mass and → massive stars. This is of considerable importance for galactic evolution, since the low-mass stars lock up mass and are long-lived, low luminosity survivors to the present epoch, whereas massive stars are short-lived, recycle and enrich interstellar gas, and leave dark remnants while producing a high luminosity per unit of mass (Silk, J., 1988, in Galactic and Extragalactic Star Formation, p. 503, eds. R. E. Pudritz and M. Fich).

See also: → bimodal; → star; → formation.

The quality or state of being → bimodal.

See also: → bimodal; → -ity.

1a) General:A box or enclosed space for storing grain, coal, or the like.

1b) Electronics: In a → CCD detector, a square or rectangular group of adjacent → pixels
used collectively to transport the → electron charges.

1c) In a statistical → histogram, the range of → distribution → intervals. The bin widths (or the number of bins) affect a histogram.

2. To place in a → bin; to create → binning.

Etymology (EN): M.E. binne, O.E. binn(e) “manger, crib,” perhaps from O.Celt. *benna, akin to Welsh benn “a cart, especially one with a woven wicker body.” The same Celtic word seems to be preserved in It. benna “dung cart,” Fr. benne “a sort of box for transporting materials, especially in mines,” Du. benne “large basket,” from L.L. benna.

Etymology (PE): Bâvin “a basket, more precisely a small basket which contains the cotton to be spun;” bâvinidan infinitive from bâvin.

The quality or condition of being binary.

See also: → binary; → -ity.

General: Characterized by or consisting of two parts or components.
Math.: Of or relating to a system of numeration having 2 as its base.
Astro.: Physically associated pair of astronomical objects.

Etymology (EN): Binary, from L.L. binarius, from bini “two-by-two,” from bis “twice, two times;” cf. Av. biš- “twice,” bi- “two,” Mod.Pers. do “two,” PIE *dwo- “two.”

Etymology (PE): Dorin, from Mid.Pers. dorin “double, pair,” from do (Av. dva-, Skt. dvi-, Gk. duo, L. duo, E. two, Der. zwei, Fr. deux) “two” + rin “time, turn.”
Dodoyi “two-by-two,” from do, → two.

A system of calculation in which the only numerals used are 0 and 1. All the real numbers are represented in terms of powers of 2.

See also: → binary; → arithmetic.

A member of a population of double objects in the main → Asteroid Belt or the → Kuiper Belt which are gravitationally bound together. So far about 200 such binary systems have been identified, while their number is increasing.
243 Ida was the first binary asteroid to be discovered during the Galileo spacecraft flyby in 1993. Other examples are
→ Antiope and Kalliope in the main belt and QG₂₉₈ in the Kuiper Belt.
The importance of these objects resides in the fact that
systems with well measured orbital parameters allow the total mass to be estimated. If the sizes of the components are known then their densities can be accurately calculated. Density is an important parameter since it yields information about composition and internal structure.

See also: → binary; → asteroid.

A → binary system consisting of two → black holes in close orbit around each other. Same as → black hole binary.

See also: → binary; → black; → hole.

Either of the digits 0 or 1, used in the → binary number system.

See also: → binary, → digit.

The fraction of stars that have at least one → companion. It is at least 50%. The binary fraction appears to increase with increasing → primary star mass, at least among the more massive stars: the → O stars and → B stars have a companion frequency of at least 70%, while for the → G stars the binary frequency is around 50% and the → M stars may have an even lower binary frequency of around 30-40%. Brown dwarfs are rare as companions to lower-main-sequence stars, although brown-dwarf binaries appear not to be rare. An increase in binary frequency with mass would be expected if most stars form in → multiple systems that disintegrate, since the more massive stars would then preferentially remain in binaries while the less massive ones would preferentially be ejected as single stars (see Richard B. Larson, 2001, in IAU Symposium 200, p. 93 and references therein).

See also: → binary; → frequency.

A pair of galaxies in orbit around each other.

Etymology (EN): → binary; → galaxy.

Etymology (PE): Kahkašân, → galaxy; dorin→ binary.

A → numeral system that has 2 as its base and uses only two digits, 0 and 1. The positional value of each digit in a binary number is twice the place value of the digit of its right side. Each binary digit is known as a bit. The decimal numbers from 0 to 10 are thus in binary 0, 1, 10, 11, 100, 101, 110, 111, 1000, 1001, and 1010. And, for example, the binary number 11101₂ represents the decimal number (1 × 2⁴) + (1 × 2³)

• (1 × 2²) + (0 × 2¹) + (1 × 2⁰), or 29. In electronics, binary numbers are the flow of information in the form of zeros and ones used by computers. Computers use it to manipulate and store all of their data including numbers, words, videos, graphics, and music.

See also: → binary; → number; → system.

A mathematical operation that combines two numbers, quantities, sets, etc., to give a third. For example, multiplication of two numbers is a binary operation.

A binary operation * on a set S is → commutative if a * b = b * a for all a, b∈ S.

A binary operation * on a set S is → associative if (a * b) * c = a * (b * c) for all a, b, c∈ S.

See also: → binary; → operation.

A pulsar in a → binary system, the companion of which often being a → neutron star or a → white dwarf. The only known binary system with two pulsars
components is the → double pulsar. As of 2010 about 70 binary pulsars have been identified. They are ideal laboratories for testing and studying the effects predicted by → general relativity, such as → spin precession, → Shapiro time delay, and → gravitational waves. The prototype, called PSR 1913+16, was discovered in 1974 by
Russell A. Hulse and Joseph H. Taylor, Jr., who received the Nobel Prize for Physics in 1993. → Hulse-Taylor pulsar.

See also: → binary; → pulsar.

Two stars gravitationally bound to each other, so that they revolve around their common center of gravity. → double star; → spectroscopic binary.

Etymology (EN): → binary; → star.

Etymology (PE): Setâré, → star; dorin, → binary.

A → dual supermassive black hole whose components are separated by a few parsecs.

See also: → binary; → supermassive; → black; → hole.

Two astronomical objects revolving around their common center of mass.

Etymology (EN): → binary; → system.

Etymology (PE): Râžmân, → system; dorin→ binary.

In → graph theory, an → ordered tree with all → nodes having at most two → children.

See also: → binary; → tree.

To tie, to fasten, to cause ti stick together.

Etymology (EN): O.E. bindan “to tie up with bonds,”
PIE base *bhendh- “to bind;” cf. Av./O.Pers. band- “to bind, fetter,” banda- “band, tie,” Skt. bandh- “to bind, tie, fasten,” bandhah “a tying, bandage.”

Etymology (PE): Bandidan “to bind, confine” [Mo’in, Dehxodâ], from band “band, tie” + -idan infinitive suffix; cognate with E. bind, as explained above.

1. Of a gravitational system, the difference in energies between the hypothetical state where all bodies of the system are infinitely separated from each other and the actual bound state.
   
2. The energy which must be supplied to a nucleus in order to cause it to decompose into its constituent neutrons and protons.

Etymology (EN): Binding, noun from → bind; → energy.

Etymology (PE): Kâruž, → energy; bandeš noun from bandidan, → bind.

Combining a few adjacent CCD pixels in bins, during readout; the method used to assemble the bins and transfer the charge by means of an electronic clock. Binning improves signal-to-noise ratio at the expense of spatial resolution.

Etymology (EN): Binning, from → bin.

Etymology (PE): Bâvineš, from bâvin, → bin.

A small optical instrument with two tubes that is used to magnify the view of distant or astronomical objects. → prism binoculars.

Etymology (EN): From Fr. binoculaire, from binocle, from L. bini “double” (L. bis, bi- “twice,” Av. biš “twice”) + ocularis “of the eye,” from oculus “eye” (compare with Av. axš-, aš- “eye,” Skt. akshi- “eye,” Gk. ops “eye,” opsis “sight, appearance,” from PIE okw- “to see;” also O.E. ege, eage, from P.Gmc. *augon, Goth. augo, Lith. akis, Armenian aku).

Etymology (PE): Docašmi “binocular,” from do, → two

• cašm, → eye, + -i adj. suffix; durbin, → telescope.

1a) An algebraic expression containing 2 terms, as x + y and 2x² - 3x. In other words, a → polynomial with 2 terms.

1b) Biology: A pair of Latin (or latinized) words forming a scientific name for organisms. The first word represents the genus, and the second the species.

2. Of, pertaining to, or consisting of a binomial.

See also: From L.L. binomi(us) “having two names,” + → -al, → nominal.

The factor multiplying the variable in a term of a → binomial expansion. For example, in (x + y)⁴ = x⁴ + 4x³y + 6x²y² + 4xy³ + y⁴ the binomial coefficients are 1, 4, 6, 4, and 1. In general, the r-th binomial coefficient in the expression
(x + y)ⁿ is: (n,r) = n!/[r!(n - r)!].

See also: → binomial; → coefficient.

An expression of the form x^m(a + bxⁿ)^pdx, where m, n, p, a, and b are constants.

See also: → binomial; → differential.

A probability distribution for independent events for which there are only two possible outcomes i.e., success and failure.

The probability of x successes in n trials is: P(x) = [n!/x!(n - x)!] p^x.q^{n - x}, where p is the probability of success and q = 1 - p the probability of failure on each trial. These probabilities are given in terms of the → binomial theorem expansion of (p + q)ⁿ.

See also: → binomial; → distribution.

A rule for the expansion of an expression of the form (x + y)ⁿ. The variables x and y can be any → real numbers and n is an → integer. The general formula is known as the → binomial theorem.

See also: → binomial; → expansion.

A system introduced by Carl von Linné (1707-1778), the Swedish botanist, in which each organism is identified by two names. The first is the name of the genus (generic name), written with a capital letter. The second is the name of the species (specific name). The generic and specific names are in Latin and are printed in italic type. For example, human beings belong to species
Homo sapiens.

See also: → binomial; → nomenclature.

A rule for writing an equivalent expansion of an expression such as (a + b)ⁿ without having to perform all multiplications involved. → binomial expansion. The general expression is

(a + b)ⁿ = &Sigma (n,k)a^kb^{n - k}, where the summation is from k = 0 to n, and (n,k) = n!/[r!(n - k)!]. For n = 2, (a + b)² = a² + 2ab + b².

Historically, the binomial theorem as applied to (a + b)² was known to Euclid (320 B.C.) and other early Greek mathematicians. In the tenth century the Iranian mathematician Karaji (953-1029) knew the binomial theorem and its accompanying table of → binomial coefficients, now known as → Pascal’s triangle. Subsequently Omar Khayyam (1048-1131) asserted that he could find the 4th, 5th, 6th, and higher roots of numbers by a special law which did not depend on geometric figures. Khayyam’s treatise concerned with his findings is lost. In China there appeared in 1303 a work containing the binomial coefficients arranged in triangular form.
The complete generalization of the binomial theorem for all values of n, including negative integers, was established by Isaac Newton (1642-1727).

See also: → binomial; → theorem.

Etymology (EN): Bio-, Gk., from bios “life,” from PIE base *gweie- “to live;” cf. O.Pers./Av. gay- “to live,” Av. gaya- “life,” gaeθâ- “being, world, mankind,” jivya-, jva- “aliving, alive,” Skt. jivah “alive, living;” Mid.Pers. zivastan “to live,” zivik, zivandag “alive, living,” L. vivus “living, alive,” vita “life,” O.E. cwic “alive,” E. quick, Lith. gyvas “living, alive.”

Etymology (PE): Zist “life, existence,” from zistan “to live,” Mid.Pers. zivastan “to live,” zivižn “life,” O.Pers./Av. gay-, as explained above.

A common branch of astronomy and biology dealing with the study of life throughout the Universe; synonymous with → astrobiology and → exobiology.

Etymology (EN): Bioastronomy, from → bio- + → astronomy.

Etymology (PE): Zistaxtaršenâsi, from zist-, → bio-,

• axtaršenâsi, → astronomy.

The → variety of → plant and → animal → species in a particular → environment.

See also: → bio-; → diversity.

The retrieval and analysis of biochemical and biological data using mathematics and computer science, as in the study of genomes (Dictionary.com).

See also: → bio-; → informatics.

An expert or specialist in biology.

See also: Biologist, from → biology + → -ist.

The study of living organisms and their interactions with the non living world.

See also: Biology, from → bio- + → -logy.

The production and emission of light by a living organism as the result of a chemical reaction (→ chemiluminescence). In other words, bioluminescence is chemiluminescence from living organisms. It is widespread in the marine environment, but rare in terrestrial and especially freshwater environments.

See also: → chemi-; → luminescence.

A biologic feature that is measured and evaluated as an indicator of normal biological processes, pathogenic processes, or pharmacological responses to a therapeutic intervention. For example, prostate specific antigen (PSA) is a biomarker for cancer of the prostate.

See also: → bio-; → marker.

A specialist in → biophysics.

See also: → bio-; → physicist.

The science that deals with biological structures and processes involving the application of physical principles and methods.

Etymology (EN): Biophysics, from → bio- + → physics.

Etymology (PE): Zistfizik, from zist-, → bio- + fizik→ physics.

A substance or phenomenon whose presence in an object such as a → meteorite or an → exoplanet indicates the existence of life.

See also: → bio-; → signature.

The part of a planet or moon within which life can occur. It may include the crust, oceans, and atmosphere.

Etymology (EN): Biosphere, from → bio- + → sphere.

Etymology (PE): Zistsepehr, from zizt-, → bio-, + sepehr→ sphere.

The → magnetic field due to → electric current flowing in a long straight conductor is directly proportional to the current and inversely proportional to the distance of the point of observation from the conductor. The law is derivable from → Ampere’s law, but was obtained experimentally by the authors.

Etymology (EN): Named after the French physicists Jean-Baptiste Biot (1774-1862) and Félix Savart (1791-1841); → law.

Ecology: The environment consisting of living organisms, which interact with each other and with their non-living surroundings.

See also: Biotic adj. of → biotics; of → environment.

The science concerned with the functions of life, or vital activity and force.

See also: From biotic, from Gk. biotikos “of or pertaining to life,” from → bio- + -tic a suffix equivalent in meaning to → -ic.

Having two poles; having two opposite main structures or components.

Etymology (EN): Bipolar, from → bi- + → polar, from → pole.

Etymology (PE): Doqotbi, from do-, → bi-, + qotbi,
→ polar, from qotb, → pole.

Same as → bipolar outflow.

See also: → bipolar; → flow.

One of two beams of high-temperature, ionized gas ejected in two opposite directions associated with a → protostar. The collimated jets, a consequence of the → accretion process, can extend over distances of several
→ light-years.

See also: → bipolar; → jet.

An interstellar cloud of ionized gas with two main lobes which lie symmetrically on either side of a central star. The bipolar shape is generally due to the ejection of material by the central star in opposing directions.

Etymology (EN): → bipolar; → nebula.

A flow of gaseous material in two opposite directions emanating from protostellar regions or from → evolved stars
during the early post-→ AGB evolution. In protostellar regions → molecular outflows are pushed by → bipolar jets.

Etymology (EN): → bipolar; → outflow.

Etymology (PE): Ostacân, → outflow; doqotbi→ bipolar.

A property of some crystalline materials (e.g. calcite, quartz) which have different indices of refraction associated with different crystallographic directions. Therefore, the crystal splits incident transmitted light into two beams, each polarized perpendicularly to the other. Also called double refraction.

Etymology (EN): Birefringence, from → bi- + refringence, from L. refringere “to break up,” from → re- “back” + combination form of frangere “to break.”

Etymology (PE): Došekast, from do- “two,” → bi- + šekast “breaking,” from šekastan “to break up,” Mid.Pers. škastan, Av. skand- “to break.”

Of or relating to → birefringence.

A type of narrow-band filter that uses the birefringence to produce selective absorption of polarized light.

See also: → birefringent; → filter.

Empty space undergoing → vacuum birefringence.

See also: → birefringent; → vacuum.

For a four dimensional → space-time, the → Schwarzschild metric is the only solution of → Einstein’s field equations which describes the gravitational field created by a spherically symmetrical distribution of mass. The theorem implies that the gravitational field outside a sphere is necessarily static, and that the metric inside a spherical shell of matter is necessarily flat.

See also: The theorem was first demonstrated in 1923 by George David Birkhoff (1884-1944), an American mathematician; → theorem

1. The act or process of bearing or bringing forth offspring.
   

2. The act or fact of being born.
   

3. The coming into existence of something.

Etymology (EN): M.E. byrthe; O.E. gebyrd; cf. O.H.G. giburt,
Ger. geburt; PIE *bhrto, from *bher- “to bear.”

Etymology (PE): 1) Zâymân, from zây present stem of zâyidan, zâdan “to give birth”
(Mid.Pers. zâtan; Av. zan- “to bear, give birth to a child, be born,” infinitive zazāite, zāta- “born;” cf. Skt. janati “begets, bears;” Gk. gignesthai “to become, happen;” L. gignere “to beget,” gnasci “to be born;” PIE base *gen- “to give birth, beget”) + -mân a suffix forming verbal nouns, → organization.

2. Zâd, past stem of zâyidan, zâdan, as above; zâdmân with prefix -mân, as above.
   

3. Zâyeš, from zây, present stem of zâyidan + -eš suffix of verbal nouns, → -tion.

In star formation models, the population of binary components formed via random pairing of stars distributed according to the → canonical IMF.

See also: → birth; → binary; → population.

The path in the → H-R diagram that continuously → accreting stars follow. Stars on the birthline are difficult to observe, because they are hidden by the → accretion disk and a dense → cocoon of → interstellar matter. The star becomes visible once a sufficient amount of the surrounding matter has been dispersed. It follows that the young starsare observed generally between the birthline and the → zero-age main sequence (ZAMS). Before hydrogen ignition, gravitational contraction and deuterium burning are the main energy sources for the star. The localization of the birthline depends on the deuterium → accretion rate.
First introduced by R. Behrend & A. Maeder, 2001 (A&A 373, 190).

See also: → birth; → line.

A method devised by the Iranian astronomer Biruni (973-1048) to measure the Earth radius, using trigonometric calculations. In contrast to foregoing → Eratosthenes’ method and → Mamun’s method, which required expeditions to travel long distances, Biruni’s method was on-site. He carried out the measurement when he was at Nandana Fort (at the southern end of the pass through the Salt Range, near Baghanwala in the Punjab).

He first calculated the height of a hill (321.5 m). To do this he used the usual method of observing the summit from two places in a straight line from the hill top. He measured the distance, d, between the two places and the angles θ₁ and θ₂ to the hill top from the two points, respectively. He made both measurements using an astrolabe. The formula that relates these angles to the hill height is: h = (d. tan θ₁ . tan θ₂) / (tan θ₂ - tan θ₁).
He then climbed to the hill top, where he measured the → dip angle (θ), that is the angle of the line of sight to the horizon. He applied the values he obtained for the dip angle and the hill’s height to the following trigonometric formula to derive the Earth radius:

R = h cosθ / (1 - cos θ).

The result for the Earth radius was 12,851,369.845 cubits (or 6335.725 km, using favorable conversion units). Despite the fact that the method is very ingenious, such a precise value is only by chance, because of several drawbacks: The plane was not perfectly flat to serve as the smooth surface of the sea. A measuring instrument more accurate than the alleged 5 arc minutes was needed. And the method suffered from the → atmospheric refraction (See, e.g., Gomez, A. G., 2010, Journal of Scientific and Mathematical Research).

See also: Abu Rayhân Mohammad Biruni (973-1048 A.D.), one of the greatest scholars of the medieval era, was an Iranian of the Khwarezm region; → method.

A white, crystalline, brittle metallic chemical element with a pinkish tinge; symbol Bi. → Atomic number 83; → atomic weight 208.9804; → melting point 271.3°C; → boiling point about 1,560°C; → specific gravity 9.75 at 20°C; → valence +3 or +5.

Bismuth is the most → diamagnetic of all metals. Its
thermal conductivity is lower than any metal, except → mercury.
There is only one naturally occurring → isotope of bismuth, ²⁰⁹Bi. Bismuth is used in a number of very different applications, chiefly in bismuth alloys, and in pharmaceuticals and chemicals.

See also: From Ger. Bismuth, Wismut, Wissmuth, probably from weisse Masse
“white mass,” indicating how the element appears in nature.

Of or relating to a → leap year or to the extra day falling in a leap year.

Etymology (EN): L.L. bissextlis (annus) “year containing an intercalary day,” from bisextus, from bis “twice, two, doubled” + sextus “sixth,” because in the → Julian calendar the sixth day before the Calends of March was doubled every four years. Same as → leap and
→ intercalary day.

Etymology (PE): Andarheli, of or relating to andarhel→ intercalation.

Note. Some authors have used the Pers. term behizaki for “bissextile,” however this is problematic as far as the
calendar history is concerned. The term comes from Mid.Pers. vihezagig or vihezakik “movable,” which was the name of the additional whole month of 30 days which Iranians used in their calendar system in the Sasanid era. In fact they did not use an intercalary day every four years, but instead added a whole month after a period of 120 years. The operation was called vihezag. The term originates from Mid.Pers. vihez-
“to move, progress,” vihezag(k) “movement, progression.” Therefore, this dictionary cautions against equating behizaki and bissextile. → embolismic month.

The condition in which a physical system is capable of assuming either of two stable states.

See also: → bi-; → stable.

An abrupt discontinuity in the → stellar wind properties of → hot stars near → effective temperatures about 21,000 K and 10,000 K, corresponding to O9.5-B3 supergiants (Castor et al. 1975, ApJ 195, 157; Lamers et al., 1995, ApJ 455, 269). At these temperatures the → terminal velocity of the wind drops steeply by about a factor two and the → mass loss rate increases steeply by about a factor three to five, when going from high to low temperatures. Bistability jump is related to the degree of ionization in the wind. With a little drop in the temperature, the dominant driving element (Fe) will recombine to lower ionization stages which produces a lower terminal velocity and a relatively high density in the wind. → wind momentum. Additional bistability jumps may occur at higher temperatures where CNO may provide the dominant line driving, especially for low metallicity stars (Vink et al. 2001, A&A 369, 574). However, a recent study using a larger sample finds that there is a gradual decline in the wind terminal velocities of early B supergiants and not a “jump” (Crowther et al. 2006, A&A 446, 279).

See also: → bistability; → jump.

The mechanism that accounts for the → bistability jump.

See also: → bistability; → mechanism.

A contraction of → binary digit, either 0 or 1.

See also: Bit, from binary + digit

1. Chem.: Having a → valence of two or having two valences.
   

2. → bivalent logic.

See also: → bi-; → valence.

A logical system, such as → classical logic,
in which every declarative sentence expressing a → proposition has exactly one → truth value, either → true or → false. Bivalent logic is just a sub-set of a more powerful type of logic known as → fuzzy logic. See also → polyvalent logic.

See also: → bivalent; → logic.

A member of a family of → quasars, or extragalactic → Active Galactic Nuclei, which displays
a high radio emission and/or important optical variability over a short period of time. BL Lac objects appear star-like but their spectrum is flat, and partially polarized. Also called → blazars.

See also: BL Lac, from object BL in the constellation → Lacerta (BL Lacertae). The reason for this terminology is that it was originally thought to be an irregular variable star in our Galaxy; hence its variable star designation. In the 1970s the “star” was identified with a bright, variable → radio source and a very faint galaxy; → object.

A mechanism aimed at explaining the → disruption of a → binary system. As one component loses mass dramatically, the resulting loss of → gravitational attraction changes the orbit of, or ejects completely, the → companion star.

Etymology (EN): Adriaan Blaauw (1914-2010), 1961, Bull. Astron. Inst. Netherlands 15, 265; → mechanism.

Very dark in color;
absorbing light without reflecting any of its various rays.

Etymology (EN): Black, from O.E. blæc “black,” from P.Gmc. *blak-, from PIE *bhelg- “to shine, flash, burn” (cf. Gk. phlegein “to burn, scorch,” L. flagrare “to blaze, glow, burn,” fulgur “lightning”).

Etymology (PE): Siyâh or siyah, from Mid.Pers. siyâ, siyâk, siyâvah “black,” Av. sâma-, sayâva- “black, dark,” cf. Skt. syama-, syava- “black, brown,” Gk. skia
“shadow.”

The appearance of a band linking the solar limb to the disk of a transiting planet (Venus or Mercury) near the point of internal tangency. This effect increases the uncertainty in measuring the period from when the planet fully enters the solar disk to when it begins to depart. Historically, the black drop phenomenon limited the accuracy of the determination of the Astronomical Unit and the scale of the Solar System in the 18th and 19th centuries. While there have been many proposed theories over the years, the true cause of the effect was revealed during a transit of Mercury in 1999, which was observed by the NASA’s TRACE satellite. Two effects could fully explain the black drop: the inherent blurriness of the image caused by the finite size of the telescope (→ point spread function), and an extreme dimming of the Sun’s surface just inside the apparent outer edge (→ limb darkening). See Schneider et al. 2004, Icarus 168, 249.

See also: → black; → drop.

A fantastically → compact object, predicted by the theory of → general relativity, whose → gravity is so powerful that not even light can escape from it. A black hole forms when matter → collapses to → infinite  → density, producing a → singularity of infinite → curvature in the fabric of → space-time. Each black hole is surrounded by an → event horizon, at which the → escape velocity is the → speed of light. The → Schwarzschild radius for the Sun is about 3 km and for the Earth about 1 cm. There is observational evidence for black holes on a remarkable range of scales in the Universe: → stellar black hole, → intermediate-mass black hole, → primordial black hole, → mini black hole, → supermassive black hole, → Schwarzschild black hole, → Kerr black hole.

See also: Historically, the Newtonian concept of such a celestial body appeared at the end of the 18th century when light was shown to have particle characteristics. In fact the English geologist John Mitchell (1724-1793) and French mathematician and astronomer Pierre Simon Laplace (1749-1827), independently, suggested that regions of space, where gravitational attraction was so strong that not even light could escape, may exist in the Universe. However, the term black hole was coined in 1967 by the Princeton physicist John A. Wheeler (1911-2008); → black; → hole.

A → binary system in which each component is a → black hole. The binary’s evolution
can be divided into three stages: → inspiral, → merger, and → ringdown.

See also: → black; → hole; → binary.

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

See also: → black; → hole; → candidate.