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T Tauri star A class of very young, often flaring stars on the verge of reaching the main sequence.

telemetry The automatic measurement and transmission of data by wire, radio, or other means from remote sources, as from space vehicles, to receiving stations for recording and analysis. Telemetry is used on satellites to monitor environmental conditions, equipment parameters, the position of the satellite, etc.

telescope Instrument used to capture as many photons as possible from a given region of the sky and concentrate them into a focused beam for analysis.

temperature A measure of the amount of heat in an object, and an indication of the speed of the particles that comprise it.

thermal radiation Radiation released by virtue of an object's heat; namely, by charged particles interacting with other charged particles.

thick disk Region of a spiral galaxy where an intermediate population of stars resides, younger than the halo stars, but older than stars in the disk.

tidal bulge Elongation of the Earth caused by the difference between gravitational force on the side nearest the Moon and the force on the side farthest from the Moon. The long axis of the tidal bulge points toward the Moon. More generally, the deformation of any body produced by the tidal effect of a nearby gravitational object.

tidal force The variation in one body's gravitational force from place to place across another body - for example, the variation of the Moon's gravity across the Earth.

tides Rising and falling motion that bodies of water follow, exhibiting daily, monthly and yearly cycles. Ocean tides on Earth are caused by competing gravitational pull of the Moon and Sun on different regions of the Earth.

time dilation A prediction of the theory of relativity, closely related to the gravitational redshift. To an outside observer, a clock lowered into a strong gravitational field will appear to run slow.

total eclipse Celestial event during which one body is completely blocked from one view by another.

transmission grating A device that disperses light into a spectrum of wavelengths when it passes through a finely constructed grating. [More Info]

transverse motion Motion perpendicular to a particular line of sight, which does not result in Doppler shift in radiation received.

triangulation Method of determining distance based on the principles of geometry. A distant object is sighted from two well- separated locations. The distance between the two locations and the angle between the line joining them and the line to the distant object are all that are necessary to ascertain the object's distance.

triple-alpha process The generation of Carbon-12 from the fusion of three helium-4 nuclei (alpha particles). Helium-burning stars occupy a region of the H-R diagram known as the horizontal branch.

Tully-Fisher relation A relation used to determine the absolute luminosity of a spiral galaxy. The rotational velocity, measured from the broadening of spectral lines, is related to the total mass, and hence the total luminosity.

turbulence The disordered, irregular motion of matter, so complex as to defy description except in a statistical manner.

21-centimeter radiation Radio radiation emitted when an electron in the ground state of a hydrogen atom flips its spin to become parallel to the spin of the proton in the nucleus.

type-I supernova One possible explosive death of a star. When accretion of gas from a companion star in a binary system or merger with another white dwarf push a white dwarf star's mass over the Chandrasekhar limit of 1.4 solar masses, the temperature in the core of the white dwarf star increases, triggering explosive nuclear fusion reactions that release an enormous amount of energy. The star explodes in about ten seconds, leaving no remnant. The expanding cloud of ejecta glows brightly for many weeks as radioactive nickel produced in the explosion decays into cobalt and then iron.

type-II supernova One possible explosive death of a star, in which the massive highly evolved stellar core rapidly implodes and then explodes, destroying the surrounding star. [More Info: Field Guide]

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