T TAURI STAR - Protostar in the late stages of formation, often exhibiting violent surface activity. T Tauri stars have been observed to brighten noticeably in a short period of time, consistent with the idea of rapid evolution during this final phase of stellar formation.
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.
TERRESTRIAL PLANETS - Rocky planets: Mercury, Venus, Earth, and Mars. Given its large size, the Moon may also be considered a terrestrial planet.
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.
THIN SECTION - Thin slice or rock, usually 30 µm thick. Thin sections are used to study rocks with a petrographic microscope.
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).
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.
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 - Asteroid that lies in or near one of the Lagrange points 60° ahead or behind Jupiter along the planet's orbit; Trojan asteroids have also been found accompanying Mars and Neptune.
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 MASS - Point on the main sequence where stars in a cluster have just started to move off toward the red giant branch. The mass of these stars is the turnoff.
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.
TYPE I SUPERNOVA - One possible explosive death of a star. A white dwarf in a binary system can accrete enough mass that it cannot support its own weight. The star collapses and temperatures become high enough for C fusion to occur. Fusion begins throughout the white dwarf almost simultaneously and an explosion results.
TYPE II SUPERNOVA - One possible explosive death of a >4 Msun star. The star of undergoes core collapse as a part of its stellar evolution and produces powerful shock waves which propagate outward, triggering nuclear reactions in the outer layers, and blasting material into space.
