Glossary

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tangential (transverse) velocity  The component of a star's space velocity that lies in the plane of the sky.
temperature A measure of how fast the particles in a body are moving or vibrating in place; a measure of the average heat energy in a body.
temperature (Celsius; formerly centigrade) Temperature measured on scale where water freezes at 0 degrees and boils at 100 degrees. 
temperature (color) The temperature of a star as estimated from the intensity of the stellar radiation at two or more colors or wavelengths.
temperature (effective) The temperature of a blackbody that would radiate the same total amount of energy that a particular object, such as a star, does.
temperature (excitation) The temperature of a star as estimated from the relative strengths of lines in its spectrum that originate from atoms in different stages of excitation.
temperature (Fahrenheit) Temperature measured on a scale where water freezes at 32 degrees and boils at 212 degrees.
temperature (ionization) The temperature of a star as estimated from the relative strengths of lines in its spectrum that originate from atoms in different stages of ionization.
temperature (Kelvin) Absolute temperature measured in Celsius degrees, with the zero point at absolute zero.
terrestrial planet  Any of the planets Mercury, Venus, Earth, or Mars; sometimes the Moon or Pluto are included in the list.
theory A set of hypotheses and laws that have been well demonstrated to apply to a wide range of phenomena associated with a particular subject.
thermal equilibrium A balance between the input and outflow of heat in a system.
thermal radiation The radiation emitted by any body or gas that is not at absolute zero.
transition region The region in the Sun's atmosphere where the temperature rises very rapidly from the relatively low temperatures that characterize the chromosphere to the high temperatures of the corona.
triple-alpha process A series of two nuclear reactions by which three helium nuclei are built up into one carbon nucleus. 
Tropic of Cancer The parallel (circle) of latitude 23.50 N.
Tropic of Capricorn The parallel (circle) of latitude 23.5' S.
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