T TAURI STAR - Protostar in the late stages of formation, often exhibiting both periodic and random fluctuations in brightness. T Tauri stars are newly-formed (<10 Ma) low to intermediate mass stars (< 3 Msun) with central temperatures too low for nuclear fusion to have started. For the first ~100 Ma, all emitted radiation comes from gravitational energy released as the star contracts under its own self-gravity. T Tauri stars represent an intermediate stage between true protostars (e.g. YY Orionis stars) and low-mass main sequence (hydrogen burning) stars like the Sun.
The nearest T Tauri stars are in the Taurus and r Ophiuchus molecular clouds, both ~400 light years away. Indications of stellar winds and jets show that at least some T Tauri stars are interacting with their environments. Both the winds and jets of are probably powered by material falling onto the central star from the accretion disk (or protoplanetary disk) observed around many of them. The random variability of T Tauri stars (with time-scales from minutes to years) are probably caused by instabilities in the accretion disk (which also produce the “bullets” of material seen in the jet of HH-30), flares on the stellar surface, or simple obscuration by nearby dust and gas clouds. Periodic (regular) variations (days) are almost certainly associated with huge sunspots on the stellar surface which pass into and out of view as the star rotates.
TACHYON - Speculative particle presumed to travel faster than light. According to Einstein's equations of special relativity, a particle with an imaginary rest mass and a velocity greater than the speed of light has a real momentum and energy. However, the greater the kinetic energy of a tachyon, the slower it travels (approaching the speed of light asymptotically from above). Alternatively, a tachyon losing kinetic energy travels faster and faster, until as the kinetic energy approaches zero, the speed of the tachyon approaches infinity; such a tachyon with zero energy and infinite speed is called transcendent. Special relativity does not seem to specifically exclude tachyons, so long as they do not cross the lightspeed barrier and do not interact with other particles to cause causality violations. Quantum mechanical analyses of tachyons indicate that even though they travel faster than light they would not be able to carry information faster than light, thus not violating causality.
TALC - A 1:2 Mg phyllosilicate, Mg3Si4O10(OH)2, best known for its extreme lark of hardness; crystals are rarely seen. The is little variation in the composition. Small amounts of Al3+ may replace S4+ and Fe2+ or Mn2+ may replace Mg2+. The Fe-rich end-member, minnesotaite, Fe3Si4O10(OH)2, may be isostructural with talc and a complete solid solution probably exists between it and talc.
TAENITE - Less common of the two Ni-Fe alloys found in iron meteorites; the other is kamacite. Taenite, γ-(Fe,Ni), has 27-65 wt. % Ni, and forms small crystals that appear as highly reflecting thin ribbons on the etched surface of a meteorite; the name derives from the Greek word for "ribbon."
TEKTITE - Natural, silica-rich glasses produced by melting of target rocks and dispersal as droplets during terrestrial impact events. In contrast to most impact glasses, which are found inside or within the immediate vicinity of impact structures, tektites are distal impact ejecta. They range in color from black or dark brown to gray or green and most are spherical in shape. Many specimens exhibit aerodynamic shaping caused by atmospheric passage while molten. Tektites have been found in four geographically extended deposits or "strewn fields" on Earth's surface: North America, Central Europe, Ivory Coast, and Australasia. Source craters are known for three of the four tektite strewn fields. The 11-km diameter Bosumtwi Crater (1.07 Ga) is linked to the Ivory Coast tektites, the 24-km diameter Ries Crater (15 Ma) is the source of the Central European tektites, and the 85-km diameter Chesapeake Bay impact structure (35 Ma) is the source crater of the North American tektites.
TEMPERATURE - Measure of the average energy of random motion of the constituents (e.g., molecules, atoms, photons) of a system.
TERMINATOR - Dividing line between the illuminated and shadowed portions of the lunar or planetary disk.
TERRESTRIAL PLANETS - Rocky planets: Mercury, Venus, Earth, and Mars. These planets have physical characteristics, chemical composition and internal structure similar to the Earth. The terrestrial planets have 0.4% of the total mass of all the planets in the Solar System. Some large satellites of planets are also similar to the characteristics of the terrestrial planets, and may be considered to be members of this group that are in orbit about a larger planet (e.g., Moon, Io).
TESSERA - Elevated area of complex ridged terrain found on Venus, crossed by lineations in two or more directions, having the appearance of a parquet tile pattern (pl. tesserae). There are no morphological analogs for these areas on Earth.
THERMAL DOPPLER BROADENING - Spreading of a spectral line due to the temperature of the emitting medium. In a gas, the individual atoms, elements or molecules are continuously moving in random directions, with an average speed proportional to the temperature of the gas:
The term on the left-hand side is the mean kinetic energy. The individual gas particles (with mass m) follow a Maxwellian velocity distribution, resulting in a spread of velocities (v) about the average value. On the right hand side of the equation, T is the temperature of the gas and σ is the Stefan-Boltzmann constant.
At a given time, any particular atom, element or molecule could be moving along the line-of-sight, perpendicular to the line-of-sight, or some combination of both. As a consequence, every spectral line emitted is Doppler shifted slightly higher or lower relative to the observer, broadening the spectral line. Thermal Doppler broadening is also possible for absorption lines, where particles in the absorbing medium have random motions, so Doppler shifts in the absorbed wavelengths can occur.
THERMAL RADIATION - Emitted radiation, the characteristics of which depend on the temperature of the source. Blackbody radiation emitted by stars and heated dust, and thermally excited spectral line emission are the two primary sources of thermal radiation in galaxies.
THERMODYNAMICALLY FAVORED - Substance that of the possible products for a reaction is energetically the most stable. Sometimes, the thermodynamically favored product is not observed because a less stable product is kinetically favored and exists metastabily.
THICK DISK - Features believed to exist in some, but not all disk galaxies; it clear whether the thick disk of the Milky Way Galaxy is typical of thick disks in other spiral galaxies. The thick disk contains about 10% of the stellar mass of the Milky Way and has a scale height of ~1,000 light years (the thin disk has a scale height of only ~400 light years). The greater scale height of the thick disk results from it being 'kinematically hotter' than the thin disk; thick disk stars are, on average, moving ~20 km/sec faster in a vertical direction with respect to the galactic plane than thin disk stars. In contrast, thick disk stars rotate about the Galactic center more slowly than the thin disk stars by ~40 km/sec.
The stars in thick disks are almost all older than 10 Ga. Metallicities of thick disk stars range from ~1/10–1/2 that of the Sun, with a typical value of ~1/4 solar (thin disk values range from 1/3–3 times solar. Both the differing age and chemical properties of the stellar populations suggests that the formation scenarios for the thin and thick disks were different. The thick disk probably formed early in the Galaxy's history and then ceased all star formation. The thin disk formed later and continues to form stars to this day.
The thick disk may have formed as the result of a significant merger event early in the Milky Way's history. If this is the case, the event must have taken place before the thin disk had fully formed, as both observations and N-body modeling indicate such events disrupt the disk and consume a significant fraction of the cold gas in a burst of new star formation. The presence of a substantial, uniform disk in modern times therefore indicates that any major merger must have taken place long ago. As an alternative, the thick disk formed relatively slowly by the actions of multiple minor mergers. The gas would collapse back into the galactic plane forming a thin disk, but the stars would retain the larger scale height of a thick disk. The Milky Way is currently undergoing at least two minor mergers, with the Sagittarius and Canis Major dwarf galaxies, suggesting that this could be a real possibility. However, the lack of young stars in the thick disk would again suggest that such processes must have stopped, or at least significantly slowed, a long time ago.
THIN DISK - The defining morphological component of disk galaxies in general and spiral galaxies in particular. The thin disk contains stars, star clusters, gas and dust confined to the galaxy's plane of rotation. There is much more information on the disk of the Milky Way than for spiral galaxies, but the disk of the Milky Way is considered typical.
Thin disks contain the majority of the baryonic material in spiral galaxies (on the order of 80% of the baryonic material in the Milky Way is in the thin disk). The thin disk of the Milky Way has a scale height of ~400 light years and scale length of ~10,000 light years and rotates about the galactic center at ~220 km/sec. Its outer regions appear to be warped, a phenomenon observed in ~50% of spiral galaxies. Although the origin of warps is uncertain, it is thought that they are probably the result of galaxy interactions.
The thin disks of spiral galaxies contain a lot of gas and dust, and an active site for ongoing star formation, especially in the spiral arms. For this reason, stars in the thin disk tend to be relatively young (average age ~6 Ga), although individual ages range from 0–10 Ga. This is evidence for secular evolution in thin disks. Thin disk stars also tend to be metal-rich compared to thick disk and halo stars, and typically have similar metallicities to the Sun (a thin disk star).
THIN SECTION - Thin slice or rock, usually 30 µm thick. Thin sections are used to study rocks with a petrographic microscope.
THYMINE - One of the five nitrogen-containing bases occurring in nucleotides.
TIDAL(TIDE-GENERATING) FORCE - Differential gravitational force arising because the gravitational force exerted on one body by a second body is not constant across its diameter. The side nearest to the second body experiences a greater force, while the opposite side experiences a lesser force.
TIDAL TAIL - Long tails of material that are the result of gravitational interactions between galaxies. During the interaction, gas and stars are stripped from the outer regions of the galaxies to form two tidal tails: one trailing and one preceding each galaxy. These tails may persist long after the galaxies have merged and are a signature of recent merger activity. Perhaps most famous tidal tails are those of the Antennae (NGC4038/4039), which are ~350,000 light years long. The central regions of the merging galaxies have hundreds of star forming knots similar in size to globular clusters (some are as large as dwarf galaxies). Tidal tails will persist for several billion years after the galactic merger.
The motions of tidal tails can be used as powerful probes of the shape of dark matter halos within galaxies. For example, the tidal stream from the Sagittarius Dwarf Galaxy has been used to show that the dark halo of the Milky Way is close to spherical.
TOP-DOWN STRUCTURE FORMATION - Formation of large structures, such as galaxy superclusters or perhaps even the vast filaments and voids, prior to the formation of smaller structures such as individual galaxies. Top-down structure formation occurs in cosmological models with Hot Dark Matter.
TRANSPORT PHENOMENON - Various mechanisms by which particles or quantities move from one place to another. Three common examples of transport phenomena are diffusion,convection, and radiation. There are three main types of transport phenomena: heat transfer, mass transfer, and fluid dynamics (or momentum transfer). An important principle in the study of transport phenomena is analogy between phenomena. For example, mass, energy, and momentum can all be transported by diffusion. The spreading and dissipation of an odor in air is an example of mass diffusion. Heat conduction in a solid material is an example of heat diffusion. The drag experienced by a rain drop as it falls in the atmosphere is an example of momentum diffusion (the rain drop loses momentum to the surrounding air through viscous stresses and decelerates).
TRANSISTOR - Electronic switch that allows a (relatively) large amount of current to flow when a (relatively) small voltage is applied (just like a light switch can provide a large amount of electric energy to a lamp when a small amount of mechanical energy is expended).
Transistors in modern electronics are made from layered p-type semiconductors and n-type semiconductors.
TRANSIT - When a small celestial body moves in front of a much larger one (as when Mercury or Venus appears in silhouette against the solar disk or when a satellite passes in front of Jupiter or Saturn). The shadow of a satellite may also transit the disk of its primary.
TRANSITION METALS - Elements whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell. By this definition, Zn, Cd, and Hg are excluded from the transition metals, as they have a d10 configuration.
TRANSITION STATE - Maximas (peaks) on an energy diagram. A transition state represents the highest energy arrangement of atoms between two more stable forms. The energy required to pass through a transition state may be called the activation energy, ΔGz.
TRANSLATION - Symmetry operation of shifting an object a given distance in a given direction. Translation may occur in 3 dimensions along axes that may or may not be perpendicular to one another. Crystal structures result from translation of a unit cell by the lengths of its edges along its crystallographic axes.
TRANS-NEPTUNIAN OBJECT (TNO) - Any object with a mean orbital diameter larger than that of Neptune. Pluto and its moon Charon are both TNOs, as are all the objects in the Kuiper Belt and the Oort Cloud. A list of the known TNOs is given at: http://www.cfa.harvard.edu/iau/lists/TNOs.html.
TRIDYMITE - Silica group mineral in which the tetrahedra occur in sheets. Tetrahedra alternately point up or down to share oxygen with tetrahedra of other sheets, forming six-sided rings perpendicular the sheets. Tridymite has a fairly open structure and accommodates Na+, K+ and Ca2+; charge balance is achieved by Al3+ ↔ Si4+. Tridymite is found in terrestrial volcanic rocks, lunar rocks and martian meteorites, IVA and lIE iron meteorites, eucrites, mesosiderites, and enstatite chondrites.
TRIPLE-ALPHA PROCESS - Production of 12C by fusion of 3 4He nuclei (α particles). Helium produced in hydrogen burning cannot undergo fusion reactions because of a stability bottleneck. The two most likely reactions for 4He fusion are:
However, both products are unstable, and decay before they can undergo any further reactions. Only at extremely high temperatures (~108 K), can this stability bottleneck be circumvented by a highly improbable reaction. At these temperatures, 8Be (t½ = 0.067 seconds) is formed at a high enough rate that there is always a very small equilibrium concentration at any one instant. This small concentration of 8Be fuses with 4He yielding an excited state of 12C (indicated as 12C*) that is also unstable. However, a few 12C* emit a γ-ray quickly enough to become stable before disintegrating. This extremely improbable sequence is called the "triple-alpha process" because the net reaction is:
The triple-alpha process does not occur in main sequence stars like the Sun because their central temperatures are too low. However, in the red giant phase, after many main sequence stars have consumed their H fuel, central temperatures rise high enough to initiate the triple-alpha process. (Further helium capture reactions then convert some C into O.) Thus, much of the energy for red giants comes from fusion of He into C. Helium burning stars occupy a region of the Hertzsprung-Russell diagram known as the horizontal branch.
TROILITE - Brass colored non-magnetic Fe sulfide, FeS, found in a variety of meteorites.
TROJAN ASTERIOD - Asteroid (or satellite) that orbits a body at one of the Lagrange points 60º ahead (L4) or behind (L5) a secondary body. Although Trojan asteroids have been discovered for Mars (5, all at L5) and Neptune (2), the term generally refers to the ~1800 asteroids accompanying Jupiter (~60% at L4). Although the Trojans are stabilized at the Lagrangian points by gravitational interactions with Jupiter and the Sun, their actual distribution is elongated along the orbit. Perturbations from other planets (primarily Saturn) cause the Trojans to oscillate around L4 and L5 by 15-20° and inclinations up to 40° to the orbital plane. These oscillations generally take 150-200 years to complete. The absence of Trojans at the Lagrangian points of Saturn most likely results from Jupiter removing them from these stable orbits through gravitational perturbations. Other examples of Trojan bodies in the solar system include daughter satellites that orbit in the Trojan points of the Saturn-Tethys system, and an additional satellite (Helene) which orbits in the forward Trojan point of Saturn and Dione.
TROPOSPHERE - Lowermost region of a planetary atmosphere where convection keeps the gas mixed and maintains a steady decrease of temperature with height. Most clouds are in the troposphere.
TULLY-FISHER RELATIONSHIP - Rotational velocity (vrotation) of a spiral galaxy as an indicator of its luminosity. The relation is approximately:
TURN-OFF POINT - Point on the main sequence where stars in a cluster have just started to move off toward the red giant branch.
TWENTY-ONE CENTIMETER EMISSION - Radio wavelength emission that originates with a neutral 1H atom. The proton and the electron each have a quantum "spin," which points either "up" or "down." Spins can be parallel (both of them "up" or "down"), or antiparallel (opposite states). The antiparallel state has slightly less energy than the parallel state, so if an atom in the parallel state changes to antiparallel, a 21-cm radio photon is emitted.