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Spectral type for blue stars, such as Rigel, Spica, and Regulus. B-type stars are hot, but even hotter blue stars are designated spectral type O. [C95]

B Band

See Fraunhofer lines. [H76]


See Einstein coefficient. [H76]

B Galaxy

In Morgan's Classification, a barred spiral.


A triplet of spectral lines of neutral magnesium lambdalambda 5167-5184.

B Star

Stars of spectral type B are blue-white stars with surface temperatures of about 11,00-28,000 K, whose spectra are characterized by absorption lines of neutral helium which reach their maximum intensity at B2. The Balmer lines of hydrogen are strong, and lines of singly ionized oxygen and other gases are also present. Examples are Rigel and Spica. [H76]

Ba or Barium Stars

Late type giants (G2 to K4) with a very strong BaII 4554 line. Main sequence stars with strong BaII lines have also been discovered recently. [JJ95]

Ba II Stars (also called Barium Stars)

Peculiar low-velocity, strong lined red-giant stars of spectral types G, K, and M, with abnormally large abundances of heavy s-process (but not r-process) elements. They are usually regarded as old disk stars of ~ 1-2 Msmsun. [H76]

Baade's Window

A clearing in the dust clouds of the constellation Sagittarius where astronomers can view stars in the Galactic bulge. Baade's window lies four degrees south of the Galactic center, so an observer's line of sight passes within 1800 light-years of the Milky Way's center. [C95]

Back Focal Length

The distance between the last surface of a compound optical system and the focal plane of the system. This distance may be quite different from the actual focal length. [McL97]

Background Count

Unwanted counts due to background noise that must be subtracted from an observed number of counts in an experiment where atomic or nuclear particles coming from a source are being enumerated. [H76]

Background Noise

All the interference effects in a system which is producing, measuring, or recording a signal. Natural background noises arise from (a) galactic noise (synchrotron radiation), (b) thermal noise (receiver and isotropic background noise), (c) quantum noise (spontaneous emission or shot noise), and (d) star noise. [H76]

Background Radiation

Or background blackbody radiation, is the isotropic residual microwave radiation in space left from the primordial big bang. At a wavelength of 7 cm it represents a temperature of about 3K. [A84]. See Cosmic Background Radiation.


Scattering of radiation (or particles) through angles greater than 90° with respect to the original direction of motion. [H76]

Back Warming

Heating of deeper layers in a star due to overlying opacity. [H76]

BAD Nuclei

Balmer-Absorption Dominated (BAD) Nuclei. Presumably post-starburst nuclei seen sufficiently long after the starburst event that the original OB stars have evolved, but recently enough that the A supoergiants are still in evidence. The spectra are dominated by A-typer supergiants with charcteristically strong Balmer absorption lines. (ref. Taniguchi et al., 1996, ApJ, 467, 21)

Bahcall-Soneira Model

A model for the Galaxy first published by John Bahcall and Raymond Soneira in 1980. In its original form, it sought to reproduce star counts in different parts of the sky by employing only a (thin) disk and a halo; it had no thick disk. [C95]

Bailey Types

A classification of RR Lyrae stars according to the shape and amplitude of their light variation (a, b, and c, although today types a and b are usually combined). The c-type stars have the smallest amplitude. (RRa: sharp rise to maximum; slow fall to minimum. RRc: Rise and fall equally long.) [H76]

Baily's Beads

Small "beads" of sunlight (the "diamond ring" effect) which shine through the valleys on the limb of the Moon in the instant before (or after) totality in a solar eclipse. Named after the English astronomer Francis Baily who first observed them in 1836. [H76]

Baldet-Johnson Bands

Spectral bands of the CO+ radical. [H76]

Baldwin Effect

The empirical observation that the equivalent width of the CIV 1549 line anti-corellates with increasing luminosity for high-luminosity quasars.

Ballik-Ramsay Bands

Spectral bands of the C2 radical in the near infrared (0-0 at 1.7625 µ). [H76]

Balloon Metaphor

An analogy in which the expansion of the universe is likened to the surface of an expanding balloon with dots painted on it. Each dot represents a galaxy. From the point of view of any one dot, all the other dots are moving away from it, and this view is the same from any dot. The analogy applies only to the surface of the balloon, not to its interior or exterior. Since the surface is 2-dimensional and actual space is 3-dimensional, the geometry of the analogy is one dimension lower than that of the actual universe. Furthermore, the analogy applies only to closed universes, since the surface of a balloon has a finite size. (See closed universe.) [LB90]

Balmer Formula

A formula which represents the wavelengths of the various spectral series of hydrogen: lambda- = R(m-2 - n-2). The Balmer series is obtained by putting m equal to 2: the Lyman series by putting m equal to 1 (see Rydberg formula). [H76]

Balmer Jump

The sudden decrease in the intensity of the continuous spectrum at the limit of the Balmer series of hydrogen at 3646 Å, representing the energy absorbed when electrons originally in the second energy level are ionized. (also called Balmer Discontinuity ) [H76]

Balmer Series

(a) A series of lines in the spectrum of radiation emitted by excited hydrogen atoms. The lines correspond to the atomic electrons falling into the second lowest energy level, emitting energy as radiation. The wavelengths (lambda) of the radiation in the Balmer series are given by:

1 / lambda = R(1 / 22 - 1 / n2)

where n is an integer and R is the Rydberg constant. See also spectral series. [DC99]
(b) The spectral series associated with the second energy level of the hydrogen atom. The series lies in the visible portion of the spectrum. The transition from the third level to the second level yields the red Halpha emission line at 6563 Å'; Hbeta is at 4861 Å; Hgamma, at 4342 Å; Hdelta, at 4101 Å. (Deuterium Halpha is 1.785 Å short-ward of hydrogen Halpha.) He II Halpha is at 1640 Å. [H76]


Asteroid 324 (a = 2.80 AU, e = 0.36, i = 11°.2). It is among the darkest known surfaces in the solar system. It is the only minor planet known to have an albedo less than 5 percent, and some astronomers think it may be larger than Pallas. Mean opposition magnitude +11.41, absolute magnitude +8.14. Rotation period 8h(?). Meteorite class: carbonaceous chondrite. [H76]

Band (molecular)

A series of closely spaced, often unresolved, emission or absorption lines found in the spectra of molecules. Each line represents an increment of energy due to a change in the rotational state of the molecule. [H76]

Band Head

The conspicuous sharp boundary which usually occurs at the head of a molecular band and which fades gradually toward either longer or shorter wavelengths, depending on the quadratic relation between frequency and rotational quantum number. [H76]

Band Spectrum

A spectrum that appears as a number of bands of emitted or absorbed radiation. Band spectra are characteristic of molecules. Often each band can be resolved into a number of closely spaced lines. The bands correspond to changes of electron orbit in the molecules. The close lines seen under higher resolution are the result of different vibrational states of the molecule. [DC99]

Band Width

An indication of the range of frequencies, or wavelengths (wave band) that:

  1. an antenna can receive efficiently
  2. a radio receiver or amplifier can efficiently handle
  3. exist in a radio transmission above and below the carrier-wave frequency
Bandpass Filter

A device used in radio astronomy for suppressing signals of unwanted frequencies without appreciably affecting the desired frequencies. [H76]


(a) The width of the portion of the electromagnetic spectrum (the range of frequencies) that is permitted to pass through an electronic device (measured in cycles per second). [H76]
(b) Refers to either a wavelength interval (or band) or a frequency interval. [McL97]


(a) The absolute cgs unit of pressure equal to 106 dyn cm-2. [H76]
(b) A unit of pressure defined as 105 pascals. The millibar (mb) is more common; it is used for measuring atmospheric pressure in meteorology. [DC99]


A dense, low-melting reactive metal. The electronic configuration is that of xenon with two additional outer 6s electrons. Barium metal is used as a `getter', i.e., a compound added to a system to seek out the last traces of oxygen; and as an alloy constituent for certain bearing metals. Metallic barium has the body-centered cubic structure.
Symbol: Ba; m.p. 729°C; b.p. 1640°C; r.d. 3.594 (20°C); p.n. 56; r.a.m. 137.327. [DC99]

Barium Stars

Peculiar low-velocity, strong lined red-giant stars of spectral types G, K, and M, with abnormally large abundances of heavy s-process (but not r-process) elements. They are usually regarded as old disk stars of ~ 1-2 Msmsun (also called Ba II stars) [H76]


(a) Symbol: b A unit of area defined as 10-28 square meter. The barn is sometimes used to express the effective cross-sections of atoms or nuclei in the scattering or absorption of particles. [DCC99]
(b) A unit of area equal to 10-24 cm2 used in measuring cross sections. [H76]
(c) The unit of nuclear cross-section equal to 10-28m2. The radius of a nucleus is of the order 10-14m, which leads to a figure of the order of 10-28m2 for the cross-sectional area. The unit gives a measure of the probability of a particular nuclear process (e.g. absorption, fission, scatter) occurring when nuclear projectiles pass through matter by giving the effective target area of the bombarded nucleus for that particular area. The cross-sectional areas vary from 10-4 barn for certain nuclear reactions to 10-11 barn for electron bombardment experiments. The name barn was made up by H. G. Holloway and C. P. Baker in Chicago in 1942[9]. They used it originally as a code word to describe the probability of certain reactions. It is possible that the authors had the idea the barn provided a target sufficiently large so that even a temporary war-time rifleman could not miss it. The unit was recommended by IUPAP in 1960 but in July 1976, the Council of Ministers of the EEC proposed the abolition of the barn and this has now become effective by writing the area as 100fm2. [JM92]

Barnard's Loop

A huge nebular shell around the central portion of Orion. [H76]

Barnard's Satellite

also known as Amalthea.

Barnard's Star

(a) Star that had - until 1968 - the greatest known proper motion of any. Seen from the Earth it moves just over 10 seconds of arc per year, but even this is deceptive because it is approaching Earth at a rate estimated to be more than 100 km/sec. It is also one of the very few stars known (fairly conclusively) to have planets. [A84]
(b) Discovered in 1916 by Edward Emerson Barnard, this red dwarf lies 5.96 light-years away and is the second nearest star system to the Sun. Barnard's Star has the largest proper motion of any star, 10.3 arcseconds per year, which means that the star moves the equivalent of a lunar diameter every 180 years. [C95]
(c) A faint M5 V optical binary (period about 25 years) about 1.83 pc distant (pi = 0'.548) in the constellation of Ophiuchus. It has the largest proper motion known (10'.25 per annum). Long-term observations of its light curve suggest a possible third component with a mass about 1.2 that of Jupiter, although this observation has been challenged. [H76]

Barometric Law

The density distribution of gas in a plane-parallel, isothermal layer acted on by a uniform gravitational field: rho(z) = rho(0) exp ( - mg / kT). [H76]

Barotropic Bas

A gas in which the pressure is a function of the density only. [H76]

Barred Spiral Galaxy

(in Hubble's classification, SB: in Morgan's classification, B) A spiral galaxy whose nucleus is in the shape of a bar, at the ends of which the spiral arms start. About one-fifth of spiral galaxies are barred spirals. First categorized by Hubble in 1936. [H76]


The center of mass of a system of bodies; e.g., the center of mass of the solar system or the Earth-Moon system. [S92]

Barycentric Dynamical Time (TDB)

The independent argument of ephemerides and equations of motion that are referred to the barycenter of the solar system. A family of timescales results from the transformation by various theories and metrics of relativistic theories of Terrestrial Dynamical Time (TDT). TDB differs from TDT only by periodic variations. In the terminology of the general theory of relativity, TDB may be considered to be a coordinate time. (See dynamical time.) [S92]


(a) The process by which the Universe's net baryon number was generated. This explains why the Universe is made predominantly of baryons and not antibaryons. [CD99]
(b) The production of baryons, a type of subatomic particle. It is believed that baryons were produced in the early universe. (See baryons.) [LB90]
(c) The creation of matter in excess of antimatter in the early universe. Only the relatively few unmatched matter particles survived to make up all subsequent structures. [HH98]
(d) The hypothetical process by which the early universe acquired a large positive baryon number. [G97]


(a) A particle made of three quarks. Baryons are a subset of the hadrons. Neutrons and protons are baryons, as are the heavier but shorter-lived particles such as the lambda and the omega. [C97]
(c) The generic term for any strongly-interacting particle with half-integer spin in units of hbar (e.g. the proton, neutron and all their more massive excited resonance states). [CD99]
(d) nuclear particle. e.g. proton, built from three quarks. [D89]
(e) Heavy subatomic particle that interacts strongly in nuclei and that has a half-integral spin. Baryons obey the Fermi-Dirac statistics and include the nucleons and the so-called strange particles. Formerly, baryons heavier than the neutron were called hyperons; this term is seldom used today. All free baryons heavier than the proton are unstable and decay into end products, one of which is a proton. [H76]
(f) A fermionic particle consisting of three quarks. The most important baryons are the proton and the neutron. [HH98]
(g) Protons and neutrons, which comprise most of the mass of ordinary matter, as well as a number of short-lived particles such as the lambda, sigma and delta, are all called baryons. These particles have in common the fact that they are each composed of three quarks. [G97]
(h) A baryon is a composite particle made of three quarks, any three of the six. Protons and neutrons are baryons. [K2000]


Consisting of baryons - protons and neutrons - baryonic matter is "normal" matter. The Sun and the Earth are made of baryonic matter. [C95]

Baryon Catastrophe
Baryon Number

(a) The total number of baryons in the universe, minus the total number of antibaryons. An index, therefore, of the cosmic matter-antimatter asymmetry. [F88]
(b) The total number of baryons in a system, minus the number of antibaryons, is called the baryon number of the system. In particle physics experiments the total baryon number of a system is always found to be conserved, but our theories predict that baryon number would not be conserved at the extraordinarily high temperatures that prevailed in the early universe. [G97]
(c) A quantity assigned to elementary particles: quarks are assigned a baryon number 1/3, and antiquarks -1/3; protons and neutrons, as well as a number of unstable particles, are each composed of three quarks and hence have baryon number 1, while antiprotons and antineutrons have baryon number - 1: particles not composed of quarks, such as the electron or photon, have baryon number 0. Baryon number is conserved in all observations, but theories such as grand unified theories imply that it may not always be conserved, particularly at the very high energies achieved in the early universe. [D89]
(d) A property of an elementary particle, equal to +1 for a baryon and -1 for an antibaryon. Gauge bosons, leptons, and mesons have a baryon number of 0. The baryon number is conserved in all types of particle-particle interaction. [DC99]

Baryon Number Conservation

The principle that the number of baryons must remain the same in any nuclear reaction. [HH98]

Baryon-to-Photon Ratio

(See photon-to-baryon ratio.) [LB90]


The baud is a unit of telegraph signalling speed; one baud is equal to one pulse per second. The unit was proposed at an International Telegraph Conference in Berlin in 1927. It is named after J. M. E. Baudot (1845-1903), the French telegraph engineer who made the first successful teleprinter. [JM92]


Blue Compact Dwarf Galaxy


Blue Compact Galaxy


Bureau des Longitudes. [LLM96]

Be Stars

Irregular variables of spectral type B (or occasionally O or A) with hydrogen emission lines in their spectra. The Be phenomenon involves rapid stellar rotation, circumstellar shells, and mass loss. (also called Early-Type Emission Stars )[H76]

Be or B Emission Line Stars

Non-supergiant B-type stars, which have shown emission in at least one of the Balmer lines at some time. [JJ95]

B[e] Stars

Be stars exhibiting forbidden lines in emission. [JJ95]


One billion (109) electron volts. See GeV. [F88]


One way to learn more about particles is to cause them to collide with one another and see what happens. Beams of electrons and protons can be made by knocking apart hydrogen atoms and applying electric fields. Positrons and antiprotons don't exist naturally, because they annihilate as soon as they encounter an electron or proton; they can be made by hitting a target with energetic protons or electrons and then collected by placing magnets after the target, arranged so as to bend each kind of particle in a different path. Then bunches of them are accelerated to higher energies. When a particle hits a target, every kind of particle is made with a certain probability, so other beams of particles can also be made (neutrons, muons, kaons, neutrinos, etc.) by the judicious arrangement of magnets and material. [K2000]

Beam Efficiency

Fraction of the total received energy contained in the main beam of an antenna. (see Aperture Efficiency ). [H76]

Beat Cepheids

Dwarf Cepheids in which two or more almost identical periods exist which cause periodic amplitude fluctuations in their light curves. The "beat" period averages about 2 to 21/2 hours. [H76]

Becklin-Neugebauer Object

An unresolved infrared point source (color temperature about 600 K) in the Orion Nebula. It is the brightest infrared object known at lambda < 10 µ, and is not coincident with any distinctive optical or radio continuum feature. Probably a collapsing protostar of 5-10 Msmsun. Discovered in 1966. (also known as the BN Object ) [H76]

Becquerel (Bq)

The becquerel is the SI unit of radioactivity; 1 becquerel represents one disintegration, or other nuclear transformation, per second, i.e. 1 becquerel = 2.703 × 10-11 curies or 1 curie = 37 × 109 becquerel. It is named after A. H. Becquerel (1852-1908), the French physicist who discovered radioactivity in 1896. The unit was approved by the 15th CGPM in May 1975. [JM92]

Beehive Cluster

See Praesepe. [H76]


A number used mainly in English-speaking countries to express the ratio of two powers as a logarithm to the base ten. It is defined as N = log10 P1/P2, where N is the number in bels and P1 and P2 represent the values of the powers. The unit is named after the telephone pioneer Alexander Graham Bell (1847-1922). The unit was first used by the Bell telephone network in the USA in 1923 when it replaced the standard cable as the unit describing the attenuation on a telephone line; it was originally called the TU or transmission unit (q.v.) and was renamed bel in 1923; the name was given international recognition in 1928. The size of the bel makes it a somewhat inconvenient unit so the decibel is frequently used instead: this, as its name implies, is equal to 1/10 bel. In continental Europe the neper is used instead of the bel. [JM92]

Bell's Inequality

one of a family of inequalities concerning the probabilities of joint occurrence of certain events in the two well separated parts of a composite system, implied by any hidden variables theory which satisfies an appropriate locality condition. The first example was derived by J.S. Bell in 1964. [D89]

Bell's Paradox

A supposed paradox arising from theoretical work on quantum mechanics by the Irish physicist John Bell. The work concerns the interpretation of quantum mechanics put forward by Niels Bohr, who argued that quantum mechanics depended on probabilities and that particles had an indeterminate existence until they were observed. Einstein never accepted this idea - he believed that there was some underlying deterministic mechanism governed by so-called hidden variables.
As an attack on Bohr's theories he (with others) postulated a thought experiment known as the Einstein-Podolsky-Rosen experiment (or EPR experiment). One simple form of it is to think of a particle of zero spin decaying into two particles with spin, which fly apart. Because spin is conserved, the particles must have opposite values; if one has a spin `up' the other must have spin `down'. In the experiment, one waits until the particles are several meters apart and then measures the spin of one particle. One instantly knows the spin of the other (because it must be opposite). But according to Bohr, the spin is neither `up' nor `down' until it is measured but is in an indeterminate state. Einstein argued that this could not be the case. Otherwise, one particle would have to communicate instantly across space. In Einstein's interpretation, the spins would be determined at the time of decay of the original particle and would be governed by hidden variables.
In 1961 Bell proved a theorem, Bell's theorem, concerning measurements of spin in different directions for two particles. He showed that a certain set of inequalities (Bell's inequalities) would hold if hidden variables operated and Einstein was correct. If Bohr was correct, they would not hold.
The theorem opened the way for a real experimental test of the theories. In 1985 Alain Aspect did such an experiment in Paris, making simultaneous measurements on photons separated by 12 meters. The results of the Aspect experiment supported Bohr's interpretation of quantum mechanics rather than Einstein's.
The consequence of this is very mysterious (hence the 'paradox'). It seems that two particles can be a large distance apart and still be part of a single system with neither one state nor another but a superposition of both. This phenomenon is known as quantum entanglement. [DC99]

Bell's Theorem

the theorem that no hidden variables theory satisfying an appropriate locality condition can make statistical predictions in complete agreement with those of quantum mechanics. Specifically, there are situations in which quantum mechanics predicts a violation of Bell's inequality. [D89]

Bellatrix (gamma Orionis)

A B2 III star 80 pc distant. [H76]


A silvery radioactive transuranic element of the actinoid series of metals, not found naturally on Earth. Several radioisotopes have been synthesized.
Symbol: Bk; m.p. 1050°C; p.n. 97; r.d. 14.79 (20°C); most stable isotope 247Bk (half-life 1400 years). [DC99]

Bernoulli Effect

The relation between the pressure in a steadily flowing fluid, and its velocity. The pressure is less where the velocity is higher as, for example, where water flows through a narrower section in a pipe. The pressure that lifts an aircraft also depends on this effect. For horizontal flow, provided frictional resistance is negligible

p1 - p2 = 1/2 rho (v22 - v12)

where p1 is the pressure where the speed is v1, and p2 is the pressure where the speed is v2. rho is the density of the fluid. The principle is used in instruments for measuring the speed of flow, such as the Pitot tube. [DC99]

Bernoulli Probability

See binomial probability. [H76]

Bernoulli's Theorem

Along a streamline the total energy per unit mass (including the internal energy and the pressure head p/rho) of an element of fluid remains constant as it moves. [H76]


(a) Element with atomic number four. Beryllium is rare and fragile, and nuclear reactions in stars destroy it. Most and possibly all beryllium originated when cosmic rays smashed into heavier atoms in space and split them into lighter ones, such as beryllium. [C95]
(b) A light metallic element, similar to aluminum but somewhat harder. It has the electronic configuration of helium with two additional outer 2s electrons. It is used as an antioxidant and hardener in some alloys, such as copper and phosphor bronzes. Beryllium is extremely toxic.
Symbol: Be; m.p. 1278±5°C; b.p. 2970°C; r.d. 1.85 (20°C); p.n. 4; r.a.m. 9.012182. [DC99]

Bessel Equation

A linear second-order differential equation, the solutions to which are expressible in mathematical functions known as Bessel functions. [H76]


(a) Emission of an electron and an antineutrino (or a positron and a neutrino) by a radioactive nucleus by any one of several processes. e.g., the spontaneous beta-decay of a free neutron (n -> p + e- + nubar). The A-number is unchanged, but the Z-number is increased (or decreased) by 1. Beta-decay is a so-called weak interaction (q.v.). Since electrons of all energies (up to a certain maximum) are emitted in beta-decay, this process exhibits a continuous spectrum (unlike alpha-particle emission, which exhibits a line spectrum). [H76]
(b) Decay of a radioactive nucleus with production of an electron (beta-particle). The underlying process is the transmutation of a neutron into a proton with electron and neutrino produced as a consequence. This process is controlled by the weak interaction and was its first known manifestation. [D89]
(c) A type of radioactive decay in which a nucleus emits, for instance, an electron. The result is a nuclide with the same mass number but a proton number one greater (electron emission) than the original nuclide. An example of beta decay is:

13H rightarrow 23 He + e- + nu

The particles emitted in beta decay are beta particles. Streams of beta particles are beta rays or beta radiation. High-energy particles may penetrate metal sheets of mass/area a few gram/cm2, or tens of meters of air at STP. The lowest energy particles may be absorbed in a few millimeters of air.

Beta particles may have a range of energies up to a maximum value characteristic of the nucleus concerned. The total energy is constant; it is carried by the beta particle and an antineutrino emitted at the same time. In another type of beta decay, positrons are emitted. In such cases the excess energy is carried by a neutrino. An example is:

713 N rightarrow 613 C + e+ + nu

See also Alpha Decay. [DC99]


(a) An electron or a positron emitted from an excited nucleus when it returns to its ground state via beta-decay. [H76]
(b) Particles first discovered in radioactive beta decay - later identified as electrons. [CD99]


see spectral lines. [H76]

Beta Decay

(a) The process in which a neutron disintegrates into a proton, an electron, and an antineutrino. The escaping electron is sometimes called a beta ray. [LB90]
(b) The disintegration of an atomic nucleus, in which an electron (which was historically called a beta particle) and an antineutrino are emitted. Since the electron carries away one unit of negative charge, the final nucleus has a charge one greater than the initial nucleus. [G97]
(c) Spontaneous emission by a heavier element (such as uranium) of negatively charged electrons - beta particles. The result of this radioactive decay is that the original element is very gradually converted into another element. Beta particle emission may be simultaneous with alpha particle decay. [A84]

Beta Transformation

The transformation of a nucleus by beta decay. Also the decay of a neutron to a proton, an electron, and an antineutrino:

n rightarrow p + e- + nu

Betelgeuse (alpha Ori)

(a) A red supergiant star in the constellation Orion and the brightest red supergiant in Earth's sky. [C95]
(b) A red semiregular variable supergiant (M2 Iab) about 500 pc distant. Betelgeuse is also a strong infrared emitter - at 2 µ the brightest in the sky. [H76]

Bethe-Weizsäcker Cycle

(a) See proton-proton cycle. [A84]
(b) see carbon cycle. [H76]


A b-factory is a facility designed to produce and detect large numbers of b-quarks, at least 100 million a year. Planned b-factories are electron-positron colliders, but a proton collider could also be used if an appropriate detector could be made. The main goal of b-factories is to study CP violation. [K2000]

BF3 Counter

A proportional counter, filled with the gas boron trifluoride, designed to count neutrons. [H76]


An epic paper, published in 1957, by Margaret Burbidge, Geoffrey Burbidge, William Fowler, and Fred Hoyle, who described in detail how the stars had created nearly every element in the universe. [C95]

Bianchi Cosmology

A cosmology which, unlike the Friedmann cosmology, dispenses with the notion of isotropy and considers homogeneous spaces. The different forms of homogeneous cosmologies that are possible are classified, according to the structure parameters of the associated groups, into nine (or 10 if a special case is included) classes. [H76]


(a) A potential applied to an electrode in an electronic device to produce the desired characteristic. [DC99]
(b) The offset from zero in the input of an array detector. [McL97]
(c) A voltage applied across a detector. [McL97]

Biased Galaxy Formation

The theory that bright galaxies form preferentially from anomalously overdense perturbations in the early universe. [HH98]


A hypothesized feature of the condensation of galaxies out of a background medium of gas. In any theory of galaxy formation, galaxies form in the places where gas is bunched up and concentrated, like ripples on a pond. According to the assumption of biasing, galaxies form only where the mass concentrations are very large and do not form at all where the concentrations are moderate. [LB90]


Blocked Impurity Band. [LLM96]


Describing a lens with two concave faces. Compare biconvex. [DC99]


Describing a lens with two convex faces. Compare biconcave. [DC99]


a phenomenon whereby the number of solutions of certain type presented by a dynamical system changes abruptly, as one of the parameters defining the dynamics crosses a critical value. [D89]

Big Bang

(a) According to standard cosmology, the explosion that started the universe expanding 10 to 15 billion years ago. [C95]
(b) The initial point of creation. [C97]
(c) The state of extremely high (classically, infinite) density and temperature from which the universe began expanding. [HH98]

Big-Bang Model

(a) A model of the Universe which started with an initial singularity. The Friedmann model of a homogeneous, isotropic universe (composed of adiabatically expanding matter and radiation, as a result of a primeval explosion) is the standard example. [H76]
(b) An evolutionary model of cosmology in which the universe began about 10 billion years ago, in a state of extremely high density and temperature. According to this model, the universe has been expanding, thinning out, and cooling since its beginning. It is an observational fact that distant galaxies are all moving away from our own galaxy, as predicted by the big bang model. (See closed universe; flat universe; Friedmann models; open universe.) [LB90]
(c) Model of cosmic history in which the universe begins in a state of high density and temperature, both of which decrease as the universe expands. Less a theory than a school of theories that attempt to trace how the universe evolved. [F88]
(d) Theory originally proposed by Georges Lemaître but elaborated by George Gamow and the alpha- beta-hypothesis- gamma that the Universe began with the Big Bang, the superexplosion of all the matter now dispersing in the Universe. Since the nuclear physics involved has been explained, and various supporting evidence - notably helium abundance and the sources of radio emission - has been discovered, the theory is almost universally accepted (although at one time the steady state theory rivaled it in popularity). [A84]

Big-Bang Nucleosynthesis

The process, which took place between one second and 3-4 minutes after the beginning, in which the protons and neutrons of the primordial soup condensed to form the lightest atomic nuclei: Deuterium, Helium-3, Helium-4, and Lithium-7. See isotope and Lithium. [G97]

Big-Bang Theory

(a) The most widely accepted theory of the origin of the Universe. It asserts that the Universe began some 1010 years ago from a space-time point of infinite energy density (a singularity). The expansion of the Universe since that time is akin to the expansion of the surface of an inflating balloon: every point on the balloon's surface is moving away from every other point. So, microbes living on the surface see their two-dimensional world expanding, yet there is no center to the expansion which is everywhere uniform. [CD99]
(b) The theory, which has been generally accepted since the 1960s, that the universe began approximately 10 to 20 billion years ago in a state of enormous temperature and density and has since expanded and cooled to its present state. [D89]

Big Blue Bump

The flux densities of most Seyfert 1 galaxy and quasar continua have an average slope of -1 (i.e. fnu ~ nu-1) extending from the visual to a far-infrared turnover (around (80 um). Relative to this red "power law", there is generally an excess of flux in the blue and ultraviolet, which constitutes the so-called "Big Blue Bump".

Big Crunch

(a) The state of extremely high density and temperature into which a closed universe will recollapse in the distant future. [HH98]
(b) If the universe has a mass density exceeding the critical mass density, then gravity will eventually reverse the expansion, causing the universe to recollapse into what is often called the big crunch. See also closed universe. [G97]
(c) One hypothesized future for the universe in which the current expansion stops, reverses, and results in all space and all matter collapsing together; a reversal of the big bang. [G99]


Bureau International de l'Heure. [LLM96]


(a) For the purposes of cosmology, one billion is defined as an American billion, i.e. 1,000 million. [C97]
(b) The American billion, equal to one thousand million or 109. [F88]. The British billion is 1012.

Binary Galaxies

Two galaxies orbiting each other owing to their mutual gravitational attraction. [LB90]

Binary Stars

in binary stars, the two stars form a physically bound pair under their mutual gravitational attraction. The stars move in elliptical orbits about their common centre of mass. As many as 50% of all stars may be members of binary star systems. [D89]

Binary System

A system of two stars orbiting around a common center of gravity. Visual binaries are those whose components can be resolved telescopically (i.e., angular separation > 0'.5) and which have detectable orbital motion. Astrometric binaries are those whose dual nature can be deduced from their variable proper motion; spectroscopic binaries, those whose dual nature can be deduced from their variable radial velocity. At least half of the stars in the solar neighborhood are members of binary (or multiple) systems. (See photometric binaries; optical pairs.)

Binding Energy

(a) The energy required to break up a system. In particular, the binding energy of an atomic nucleus is the energy released in the formation of the nucleus. The most strongly bound nuclei are those with atomic weights between about 50 and 65 (the iron group). Lighter nuclei are less strongly bound because of their larger surface-to-volume ratios; heavier nuclei, because the effects of Coulomb repulsion increase with the nuclear charge. [H76]
(b) The energy equivalent to the difference between the mass of the nucleus and the sum of the masses of its constituent nucleons. An example of calculating the binding energy of 73Li, with 4 neutrons and 3 protons is shown.
A useful measure is binding energy per nucleon. In the example the binding energy per nucleon is 39.2501/7 = approximately 5.6 MeV. For most nuclei, binding energy lies between about 7 and about 9 MeV per nucleon, reaching a maximum of about 9 MeV for nuclei of mass number about 60. The difference in mass in the example (i.e. the mass equivalent to the binding energy) is the mass defect [DC99]

Calculation of Mass Defect and Binding Energy

 mass of neutrons
 (4 × 1.008 983 amu)    4.035 932 amu
 mass of protons
 (3 × 1.008 144 amu)    3.024 432 amu
total mass of
  constituents          7.060 364 amu
 mass of 7Li nucleus    7.018 222 amu
mass defect            0.042 142 amu
 binding energy
  (1 amu = 931.14 MeV)  39.240 MeV

On-chip binning. Combining data from a rectangular group of pixels on the CCD itself by appropriate clocking. An improvement in signal-to-noise ratio is achieved at the loss of some picture detail (or spatial resolution). [McL97]

Binomial Probability

The probability that a particular result will be obtained in a given number of trials. (also called Bernoulli Probability ) [H76]


The scientific study of life and living matter. [F88]


Bureau International des Poids et Mesures. [LLM96]

Biprism, Fresnel's

A glass prism with a large angle, used to produce two coherent (virtual) sources for light interference experiments. As with Young's double slit arrangement, the wavelength lambda of the incident monochromatic radiation is given by:

lambda = y d / D

where y is the fringe separation, d is the source separation, and D is the source-screen distance. The fringes obtained with this arrangement are brighter than those in Young's experiment. [DC99]
Biprism, Fresnel's


Also birefringence. Having two values of the index of refraction. See refractive index. [McL97]

Birefringent Crystal

A crystal that splits incident transmitted light into two beams, each polarized perpendicularly to the other. The effect (called birefringence or double refraction) is particularly well-known in calcite (Iceland spar). It depends on the angle of incidence relative to the crystal axes, along which the speed of the light differs. The ordinary ray obeys the laws of refraction: it is polarized perpendicularly to the crystal axis. The extraordinary ray does not obey the laws of refraction (in the usual sense); hence its name. The study of the polarization properties of crystals is of great significance in geology, for the identification of minerals. [DC99]

Birkhoff's Theorem

(a) Every centrally symmetric geometry which is free of mass-energy is static and identical up to a coordinate transformation with the geometry defined by the Schwarzschild metric. [H76]
(b) A theorem of general relativity that states that all spherical gravitational fields, whether from a star or from a black hole, are indistinguishable at large distances. A consequence of this is that purely radial changes in a spherical star do not affect its external gravitational field. [HH98]


A brittle pinkish metallic element. Bismuth is widely used in alloys, especially low-melting alloys. The element has the property of expanding when it solidifies.
Symbol: Bi; m.p. 271.35°C; b.p. 1560±5°C; r.d. 9.747 (20°C); p.n. 83; r.a.m. 208.98037. [DC99]


(a) In computer terminology, a shortened form for binary digit (0 or 1). [H76]
(b) The binary unit of information used in digital computers; it represents the smallest piece of addressable memory within a computer. The binary system of notation uses only two symbols, zero (0) and unity (1), to represent specific values. These two symbols are properly called the 0 bit and 1 bit but are commonly called bits. The name, which is derived from the first and last two letters of the phrase binary digit made its first appearance in the Bell System Technical Journal of July 1948. [JM92]


Broad Line Region

Blaauw Mechanism

A mechanism advanced to explain the disruption of a binary system by the decrease in the gravitational binding force when an ejected shell overtakes the secondary component. [H76]


(a) An idealized body that absorbs radiation of all wavelengths incident on it. It is a perfect absorber and therefore a perfect emitter. The radiation emitted by a blackbody depends only on the temperature of the blackbody. [Silk90]
(b) In astronomy, a body with ideal properties of radiation absorption and emission, against which less perfect actual stars and celestial objects can be measured. Black-body radiation has a continuous spectrum governed solely by the body's temperature: for any particular temperature there is a specific wavelength at which radiation emission is greatest. (This can be depicted - and used - graphically.) [A84]
(c) A perfectly absorbing (and perfectly emitting) body. [HH98]
(d) This is the name given to an object which absorbs all radiation incident on it. Such a body is also the most efficient emitter of radiation. The simplest model of a blackbody is a small hole in a matt-black, heated hollow chamber. Since radiation entering the hole has minimal chance of re-emerging, radiation leaving the hole will be characterized by the temperature of the chamber. The amount of radiant energy emitted by a blackbody at any wavelength depends only on the absolute temperature of the body, and is given mathematically by the Planck function. Certain astronomical sources are almost blackbody emitters, but most objects are not quite black enough and therefore have an emissivity of less than 100% of ideal. [McL97]

Blackbody Curve

Plot of energy level against wavelength for heat or other radiation emitted by an object capable of absorbing all the energy that strikes it. The curve has a pronounced hump that moves toward shorter wavelengths as the temperature increases. The cosmic background radiation, thought to consist of photons emitted during the big bang, conforms to a black-body curve. [F88]

Blackbody Radiation

(a) Radiation whose spectral intensity distribution is that of a blackbody in accordance with Planck's law. (sometimes called thermal radiation) [H76]
(b) A unique type of radiation whose spectrum and other properties are completely characterized in terms of a single quantity, temperature. Blackbody radiation is produced after a group of particles and photons have come into thermal equilibrium with each other, with every reaction between the particles balanced by the reverse reaction, so that the system as a whole has stopped changing. In this situation, all parts of the system, including the radiation, have come to the same temperature. Blackbody radiation would be produced, for example, inside an oven that is maintained at a constant temperature and in which the door has been left closed for a long time. (See photon; spectrum; thermal equilibrium.) [LB90].
(c) A special case of thermal radiation, emitted by a blackbody and characterized by thermal equilibrium of the photons. A blackbody spectrum is completely determined by the temperature of the emitter. [HH98]
(d) If a closed box made of any material is heated to a uniform temperature, the interior will become filled with electromagnetic radiation (photons) with an intensity and spectrum determined by the temperature alone, independent of the composition of the box. Radiation with this intensity and spectrum is called blackbody, or thermal radiation. The intensity and spectrum are determined by the criterion of thermal equilibrium; i.e., only for this intensity and spectrum will the absorption and emission of photons by the walls be in balance for each wavelength, so the intensity for each wavelength can be independent of time. The universe today appears to be permeated with blackbody radiation at a temperature of 2.73°K - the cosmic background radiation - which we interpret as a remnant afterglow of the heat of the early universe. [G97]

Black Dwarf

The final stage in the evolution of a star of roughly 1 Msmsun. It is a mass of cold, electron-degenerate gas, and can no longer radiate energy, because the whole star is in its lowest energy state. No black dwarfs have ever been observed. Also, an object (M < 0.085 Msmsun) that is not massive enough to achieve nuclear chain reactions. [H76]

Black Hole

(a) A region of space-time which cannot be seen by distant observers because light is trapped by a strong gravitational field. The boundary of this region is called an event horizon because it separates events (i.e. those in the hole) that cannot be seen from events outside the hole, which can. Black holes might form, for example, from the gravitational collapse of a massive star. When the star shrinks inside an event horizon, it will collapse without known limit, leaving the surrounding space empty, Thus the designation `hole'. Spherical black holes (without electric charge) are known as Schwarzschild black holes. Rotating holes are nonspherical: they are known as Kerr black holes. [D89]
(b) An object that is maximally gravitationally collapsed, and from which not even light can escape. [HH98]
(c) An object with such a strong gravitational field that even light cannot escape. Matter can fall into a black hole, but according to classical physics no matter or energy can leave it. (Hawking has used quantum theory to show that black holes emit blackbody radiation, but the effect is significant only for black holes much smaller than those that are expected to form by the collapse of stars, which have masses of several solar masses or more.) [G97]
(d) A mass that is sufficiently compact that not even light can escape its intense gravity. Thus it appears black from the outside. If the sun were compressed to a sphere about four miles in diameter, it would become a black hole. It is believed that some massive stars, after exhausting their nuclear fuel, collapse under their own weight to form black holes. [LB90]
(e) A singularity in space, surrounded by an event horizon, caused by the collapse of a small but massively dense star through the effects of its own increasing gravity. By the time the state of singularity is reached, the remnants of the star may be minimal, but the gravitational force is so strong it prevents even light from escaping. Black holes may form the "power centers" of galaxies, thus explaining infrared radiation detected in several galactic centres. The properties of matter entering a black hole are the theme of John Wheeler's no hair theorem. [A84]
(f) A gravitationally collapsed mass inside the Schwarzschild radius, from which no light, matter, or signal of any kind can escape. A black hole occurs when the escape velocity of a body becomes the velocity of light (2GM / R = c2). If an object with the mass of the Sun had a radius of 2.5 km, it would be a black hole. Black holes represent one of the possible endpoints of stellar evolution for stars very much more massive than the Chandrasekhar limit. [H76]

Black-Hole Entropy

The entropy embodied within a black hole. [G99]

Black-Hole Thermodynamics

The theory that permits a temperature and an entropy to be defined for black holes. [HH98]


(a) A highly variable active galaxy which, in general, displays no emission lines in its spectrum. [C97]
(b) A term collectively used to refer to Optically Violent Variables (OVVs) and BL Lac objects. All known blazars are radio sources [adapted from B.M. Peterson An Introduction to Active Galactic Nuclei, Cambridge University Press, (1997)].

Blaze Angle

The tilt of the facets or grooves of a diffraction grating. [McL97]

Blazed Grating

Diffraction grating so ruled that the reflected light is concentrated into only a few orders, or even a single order, of the spectrum. [H76]

BL Lac

BL Lacertae
An extreme form of active galactic nucleus in which the light is dominated by nonthermal continuum emission and there are no emission lines present in the spectrum, in contrast to the case of quasars. These active nuclei are highly variable in intensity and often are very highly polarzed. They are named after the brightest prototype of this class, the variable object BL-Lacertae. [D89]

Blocking Layer

An undoped layer in an extrinsic infrared detector which converts the action of the photoconductor to a behavior more like a photovoltaic detector. [McL97]


A method of coating lenses to reduce back-reflection from their surfaces. It involves destructive interference in the thin layer. Each such layer can completely prevent reflection at only one wavelength (lambda). lambda is four times the layer thickness (t). For best effects the coating medium used should have a refractive constant n = [(n1n2)]^1/2, where n1 and n2 are the refractive constants of the media on each side. Single-layer blooming is normally used to prevent reflection of yellow light; bloomed surfaces thus reflect reds and blues, and appear purple. Multilayer blooming is sometimes employed, but is very costly. [DC99]


Broad-line Radio Galaxy


Broad Line Seyfert 1

Dlue Dwarf, Blue Giant

High-temperature stars (as opposed to red stars). Blue giants are generally on or near the main sequence of the hertzsprung-russell diagram; blue dwarfs represent the very dense, but very small, near-final form of what was once a red giant. [A84]

Blue Edge

(of the RR Lyrae instability strip) The curve on the H-R diagram that is traced out by the maximum temperature at which a stellar model is unstable against small-amplitude pulsations as the luminosity is varied. The position of this borderline is a function of mass and composition. Iben defines the blue edge as that point where the growth rate for pulsation is zero. [H76]

Blue Giant

A giant star with spectral type O or B. [C95]

Blue Halo Stars

Hot stars that are in the horizontal-branch, post-horizontal-branch, and post-asymptotic branch phases of evolution. [H76]

Blue Haze

A condition in the Martian atmosphere which sometimes makes it opaque to radiation in the blue-violet end of the visible spectrum. [H76]

Blue Horizontal-Branch Stars

Population II stars (B3-A0) in the galactic halo, characterized by strong, sharp hydrogen lines and large Balmer jump, and very weak lines of all other elements (see also HZ stars). [H76]

Blue Shift

(a) The shift to the blue of an object's spectrum. A blueshift arises when an object moves toward us: its light waves get compressed and reduced in wavelength, so that the entire spectrum is shifted to shorter, or bluer, wavelengths. The greater a star's blueshift, the faster the star is moving toward us. A few galaxies also show blueshifts, the most famous being Andromeda, but most show redshifts, due to the expansion of the universe. [C95]
(b) The increase in frequency of electromagnetic radiation, brought about by the relative distance decreasing between the source and observer. [C97]
(c) A shift in the frequency of a photon toward higher energy. [HH98]
(d) If a star or galaxy is moving towards us, the radiation from the star or galaxy appears shifted towards shorter wavelengths, or towards the blue end of the spectrum. See Doppler shift. [G97]

Blue Stragglers

(a) Stars (in a cluster) which fall close to the cluster's extrapolated main sequence but which occur a few magnitudes above its turnoff point. [H76]
(b) Globular or open cluster stars, which are located on the main sequence but at such a place that they are disconnected from the cluster main sequence. [JJ95]

Blue Supergiant

A supergiant star with spectral type O or B. All blue supergiants are hot and young. Rigel, in the constellation Orion, is the best example. [C95]

BN Object

Becklin-Neugebauer Object
One of the first major discoveries of infrared astronomy. Eric Becklin and Gerry Neugebauer at the California Institute of Technology found a very luminous infrared source with no visible counterpart buried deep in the Orion Molecular Cloud. [McL97]

Bode's Law

(a) A prescription for calculating planetary distances: the distance to the nth planet is 0.4 + (0.3)n Astronomical Units. Bodes law works surprisingly well out to Uranus but then breaks down. [Silk90] (b) Or the Titius-Bode Rule, was first devised in 1772 and comprised the series 0 + 4/10, 3 + 4/10, 6 + 4/10, 12 + 4/10, 24 + 4/10 and so on, which was found to describe fairly accurately the distance in astronomical units of the then known planets from the Sun. After the discovery of Neptune (to which the "Law" does not apply at all), the Law was somewhat discredited, although later still, the positioning of Pluto (which corresponds approximately) made it seem possibly more than coincidental. [A84]

Bode's Relationship

Bode's relationship gives an approximate indication of the comparative distances of the planets from the sun. It is obtained by adding 4 to the appropriate numbers in the series 0, 3, 6, 12, 24... as shown in Table 2.

The relationship, considered by many to be coincidental, was promulgated by J. T. Titius (1729-1796) at Wittenberg in 1792 but was publicized by his fellow countryman, the astronomer J. E. Bode (1747-1826), after whom it is known today.

Neptune and Pluto were not observed until after the death of Bode.

 Table 2. Comparative distance from the

Planet Bode Average
Mercury 4 + 0 = 4 3.9 Venus 4 + 3 = 7 7.9 Earth 4 + 6 = 10 10.0 Mars 4 + 12 = 16 15.2 Asteroids 4 + 24 = 28 - Jupiter 4 + 48 = 52 52.0 Saturn 4 + 96 = 100 95.4 Uranus 4 + 192 = 196 198.0 Neptune 4 + 384 = 388 300.0 Pluto 4 + 768 = 772 394.0
Bohr Atom

The model of an atom whose electrons are pictured as describing "Keplerian" orbits about the central nucleus. [H76]

Bohr Magneton

(µo or µB)
Magnetic moment of an electron in the first Bohr orbit (µo = eh / 4pi mc). It is a unit representing the minimum amount of magnetism which can be caused by the revolution of an electron around an atomic nucleus. 1 Bohr magneton = 0.92 × 10-20 ergs per gauss. [H76]

Bohr Radius

(a0 = hbar2 / me2)
(a) The radius of the orbit of the hydrogen electron in its ground state (0.528 Å). The electron makes 6.6 × 1015 revolutions s-1 (velocity of electron 2.19 × 108 cm s-1). [H76]
(b) The radius of the first Bohr orbit of the hydrogen atom is approximately 53 × 10-12 m and it was proposed as a unit of length by Hartree in 1928[24]. [JM92]


A synthetic radioactive element first detected by bombarding a bismuth target with chromium nuclei. Only a small number of atoms have ever been produced.
Symbol: Bh; p.n. 107; most stable isotope 262Bh (half life 0.1s). [DC99]

Bok Globule

A compact, spherical dark nebula. Bok globules have characteristic radii of 103-105 AU. Estimates of their mass suggest that their density is too low for gravitational collapse. They tend to lie in regions of much dust but less gas than would be expected for star-forming regions. [H76]


Extremley bright meteor. (see meteor)


(a) An instrument for measuring small amounts of radiant heat or microwaves. It depends on the change in resistance of a piece of metal foil or a superconductor when it absorbs radiant energy. [DC99]
(b) A device for measuring the total amount of radiant energy received from a celestial object. [McL97]

Bolometric Absolute Magnitude (Mbol)

A measure of the total amount of energy radiated by a star at all wavelengths. Mbol of Sun = 4.72 mag. The fraction of total energy emitted by a very blue or very red star that lies in the visible range may differ from the total energy by 4 or 5 mag - i.e., only a few percent of the energy lies in the visible. [H76]

Bolometric Correction (B.C.)

The visual (or photovisual) magnitude minus the bolometric magnitude of a star. It is always a positive number. [H76]

Boltzmann Constant

Symbol: k
(a)The constant 1.380 54 x 10-23 J K-1, equal to the gas constant (R) divided by the Avogadro constant (NA). See also degrees of freedom. [DC99]
(b) The constant of proportionality relating the mean kinetic energy of an atom to its absolute temperature: k = 1.38 × 10-16 ergs per kelvin. [H76]
(c) The average translational kinetic energy of any gas molecule depends only on temperature and is linearly proportional to its absolute temperature given in kelvins. The constant of proportionality is the Boltzmann constant. It also equals the gas constant R per mole which is R(8.314512 J mol-1 K-1) divided by the Avogadro number NA(6.0221367 × 1023 mol-1), giving k = 13.80558 × 10-24 J K-1. [JM92]

Boltzmann Factor

The factor e-E / kT involved in the probability for atoms having an excitation energy E at temperature F. [H76]

Boltzmann-Saha Theory

A theory describing the distribution of atoms of partially ionized matter over possible excitation and ionization states, in the limit of low density (see Thomas-Fermi theory). [H76]

Bond Albedo

Fraction of the total incident light reflected by a spherical body. It is equal to the phase integral, multiplied by the ratio of its brightness at zero phase angle to the brightness it would have if it were a perfectly diffusing disk. [H76]

Bond, Covalent

The connection between two atoms, consisting of a shared orbital. [SEF01]

Bond, Ionic

The connection between two ions, consisting of electrostatic attraction (e.g., Na+ to OH-). [SEF01]

Boost Factor

The quantity Gamma = 1 / sqrt(1 - v2 / c2) in the special theory of relativity that relates measurements in two inertial frames. [HH98]

alpha Bootis

see Arcturus. [H76]

lambda Bootis Stars

A type of young (usually early A), weak-lined, metal-poor stars with low radial velocities. [H76]


A hypothesis about the nature of the elementary particles derived from S-matrix theory. It suggests that no particular particles are more elementary than any others but that they all arise out of each other in a democratic and self-consistent fashion. That is, the elementary particles pull themselves up by their own bootstraps. [P88]

Born Approximation

An approach to collision problems by using perturbation methods. In collisional excitation the Born approximation becomes valid when the incident energy is some 50 times larger than the excitation energy. [H76]

Born-Oppenheimer Approximation

An approximation that treats the motion of an electron under the attraction of two free nuclei by regarding the nuclei (because of their greater mass and consequent smaller velocities) as fixed. [H76]


(a) Element with atomic number five. It is rare and fragile, and nuclear reactions in stars destroy it. Most boron is created in space, by cosmic rays that smash into heavier atoms and split them. [C95]
(b) A hard rather brittle metalloid element. It has the electronic structure 1s22s22p1. Only small quantities of elemental boron are needed commercially; the vast majority of boron supplied by the industry is in the form of borax or boric acid. Natural boron consists of two isotopes, 10B (18.83%) and 11B (81.17%). These percentages are sufficiently high for their detection by splitting of infrared absorption or by n.m.r. spectroscopy.
Symbol: B; m.p. 2300°C; b.p. 2658°C; r.d. 2.34 (20°C); p.n. 5; r.a.m. 10.811. [DC99]

Bose Condensation

a phenomenon occurring in a system of bosons whose total number cannot change, at a temperature of the order of the degeneracy temperature, in which a finite fraction of all the particles begin to occupy a single one-particle state. [D89]

Bose Statistics

the form of statistics applicable to bosons. It favors configurations in which many particles occupy the same state. [D89]

Bose-Einstein Condensation

A phenomenon in which several thousand atoms of certain elements are able to combine to form a single entity (a superatom) at very low temperatures. The phenomenon is important in the theory of superfluids. [DC99]

Bose-Einstein Nuclei

Nuclei of even A-number (i.e., those with integral spin) (cf. Fermi-Dirac nuclei). Bose-Einstein nuclei do not obey the exclusion principle, and their ground state has zero angular momentum. [H76]

Bose-Einstein Statistics

The statistical rules for studying systems of identical bosons. It is assumed that (a) all identical particles are to be regarded as absolutely indistinguishable; and (b) any number of identical bosons can have the same set of quantum numbers in a given system. These rules were first introduced by Bose (1923) in his proof of Planck's radiation law, treating photons as quasi-particles. See Fermi-Dirac statistics, Maxwell-Boltzmann statistics. [DC99]


(a) A particle which does not obey Pauli's exclusion principle. It is denoted by an integer (or zero) spin. [C97]
(b) Any particle with integer spin: 0, hbar, 2hbar, etc. [CD99]
(c) Generic name (after Satyendra Bose) for a class of particles possessing whole number units of intrinsic spin, measured in terms of Planck's constant divided by 2pi. All known particles that are not bosons possess half-integer units of spin, and are called fermions. The particles associated with the transmission of forces - photons, gravitons, gluons, W and Z - are bosons. Certain assemblages of fermions, such as the helium-4 nucleus, in which the combined spin is an integer, behave as bosons. [D89]
(d) A class of elementary particles whose spin is an integer multiple of a fundamental quantized value. The major function of bosons is to mediate the fundamental forces. The best-known boson is the photon. [HH98]
(e) A subatomic particle whose spin is an integral multiple of hbar (cf. fermion). Bosons include the photons, the pions, the gravitons, and all Bose-Einstein nuclei. Boson number is not conserved. [H76]
(f) Elementary particles with integer spin that do not obey the Pauli exclusion principle. They include the photons and the W and Z particles, carriers of the electromagnetic and the electroweak forces respectively. [F88]
(g) Elementary particles that have integral spins. Force particles such as the photon, gluon, and vector bosons are all bosons. But note that there can also exist composite particles formed out of collections of fermions such as a helium atom-which act collectively as bosons. [P88]
(h) A particle, or pattern of string vibration, with a whole number amount of spin; typically a messenger particle. [G99]
(i) Bosons are any particles that carry an integer unit of spin (0, 1, ...). They have different properties from particles with half a unit of spin (fermions). In particle physics, boson has a more specific meaning: Bosons (photons, gluons, and W and Z bosons) are particles that are the quanta of the electromagnetic, strong, and weak fields. They transmit the effects of the forces between quarks, leptons, and themselves. Higgs bosons are quanta of a Higgs field; as this book is being written, the evidence for the existence of Higgs bosons is still indirect. [K2000]

Bosonic String Theory

First known string theory; contains vibrational patterns that are all bosons. [G99]


A flavor of quark. [G97]

Bottom-Up Scenario

A galaxy-formation scenario in which small galaxies form first. Larger and larger structures are then formed in due course. [C97]

Bottom-Up Structure Formation

The idea that small structures, perhaps galaxies or even smaller substructures, form first in the universe, followed later by larger structures. [HH98]

Boundary Condition

(a) Restriction on the limits of applicability of an equation. Examples include the definition of a "closed system" in thermodynamics, and the theater within which one collapses the wave function in quantum mechanics. Every equation in physics may in principle be reduced to two fundamentals - the initial conditions and the boundary conditions. [F88]
(b) Conditions needed to determine the evolution of a physical system, given the laws of nature. For example, the swing of a pendulum is determined both by the laws of mechanics and gravity and by the initial height at which the pendulum is released. This latter is called a boundary condition, or an initial condition. [LB90]

Boundary Layer

A thin layer of fluid, such as the one next to a solid surface past which the fluid is moving. Friction with the surface slows flow within the boundary layer so that next to the surface the fluid is stationary. At the other edge of the boundary layer, the velocity approaches that of the main flow. Within it the effects of viscosity are significant, whereas in the main stream they can often be neglected. [DC99]

Bound-Bound Transitions

Transitions between energy levels of an electron bound to a nucleus (the electron is bound both before and after the transition). [H76]

Bound-Free Transitions

Transitions in which a bound electron in any energy level is liberated. [H76]

Boussinesq Equations

Hydrodynamic equations often used to analyze the onset of convection in a fluid by allowing for the variations of density only insofar as buoyancy forces are concerned. [H76]

Bowen Fluorescence Mechanism

A mechanism first discovered by Bowen which explains the anomalously strong lines of O III in the spectra of some planetary nebulae as fluorescence involving the radiative excitation of the 2p3d 3Po2 level of O2+ (54.71 eV) from the 2p2 3P2 state in the ground term by He II Lyman-alpha photons (54.17 eV). [H76]

Boyle's Law

(a) At a constant temperature, the pressure of a fixed mass of a gas is inversely proportional to its volume: i.e. pV = K, where K is a constant. (A graph of p against 1/V is a straight line.) The value of K depends on the temperature and on the gas. The law holds strictly only for ideal gases. Real gases follow Boyle's law at low pressures and high temperatures. [DC99]
(b) The pressure of an ideal gas kept at constant temperature varies inversely as the volume, i.e., directly as the density. [H76]

Bp Stars

Peculiar B stars whose spectra show a deficiency in helium. [H76]

Bp, Ap Stars

Or peculiar B-type or A-type Stars.
B-type or A-type stars in which the lines of one or several elements are abnormally enhanced. Traditionally the most important subgroups are Si lambda4200, Hg-Mn and Cr-Eu-Sr stars. The latest objects of the latter group correspond to early F-type. [JJ95]

BPS States

Configurations in a supersymmetric theory whose properties can be determined exactly by arguments rooted in symmetry. [G99]

BPT Diagram

Baldwin, Phillips, and Terlevich Diagram
Diagnotic diagram for discriminatinmg between therally excited and non-thermally heated sources of line emission.

Brackett Series

The spectral series associated with the fourth energy level of the hydrogen atom. Balpha is at 40512 Å. (He II Balpha is at 10124 Å; see Pickering series). [H76]

Bragg Angle

Glancing angle between an incident X-ray beam and a given set of crystal planes for which the secondary X-radiation from the planes combines to give a single reflected beam. [H76]

Bragg's Law

If a beam of x-rays of wavelength lambda is directed at a crystal with parallel crystal planes that are distance d apart, then the reflected x-rays from each plane undergo interference. Constructive interference occurs at angles theta where nlambda = 2d sin theta, n being an integer (1, 2, 3, etc.). theta is the angle between the crystal plane and the incident beam (called the Bragg angle). The equation is used in determining crystal structure from interference patterns produced by monochromatic x-rays. [DC99]

Braking Parameter (of a pulsar)

A quantity describing the rate of slowdown of a pulsar, dE / dt = Aomega n, where the braking index n = omega (omega dot dot) / (omega dot)2. [H76]

Branching Ratio

Ratio between the numbers of atoms starting from a given initial state which undergo two different types of transitions, perhaps, or between different bound states. [H76]


Any of the extended objects that arise in String Theory. A one-brane is a string, a two-brane is a membrane, a three-brane has three extended dimensions, etc. More generally, a p-brane has p spatial dimensions. [G99]

Brans-Dicke Theory

(a) Alternative theory of gravity to general relativity devised by Carl H. Brans and Robert H. Dicke around 1960. A scalar gravitational field is postulated in addition to the curved space-time metric. Predictions for observable effects differ slightly from those of general relativity. [D89]
(b) A theory of gravity formulated by Robert Dicke and Carl Brans in 1961 and differing from Einstein's theory of gravity, general relativity. In Einstein's theory, Newton's "gravitational constant", which measures the strength of the gravitational force, is a constant. In the Brans-Dicke theory, the magnitude of Newton's gravitational constant depends on the distribution of distant masses in the universe. (See general relativity.) [LB90]
(b) A scalar-tensor modification of the general theory of relativity by the introduction of a scalar field (in Einstein's theory, gravitation is described by a single field quantity, a tensor). [H76]

Breit-Wigner Equation

An equation relating the cross section in a nuclear reaction to the energy of the incident particle. [H76]


(a) X-rays emitted when fast electrons are slowed down violently, as when electrons strike the target in an x-ray tube. The word translates as 'braking radiation'. Bremsstrahlung is caused when an electron passes through the electric field of a nucleus and constitutes the continuous x-ray spectrum. [DC99] (literally, "deceleration radiation")
(b) Radiation emitted or absorbed when a free electron is accelerated in the field of an atomic nucleus but remains in a hyperbolic orbit without being captured. Since bremsstrahlung is not quantized, photons of any wavelength can be emitted or absorbed. (Also called a free-free transition because the electron is free both before and after the transition.) [H76]

Bremsstrahlung Radiation

Electromagnetic radiation given out by electrons interacting with the ions in an ionized gas. [C97]

Brewster Angle

Symbol: iB The angle of incidence, on a partially reflecting surface, at which the reflected radiation is fully plane-polarized. It is also the angle of incidence at which the reflected and refracted beams are perpendicular. Polarization by reflection is a refractive property of the surface.

1n2 = tan iB

The plane of polarization is parallel to the surface. The refracted radiation is partly polarized parallel to the normal. Formerly, the Brewster angle was called the angle of polarization or the polarizing angle. [DC99]

Bright Blue Variables

Early-type high-luminosity stars with peculiar spectra and large-amplitude light variations over a long time scale (e.g. eta Car). [JJ95]

Bright Points

Bright regions (in X-ray and XUV) observed on the Sun during Skylab missions. They are distributed fairly uniformly over the disk, and have diameters of about 20,000 km, mean lifetimes of about 8 hours, and electron temperatures of about 1-2 × 106 K. [H76]

Bright Ring

see Saturn's rings. [H76]


(a) Refers to the amount of light coming from an object. Apparent brightness is the light we see; intrinsic brightness, which has more importance but is more difficult to measure, is the light an object actually gives off - also known as luminosity. By itself, the word "bright" can mean either apparent or intrinsic brightness, depending on the context. [C95]
(b) A vague term describing the intensity of light. It can be applied to a source of light, to light itself, or to an illuminated surface. The brightness or intensity of light, in any of these three cases, relates to the rate of supply of energy (i.e. the power). The relation is complicated as it must take account of the sensitivity of the eye (or other detector) at different frequencies. [DC99]

Brightness Distribution

A statistical distribution based on brightness, or the distribution of brightness over the surface of an object. [H76]

Brightness Temperature

(a) The temperature that a blackbody would have to have to emit radiation of the observed intensity at a given wavelength. Brightness temperature in radio astronomy is equivalent to specific intensity in optical astronomy. [H76]
(b) A statement of the brightness of a radio object in which power is converted to a temperature using the Rayleigh-Jeans law, irrespective of whether or not the source is thermal. [McL97]


For Mercury and Venus the quantity ks2 / r2, where k = 0.5(1 + cos i), i is the phase angle, s is the apparent semidiameter, and r is the heliocentric distance. [S92]

Brillouin Scattering

Slight changes in the frequency of radiation, caused by reflection or scattering from the high-frequency sound waves that arise from thermal vibrations of atoms in the medium. [H76]

Brillouin Zone

A continuous ensemble of all energies and wave functions which may be obtained from one atomic energy level in a metallic-crystal lattice. [H76]

Broken Symmetry

In cosmology and particle physics, a state in which traces of an earlier symmetry may be discerned. A broken symmetry condition differs from chaos in that its parts can in theory be united in a symmetrical whole, like the pieces of a jigsaw puzzle. [F88]


A deep red, moderately reactive element belonging to the halogens. Bromine is a liquid at room temperature (mercury is the only other element with this property).
Symbol: Br; m.p. -7.25°C; b.p. 58.78°C; r.d. 3.12 (20°C); p.n. 35; r.a.m. 79.904. [DC99]

Brown Dwarf

(a) A star with too little mass to ignite its hydrogen-1 fuel. If brown dwarfs exist, they shine faint red for a time, as they convert gravitational energy into heat, and then fade and cool. Their masses range from 1 to 8 percent of the Sun's mass. [C95]
(b) A term used to describe very low mass stars, less than about one-tenth the mass of the Sun, in which the central temperature is too low for the nuclear burning of hydrogen into helium to take place. They may derive their internal energy from the initial energy of collapse. They are expected to be very cool stars and hence strong infrared emitters. They could make an important contribution to the hidden or dark matter in galaxies and clusters of galaxies. [D89]
(c) A substellar object that is below the minimum mass required for nuclear fusion reactions to occur in its core. [HH98]
(d) A self-gravitating, self-luminous gaseous object which is not sufficiently massive to result in thermonuclear hydrogen fusion reactions in its core and cannot therefore be considered a star. Such objects are expected to have a mass less than 7% of the Sun's mass and represent a "missing link" between low-mass stars and gas giant planets like Jupiter (at 0.1% of the Sun's mass). [McL97]

B-type Star

Having a spectral type of B, that is, hot and blue, such as Rigel and Regulus. [C95]


The false vacuum decays in a manner similar to the way water boils, forming bubbles of normal matter in the midst of the false vacuum, just as bubbles of steam form in the midst of water heated past its boiling point. (The bubbles that form when the false vacuum decays should not be confused with false vacuum bubbles, which have false vacuum rather than normal matter on the inside.) [G97]
(b) Regions of empty space surrounded by a thin wall of energy. According to an early version of the inflationary universe model, bubbles condensed out of the surrounding medium of energy in the manner that chunks of ice might condense out of a liquid carefully cooled to below the freezing point. [LB90]
(c) The name for the structures formed by the observed distribution of galaxies in space. Some surveys of nearby galaxies show that galaxies are located on roughly spherical shells, called bubbles, of about a hundred million light years in diameter (about a thousand times the diameter of a single galaxy). Few galaxies reside in the interior of a "bubble". (also known as Voids) [LB90]


The stellar population that lies within several thousand light-years of the Galactic center. The bulge is old, dense, and metal-rich. [C95]

Buried Channel

A construction in a silicon CCD which results in a collection zone for photo-generated electrons which is buried well below the surface of the silicon. The zone is produced by a combination of a pin junction within the silicon and a metal electrode on the surface of the silicon. [McL97]

Burst (Cosmic-Ray)

A sudden intense ionization, apparently due to the effect of cosmic rays on matter, and often giving rise to great numbers of ion pairs at once. [H76]

Burst (Solar)

Suddenly enhanced nonthermal radio emission from the high solar corona immediately following a solar flare, probably due to energetic electrons trapped in the coronal magnetic field. Bursts are divided into several types, depending on their time frequency characteristics (type III is the most common). They are classified on a scale of importance ranging from -1 (least important) to +3. Bursts are generally attributed to a sudden acceleration of some 1035-36 electrons to energies greater than 100 keV in less than 1 second. [H76]


The general term for hardware for dealing with the input-output pathway and backplane of a computer. There are many types, e.g. IEEE-488 bus, Q-bus, VME-bus. [McL97]

Butterfly Diagram

Plot of heliographic latitude of sunspots versus time, developed by Maunder in 1904 to illustrate the solar cycle. [H76]

Butterfly Effect

Any effect in which a small change to a system results in a disproportionately large disturbance. The term comes from the idea that the Earth's atmosphere is so sensitive to initial conditions that a butterfly flapping its wings in one part of the world may be the cause of a tornado in another part of the world. See chaos theory. [DC99]

Bw Stars

B stars with weak helium lines - i.e., B stars which, if classified according to their colors, would have helium lines too weak for the classification, and which, if classified according to their helium lines, would have colors too blue for their spectral type. [H76]


A group of eight "bits" or binary digits (ones or zeros). Two bytes (16 bits) make a word. A megabyte is one million bytes and a gigabyte is one thousand megabytes. [McL97]

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