1 A Spectral type for white stars, such as Sirius, Vega, Altair, Deneb, and Fomalhaut. [C95]
2 AAT Anglo-Australian Telescope
3 A band One of about a dozen of the strongest Fraunhofer lines seen in the Solar spectrum, the A band at 7600 angstoms is due to telluric lines of molecular oxygen in the Earth's atmosphere.
4 A-coefficient Einstein coefficient, where Aji is the coefficient of spontaneous emission from upper level j to lower level i.
5 A-number Atomic Mass Number: The total number of protons and neutrons in an atom's nucleus. For example, Oxygen-16 has a mass number of sixteen, because it has eight protons and eight neutrons. [C95]
6 A Shell Stars A-type stars in which two different types of line profiles co-exist. [JJ95]
7 A Star A star of spectral type A with a surface temperature of about 10,000 K, in whose spectrum the Balmer lines of hydrogen attain their greatest strength. Helium lines can no longer be seen. Some metallic lines are present; in late A stars the H and K lines of ionized calcium appear. A0 stars have a color index of zero. Examples of A stars are Vega and Sirius. [H76]
8 ab Variables A sub-class of Bailey type RR Lyrae variables, having asymmetric lioght curves of large amplitude. [H76]
9 AB Magnitude System The AB magnitude system is defined such that for any bandpass or filter being considered, the magnitude zero-point corresponds to a flux density of 3631 Jy (1 Jy = 1 Jansky = 10-26 W Hz-1 m-2 = 10-23 erg s-1 Hz-1 cm-2) [BFM03].
10 ABT Abbreviation employed in this book to mean After the Beginning of Time, which is here defined as the beginning of the expansion of the Universe. [F88]
11 Ae or A Emission Stars A-type stars with emission in one or several Balmer lines. [JJ95]
12 Am Stars (a) Peculiar stars whose metallic lines are as strong as those of the F stars but whose hydrogen lines are so strong as to require that they be classed with the A stars. They are generally short-period (<300d) spectroscopic binaries with high atmospheric turbulence and variable spectra, and are slower rotators than normal A stars. [H76] (b) A-type or F-type objects to which no unique spectral type can be assigned. Usually the classifier provides a classification according to the hydrogen, metallic and calcium lines. Also call metallic-line stars. [JJ95]
13 Ap Stars Peculiar A-type stars ("magnetic" A stars) that show abnormally strong lines, sometimes of varying intensity, of certain ionized metals. Recent evidence indicates that all Ap stars are slow rotators compared with normal A stars. [H76]
14 Abelian Group A mathematical group of transformations with the property that the end result of a series of transformations does not depend on the order in which they are performed. [CD99]
15 Aberration (a) Defect in the image formed by a lens, mirror or optical system. Spherical aberration results when different rays of light are brought to more than one focus, producing a blurred image or coma; chromatic aberration when different wavelengths within a ray of light are brought to more than one focus, producing an image distorted by colored fringes. Aberration in lenses can be overcome by the use of an achromatic lens or a combination of lenses made of glasses of different refractive indices. [A84] (b) The apparent angular displacement of the observed position of a celestial object from its geometric position, caused by the finite velocity of light in combination with the motions of the observer and of the observed object. see Aberration, Planetary [S92]
16 Aberration A defect in an optical system such that the image is not a true picture of the object. For instance, colored fringes may appear, the image may not be focused, or the shape may show distortion. Techniques of aberration correction exist; these can, however, be complex and costly. Chromatic (color) Aberration is found with a single lens; mirrors do not suffer from chromatic aberration. Because dispersion always accompanies refractive deviation, the `red' image will be farther from the lens than the `blue'. Consequently, the image is surrounded by colored fringes. Chromatic aberration is corrected by forming a compound lens, whose elements have different refractive constants. Spherical Aberration always occurs with rays that are distant from the axis and incident on a spherical mirror or lens. It is the cause of the caustic curve. Spherical aberration is corrected by using parabolic reflecting and refracting surfaces. Astigmatism affects rays neither close nor parallel to the axis. The cone of rays through a lens from an off-axis object does not focus at a point. Instead, two images in the form of short lines are formed at different distances from the lens. Between the two the image appears blurred. Mirrors forming images of off-axis points show a similar defect. The best method of minimizing astigmatism is to reduce the aperture with stops, thus allowing light only through the center of the lens. Coma is rather similar in cause, effect, and correction to astigmatism. After refraction by a lens, a cone of rays from an off-axis object tends to have a tadpole-shaped section because of coma. Distortion is the result of differences in a lens' magnifying power between different axes. Reduction of aperture is the normal solution to both coma and distortion. [DC99]
17 Aberration Annual The component of stellar aberration (see Aberration, Stellar) resulting from the motion of the Earth about the Sun. [S92]
18 Aberration Diurnal The component of stellar aberration resulting from the observer's diurnal motion about the center of the Earth. [S92]
19 Aberration E-terms of Terms of annual aberration depending on the eccentricity and longitude of perihelion of the Earth (abbreviation for Ecliptic Aberration). [S92]
20 Aberrations effects Effects associated with the performance of optical components which give rise to imperfect optical images. [McL97]
21 Aberration Planetary The apparent angular displacement of the observed position of a celestial body produced by the motion of the observer and the actual motion of the observed object. [S92]
22 Aberration Secular The component of stellar aberration resulting from the essentially uniform and rectilinear motion of the entire Solar System in space. Secular aberration is usually disregarded. [S92]
23 Aberration Spherical Always occurs with rays that are distant from the axis and incident on a spherical mirror or lens. It is the cause of the caustic curve. Spherical aberration is corrected by using parabolic reflecting and refracting surfaces.
24 Aberrations Effects associated with the performance of optical components which give rise to imperfect optical images. [McL9
25 Ablation Erosion of an object (generally a meteorite) by the friction generated when it passes through the Earth's atmosphere. [H76]
26 Absolute Brightness The total luminosity radiated by an object. [Silk90]
27 Absolute Magnitude (a) A measure of the intrinsic brightness of a star or galaxy. Absolute magnitude is defined as the apparent magnitude the star or galaxy would have if it were 32.6 light-years (10 parsecs) from Earth. The lower an object's absolute magnitude, the greater its intrinsic brightness. For example, the Sun has an absolute magnitude of +4.83, while Sirius, whose intrinsic brightness is greater, has an absolute magnitude of +1.43. A star that is one absolute magnitude brighter than another (e.g., +4 versus +5) is 2.5 times intrinsically brighter; a star that is 5 absolute magnitudes brighter is 100 times intrinsically brighter; and a star that is 10 absolute magnitudes brighter is 10,000 times intrinsically brighter. [C95] (b) The absolute magnitude (g) of a Solar-System body such as an asteroid is defined as the brightness at zero phase angle when the object is 1 AU from the Sun and 1 AU from the observer. [H76]
28 Absolute Space Newtonian space, hypothesized to define a cosmic reference frame independent of its content of matter or energy. The existence of absolute space, enshrined in aether theory, was denied in relativity. [F88]
29 Absolute Unit A unit defined in terms of fundamental quantities (such as length, mass, time, and electric charge). [DC99]
30 Absolute Zero (a) The zero value of thermodynamic temperature; 0 kelvin or -273.15?C. [DC99] (b) The lowest possible temperature, attained when a system is at its minimum possible energy. The Kelvin temperature scale sets its zero point at absolute zero (-273.15? on the Celsius scale, and -434.07? on the Fahrenheit scale). [HH98] (c) The temperature at which thermal disorder completely disappears, and which is therefore the ultimate limit of `coldness'. On the Kelvin (absolute) temperature scale it is by definition the zero of temperature: on the Celsius scale it lies at about -273 degrees. [D89]
31 Absorptance Symbol:???The ratio of the radiant or luminous flux absorbed by a body or material to the incident flux. It was formerly called the absorptivity. [DC99]
32 Absorption (a) A process in which a gas is taken up by a liquid or solid, or in which a liquid is taken up by a solid. In absorption, the substance absorbed goes into the bulk of the material. Solids that absorb gases or liquids often have a porous structure. The absorption of gases in solids is sometimes called sorption. Compare adsorption. [DC99] (b) Decrease in the intensity of radiation, representing energy converted into excitation or ionization of electrons in the region through which the radiation travels. As contrasted with monochromatic scattering (in which reemission occurs in all directions at the same frequency), the inverse process of emission refers to radiation that is reemitted in general in all directions and at all frequencies. [H76]
33 Absorption Band see Band Spectrum
34 Absorption Coefficient Fraction of the incident radiation absorbed at a certain wavelength per unit thickness of the absorber. The absorption coefficient is in general a function of temperature, density, and chemical composition. (or k in cm-1) see Lambert's Law. [H76]
35 Absorption Edges Sudden rises superposed on the smooth decrease of the curve of the attenuation coefficient, which cause the curve to have a typical sawtooth aspect. They generally occur at the limit of spectral lines. [H76]
36 Absorption Lines Dark lines in a spectrum, produced when light or other electromagnetic radiation coming from a distant source passes through a gas cloud or similar object closer to the observer. Like emission lines, absorption lines betray the chemical composition and velocity of the material that produces them. [F88]
37 Absorption of Radiation No medium transmits radiation without some energy loss. This loss of energy is called absorption. The energy is converted to some other form within the medium. see also Lambert's Law. [DC99]
38 Absorption Spectrum Dark lines superposed on a continuous spectrum, caused by the absorption of light passing through a gas of lower temperature than the continuum light source. [H76]
39 Absorption Trough Range of wavelengths (around 21 cm) at which atomic hydrogen absorbs (or emits) radiation; this is a concept used in the attempt to detect intergalactic matter. [A84]
40 Abundance (a) The relative amount of a given element among others; for example, the abundance of oxygen in the Earth's crust is approximately 50% by weight. (b) The amount of a nuclide (stable or radioactive) relative to other nuclides of the same element in a given sample. The natural abundance is the abundance of a nuclide as it occurs naturally. For instance, chlorine has two stable isotopes of masses 35 and 37. The abundance of 35Cl is 75.5% and that of 37Cl is 24.5%. For some elements the abundance of a particular nuclide depends on the source. [DC99] (c) The relative amounts of chemical elements. For example, hydrogen makes up about 75 of the mass of the Universe, so its "cosmic abundance" is 75%. [LB90]
41 Abundance Ratio The ratio of the number of atoms of an isotope to the number of atoms of another isotope of the same element in a sample. [DC99]
42 Acausal Initial Conditions Initial conditions that could not have been caused by any prior physical process. [LB90]
43 Acceleration (a) The SI unit is the meter per second per second (m s-2). 1. When considering motion in one dimension, and in unscientific usage, acceleration means rate of increase of speed. This is a scalar quantity, which can be positive or negative. Negative values mean that the speed is decreasing and may be called deceleration or retardation. 2. In scientific study of motion in two or three dimensions acceleration means rate of change of velocity; a = dv / dt. This is a vector quantity having magnitude (which is always positive) and direction. Whenever speed changes (increasing or decreasing), or direction changes, or both speed and direction change, this is an acceleration. By Newton's second law the net force F acting on a body of mass m gives it an acceleration a where F = ma. [DC99] (b) The rate of increase of velocity with time. [HH98]
44 Accelerator (a) A machine for speeding subatomic particles to high velocity, then colliding them with a stationary target or with another beam of particles moving in the opposite direction. (In the latter instance, the machine may be called a collider.) At velocities approaching that of light the mass of the particles increases dramatically, adding greatly to the energy released on impact. The resulting explosion promotes the production of exotic particles, which are analyzed according to their behavior as they fly away through a particle detector. [F88] (b) Accelerators are machines that use electric fields to accelerate electrically charged particles (electrons, protons, and their antiparticles) to higher energies. If accelerators are linear, they need to be very long to achieve the desired energies, so in some, magnets are used to bend the particles around and back to the starting point, giving them a little extra energy each time around. [K98]
45 Accretion (a) Collection of material together, generally to form a single body. [A84] (b) A process by which a star accumulates matter as it moves through a dense cloud of interstellar gas; or, more generally, whereby matter surrounding a star flows toward it (as in close binaries). [H76]
46 Accretion Disk A disk of gas that accumulates around a center of gravitational attraction, such as a white dwarf, neutron star, or black hole. As the gas spirals in, it becomes hot and emits light or even X-radiation. [HH98]
47 Accumulation Theory The theory by which planetesimals are assumed to collide with one another and coalesce, eventually sweeping up enough material to form the planets. [Silk90]
48 Achernar A subgiant of spectral type B5, about 35 pc distant. (
49 Achilles Asteroid No. 588, a Trojan 60? ahead of Jupiter (P = 11.98 yr, a = 5.2 AU, e = 0.15, i = 10?.3). It was the first Trojan to be discovered (in 1906). [H76]
50 Achromat An achromatic lens. [DC99]
51 Achromatic Color A color that has no hue; i.e. black, white, or gray. [DC99]
52 Achromatic Objective A lens of two or more components with different refraction indices (e.g., crown glass and flint glass), used to correct for chromatic aberration. [H76]
53 Actinic Radiation Radiation that can cause a chemical reaction; for example, ultraviolet radiation is actinic. [DC99]
54 Actinium A soft silvery-white radioactive metallic element that is the first member of the actinoid series. It occurs in minute quantities in uranium ores. It can be produced by neutron bombardment of radium and is used as a source of alpha particles. The metal glows in the dark. Symbol: Ac; m.p. 1050?50?C; b.p. 3200?300?C; r.d. 10.06 (20?C); p.n. 89; most stable isotope 227Ac (half-life 21.77 years). [DC99]
55 Action A quantity related to the momentum and position of a body or system of particles. The Principle of Least Action asserts that the integral, or sum of this action, taken over a particular path must be a minimum. This Principle of Least Action can be used instead of Newton's Laws to determine the motion of a system. [P88]
56 Action-at-a-Distance A description of a force, such as Newton's law of gravity, in which two separated bodies are said to directly exert forces on each other. In the modern description, the bodies produce a gravitational field, which in turn exerts forces on the two bodies. see Gravitational Field [G97]
57 Active Galactic Nucleus AGN -- An unusually bright galactic nucleus whose light is not due to starlight. [HH98]
58 Active Galaxy Any galaxy which is emitting large quantities of non-thermal radiation. [C97] (b) Active galactic nuclei are very luminous (1043-1046 ergs s-1). Their energy output is in two forms: nonthermal continuum and thermal emission line. [H76]
59 Active Optics Controlling the shape of a telescope mirror at a relatively slow rate. [McL97]
60 Active Sun The Sun during its 11-year cycle of activity when spots, flares, prominences, and variations in radiofrequency radiation are at a maximum. [H76]
61 Activity Symbol: A For a radioactive substance, the average number of atoms disintegrating per unit time. [DC99]
62 Acuity, Visual The ability of the eye to see separately two points close to each other. It is a measure of the resolving power of the eye's optical system and depends on the density of cells in the retina. The maximum acuity of the normal human eye is around 0.5 minutes of arc - points separated by this angle at the eye should be seen as separate. see Resolution [DC99]
63 Adaptive Optics Compensating for atmospheric distortions in a wavefront by high-speed changes in the shape of a small, thin mirror. [McL97]
64 ADC Analog-to-Digital Converter -- An electronic circuit which takes an input voltage in a given range (typically 0-10 volts) and provides a corresponding digital output by setting output lines (bits) high or low. A 16-bit ADC has 16 output lines. [McL97]
65 ADF Astrophysical Data Facility, located at the NASA Goddard Space Flight Center (GSFC), is responsible for designing, developing, and operating data systems that support the processing, management, archiving and distribution of NASA mission data. The ADF serves three broadly-defined astrophysics disciplines: high-energy astrophysics, UV/optical astrophysics, and infrared/submillimeter/radio astrophysics. The ADF collaborates with the GSFC Laboratory for High Energy Astrophysics (LHEA) and the Laboratory for Astronomy and Solar Physics (LASP) in managing data for specific missions. The ADF staff also support the astrophysics community's access to multi-mission and multi-spectral data archives in the National Space Science Data Center (NSSDC).
66 Adhesion A force of attraction between atoms or molecules of different substances. For example, adhesion between water molecules and glass creates a meniscus. [DC99]
67 Adiabatic Change A change taking place in a system that has perfect thermal insulation, so that heat cannot enter or leave the system and energy can only be transferred by work. In practice, a close approximation to an adiabatic change can be achieved by the process being too rapid for significant heat transfer, or by the large scale of the system (e.g. a large volume of air in the atmosphere). In an adiabatic expansion of a gas, mechanical work is done by the gas as its volume increases and the gas temperature falls. For an ideal gas undergoing a reversible adiabatic change it can be shown that
68 Adiabatic Index The ratio of the fractional change in pressure to the fractional change in density as an element of fluid expands (or contracts) without exchange of heat with its surroundings. [H76]
69 Adiabatic Fluctuations Fluctuations in both the matter and radiation density, as though a volume of the Universe were slightly squeezed but allowing no radiation to escape. Prior to the Decoupling Era, adiabatic fluctuations behaved like waves, on scales smaller than the horizon size. After decoupling, gravitational instability sets in on scales above about 1013 M , smaller adiabatic fluctuations having been damped at earlier eras. [Silk90]
70 Adsorption A process in which a layer of atoms or molecules of one substance forms on the surface of a solid or liquid. All solid surfaces take up layers of gas from the surrounding atmosphere. The adsorbed layer may be held by chemical bonds (chemisorption) or by weaker van der Waals' forces (physisorption). Compare absorption. [DC99]
71 ADU Analog-to-Digital Units see DN [McL97]
72 Advance of the Perihelion The slow rotation of the major axis of a planet's orbit in the same direction as the revolution of the planet itself, due to gravitational interactions with other planets and/or other effects (such as those due to general relativity). [H76]
73 Advection The transfer of matter such as water vapor or heat through the atmosphere as a result of horizontal movement of an air mass. [DC99]
74 Aeon (or Eon) In astronomical terms, 1,000 million years. [A84]
75 Aerolite A stony meteorite, composed primarily of silicates. About 93 percent of all known falls are aerolites. They include the carbonaceous chondrites, other chondrites, and achondrites. (lit. "air stone.") [H76]
76 After-Image An image seen after the eye's retina has been exposed for a time to an intense or stationary light source. It may be negative or positive, or appear in complementary colors. [DC99]
77 Aether (1) In Aristotelian physics, the fifth element, of which the stars and planets are made. (2) In Classical physics, an invisible medium that was thought to suffuse all space. [F88]
78 Age of the Universe The time elapsed since the singularity predicted by the Big Bang theory, estimated to be around 13 billion years. [BFM02]
79 AIPS Astronomical Image Processing System -- National Radio Astronomy Observatory
80 Airglow Light in the nighttime sky caused by the collision of atoms and molecules (primarily oxygen, OH, and Ne) in Earth's geocorona with charged particles and X-rays from the Sun or outer space. The airglow varies with time of night, latitude, and season. It is a minimum at zenith and maximum about 10? above the horizon. (also called nightglow)[H76]
81 Airy Diffraction Disk The central spot in the diffraction pattern of the image of a star at the focus of a telescope. Named for Sir George Airy (1801-1892), seventh Astronomer Royal. [McL97]
82 Albedo (a) The ratio of the amount of light reflected from a surface to the amount of incident light. [DC99] (b) Ratio of the total flux reflected in all directions to the total incident flux. see Bond Albedo; Geometric Albedo [H76]
83 Alchemy Art of bringing parts of the Universe to the perfect state toward which they were thought to aspire - e.g., gold for metals, immortality for human beings. [F88]
84 Alcyone (Tau) -- The brightest star in the Pleiades (spectral type B5). [H76]
85 Aldeberan (a) The brightest star in the constellation Taurus, Aldebaran is an orange K-type giant that lies 60 light-years away. [C95] (b) A K5 III subgiant (a foreground star in the Hyades) about 21 parsecs distant. It has a faint M2 V companion. It is now known to be slowly and irregularly variable. [H76]
86 Alfven number A dimensionless number characterizing steady fluid flow past an obstacle in a uniform magnetic field parallel to the direction of flow. It has a partial analogy to the Mach number. The Alfven number is given by vl(?)1/2 B-1/2 where v is the velocity of flow, l is length of obstacle,
87 Alfv?n-Klein Cosmology A cosmological model in which the early Universe is depicted as a giant collapsing spherical cloud of matter and antimatter. When a critical density is reached, the matter and antimatter begin to annihilate, the resulting release of radiation and energy causing the Universe to expand. There are many difficulties with this model of the expanding Universe, which is largely discredited on observational grounds. [Silk90]
88 Alfv?n Speed The speed at which hydromagnetic waves are propagated along a magnetic field: (VA) = B / (4 )1/2. [H76]
89 Alfv?n Waves Waves moving perpendicularly through a magnetic field. They are caused by the oscillation of magnetic lines of force by the motions of the fluid element around its equilibrium position, which in turn is caused by the interactions between density fluctuations and magnetic variations. [H76]
90 Algol Per (a) The most famous eclipsing binary, Algol was probably the first variable star discovered. It lies in the constellation Perseus and consists of two stars that orbit each other every 2.87 days. When one star passes in front of the other, the light of the system dims. [C95] (b) An eclipsing system of at least three components (B8 V, K0, Am), about 25 pc distant. Period of components A and B is about 68.8 hours; period of components A, B, and C is about 1.9 years. Long term observations also indicate a massive, unseen fourth component with a period of about 190 years. Algol is also an erratic radio source of about 0.5 AU diameter. [H76]
91 Aliasing In a discrete Fourier transform, the overlapping of replicas of the basic transform, usually due to undersampling. [H76]
92 Allotropy The existence of a solid substance in different physical forms. Tin, for example, has metallic and non-metallic crystalline forms. Carbon has two crystalline allotropes: diamond and graphite. [DC99]
93 Almagest Arabic title for Ptolemy of Alexandria's Syntaxis, the writings in which he combined his own astronomical researches with those of others. Although much of the work is inaccurate even in premise, until Nicolaus Copernicus published his results fourteen centuries later the Almagest remained the standard reference source in Europe. [A84]
94 Alph -Particle The nucleus of a 4He atom, consisting of two protons and two neutrons. Mass of -Particle 4.00260 amu. [H76]
95 Alpha -Process A hypothetical process of nucleosynthesis (now considered obsolete terminology), which consisted of redistributing alpha-particles in the region from 20Ne to 56Fe (and perhaps slightly higher). The -process has been replaced by explosive and nonexplosive C, O, and Si burning occurring in rapidly evolving or even explosive stages of stellar evolution which at higher temperatures and densities becomes the e-process. [H76]
96 Alpha Centauri (a) Bright binary star in which both components contribute to a magnitude of -0.27: it is also the nearest of the bright stars (at a distance of 4.3 light years). [A84] (b) The nearest star system to the Sun and the third brightest star in the night sky. Unfortunately, Alpha Centauri is so far south that it is visible only from latitudes below 25 degrees north. The system consists of three stars: Alpha Centauri A, the brightest, which is a yellow G-type main-sequence star like the Sun; Alpha Centauri B, the second brightest, which is an orange dwarf; and Alpha Centauri C, by far the faintest, which is a red dwarf. Alpha Centauri A and B both lie 4.35 light-years from Earth and orbit each other every 80 years; Alpha Centauri C lies far from its mates and 4.25 light-years from Earth. Because it is closer to Earth than are A and B, Alpha Centauri C is usually called Proxima Centauri. [C95]
97 Alpha Decay (a) A type of radioactive decay in which the unstable nucleus emits a helium nucleus. The resulting nuclide has a mass number decreased by 4 and a proton number decreased by 2. An example is: 88226 Ra 86222 Rn + 24 He
98 Alpha decay The particles emitted in alpha decay are alpha particles. Streams of alpha particles are alpha rays or alpha radiation. They penetrate a few centimeters of air at STP or a metal foil of mass/area a few milligram/cm2. see also Beta Decay [DC99] (b) The disintegration of an atomic nucleus, in which the final products are an alpha particle and a nucleus with two fewer protons and two fewer neutrons than the original. [G97] (c) Spontaneous emission by a heavier element (such as uranium) of positively charged helium nuclei - alpha particles - comprising 2 protons and 2 neutrons. The result of this radioactive decay is that the original element is very gradually converted into another element, with a decreased atomic number and mass. Alpha particle emission may be simultaneous with beta particle decay. [A84]
99 Alpha Particles Particles first discovered in radioactive decay, and later identified as helium nuclei (two protons and two neutrons bound together). [CD99]
1 B 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.
2 B Galaxy In Morgan's Classification, a barred spiral.
3 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.
4 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.
5 Ba II Stars (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 M(sun)
6 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.
7 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.
8 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.
9 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.
10 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. See Cosmic Background Radiation.
11 Backscattering Scattering of radiation (or particles) through angles greater than 90 with respect to the original direction of motion.
12 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 supergiants are still in evidence. The spectra are dominated by A-typer supergiants with charcteristically strong Balmer absorption lines.
13 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.
14 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.)
15 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.
1 C Galaxies In the Yerkes 1974 system, small, high surface-brightness galaxies which are slightly resolved on medium- and large-scale photographs.
2 C Stars A class of carbon stars ( q.v.), defined by Morgan and Keenan to replace the Harvard R and N spectral classes.
3 C-S Stars Group characteristics are: strong bands of CN, outstandingly strong absorption near the Na D lines, usually sufficient structure in the 6400-6500 region to suggest ZrO.
4 Cabbibo Angle The measure of the probability that one flavor of quark (u) will change into other flavors (d or s) under the action of the weak force.
5 Cadmium A transition metal obtained as a by-product during the extraction of zinc. It is used to protect other metals from corrosion, as a neutron absorber in nuclear reactors, in alkali batteries, and in certain pigments. It is highly toxic.
6 Calabi-Yau Space A space (shape) into which the extra spatial dimensions required by string theory can be curled up, consistent with the equations of the theory.
7 Calcium A moderately soft, low-melting reactive metal. The electronic configuration is that of argon with an additional pair of 4s electrons. Calcium is widely distributed in the Earth's crust and is the third most abundant element. At ordinary temperatures calcium has the face-centered cubic structure with a transition at 450C to the close-packed hexagonal structure.
8 Calibi-Yau Space These six-dimensional spaces are hypothesized as arising when the ten dimensions of superstring theory are compactified down to four dimensions. They are also related to orbifold spaces.
9 California Nebula (IC 1499) An HII region ionized by Zeta Persei.
10 Callisto (a) Fifth (known) moon out from jupiter, and its second largest. [A84] (b) A Galilean satellite (J IV) of Jupiter, about 5050 km in diameter. Orbital and rotation period 16.7 days (e = 0.0075; i = 0.3). It has the lowest density (1.7 g cm-3), lowest albedo (0.15), and highest temperature (156 K) of any of the four.
11 Z Camelopardalis Stars A class of dwarf novae (q.v.) with standstills in their light curves. Z Cam itself is a semidetached binary (period 7h21m) consisting of a dG1 star and a hot white dwarf or a hot blue subdwarf which is probably degenerate. Mean time between eruptions, 20 days. Peak-to-peak amplitude, about 0.5 mag.
12 Candela Symbol: cd The SI base unit of luminous intensity; the luminous intensity in a given direction of a source that emits monochromatic radiation of frequency 540 1012 hertz and that has radiant intensity in that direction of 1/683 watt per steradian. Formerly, the unit was defined as the intensity (in the perpendicular direction) of the black-body radiation from a surface of 1/600 000 square meter at the temperature of freezing platinum and at a pressure of 101 325 pascals.
13 VY Canis Majoris A peculiar cM3e irregular variable with an extremely strong infrared excess, presumably due to a circumstellar dust shell. It is a class 2b OH emitter, and CO and H2O have been identified in its spectrum. It is a multiple star with at least six components, surrounded by a small reflection nebula, about 1.5 kpc distant, in the galactic plane. It may be a pre-main-sequence star, or it may be a highly evolved object like an extremely young planetary nebula.
14 Canonical The Canonical Approach to dynamics refers to the scheme in which the basic constituent is a space of states and the evolution of the system is described by a curve in this space parametrized by time. This approach to classical physics is in many respects the basic one to adopt when attempting to include quantum effects. In the case of a field theory, it has the disadvantage that space and time are treated on a different footing, and hence it is not always an easy matter to show that the formalism is compatible with the theory of relativity.
15 Canopus (alpha Car) (a) The brightest star in the constellation Carina and the second brightest star in the night sky. It is spectral type F and shines yellow-white. [C95] (b) A type F0 Ib supergiant, about 55 pc distant, the second brightest star in the southern sky.
16 c16 c16
17 c17 c17
18 c18 c18
19 c19 c19
20 C20 C20
21 C21 C21
22 C22 C22
23 C23 C23
24 C24 C24
25 C25 C25
26 c26 c28
27 C27 sdgsdgsdg
28 C28 dfgdfgdfg
29 c29 c29
30 c30 c30
31 c31 c31
32 c32 c32
33 c33 c33
34 c34 c34
35 c35 c35
36 c36 c36
37 c37 c37
38 c38 c38
39 c39 c39
40 c40 c40
41 c41 c41
42 c42 c42
43 c43 c43
44 c44 c44
45 c45 c45
46 c46 c46
47 c47 c47
48 c48 c48
49 c49 c49
50 c50 c50
1 d-Electron An orbital electron whose l quantum number is 2.
2 D Galaxy A supergiant radio galaxy (the most common type of radio galaxy) which has an elliptical nucleus surrounded by an extended envelope. Or, an optical galaxy with a very bright nucleus. In the Morgan classification, a galaxy with rotational symmetry but without pronounced spiral or elliptical structure (a dustless galaxy). In the Yerkes 1974 system a galaxy with an elliptical-like nucleus surrounded by an extensive envelope (see also R galaxy).
3 D Layer The lowest part of Earth's ionosphere (starts at about 100 km). This is the layer that reflects "broadcast" radio waves.
4 D Lines Two close spectral lines of neutral sodium (see Fraunhofer lines) at 5896 (D1) and 5890 (D2).
5 D Ring The innermost ring of Saturn, discovered in 1969.
6 Damped Oscillation An oscillation with an amplitude that progressively decreases with time.
7 Damping a) In any oscillating system, a decrease in the amplitude of an oscillation due to the dissipation of energy. (b) The gradual decay of an oscillation or wave due to viscous drag forces.
8 Dark Current a) The current that flows in a photoelectric cell when not illuminated. [H76] (b) Signals generated in a detector such as a CCD merely by the heat energy of the atoms which, through random jostling, results in the production of free electrons. Dark current is greatly reduced by physically cooling the detector.
9 Dark Halo (a) The massive outer region of the Milky Way that surrounds the disk and stellar halo. The dark halo consists mostly of dark matter, whose form is unknown. Though it emits almost no light, the dark halo outweighs the rest of the Galaxy.
10 Dark Matter (a) Material astronomers cannot see but whose presence they believe in either because they detect its gravitational influence or because certain theories predict its existence. For example, astronomers believe that the outer part of the Galaxy harbors dark matter, because they notice its gravitational influence on the stars they can see; and inflationary cosmologists believe that the universe is full of dark matter, because inflation predicts that the universe has a large density. (b) Any form of matter which exists in the Universe in a non-luminous form. (c) Matter whose existence is inferred on the basis of dynamical studies - e.g., the orbits of stars, in galaxies - but which does not show up as bright objects such as stars and nebulae. Its composition is unknown: It might consist of subatomic particles, or of dim dwarf stars or black holes, or a combination of various sorts of objects. (d) Matter that is invisible to us because it emits little or no light. As much as 90%-99% of the mass of the universe may be dark. (e) Matter that is detected only by its gravitational pull on visible matter. At least 90%, and possibly 99% of the matter in the universe is dark. The composition is unknown; it might consist of very low mass stars or supermassive black holes, but big-bang nucleosynthesis calculations limit the amount of such baryonic matter to a small fraction of the critical mass density. If the mass density is critical, as predicted by the simplest versions of inflation, then the bulk of the dark matter must be a gas of weakly interacting non-baryonic particles, sometimes called WIMPS (Weakly Interacting Massive Particles). Various extensions of the standard model of particle physics suggest specific candidates for the WIMPs. (f) Matter
11 Dark Nebula A relatively dense (up to 10^4 particles per cm^3) cloud of interstellar matter whose dust particles obscure the light from stars beyond it and give the cloud the appearance of a region devoid of stars.
12 Data Rate The number of pieces of digital information being returned each second from an instrument. Measured in bits per second (bps) for serial information or bytes per second for parallel data.
13 db Galaxy One of a small number of dumbbell-shaped radio galaxies. They might be called D systems with double nuclei, in which two elliptical nuclei share a common extended envelope.
14 Debye Unit of electric-dipole moment, equal to that existing between a unit of positive charge and a unit of negative charge separated by a distance of 1 cm. 1 debye = 10exp-18 statcoulomb cm.
15 Debye Length A theoretical length which describes the maximum distance that a given electron can be from a given positive ion and still be influenced by the electric field of that ion in a plasma. Although according to Coulomb's law oppositely charged particles continue to attract each other at infinite distances, Debye showed that there is a cutoff of this force where there are other charged particles between. This critical separation decreases for increased density. lD = (kTe / 4 neEexp2)exp1/2.
1 e-Folding Time The time within which the amplitude of an oscillation increases or decreases by a factor e (e = 2.718...).
2 E galaxy In both Hubble's and Morgan's classifications, an elliptical galaxy.
3 E layer The part of Earth's ionosphere (about 150 km) where the temperature gradient reverses and starts to rise. It reflects "short-wave" radio waves. (also called Kennelly-Heaviside Layer)
4 E+A Galaxy One of the most enigmatic types of galaxy, spectrally, are the "E+A" galaxies. They appear to be a combination of an old elliptical galaxy spectrum (E) with that of a significantly younger A-star population (A), thought to be formed in a recent (t < 1 Gyr) episode of star formation. "E+A" galaxies show strong Balmer absorption lines (EW > 5.5 Angstroms) and weak [OII] emission (EW[OII] < 2.5 Angstroms). The standard interpretation is that these are post-starburst galaxies in which star formation ceased within the last gigayear. The mechanisms responsible for the formation of "E+A" galaxies are not fully understood, neither has the environment in which these galaxies have been found been fully defined. These objects are also sometimes referred to as "K+A" galaxies.
5 E Line A Fraunhofer line at 5270 . It is a blend of Fe I and Ca I.
6 e-Process A hypothetical group of nuclear reactions by which the iron group is assumed to be synthesized. At temperatures > 5 109 K and densities > 3 106 g cm-3 there are great numbers of collisions between high-energy photons and nuclei. These collisions break up the nuclei, the fragments of which promptly combine with other particles. Thus, there is in effect an equilibrium between formation and breakup. Since the iron group has the largest binding energies, the particles over the long run will tend to be trapped in these nuclei. The e-process (the e stands for equilibrium) is presumed to occur in a supernova explosion.
7 Early-Type Spiral In Hubble's classification, a spiral with a large nuclear bulge and closely coiled arms.
8 Early-Type Stars Hot stars of spectral types O, B, A, and early F.
9 Earthlight Light reflected from the Earth's atmosphere onto the dark part of the Moon.
10 EB-CCD Electron-Bombarded CCD An imaging device containing a thin target material which emits electrons by the photoelectric effect when illuminated and then magnetically focuses these electrons to impact onto a silicon CCD where they generate a large charge.
1 F Spectral type for yellow-white stars, which are slightly hotter than the Sun. The brightest F-type stars in Earth's sky are Canopus and Procyon.
2 F component The outer part of the Solar corona (see K Component) which emits a continuous spectrum in which absorption lines can be seen. The F corona is caused by radiation from the photosphere scattered by interplanetary dust, and it decreases slowly with distance from the Sun. (F stands for Fraunhofer)
3 f-Electron An orbital electron whose l quantum number is 3.
4 F Layers Two layers in the Earth's ionosphere (F1 and F2 at about 200 and 300 km, respectively) immediately above the E layer. (also called Appleton Layers)
5 f Number Ratio of the focal length to the diameter of a lens.
6 F Region Region of the ionosphere above the F layers.
7 F Star A star of spectral type F with a surface temperature of about 60O0-7500 K, in which lines of hydrogen and Ca II are of about equal strength. Metal lines also become noticeable. Examples are Canopus, Procyon.
8 f-Sum Rule The sum of the f-values for all the transitions from a given state (positive for absorption and negative for emission) is unity.
9 ft-Values t = half-life of the Beta-unstable nucleus, and f stands for an integral which depends on the Beta -decay energy and the type of transition.
10 Faber-Jackson Relation An empirically observed correlation between the speeds of stars in the center of a galaxy and the intrinsic luminosity of the galaxy - the higher the random speeds, the more luminous the galaxy. Since the speeds of stars can be directly measured by the Doppler shift in their colors, the Faber-Jackson relation permits an estimate of the intrinsic luminosity of a galaxy. By comparison of this with the observed brightness, the distance to the galaxy may be inferred.
1 G Spectral type for yellow stars, such as the Sun, Alpha Centauri A, and Capella.
2 G-band A band of CH at 4303 . It is conspicuous in the spectra of G-K stars.
3 g-factor Ratio of a particle's magnetic moment to its spin angular momentum.
4 gf-values Weighted oscillator strengths. f = oscillator strength of the transition: g = statistical weight of the lower level.
5 G star Stars of spectral type G are yellowish stars with surface temperatures of about 5000 to 6000 K, in which the H and K lines of Ca II have become dominant and in which a tremendous profusion of spectral lines of both neutral and ionized metals, particularly iron, begins to show. The Balmer lines of hydrogen are still recognizable. Examples are the Sun and Capella.
6 Gadolinium A ductile malleable silvery element of the lanthanoid series of metals. It occurs in association with other lanthanoids. Gadolinium is used in alloys, magnets, and in the electronics industry. Symbol: Gd; m.p. 1313C; b.p. 3266C; r.d. 7.9 (25C); p.n. 64; r.a.m. 157.25.
7 Gain The amplification factor.
8 Galactic 1. When capitalized, the word refers to our Galaxy. 2. When not capitalized, it refers to a galaxy.
9 Galactic Anticenter The point in the Galactic plane that lies directly opposite the Galactic center. Here we gaze toward the edge of the Galactic disk. The nearest bright star to the anticenter is El Nath, in the constellation Taurus.
10 Galactic Centers Are now thought to comprise black holes - which would explain why the centre of our Galaxy appears strangely obscure, and emits only infrared radiation.
1 H Chemical symbol for hydrogen. The most abundant chemical in the universe. H2 is the symbol for the molecular hydrogen molecule which is abundant in giant clouds in our galaxy and can be detected by its infrared spectrum. The symbols HI and HII are used to indicate neutral and ionized hydrogen respectively. HII regions are usually associated with star formation, e.g. the Orion Nebula, and are detected by their emission lines such as H-alpha at 656.3 nm. Radio emission at 21 cm wavelength can be detected from neutral hydrogen.
2 h Hubble's constant in units of 100 km s-1 Mpc-1.
3 h-line An Mg II resonance line at 2803 .
4 H and K Emission Line Stars Late objects (F4 to M), which exhibit emission features in their H and K lines of Ca II.
5 H and K lines The two closely spaced lines of singly ionized calcium at 3968 and 3934 , respectively.
6 H-magnitude The magnitude derived from infrared observations at 1.6 microns.
7 H I Neutral hydrogen gas. It emits radio waves that are 21 centimeters long.
8 H I Region Region of neutral (atomic) hydrogen in interstellar space. The temperature is about 125 K (the spin temperature of neutral hydrogen - far too low for electrons to emit radiation in the optical part of the spectrum (see 21-cm radiation). At least 95 percent of interstellar H is H I. (Density is about 10 atoms per cm3, about the same as in H I regions.)
9 H II Ionized hydrogen - that is, hydrogen with its electron missing.
11 H II Region (a) An area of ionized hydrogen. Most H II regions are red and arise from hot blue O and B stars, whose ultraviolet light can ionize all the hydrogen for dozens or even hundreds of light-years in every direction. The most famous H II region is the Orion Nebula. (b) Region of ionized hydrogen in interstellar space. H II regions occur near stars with high luminosities and high surface temperatures. The kinetic temperature of H II regions is about 10,000-20,000 K, and the density is about 10 atoms per cm3. Ionized hydrogen, of course, having no electron, does not produce spectral lines; however, occasionally a free electron will be captured by a free proton and the resulting radiation can be studied optically (see also radio recombination lines).
12 H-R Diagram Hertzsprung-Russell diagram. A diagram of stars arranged according to their luminosity (measured on the y axis) and temperature (on the x axis). In the early part of the twentieth century, the Danish astronomer E. Hertzsprung and the American astronomer H. Russell plotted known stars in such a diagram and found a definite correlation between luminosity and temperature.
13 Hadron (a) The generic name for any particle which experiences the strong nuclear force. (b) The word applied to an object made of quarks and/or antiquarks. Thus protons, neutrons, antiprotons, antineutrons and pions are examples of hadrons. Since quarks have `color' they experience the strong QCD forces and so therefore do hadrons. Examples of particles which are not hadrons are leptons (electrons, neutrinos, muons, etc.) and photons and W and Z particles. (c) Elementary particles that are influenced by the strong nuclear force. There are two sorts of hadrons - mesons, which have zero spin, and baryons, which have spin 1/2 or 3/2. (d) Strongly interacting elementary particles. The hadrons all tend to have high masses. (e) The properties of the color force and the rules of quantum theory allow certain combinations of quarks (and antiquarks) and gluons to bind together to make a composite particle; all such particles are called hadrons. When mainly three quarks bind, the resulting hadron is called a baryon. When quark and antiquark combine, the result is called a meson, and when gluons combine, it is called a glueball. Hadrons have diameters of about 10-13 cm. The proton and neutron are the most familiar baryons. Pions are the lightest mesons, so they are produced frequently in collisions. Kaons are the next lightest hadrons; their properties make them useful in many studies.
14 Hadron Barrier The interval (t 10exp-43 [10exp-23] seconds after the big bang, when = 10exp93 [10exp52] g cm-3) during which quantum and general-relativistic effects are expected to modify each other in an unknown way. The quantities in brackets are for a different equation of state.
15 Hadron Era The interval (t 10exp-5 sec after the big bang) when the Universe was matter-dominated and when kT m . It was followed by the lepton era (q.v.).
16 Hadronic Era The interval lasting until some 10exp-4 seconds after the Big Bang when the universe was matter-dominated, containing many hadrons in equilibrium with the radiation field. The Hadronic Era ended when the characteristic photon energy fell below the rest mass of a pion or -meson (270 electron masses), and very few hadrons remained (about one hadron for every 10exp8 photons).
17 Hagedorn Equation of State An equation of state for extremely degenerate matter (density greater than about 10exp15 g cm-3).
18 Halation The formation of a halo around bright star images by light reflected from the back of the photographic plate or emulsion.
19 Half-Power Beamwidth (HPBW) The angle across the main lobe of an antenna pattern between the two directions where the sensitivity of the antenna is half the value at the center of the lobe. This is the nominal resolving power of the antenna system.
20 Hall Effect When an electric current is passed through a conductor and a magnetic field is applied at right angles, a potential difference is produced between two opposite surfaces of the conductor. The direction of the potential gradient is perpendicular to both the current direction and the field direction. It is caused by deflection of the moving charge carriers in the magnetic field. The size and direction of the potential difference gives information on the number and type of charge carriers.
21 Halo (a) Nebulous quality round a celestial body (particularly round a red giant); the galactic halo, however, describes the spherical collection of stars forming a surrounding "shell" for our otherwise compact, discoid Galaxy. (b) The somewhat round population of old, metal-poor stars in the Milky Way. Also, the huge entity that surrounds the disk and contains most of the Galaxy's dark matter. To distinguish between the two, astronomers call the former the stellar halo and the latter the dark halo. Most of the stellar halo lies closer to the Galactic center than the Sun, while most of the dark halo lies farther from the Galactic center than the Sun.
22 Halo Population Old stars typical of those found in the halo of the Galaxy; also called Population II.
24 Halo Stars Stars that have high spatial velocity and low metallicity. This is not an observational definition.
25 Hamiltonian Function (H) The quantity in classical mechanics corresponding to the total energy of a system, expressed in terms of momenta and positional coordinates.
26 Hamiltonian Operator (H) The dynamical operator in quantum mechanics that corresponds to the Hamiltonian function in classical mechanics.
27 Hamiltonian Theory A theory for calculating the trajectory of a particle under an applied force. Hamiltonian theory, developed in the nineteenth century, is equivalent to Newton's laws of mechanics but is reformulated in a mathematically elegant way to allow easier solutions to some problems.
28 Hanning Method A method of smoothing out the noise in radio data. For each data point, one-half the value of that point is taken, plus one-quarter the value of the point on each side. The result is usually a smoother curve.
1 Iapetus The ninth satellite of Saturn, about 850100 km in radius; period 79d7h55m, e = 0.028, inclination to Saturn's orbital plane 14.7. It has the most extreme variation in albedo of any satellite in the solar system (0.04 for the leading side, 0.28 for the trailing side). Discovered by Cassini in 1671.
2 I.A.U. International Astronomical Union.
3 I.B.C. Impurity Band Conduction.
4 IC 4182 A nearby galaxy in which a type Ia supernova exploded in 1937.
5 Icarus Asteroid No. 1566, 1.1 km in diameter, discovered by Baade in 1948. It has the smallest orbit and highest eccentricity (a = 1.07 AU, e = 0.827, i = 23, P = 408d) of any known minor planet. It is the only asteroid known to come closer to the Sun than Mercury (perihelion distance 0.19 AU). Rotation period 2h16m.
6 Ideal Gas a) A nondegenerate gas in which the individual molecules are assumed to occupy mathematical points and to have zero volume, and in which the mutual attraction of neighboring molecules is zero. (also called Perfect Gas) b) The pressure of a gas is directly proportional to the product of its temperature and density (p = C T). The higher the temperature and the more rarefied a gas, the more closely it obeys the ideal gas laws, so the gases in most stars closely approximate ideal gases. For a degenerate gas, the pressure depends only on the density and is independent of the temperature.
7 I.D.L. Interactive Data Language.
8 I.F. Intermediate Frequency. The beat frequency between the signal and the local oscillator in a radio detection system.
9 Illumination Symbol: E A measure of the visible-radiation energy reaching a surface in unit time. Once called `intensity of illumination', it is measured in lux (lx). One lux is an illumination of one lumen per square meter.
10 I.L.R. Intermediate Line Region
11 Image Dissector Scanner A specialized television camera used as a light detector (instead of a photographic plate) in the 1970s.
12 Image Intensifier An electronic device for increasing the brightness of a faint optical image. The image is first formed on a thin metallic surface called a photocathode from which electrons are then ejected. The stream of electrons is accelerated and focused onto a phosphorescent screen which glows brightly as a result of the impact.
1 J-file A group of lines of a supermultiplet having a common lower level.
2 j-j Coupling see LS coupling
3 J magnitude The magnitude derived from the observations at an infrared wavelength of 1.3 microns.
4 J Meson name for the J- or ?-meson, mass 3 GeV, composed of a charmed quark and charmed antiquark. Its discovery in 1974 instigated a scientific revolution in the later half of the decade.
5 J-value Value of the total angular momentum (orbital plus spin). J is the rotational quantum number which specifies the rotational level of a molecule.
6 Jacobi Ellipsoid Jacobi discovered that homogeneous, self-gravitating masses rotating uniformly and sufficiently rapidly can have the shape of triaxial ellipsoids. These are the Jacobi ellipsoids.
7 Jansky Jansky A unit used in radio astronomy to indicate the flux density of electromagnetic radiation received from outer space, one jansky being equal to 10-26 W m-2 Hz-1. The unit is named after the American electrical engineer Karl G. Jansky (1905-1950), who became the first radio astronomer in the world in December 1930 when he detected electromagnetic radiation of wavelength 15 m coming from the Milky Way (see Flux unit). It was adopted in August 1973 by the International Astronomical Union. (b) Unit of flux density adopted by the IAU in 1973. 1 Jy = 10-26 W m-2 Hz-1.
8 Janus The innermost satellite of Saturn, just outside Saturn's rings. P = 0.75 days; R = 175 (?) km; i 0; e 0. It was discovered by Dollfus in 1966 and was named Janus for the first and the last.
9 Julian Day (JD) A unit of time within the Julian Dating System where the number of ephemeris days that have elapsed since 12h ephemeris time on January 1, 4713 B.C. JD for 1970 January 1 is 2440588.
10 Jeans Instability Criterion see Jeans Length
11 Jeans Length The critical wavelength ( J = cs ( / G 0)1/2, where cs is the isothermal sound speed in the medium) at which the oscillations in an infinite, homogeneous medium become gravitationally unstable. Any disturbance greater than the Jeans length will decouple by self-gravitation from the rest of the medium to become a stable, bound system. In general, J 1020 cm.
12 Jeans Mass (a) The critical mass a volume of space must contain before it will collapse under the force of its own gravity. (b) The mass enclosed within a sphere of diameter equal to the Jeans length.
13 Jet Spray of particles produced from the vacuum by the passage of a high momentum quark or gluon. The direction of the jet indicates the direction of the said quark or gluon.
14 J H K L M N Q Designations for parts of the infrared waveband transmitted by the Earth's atmosphere in the wavelength range 1 to 20 m.
15 JPL Jet Propulsion Laboratory, Pasadena, California. Funded by NASA and operated by the California Institute of Technology (Caltech).
1 K a) Spectral type for orange stars, such as Arcturus, Aldebaran, and Alpha Centauri B. K type stars are somewhat cooler than the Sun. b)A quantum number which refers to the component of angular momentum around a molecule's axis of symmetry.
2 Kalpha A spectral line in the X-ray region (alpha = 0.334 ), produced by the transition between the lowest level of the K shell and the lowest level of the L shell.
3 K capture Capture by a nucleus of one of the electrons in its innermost shell, accompanied by the emission of X-rays.
4 K component The inner part of the Solar corona (the gaseous phase) which emits a continuous spectrum without absorption lines. Physically, the K component results from Thomson scattering of photospheric radiation by free electrons in the corona. The K component is polarized and decreases rapidly with distance from the Sun. (from the German Kontinuum)
5 K corona See K component.
6 K correction In extragalactic studies, the observed luminosities and colors of objects participating in the expansion of the Universe are known to require procedural corrections for two physically distinct, but redshift-related, reasons: (1) the stretching of the restframe spectral energy distribution across the observer's (fixed) bandpass, and (2) the redshift/displacement of the restframe wavelength across and into the observer's filter. The first effect is purely a function of the redshift, z, where the effective filter width decreases in proportion to 1 / (1 + z). The second effect is primarily a function of the spectral energy distribution of the source, rising and falling with the relative continuum (or emission-line) strength being sampled for that filter and redshift combination. Failure to appreciate these "technical effects" associated with the redshift led to a very early claim of rapid evolution of nearby elliptical galaxies, the so-called Stebbins-Whitford Effect. (also called K term)
7 K edge The absorption edge of the K shell (see absorption edges).
8 K electron An electron in the K shell.
9 kline A Mg II resonance line at 2795.5 .
10 K line A spectral line of singly ionized calcium at 3933 (see Fraunhofer lines).
1 Luminous Blue Variables (LBV) A variable-star designation for the high-luminosity early type objects. Also called S Dor variables or Hubble-Sandage Variables.
2 L Component The part of the Solar corona whose spectrum consists of emission lines.
3 L Galaxy In Morgan's classification, an elongated galaxy of low surface brightness.
4 L-magnitude The magnitude derived from observations at an infrared wavelength of 3.5 microns.
5 l-number The orbital quantum number, which determines the magnitude of an electron's angular momentum.
1 M (a) Spectral type for red stars, such as Betelgeuse, Antares, and Proxima Centauri. (b) Designation of objects in the Messier Catalog of nebulae, star clusters, and galaxies, published in the Eighteenth Century.
2 M13 A great globular cluster in the constellation Hercules.
3 M31 The Andromeda galaxy, the largest member of the local group. It is a giant spiral galaxy that lies 2.4 million light-years away.
4 M32 An elliptical galaxy that orbits the Andromeda galaxy.
5 M33 The Pinwheel Galaxy, the third largest member of the Local Group, after Andromeda and the Milky Way. It is a spiral galaxy that lies 2.6 million light-years away.
1 N-Magnitude A magnitude derived from observations made at a wavelength of 10 microns.
2 N Galaxy A galaxy with a small, bright, blue nucleus superposed on a considerably fainter red background. (In the Yerkes 1974 system, a galaxy with a small nucleus containing a considerable fraction of the luminosity; N-, less pronounced N galaxies; N+, extreme examples of N galaxies.) Also, a type of radio galaxy having a brilliant, starlike nucleus containing most of the luminosity of the system. N galaxies are compact galaxies, and as a class are intermediate between Seyfert galaxies and quasars in properties of form, color, spectra, redshift, and optical and radio variability.
3 N Lines Two green forbidden lines of doubly ionized oxygen [O III]. N1 is at 5007 Angstrom; N2 at 4959 Angstrom.
4 N Star (a)Stars of spectral type N are very red stars similar to M stars except that bands of C2, CN, and CH are present instead of those of TiO. N stars are strongly concentrated toward the Galactic plane. (b) In the old terminology, the cooler C-type stars.
5 Nadir The point on the celestial sphere diametrically opposite to the zenith.
1 O Spectral type for the hottest blue stars, even hotter than B-type stars. O-type stars are rare and short-lived.
2 O Magnitude The magnitude derived from observations at 11 microns.
3 OAO Orbiting Astronomical Observatory
4 OB Spectral type O or B - that is, hot and blue.
5 O Star Stars of spectral type O are very hot blue stars with surface temperatures of about 35,000 K, whose spectra are dominated by the lines of singly ionized helium (see Pickering series). (Most other lines are from at least doubly ionized elements, though H and He I lines are also present.) O stars are useful because they are found in dust clouds and virtually define the spiral arms. Most O stars are very fast rotators. O stars have lifetimes of only 3 to 6 million years.
1 P-Branch A set of lines in the spectra of molecules corresponding to unit increases in rotational energy.
2 p-Electron An orbital electron whose l quantum number is 1.
3 P-Strong Stars A small subgroup of B-type stars in which P lines are very strong.
4 P Cyg Stars High-luminosity, early-type stars, in which all lines have a P Cyg type profile (an emission component on the red side of the absorption line).
5 pep Reaction A reaction occurring in the proton-proton chain. The first step, instead of p + p -> d + e+ + ve, is p + e- + p -> d + ve. This latter reaction occurs only once in 400 p-p reactions but produces far more energetic neutrinos (1.44 MeV as against 0.42 MeV).
1 q0 The cosmological deceleration parameter.
2 QCD Quantum Chromodynamics
3 QED Quantum Electrodynamics
4 QEH Quantum Efficiency Hysteresis An increase in QE after exposure to light.
5 QSO Quasi-Stellar Object (Quasar)
1 R-Branch A set of lines in the spectra of molecules corresponding to unit decreases in rotational energy.
2 R Galaxy In the Yerkes9 1974 system, a system showing rotational symmetry, without clearly marked spiral or elliptical structure (formerly called D galaxy).
3 r-Process The capture of neutrons on a very rapid time scale (i.e., one in which a nucleus can absorb neutrons in rapid succession, so that regions of great nuclear instability are bridged), a theory advanced to account for the existence of all elements heavier than bismuth (up to A 298) as well as the neutron-rich isotopes heavier than iron. The essential feature of the r-process is the release of great numbers of neutrons in a very short time (less than 100 seconds). The presumed source for such a large flux of neutrons is a supernova, at the boundary between the collapsing neutron star and the ejected material. However, other proposed sources have included such things as supernova shocks and black-hole-neutron-star collisions. The heavier r-process elements are synthesized at a temperature of about 109 K and an assumed neutron density of 1020-1030 per cm3. The r-process is terminated by neutron-induced fission. The existence of 244Pu (half-life 82 million years) in the early solar system shows that at least one r-process event had occurred in the Galaxy just before the formation of the solar system.
4 R Star Stars of spectral type R are stars with spectral characteristics similar to those of K stars except that molecular bands of C2, CN, and CH are present instead of TiO bands.
5 R Zones Regions in the solar corona in which short-lived radiofrequency variations are observed.
1 S Band A radiofrequency band at a wavelength of 11.1 cm.
2 s-Electron An orbital electron whose l quantum number is zero.
3 S-Factor A nuclear cross-section factor measured in keV-barns.
4 S-Matrix Scattering Matrix: A matrix representing the transitions from some initial to some final state in a given interaction. The transitions may involve changes in the number of particles in the system.
5 S-Process Slow Neutron Capture: A process in which heavy, stable, neutron-rich nuclei are synthesized from iron-peak elements by successive captures of free neutrons in a weak neutron flux, so there is time for beta-decay before another neutron is captured (cf. r-process). This a slow but sure process of nucleosynthesis which is assumed to take place in the intershell regions during the red-giant phase of evolution, at densities up to 10exp5 g cm-3 and temperatures of about 3 10exp8 K (neutron densities assumed are 10exp10 cm-3). The s-process slowly builds stable nuclear species up to A = 208 (time between captures about 10-100 years). It ends there, because any further capture of neutrons leads immediately to alpha-decay back to lead or thallium. The most likely source of neutrons for the s-process is linked to thermal instabilities in the helium shell during double shell burning after core He exhaustion. The s-process probably occurs in stars where M < 9 Msun.
1 T Associations Stellar associations containing many T Tauri stars. About 20 are known.
2 T Tauri Stars Late type irregular variables associated with bright or dark nebulosity. The spectrum exhibits emission in both CaII and H lines.
3 t-Time A time scale in which the relative motion of two observers is nonzero but unaccelerated (see -Time).
4 Tachyon (a) A hypothetical subatomic particle that can travel faster than the speed of light. (b) Particle whose mass (squared) is negative; its presence in a theory generally yields inconsistencies.
5 TAI International Atomic Time
6 Tail The long streamer (about 107 km long; density about 10-18 atm) behind the comet head. Type I Tails are straight (ionic tails): Type II Tails are curved (dust tails, little or no charge). Dust tails are usually driven by radiation pressure; ionic (gas) tails are driven by the solar wind. Comet tails do not usually appear until the comet is inside the orbit of Mars.
7 Tangential Velocity A star's velocity across an observer's line of sight. To calculate a star's tangential velocity, one must know the star's distance and proper motion.
8 Tantalum A silvery transition element. It is strong, highly resistant to corrosion, and is easily worked. Tantalum is used in turbine blades and cutting tools and in surgical and dental work. Symbol: Ta; m.p. 2996C; b.p. 5425 100C; r.d. 16.654 (20C); p.n. 73; r.a.m. 180.9479.
9 Tarantula Nebula 30 Doradus Nebula.
10 tau-Time A time scale in which there is no relative motion between two observers (cf. t-time).
1 U(1) the symmetry group associated with electromagnetic gauge invariance.
2 U line A sodium line at 3302 Angstrom
3 UBV Photometric System (a) Measurement of the astronomical color index of a star, utilizing the ultraviolet, blue and yellow visual images over two pre-set wavelengths obtained by photoelectric filtering. Other standardized filter wavebands are also used. (b) A system of stellar magnitudes devised by Johnson and Morgan at Yerkes which consists of measuring an object's apparent magnitude through three color filters: the ultraviolet (U) at 3600 Angstrom; the blue (B) at 4200 Angstrom; and the "visual" (V) in the green-yellow spectral region at 5400 Angstrom. It is defined so that, for A0 stars, B - V = U - B = 0; it is negative for hotter stars and positive for cooler stars. The Stebbins-Whitford-Kron six-color system (U, V, B, G, R, I) is defined so that B + G + R = 0.
4 UCD Ultra-Compact Dwarf galaxy.
5 UBVRI Designations for parts of the optical waveband, isolated by means of special glass filters which eliminate the unwanted regions, and used for standard astronomical intensity measurements.
6 UHF Ultra-High Frequency A radio frequency in the range between 3 GHz and 0.3 GHz (wavelength 10 cm-1 m).
7 Uhuru A satellite devoted entirely to the study of cosmic X-ray sources. It was launched off the coast of Kenya on 1970 December 12.
8 ULIRG Ultra-Luminous InfraRed Galaxy
9 Ultra-High Frequency UHF A radio frequency in the range between 3 GHz and 0.3 GHz (wavelength 10 cm-1 m).
10 Ultramicroscopic Length scales shorter than the Planck length (and also time scales shorter than the Planck time).
1 Vacancy A site on a lattice on which there is no atom present.
2 Vacuum (a) A space containing gas below atmospheric pressure. A perfect vacuum contains no matter at all, but for practical purposes soft (low) vacuum is usually defined as down to about 10-2 pascal, and hard (high) vacuum as below this. Ultra-high vacuum is lower than 10-7 pascal. (b) Roughly speaking the vacuum is a space devoid of matter, but this definition suffers from the ambiguity of the word "matter." Particle physicists therefore define the vacuum as the state of lowest possible energy density. The vacuum is not simple, since the inherently probabilistic nature of quantum theory implies that unpredictable events, such as the chance materialization of an electron and its antiparticle, the positron, can occur at any time. Such pairs have a fleeting existence of perhaps 10-21 seconds, and then annihilate into nothingness. In addition, particle theories suggest that there are Higgs fields which have nonzero values in the vacuum. (c) The state of minimum energy (or ground state) of a quantum theory. It is the quantum state in which no real particles are present. However, because of Heisenberg's uncertainty principle, the vacuum is actually seething with virtual particles which constantly materialize, propagate a short distance and then disappear. (d)Any physical system will settle into the lowest-energy state it can, which in particle physics we call its vacuum state. For most systems, this is the state where the fields making up the system are zero, but theorists hypothesize that for systems containing Higgs fields, the lowest energy occurs when the Higgs field takes on a constant value different from zero. The value of the Higgs field in that system is called its vacuum expectation value.
3 Vacuum Expectation Value The value of the Higgs field (a constant value different from zero) is called a system's Vacuum Expectation Value.
4 Vacuum Fluctuation An unpredictable event occurring in the vacuum as a result of the inherently probabilistic nature of quantum theory. Particles can materialize in conjunction with their antiparticles, and fields can undergo fluctuations in their values. (see Vacuum)
5 Vacuum Genesis Hypothesis that the universe began as nothingness, from which matter and energy arose by a process analogous to the appearance of virtual particles from a vacuum.
6 Vacuum Higgs Value The value that the Higgs field(s) have in the vacuum. (see Higgs Fields)
7 Valence Also valence band or valence electrons; the electrons in the outermost orbit.
8 Valence Band The highest completely filled energy band of a solid. In an insulator or semiconductor empty states in the valence band can carry an electric current as positively charged `holes'.
9 Valence Electron In an atom, an electron in an incompletely filled (usually outer) shell, available for chemical bonding to form a molecule.
10 Van Allen (Radiation) Belts (a) Two doughnut-shaped belts in the Earth's magnetosphere (inner belt some 3000 km above the surface; outer belt, 18,000-20,000 km above the surface), where many energetic charged particles from the solar wind are trapped in Earth's magnetic field. The energy of the particles is highest in the inner belt. (b) Two toroidal zones of high radiation in Earth's upper atmosphere, above the equator, caused by the trapping of charged particles in the magnetosphere. The outer zone is composed chiefly of electrons, the inner of protons.
1 W3 A dense cloud of gas about 3 kpc distant in the Perseus arm.
2 W44 A radio source. It is a supernova remnant about 3 kpc distant and less than 0.5 deg from the galactic plane.
3 W49 A radio source (a giant H II region) about 14 kpc distant. It is the most powerful thermal radio source known in our Galaxy.
4 W51 A radio source, a supernova remnant. PSR 1919+14 lies within its radio contours.
5 W and Z Particles Particles that transmit the unified electromagnetic and weak nuclear forces. These particles were predicted by the Weinberg-Salam theory of the 1960s and later discovered in the 1980s. (see Electroweak Theory; Weinberg-Salam Theory.)
6 W Boson see Intermediate Vector Boson.
7 Warp The deviation from flatness in the outer Galactic disk. Some parts of the outer disk lie above the Galactic plane; others lie below it.
8 Watt The SI unit of power. 1 W = 10 EXP7 ergs s EXP-1.
9 Wave A propagating pattern of disturbance. One example is a sound wave, in which a pattern of alternating high and low pressures propagates through air. Another example is an electromagnetic wave, in which a pattern of electric and magnetic fields propagates through empty space.
10 Wave Function (a) The mathematical object in quantum theory which determines probabilities of different results of experiments. It is a complex quantity, so it has an amplitude (whose square gives the probability) and a phase-angle. The phase-angle has no direct physical interpretation, but is important in interference effects, where two wave-functions are added together. (b) The mathematical description of a physical system according to the laws of quantum mechanics. The wave function tells what possible states the physical system could be in and what is the probability of being in any particular state at any given moment. (c) A mathematical function that describes the wave-mechanical state of a system (atomic or nuclear). In a one-electron atom, it yields the likelihood that the electron will be found in the neighborhood of that point (per unit volume). This interpretation can be generalized to more complicated systems.
1 X-Band A radio band at a wavelength of 3.7 cm (8085 MHz).
2 XBL X-Ray selected BL Lac Object
3 XBONG X-Ray Bright Optically Normal galaxy. X-ray satellites have now detected a sizeable number of X-ray sources spectroscopically identified with "normal" galaxies otherwise having no any obvious signs of nuclear activity in their optical spectra. The large X-ray-to-optical flux ratio exceeds the average value for early-type galaxies of similar optical lumisosity, by more than an order of magnitude. The hardness of the X-ray spectrum suggests that highly obscured AGN activity is taking place in their nuclei. However, the lack of optical emission lines could also be explained if the nuclear light is being overwhelmed by either the stellar continuum or a non-thermal component, or if the lines are not efficiently produced.
4 XMM X-ray Multi-mirror Mission.
5 Xenon A colorless odorless monatomic element of the rare-gas group. It occurs in trace amounts in air. Xenon is used in thermionic tubes and strobe lighting. Symbol: Xe; m.p. -111.9 C; b.p. -107.1C; d. 5.8971 (0C) kg m-3; p.n. 54; r.a.m. 131.29.
6 X-ogen An unidentified molecular transition at 3.36 mm (89.19 GHz) discovered in 1970.
7 X Particle Exceedingly massive (hypothetical) particle predicted by grand unified theories to convey a very short-ranged interaction between quarks and leptons. An X particle would be able to change a quark into a lepton or an antiquark.
8 X Process The unknown nucleosynthetic process that Burbidge, Burbidge, Fowler and Hoyle said had formed the light nuclei Deuterium, Lithium, Beryllium, and Boron.
9 X-Rays (a)Photons of wavelengths between about 0.1 Angstrom and 100 Angstrom - more energetic than ultraviolet, but less energetic than -rays. (b) A large band of electromagnetic radiation with wavelengths smaller than extreme ultraviolet light. A typical X-ray photon has over one thousand times as much energy as a photon of visible light.
10 X-Ray Astronomy (a) Detection of stellar and interstellar X-ray emission. Because X-rays are almost entirely filtered out by the Earth's upper atmosphere, the use of balloon- and rocket-borne equipment is essential. (b) Astronomy carried out in the waveband roughly 0.1-100 keV. The atmosphere is opaque to radiation at these wavelengths and so observations have to be carried out from above the Earth's atmosphere. It has been found that many classes of object are X-ray emitters, including stars, supernovae and active galaxies.
1 Yang-Mills Theories Also known as gauge theories, these theories were invented in 1954 by Chen Ning Yang and Robert Mills. In 1973 David Gross, Frank Wilczek, and David Politzer showed that these theories possess a property called asymptotic freedom, just what was needed for a theory of how quarks bind to form protons and neutrons. The new theory, dubbed quantum chromodynamics or QCD, proposed that the color of the quarks acts as the charge of the Yang-Mills interactions.
2 Year A period of time based on the revolution of the Earth around the Sun. The calendar year (see Gregorian calendar) is an approximation to the tropical year (see year, tropical). The anomalistic year is the mean interval between successive passages of the Earth through perihelion. The sidereal year is the mean period of revolution with respect to the background stars. (See Julian year; year, Besselian.)
3 Year (Besselian) The period of one complete revolution in right ascension of the fictitious mean sun, as defined by Newcomb. The beginning of a Besselian year, traditionally used as as standard epoch, is denoted by the suffix ".0". Since 1984 standard epochs have been defined by the Julian year rather that the Besselian year. For distinction, the beginning of the Besselian year is now identified by the prefix B (e.g., B1950.0).
4 Year (Tropical) The period of one complete revolution of the mean longitude of the sun with respect to the dynamical equinox. The tropical year is longer than the Besselian year (see year, Besselian) by 0s.148 T, where T is centuries from B1900.0.
5 Yellow Giant A giant star with a spectral type of G. The nearest and brightest yellow giants are the two composing the double star Capella.
6 Yellow Spot (Macula Lutea) An area a few millimeters across in the human retina. It has a high concentration of rods, giving high visual acuity and color vision but low sensitivity to dim light.
7 Yellow Supergiant A supergiant star with a spectral type of G
8 Yerkes System A spectral classification system for stars; see MKK system.
9 Ylem a) The word used by Gamow and his collaborators for the primordial material of the Big Bang. In most of his work Gamow assumed that the ylem consisted entirely of neutrons. In inflationary cosmology, the role of the ylem is played by the false vacuum. (b) Primordial state of matter - neutrons and their decay products (protons and electrons) - before the Big Bang. The term was taken from Aristotle and used for the alpha-beta-gamma theory.
10 Young Disk Cepheids Population I Cepheids.
1 z Notation for redshift. Objects of higher z are further away. A redshift of z near 0 corresponds to objects nearby; a redshift of z = 1 corresponds to objects at a distance of about 10 billion light years.
2 Z-number See atomic number.
3 z pinch A diffuse toroidal pinch in which the magnetic field runs around the plasma column.
4 Zanstra's theory A theory of emission lines in planetary nebulae which supposes that the emission lines in hydrogen (and helium) arise from a process of ionization (by the ultraviolet radiation of the central star) and recombination, and that the forbidden lines arise from the collisional excitations of the metastable state.
5 Zeeman effect (a) The splitting of atomic spectral lines into two or more components in a transverse magnetic field. (b) The splitting of spectral lines by a magnetic field. (c) Line broadening due to the influence of magnetic fields. A multiplet of lines is produced, with distinct polarization characteristics. The Zeeman effect is measured by measuring the difference between right-hand and left-hand polarization across a spectral line.
6 Zel'dovich Spectrum A particular prescription for how much clumping of matter should occur on each length scale. Specifically, the Zel'dovich spectrum proposes that the strength of inhomogeneity, of clumping, should be the same for each length scale at the moment when that length scale is equal to the size of the horizon. (See horizon.)
7 Zener Diode A semiconductor diode with high doping levels on each side of the junction. If the junction is reverse-biased, breakdown occurs at a well-defined potential, giving a sharp increase in current. The effect is called Zener breakdown; it occurs because electrons are excited directly from the valence band into the conduction band. Zener diodes are used as voltage regulators. See also diode.
8 Zenith (a) The point in the sky directly overhead. (b) The point on the celestial sphere directly above the observer's head - i.e., opposite to the direction of gravity (cf. nadir).
9 Zenith Distance Angular distance on the celestial sphere measured along the great circle from the zenith to the celestial object. Zenith distance is 90 deg minus altitude.
10 Zeolite An absorbent material (in the form of small pellets) used in low-temperature cryostats to trap gases released gradually after active pumping has ceased. See outgassing.
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