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T Associations

Stellar associations containing many T Tauri stars. About 20 are known. [H76]

T Tauri Stars

Late type irregular variables associated with bright or dark nebulosity. The spectrum exhibits emission in both CaII and H lines. [JJ95]


A time scale in which the relative motion of two observers is nonzero but unaccelerated (see tau-Time). [H76]


(a) A hypothetical subatomic particle that can travel faster than the speed of light. [DC99]
(b) Particle whose mass (squared) is negative; its presence in a theory generally yields inconsistencies. [G99]


International Atomic Time [LLM96]


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. [H76]

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. [C95]


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. 2996°C; b.p. 5425 ± 100°C; r.d. 16.654 (20°C); p.n. 73; r.a.m. 180.9479. [DC99]

Tarantula Nebula

30 Doradus Nebula. [H76]


A time scale in which there is no relative motion between two observers (cf. t-time). [H76]

alpha Tauri

Also know as Aldebaran. [H76]


An infrared source (an M-type Mira variable with a period of 465 days) discovered by Neugebauer, Martz, and Leighton in 1965. ( IK Tau) [H76]

Tau Ceti

A G-type main-sequence star that lies in the constellation Cetus, 11.4 light-years away. It is a single star like the Sun. [C95]

RV Tauri Stars

A class of about 100 semiregular variable yellow supergiants of late spectral type (G-K), similar to W Virginis stars but with longer periods. Their spectra often contain emission lines, and their light curves have alternating deep and shallow minima. They have a large infrared flux. RVa stars maintain an approximately constant mean brightness; RVb stars have long-term (on the order of 1000 days) periodicity. [H76]

T Tauri Stars

Eruptive variable subgiant stars associated with interstellar matter and believed to be still in the process of gravitational contraction on their way to the main sequence. They are found only in nebulae or very young clusters. They have low-temperature (G-M) spectra with strong emission lines and broad absorption lines. Their absolute magnitudes are brighter than those of main-sequence stars of similar spectral types. They have a high lithium abundance. T Tau itself is dG5e. (sometimes called RW Aurigae stars) [H76]

Tau Lepton

negatively charged lepton in the third generation of particles. A heavier analogue of the electron and muon. [D89]

Taurus A

Radio source designation for Crab Nebula.

Taylor Column

A column that occurs over a fixed region in a rotating fluid because of the two-dimensional character of the motion in the absence of viscosity. This phenomenon is used to interpret the Red Spot of Jupiter. [H76]

Taylor Instability

A hydrodynamic instability which occurs whenever there is a density inversion. This configuration is said to be Taylor unstable (or Rayleigh-Taylor unstable) against perturbations that would cause mixing of layers of unequal densities. [H76]


Transmission Control Protocol. [LLM96]


Barycentric Dynamical Time


Tidal Dwarf Galaxy. A self-gravitating entity of dwarf-galaxy mass, built from tidal material expelled during interactions. Examples include AM0547-244, AM1054-325 and AM1353-272 (see Weilbacher, et al., Astron. Astrophys., 358, 819, 2000)


Time Delay and Integration or drift-scan. Methods for averaging the response of a CCD along columns by reading out at the same rate as a mechanical motion is shifting the optical image along the CCD. [McL97]


Transmission Data and Relay Satellite System. [LLM96]


Terrestrial Dynamical Time. The independent argument for apparent geocentric ephemerides. At 1977 January 1d00h00m00s TAI, the value of TDT was exactly 1977 January 1d.0003725. The unit of TDT is 86400 SI seconds at mean sea level. For practical purposes TDT = TAI + 32s.184. (see Barycentric Dynamical Time; Dynamical Time; International Atomic Time.) [S92]


(a) Radioactive element with atomic number 43. It was seen in red giants in 1952; because it is unstable, its presence indicated that the stars themselves had made it. [C95]
(b) A transition metal that does not occur naturally on Earth. It is produced artificially by bombarding molybdenum with neutrons and also during the fission of uranium. It is radioactive.
Symbol: Tc; m.p. 2172°C; b.p. 4877°C; r.d. 11.5 (est.); p.n. 43; r.a.m. 98.9063 (99Tc); most stable isotope 98Tc (half-life 4.2 × 106 years). [DC99]
(b) An unstable element which does not exist naturally on Earth. The longest-lived isotope 97Tc has a half-life of 2.6 million years. (Only 99Tc, half-life 2.1 × 105 years, can be produced by the s-process.) Technetium is found only in the spectra of MS, S, and N variable stars. [H76]


A small glassy body containing no crystals, probably of meteoritic origin and bearing no antecedent relation to the geological formation in which it is found. [H76]


A device for gathering and amplifying light or other energy. Refracting telescopes gather light by means of a lens, reflecting telescopes by means of a mirror. Radiotelescopes gather radio energy typically by using a metallic dish antenna. Telescopes have also been built that can gather X rays, gamma rays, and other forms of energy. [F88]

Telluric Lines

Spectral lines or bands that originate from absorption by gases such as O2, H2O, or CO2 in the Earth's atmosphere. [H76]


A brittle silvery metalloid element belonging to group 16 of the periodic table. It is found native and in combination with metals. Tellurium is used mainly as an additive to improve the qualities of stainless steel and various metals.
Symbol: Te; m.p. 449.5°C; b.p. 989.8°C; r.d. 6.24 (20°C); p.n. 52; r.a.m. 127.6. [DC99]


A measure of the average kinetic energy of the particles of a system. [H76]


Celsius. This mercury-in-glass scale was devised as early as 1710 and was used by Linnaes at Uppsala certainly before 1737. The zero of the scale represents the melting point of ice and the boiling point of water is taken to be 100 degrees. In continental Europe the scale has always been known as the Celsius scale in the mistaken belief that it was invented by Anders Celsius (1701-1744), whereas Celsius proposed a scale which had zero for the boiling point of water and 100 for the melting point of ice. The scale was inverted by J. P. Christen (1683-1755) in 1743. In England the scale was originally called the Centigrade scale but this name was abandoned in favour of Celsius in 1948.

Fahrenheit. This scale was composed by G. D. Fahrenheit (1686-1736) between 1710 and 1714. Three fixed temperature points were used - the temperature of an ice and salt mixture, the freezing point of water and normal human temperature - which were taken to be 0, 32 and 96 respectively. It is mere coincidence that the temperature interval between the freezing (32°F) and boiling (212°F) points of water is 180° when expressed in the Fahrenheit scale.

Reaumur. An arbitrary scale in which the freezing and boiling points of water are taken to be 0 and 80°R respectively. R. A. F. Reaumur (1683-1757) deduced his scale in 1730 from the thermal expansion of an alcohol and water mixture. When he considered the `length' to be 1000 units at the ice point he found that at the boiling point the length had expanded to 1080 units, hence the peculiar figure of 80 in his scale.

Thermodynamic. In this use is made of the fact that the temperature of a body is considered to be a measure of the average kinetic energy per degree of freedom of the constituent molecules. This concept of temperature can be applied only to a body consisting of a large number of molecules. For the purpose of establishing a scale, two points are used to determine it. The first is zero temperature corresponding to zero energy. The second is the temperature of the triple point of water (0.0 1°C), i.e. the point at which the solid, liquid and gaseous phase of pure water are in equilibrium and this is defined as being equal to 273.16 degrees when these are degrees on the Celsius temperature scale. The base unit of the thermodynamic scale is the kelvin (K) defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. Kelvin temperatures represent the same temperature interval as those on the Celsius scale and are obtained by adding 273.15 to the Celsius temperature. Also used occasionally to indicate the thermodynamic temperature is the degree Rankine (°R), defined as 5/9 of the kelvin. Rankine temperatures have the same temperature interval as those on the Fahrenheit scale and are obtained by adding 459.69 to the Fahrenheit temperatures. The scales are named after Lord Kelvin (1824-1907) and W. J. M. Rankine (1820-1872), both of whom held chairs at Glasgow University and who, along with Joule, established the thermodynamic temperature scale in the middle of the nineteenth century. In Glazebrook's Dictionary of Applied Physics (1923) there is a suggestion that the present Kelvin and Rankine scales should be called the tercentesimal (273 approx 300) and quinquentesimal (1460 approx 500) scales respectively. The Kelvin scale is also called the Absolute scale. In 1968 the CGPM decided to replace the term degree Kelvin (°K) by the single word kelvin, for which the abbreviation is K.

The fundamental points on the thermodynamic temperature scales are given in below.

Fundamental Points of the Thermodynamic Temperature Scales

Temperature Value at
Unit                Symbol      Absolute Zero   Triple Point of Water

Degree Kelvin       °K            0             273.16
Degree Celsius      °C         -273.15            0.01
Degree Rankine      °R            0            491.688
Degree Fahrenheit   °F         -459.67          32.018

International Temperature Scale (ITS-90). This scale was introduced in 1927 to overcome the practical difficulties of the direct realization of temperature by gas thermometry and to provide a practical temperature scale which could be easily and accurately reproduced. Initially it had six fixed points, but these were increased at its revision in 1948. The scale was reissued as the International Practical Scale (IPTS-68) in 1968 when its lowest fixed point was reduced from 54.261°K (the triple point of oxygen) to 13.81°K (the triple point of hydrogen). However, its upper limit remained unchanged at 1064.43°C (1337.58°K), the freezing point of gold. Between these two limits there were nine accurately defined fixed points, one of which is the triple point of water, 0.01°C. The scale was revised in 1975 and, in 1989, the CIPM approved the issue of a new scale to be known as the International Temperature Scale (ITS-90). This covers temperatures from 0.65°K to the highest temperatures practicably measurable in terms of the Planck radiation law using monochromatic radiation. Between these two limits there are 17 fixed points. Temperatures below 5°K are defined in terms of the vapour pressures of 3He and 4He and the fixed points between 5°K and the triple point of water are marked by the ° triple points of certain elements, e.g. neon (24.5561 K). Those above 0.01°C are defined with reference to the freezing points of specified metallic elements, one of which is gold. However, in the ITS-90 scale this is given as 1064.18°C, which is 0.25°C less than specified in the IPTS-68 scale. Finally, the unit of the fundamental physical quantity known as thermodynamic temperature is defined as the kelvin (K), which is given as 1/273.16 of thermodynamic temperature of the triple point of water. Furthermore, ITS-90 gives the relationship between the thermodynamic and the Celsius scales as t90 = T90 - 273.16, where T90 and t90 are in kelvins and degrees Celsius respectively.

A brief account of the IPTS and the Chappius constant volume scale which it replaced was given by J. A. Hall in 1967.

Helium Scale. In 1958 international agreement was obtained to use the vapour pressure of Helium-4 as an indication of temperature in the region 1 to 5.2°K.

Curie Temperature Scale. This is sometimes used for indicating temperature in the vicinity of absolute zero. It is based on Curie's law, which states that the susceptibility of a paramagnetic material is approximately proportional to its absolute temperature. Curie Temperatures are sometimes called Magnetic Temperatures. [JM92]


A unit of time equal to the length of time it takes light to cross the classical radius of an electron (about 10-23) seconds). [H76]


A prefix meaning 1012. [H76]


A soft ductile malleable silvery rare element of the lanthanoid series of metals. It occurs in association with other lanthanoids. One of its few uses is as a dopant in solid-state devices.
Symbol: Tb; m.p. 1356°C; b.p. 3123°C; r.d. 8.229 (20°C); p.n. 65; r.a.m. 158.92534. [DC99]

Terminal Velocity

The steady final velocity reached by a body in a fluid when the resultant force on it is zero.[DC99]


The boundary between the illuminated and dark areas of the apparent disk of the Moon, a planet or a planetary satellite. [S92]


Pertaining to the Earth.

Terrestrial Dynamical Time

TDT The independent argument for apparent geocentric ephemerides. At 1977 January 1d00h00m00s TAI, the value of TDT was exactly 1977 January 1d.0003725. The unit of TDT is 86400 SI seconds at mean sea level. For practical purposes TDT = TAI + 32s.184. (see Barycentric Dynamical Time; Dynamical Time; International Atomic Time.) [S92]

Tertiary (mirror)

The third mirror to be encountered by the light in a telescope system. A tertiary mirror is required on alt-az telescopes to direct light to the stationary Nasmyth foci. [McL97]


The derived SI unit of magnetic flux density. 1 Tesla = 1 Wb m-2 = 104gauss. [H76]


Fourth satellite of Saturn, discovered by Cassini in 1684. Diameter about 1000 km; P = 1.87 days. [H76]


Equal to one teraelectron volt, or 1,000 GeV. [F88]


A particle accelerator capable of attaining an energy of 1 TeV. [F88]


A soft malleable grayish metallic element belonging to group 13 of the periodic table. Thallium is highly toxic and was used previously as a rodent and insect poison. Various compounds are now used in photocells, infrared detectors, and low-melting glasses.
Symbol: Tl; m.p. 303.5°C; b.p. 1457°C; r.d. 11.85 (20°C); p.n. 81; r.a.m. 204.3833. [DC99]


A satellite of Saturn discovered by Pickering in 1900, but since lost. [H76]


An optical instrument (using prisms and lenses) for measuring horizontal and vertical angles with great accuracy. It is an essential tool in surveying. [DC99]


(a) A rationally coherent account of a wider range of phenomena than is customarily accounted for by a hypothesis. [F88]
(b) The word theory is usually used precisely in physics. Theories are not conjectures but sets of equations whose solutions describe physical systems and their behavior. [K2000]

Theory of Everything

A "Theory of Everything" would not only describe how thing s work but also explain why things are the way they are. The name is unfortunate in one way, because it does not tell how to deduce the behavior of complex systems from a knowledge of their components. In this book I have used the name primary theory instead. [K2000]

Thermal Background

The radiation emitted by the telescope and the atmosphere at infrared wavelengths due to the heat (temperature) of the source. [McL97]

Thermal Bremsstrahlung

A mode of X-ray production by very energetic electrons accelerated in the field of a positive ion. [H76]

Thermal Convection

The energy transfer in a fluid by a mechanism of bulk hydrodynamic movement. [D89]

Thermal Diffusion

A method of separating gas molecules of different masses by maintaining one part of the gas at a lower temperature than the other (i.e. producing a temperature gradient along a column of gas) - the more massive molecules tend to stay at the low-temperature end. It can be used for the separation of isotopes. [DC99]

Thermal Energy

Energy associated with the motions of the molecules, atoms, or ions in a substance. [H76]

Thermal Equilibrium

(a) A state in which there is no net flow of heat. If two bodies are in thermal equilibrium, then they have the same temperature. (see also Equilibrium) [DC99]
(b) A simple theoretical description of the early universe is possible because the primordial soup had time to reach thermal equilibrium, the state for which the relative abundances of different constituents is determined not by the initial conditions, but instead by the criterion that the production and annihilation rates for any particular constituent are in balance. As long as there are reactions that can convert one constituent to another, the ratios of those two constituents will be determined by thermal equilibrium. For example, protons and neutrons can interconvert by the reactions

proton + electron <-> neutron + neutrino,

where the double arrow indicates that either the left-hand side or the right-hand side can represent the initial particles. If we imagine that the universe began with only protons, then this reaction would cause some to convert to neutrons. As the density of neutrons increased, the rate at which neutrons are converted back to protons would increase. The reactions would continue until there were just enough neutrons so that the number of neutrons converting to protons each second would equal the number of protons converting to neutrons. As another example, thermal equilibrium implies that the photons in the early universe did not have some arbitrary distribution, but were instead described by the intensity and spectrum of black body radiation. [G97]
(c) The condition of a system in which all its parts have exchanged heat and come to the same temperature. An isolated system in thermal equilibrium does not change over time. This is also a state of maximum disorder. [LB90]

Thermal Equilibrium, Law of

The temperature of a body in equilibrium is the same at all points (also called zeroth law of thermodynamics). [H76]

Thermal Noise

Low-frequency electromagnetic radiation associated with thermal fluctuations, which is emitted by all bodies whose temperature is above 0 K. Also known as Johnson noise. [H76]

Thermal Radiation

Blackbody radiation; radiation caused by the high temperature of the radiating objects, as opposed to nonthermal radiation, which is caused by energetic (not necessarily hot) electrons. [H76]


An atomic or molecular transition is thermalized when the Boltzmann factor for the two levels of the transition takes on the value it would have in thermodynamic equilibrium. [H76]


An ion, either positive or negative, which has been emitted by a heated body. Negative thermions are electrons. [H76]

Thermodynamic Equilibrium

(a) The state reached ultimately by an isolated system.[D89]
(b) The condition of a system whose members have conformed to the principle of equipartition of energy, so that there is no net exchange of energy. [H76]

Thermodynamic Potential

a function of the state of a system which takes its extreme value on the asymptotically stable state reached by the system in the course of time. [D89]


(a) The study of the behavior of heat (and, by implication, other forms of energy) in changing systems. [F88]
(b) Laws developed in the nineteenth century to describe aspects of heat, work, energy, entropy, and their mutual evolution in a physical system. [G99]

Thermodynamics Laws

The first is the law of conservation of energy; the second is the law of entropy. (see also Nernst Theorem) [H76]

Thermohaline Convection

A type of hydrodynamic instability. [H76]

Theta Pinch

A fusion device in which the magnetic field runs parallel to the plasma column. It is a long cylindrical tube enclosed in a one-turn magnet coil. [H76]

Thick Disk

The stellar population that contains Arcturus and about 4 percent of the other stars near the Sun. It has a scale height of about 3500 light-years and consists of old stars. [C95]

Thin Disk

The stellar population that contains the Sun and most other nearby stars. Most of its stars have a scale height of 1000 light-years and orbit the Galaxy on fairly circular orbits. The stars of the thin disk range in age from 0 to about 10 billion years. The thin disk breaks into two subpopulations, the young thin disk (ranging in age from 0 to 1 billion years) and the old thin disk (ranging in age from 1 to about 10 billion years). The young thin disk has a smaller scale height than the old thin disk, and the former's stars have more circular orbits. [C95]

Thin-Screen Model

A model in which Gaussian angular scattering is concentrated near one point along the path. [H76]

Thirring Effect

An effect predicted by general relativity, which causes the dragging of the inertial frame outside a rotating mass. As a pulsar, for example, rotates, it drags along the inertial frames, both inside and outside. (see also Lense-Thirring Effect) [H76]

Thomas-Fermi Theory

A theory of the energy of partially ionized matter in the limit of high density (see also Boltzmann-Saha Theory) [H76]

Thomson Scattering

The limit of Compton scattering at low energies. [H76]


A toxic radioactive element of the actinoid series that is a soft ductile silvery metal. It has several long-lived radioisotopes found in a variety of minerals including monazite. Thorium is used in magnesium alloys, incandescent gas mantles, and nuclear fuel elements.
Symbol: Th; m.p. 1780°C; b.p. 4790°C (approx.); r.d. 11.72 (20°C); p.n. 90; r.a.m. 232.0381. [DC99]

Thought Experiment

An experiment that cannot be or is not carried out in practice, but can, given sufficient imagination and rigor, be reasoned through by thought and intuition alone. [F88]

3alpha Process

A nuclear reaction (3 4He rightarrow 12C + gamma + 7 MeV) by which helium is transformed into carbon. The process is dominant in red giants. At a temperature of about 2 × 108 K and a density of 105 g cm-3, after core hydrogen is exhausted, three alpha-particles can fuse to form an excited nucleus of 12C, which occasionally decays into a stable 12C nucleus. The overall process can be looked upon as an equilibrium between three 4He nuclei and the excited 12C*, with occasional irreversible leakage out of the equilibrium into the ground state of 12C. Further capture of alpha-particles by 12C nuclei produces 16O and 20Ne. (also called the triple-alpha process) [H76]

3-Kpc Arm

A component of the Sagittarius Arm with noncircular gas motions. It is seen in absorption against Sgr A with a velocity of -53 km s-1, implying that at least part of the arm is expanding away from the galactic center. The nearest "edge" is presently at a radius of 4 kpc from the Galactic center. [H76]

Three-Body Problem

Eighteenth- and nineteenth- century problem in celestial mechanics to analyze the gravitational effects of three celestial bodies in finding a stable orbital configuration. [A84]

Three-Phase CCD

A CCD construction in which three overlapping metal electrodes are used to define a pixel and effect the transfer of charge, in either direction along a column, by the charge-coupling method. If only two electrodes are used then the device is two-phase. [McL97]

Threshold Energy

Difference between the energy at the first excited level and that of the ground state. [H76]


A measure of the efficiency of an optical system. Formally, the product of the solid angle accepted and the effective collecting aperture. (also referred to as étendue.) [McL97]


(alpha Dra) A fourth-mag A0 star. It was the "Pole Star" at the time the Egyptians built the Pyramids. [H76]


A soft malleable ductile silvery element of the lanthanoid series of metals. It occurs in association with other lanthanoids.
Symbol: Tm; m.p. 1545°C; b.p. 1947°C; r.d. 9.321 (20°C); p.n. 69; r.a.m. 168.93421. [DC99]


TeraHertz. [LLM96]

Tidal Force

The differential gravitational pull exerted on any extended body in the gravitational field of another body. [Silk90]

Tidal Theory

The theory of the origin of the solar system involving the near collision of a massive body with the sun. The original version of a tidal theory, due to Buffon (1785), considered passage of a comet, but modern versions of this theory invoke a passing star. The gaseous debris torn from the sun by tidal forces is supposed to have condensed into the planets; however, this theory has been replaced by the nebular theory. [Silk90]


(a) A differential gravitational force. In the Galaxy, a tide results because the Milky Way's gravity pulls more strongly on the side of an object facing the Galactic center than on the object's other side, so the object may get torn apart. [C95]
(b) Effect of the Moon's gravity on Earth's seas, such that an oceanic "bulge" each side of the Earth follows the Moon's progress around the planet. [A84]


A dimension distinguishing past, present, and future. In relativity, time is portrayed as a geometrical dimension, analogous to the dimensions of space. [F88]

Time Delay

see Dispersion. [H76]

Time Dilation

Feature emerging from special relativity in which the flow of time slows down for an observer in motion. [G99]

Time-Like Path

A path whose tangent obeys U · U < 0. In relativity, all material particles travel along timelike paths. [H76]

Timing Waveform

A diagram showing the time sequence and voltage levels of a stream of pulses required, for instance, to perform charge-coupling in a CCD. [McL97]

A white lustrous metal of low melting point. Its electronic structure has outer s2p2 electrons ([Kr]4d105s25p2). The element is of low abundance in the Earth's crust (0.004%) but is widely distributed. Tin has three crystalline modifications or allotropes, alpha-tin or `gray tin' (diamond structure), beta-tin or `white tin', and gamma-tin; the latter two are metallic with close packed structures. Tin also has several isotopes. It is used in a large number of alloys including Babbit metal, bell metal, Britannia metal, bronze, gun metal, and pewter as well as several special solders.
Symbol: Sn; m.p. 232°C; b.p. 2270°C; r.d. 7.31 (20°C); p.n. 50; r.a.m. 118.710. [DC99]
Tincle The track of a charged particle in a meteorite. (track in the cleavage)[H76]
Tired Light

The hypothesis that light may be degraded in energy, thereby increasing in wavelength and becoming redshifted, during its passage through intergalactic space. This would provide an alternative to the Big Bang model in accounting for the redshifts of distant galaxies. However, there is no evidence for any such tired-light effect. [Silk90]


(a) Seventh (known) moon out from Saturn, and its largest. It is possibly also the largest satellite in the Solar System (although Neptune's Triton may be proved to be larger). It is 20% larger than the planet Mercury and is known to have an atmosphere. [A84]
(b) The largest and brightest (albedo = 0.21) satellite of Saturn, discovered by Huyghens in 1655. R approx 2900 km (about the size of Mercury), period (orbital and spin) 15d22h41m. H2 and CH4 have been discovered in its atmosphere. [H76]


Fourth satellite of Uranus, discovered by Herschel in 1787, R approx 850 km; P = 8d17h. [H76]


A silvery transition metal. It is used in the aerospace industry as it is strong, resistant to corrosion, and has a low density. It forms compounds with oxidation states +4, +3, and +2, the +4 state being the most stable.
Symbol: Ti; m.p. 1660°C; b.p. 3287°C; r.d. 4.54 (20°C); p.n. 22; r.a.m. 47.867.[DC99]


Theory of Everything A quantum-mechanical theory that encompasses all forces and all matter. [G99]


A type of "magnetic bottle" used in experiments on controlled nuclear fusion. (the name is a Russian acronym) [H76]


The sixth flavor of quark. [CD99]

Top-Down Scenario

A scenario of galaxy formation in which large structures form first and then fragment to become galaxies. [c97]


With reference to, or pertaining to, a point on the surface of the Earth, usually with reference to a coordinate system. [S92]

Topological Defects
Topologically Distinct

Two shapes that cannot be deformed into one another without tearing their structure in some manner. [G99]


(a) The branch of mathematics that treats the `large-scale' structure of curved spaces. Topological properties of a geometrical space are those properties that are unchanged by continuous distortion of the space. An example is that in two-dimensional space a simple closed curve divides the space into two regions, one inside the curve and the other outside. [D89]
(b) The global property of a surface that indicates what other surfaces it can be deformed into without changing the number of interior holes. For example, a solid cylinder has the same topology as a sphere because a cylinder may be reshaped into a sphere without adding holes. A donut has a different topology than a sphere because it cannot be reshaped into a sphere without removing the hole in its middle. A donut has the same topology as a coffee cup with one handle. [LB90]
(c) Classification of shapes into groups that can be deformed into one another without ripping or tearing their structure in any way. [G99]

Topology-Changing Transition

Evolution of spatial fabric that involves rips or tears, thereby changing the topology of space. [G99]


An Earth-crossing asteroid (No. 1685) discovered by Wirtanen in 1948 and rediscovered in 1964, of irregular shape (about 5 × 3 km), whose closest approach takes it within 0.13 AU of Earth. Perihelion distance 0.77 AU; aphelion distance 1.96 AU; e = 0.44. Orbital period 584.2 days (8/5 that of Earth); rotation period 10h11m; a = 1.37 AU. Radar observations indicate a rocky surface, thinly covered with dust. High albedo (leq 0.15). [H76]


A unit of pressure equal to 1/760 of an atmosphere, or about 1 mm Hg. [H76]

Torsional Wave

A wave motion in which the vibrations in the medium are rotatory simple harmonic motions around the direction of energy transfer. [DC99]


The topological name for the shape of a donut. While a donut is a two-dimensional surface in a three-dimensional space, the torus can be generalized to higher numbers of dimensions. [P88]

Toy Theory

A theory which is known to be too simple to describe reality, but which is nonetheless useful for theorists to study because it incorporates some important features of reality. For example, most of what is known about magnetic monopoles in grand unified theories was discovered first in a toy theory that includes only three Higgs fields, while the simplest realistic grand unified theory includes twenty-four of them. [G97]

Transient X-Ray Sources

As of early 1974, four had been detected: Cen X-2, Cen X-1, 2U 1543-47, and Cep X-4. They resemble slow novae. [H76]


The passage of a smaller, nearer astronomical object across the face of a larger object in the background, as in a transit of Venus across the sun. [F88]

Transit Circle

Large instrument for the accurate observation and measurement of a transit. [A84]

Transit Telescope

A stationary support structure for a telescope. Motion is allowed along the meridian from the zenith to the horizon, but stars cannot be tracked east/west. Measurements are only possible when the objects "transit" the meridian due to the Earth's rotation. [McL97]

Transit-Time Effect

The time required for the radiation to travel from the source to the object which reflects, or absorbs and reemits, it to the observer. [H76]

Transition Metal

One of the metals such as iron, manganese or platinum in the centre of the periodic table. [D89]

Transition Probability

The probability that a system in one energy state will undergo a transition into another. Associated with any given pair of energy levels are three transition probabilities: the spontaneous-emission probability, the absorption probability, and the stimulated-emission probability. [H76]

Transition Radiation

Radiation emitted (in the X-ray region) when energetic charged particles pass through an interface between two media of different dielectric properties. [H76]


Able to pass radiation, but with much deviation and/or absorption. Compare transparent. [DC99]


The ratio of the transmitted energy that a substance allows through to the energy incident upon it. (Transmission Coefficient) [DC99]


Able to pass radiation without significant deviation or absorption. Note that a substance transparent to one radiation may be opaque to another. The divide between transparency and translucency is not well defined. Thus some people call a filter transparent (as it does not distort radiation): others would call it translucent (as it absorbs some of the radiation). (see also Translucent) [DC99]


A compact computer chip with a special design for linking to other transputers to make the program run faster. [McL97]

Transuranic Elements

Elements that have a proton number greater than 92 (i.e. greater than that of uranium). The transuranic elements are all unstable (radioactive) and are produced by nuclear reactions induced either by bombarding heavy elements with high-energy nuclei of light elements or by the capture of slow neutrons, followed by beta decay. In nuclear explosions, transuranic elements may be produced by numerous successive captures of neutrons by uranium nuclei. [DC99]

Transverse Velocity

see Tangential Velocity [H76]

Transverse Wave

A wave motion in which the motion or change is perpendicular to the direction of energy transfer. Electromagnetic waves are examples of transverse waves. (see Polarization) [DC99]

Transverse Waves

Waves vibrating at right angles to the direction of propagation - e.g., electromagnetic waves. [H76]


Irregularities in the silicon crystal lattice which can absorb free charges created in the semiconductor by, for instance, the absorption of light. [McL97]


(theta 1 Ori) Four very young stars (O6-B3) (there are six stars in the system) in the center of the Orion Nebula. They form the vertices of a trapezoid. [H76]

Trapped Surface

A surface (e.g., of a black hole) from which light cannot escape to infinity. [H76]


Measurement of the distance of a planet or nearby star by sighting its apparent position against background stars from two or more separate locations. (see Parallax) [F88]

Triangulum Galaxy

A Sc II-III spiral galaxy, a satellite of the Andromeda Galaxy, about 700 kpc distant. Mv = -18.9 mag (M33, NGC 598) [H76]

Trifid Nebula An emission nebula in Sagittarius, ~ 1 kpc distant. (M20, NGC 6514)[H76]

A thousand billion (1012) in American usage. [F88]

Triple-alpha process

see 3alpha process. [H76]

Triple Point

The only point at which the gas, solid, and liquid phases of a substance can coexist in equilibrium. The triple point of water (273.16 K) is used to define the kelvin. Some substances (e.g. carbon dioxide) have pressures at the triple point greater than atmospheric. Such materials can appear as liquids only when under pressure. [DC99]

Triple Star

A star system having three stars that revolve around one another. [A84]


(a) A short-lived isotope of hydrogen in which each nucleus contains one proton and two neutrons, instead of only one proton as in normal hydrogen. Hydrogen-3 the heaviest isotope of hydrogen. Deuterium is another isotope of hydrogen. Tritium does not exist naturally on Earth; it has a half-life of 12 years and a mass of 3.016 amu.
(b) Symbol: T, 3H A radioactive isotope of hydrogen of mass number 3. The nucleus contains 1 proton and 2 neutrons. Tritium decays with emission of low-energy beta radiation to give 3He. The half-life is 12.3. years. Tritium is found in the atmosphere, possibly as a result of the bombardment of nitrogen by neutrons from cosmic rays:

14N + 1n rightarrow 12C + 3H

It is useful as a tracer in studies of chemical reactions. Compounds in which 3H atoms replace the usual 1H atoms are said to be tritiated. [DC99]

Triton (tau)

The nucleus of the tritium atom. [H76]


The inner satellite of Neptune, discovered by Lassell in 1846. It is larger than the Moon (R approx 2900 km), with an almost circular retrograde orbit of 5 days 21 hours. [H76]


The asteroids located at the points of Jupiter's orbit around the Sun that are equidistant from the Sun and Jupiter (see Lagrangian Points). The first Trojan (Achilles) was discovered in 1906. About 15 are now known. [H76]

Tropical Year

The interval of time between two successive vernal equinoxes. It is equal to 365.242 mean solar days. [H76]


Upper boundary of the troposphere (about 15 km), where the temperature gradient goes to zero. [H76]


Lowest level of Earth's atmosphere, from zero altitude to about 15 km above the surface. This is the region where most weather occurs. Its temperature decreases from about 290 K to 240 K. [H76]

True Anomaly

The angle, measured at the focus nearest the pericenter of an elliptical orbit, between the pericenter and the radius vector from the focus to the orbiting body; one of the standard orbital elements (see Elements; Orbital; Eccentric Anomaly; Mean Anomaly) [S92]

True Equator and Equinox

The Celestial Coordinate System determined by the instantaneous positions of the celestial equator and ecliptic. The motion of this system is due to the progressive effect of precession and the short-term, periodic variations of nutation. (see Mean Equator and Equinox.) [S92]

True Vacuum

This phrase has the same meaning as vacuum, with the word "true" being used only to emphasize the distinction with the false vacuum. [G97]

Trumpler Stars

A class of extremely luminous (and formerly considered extremely massive) stars. [H76]

Tsytovitch Effect

An effect wherein the index of refraction of a medium is much less than unity so that the phase velocity of electromagnetic waves is greater than the speed of light in the medium. In this case, a relativistic electron can no longer keep in phase with the waves it generates, and the intensity of synchrotron radiation is very much reduced. (also called the Razin-Tsytovitch Effect.) [H76]

T Tauri Stars

Luminous variable stars with low effective temperatures and strong emission lines, associated with interstellar gas clouds and found in very young clusters. They are believed to be still in the process of gravitational contraction from their protostellar phase and have not yet arrived at the main sequence and begun to burn hydrogen. (see also Nebular Variable Stars) [Silk90]

47 Tucanae

A metal-rich globular cluster about 5.1 kpc distant. It has roughly one-quarter the solar metal abundance. It has a high galactic latitude and low reddening. [H76]

Tully-Fisher Relation

An observed relation between the intrinsic luminosity of a spiral galaxy and the rotational speed of its stars. More luminous galaxies have stars that are moving faster. [LB90]


A transition metal, formerly called wolfram. It is used as the filaments in electric lamps and in various alloys.
Symbol: W; m.p. 3410 ± 20°C; b.p. 5650°C; r.d. 19.3 (20°C); p.n. 74; r.a.m. 183.84. [DC99]

Tunnel Diode

A highly doped p-n junction diode that has a large reverse current, and, in the forward direction, a negative slope resistance over part of the voltage-current characteristic.
The `Tunnel Effect' explains the shape of the characteristic. Electrons are able to tunnel through the conduction band, which is normally forbidden to them because they do not possess sufficient energy to cross the potential barrier of the junction. (see also Diode; Semiconductor) [DC99]

Tunnel Effect

The passage of a particle through a potential barrier, even though it has not enough energy to pass the barrier on classical grounds. The tunnel effect can be explained by quantum mechanics. (see Tunnel Diode) [DC99]


A phenomenon in quantum mechanics whereby a particle has a nonzero chance of crossing a potential barrier into a region which in classical mechanics would be forbidden to it. Tunneling is a direct consequence of the wave nature of material particles. [H76]


The appearance of a star as a disk in a long-exposure photograph, due to the scattering of adjacent continuum light by emulsion grains. [H76]


a hydrodynamic flow characterized by an irregular space and time dependence. Chaotic dynamics and fractal geometry constitute the natural models capturing the essence of ae turbulence. [D89]

Turbulent Flow

Fluid flow in which the speed at any point varies rapidly in magnitude and direction. Fluid flow becomes turbulent when its speed increases beyond a critical speed. This corresponds to a critical value of the Reynolds number that depends on the geometry of the system. For highly turbulent flow, the resistance to motion is proportional to the product of the density of the fluid, the speed squared, and the square of the linear dimensions. Compare with Laminar Flow. (see also Reynolds Number) [DC99]

Turn-Off Point

The point on the H-R diagram at which stars turn off from the Main Sequence. The brighter the turn-off point, the younger the cluster age.

21-cm Radiation

The emission line (in the radio range at a frequency of 1420 MHz) of neutral (atomic) hydrogen, caused when an electron "flips" from spinning in a direction parallel to the proton's spin to the opposite direction of lower energy. Spontaneous transitions from one level to another occur only once every 11,000,000 years on the average for a single electron, but there are so many billions of electrons in the Milky Way that this radiation can be detected by radio telescopes. (The analogous deuterium line is at 91.6 cm.) First detected in 1951; 2 years later extragalactic H I was detected. [H76]


The interval of time preceding sunrise and following sunset (see Sunrise; unset) during which the sky is partially illuminated. Civil twilight comprises the interval when the zenith distance, referred to the center of the Earth, of the central point of the Sun's disk is between 90°50' and 96°, nautical twilight comprises the interval from 96° to 102°, astronomical twilight comprises the interval from 102° to 108°. [S92]

Twin-Exhaust Model

A theoretical model for radio galaxies in which a compact source in the galactic nucleus is assumed to emit twin beams of rapidly moving plasma that traverse hundreds of thousands of light-years, eventually splattering to a halt in the ambient intergalactic gas, where the resulting dissipation energizes the radio lobes. [Silk90]


A form of wiring consisting of two strands of single wire twisted together to form a transmission line. One strand carries the signal and the other is grounded to the single "star" ground point of the system. [McL97]


The twistor is a sort of generalization of a spinor, being a massless object having both linear and angular momentum. It can be defined in terms of a pair of spinors. Twistors are the coordinates of twistor space, but they also have a geometrical interpretation in space-time. Twistors with zero helicity correspond to null lines while more general twistors must be pictured as congruences of null lines. [P88]

Twistor Theory

Model of the Universe proposed by Roger Penrose, based on the application of complex numbers (involving (-1)1/2) used in calculations in the microscopic world of atoms and quantum theory to the macroscopic ordinary world of physical laws and relativity. The result is an eight-dimensional concept of reality that although complicated is possibly a more logical understanding of the constitution of the Universe. [A84]

Two-Component Model

A model of the solar wind which has two thermal components - electron and proton gases of differing temperatures. [H76]

Tycho's Star

Remnant of a Type I supernova (B Cas), 3-5 kpc distant, which Tycho observed and described in 1572. At its peak it was as bright as Venus and was visible in the daytime, reaching a magnitude of about -4. It is an X-ray source (2U 0022+63). (3C 10) [H76]

Type I String Theory

One of the five superstring theories; involves both open and closed strings. [G99]

Type IIA String Theory

One of the five superstring theories; involves closed strings with left-right symmetric vibrational patterns. [G99]

Type IIB String Theory

One of the five superstring theories; involves closed strings with left-right asymmetric vibrational patterns. [G99]

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