tail: Gas and dust left behind as a comet orbits sufficiently close to the Sun, illuminated by sunlight.
temperature-luminosity diagram: A diagram of a group of stars with temperatures on the horizontal axis and intrinsic brightness on the vertical axis; also called a temperature-magnitude diagram or a Hertzsprung-Russell diagram.
terminator: The line between night and day on a moon or planet; the edge of the part that is lighted by the Sun.
terrestrial planets: Mercury, Venus, Earth and Mars.
theory: A later stage of the traditional form of the scientific method, in which a hypothesis has passed enough of its test that it is generally accepted.
thermal pressure: Pressure generated by the motion of particles that can be characterized by temperature.
thermal radiation: Radiation whose distribution of intensity over wavelength can be characterized by a single number (the temperature). Black-body radiation, which follows Planck’s law, is thermal radiation.
thermosphere: The uppermost layer of the atmosphere of the Earth and some other planets, the ionosphere, where absorption of high-energy radiation heats the gas.
3º background radiation: The isotropic black-body radiation at 3 K, thought to be a remnant of the big bang.
tide: A periodic variation of the force of gravity on a body, based on the difference in the strength of gravitational pull from one place on the body to another (the tidal force); on Earth, the tide shows most obviously where the ocean meets the shore as a period variation of sea level.
time dilation: According to relativity theory, the slowing of time perceived by an observer watching another object moving rapidly or located in a strong gravitational field.
TRACE: The Transition Region and Coronal Explorer, a solar satellite to study the solar corona and the transition region between the chromosphere and corona at high spatial resolution.
transit: The passage of one celestial body in front of another celestial body. When a planet is in transit, we understand that it is passing in front of the Sun, Also, transit is the moment when a celestial body crosses an observer’s meridian, or the special type of telescope used to study such events.
transition zone: The thin region between a chromosphere and a corona.
Trans-Neptunian Object (TNO): One of the sub-planetary objects in the Kuiper belt; it was proposed but not accepted that Pluto should be considered as one.
transparency: Clarity of the sky.
transverse velocity: Velocity along the plate of the sky.
trigonometric parallax: see parallax.
triple-alpha process: A chain of fusion processes by which tree helium nuclei (alpha particles) combine to form a carbon nucleus.
Trojan asteroids: A group of asteroids that precede or follow Jupiter in its orbit by 60º.
Tropical year: The length of time between two successive vernal equinoxes.
troposphere: The lowest level of the atmosphere of the Earth and some other planets, in which all weather takes place.
T Tauri star: A type of irregularly varying star, like T Tauri, whose spectrum shows broad and very intense emission lines. T Tauri Stars have presumably not yet reached the main sequence and are thus very young.
tuning-fork diagram: Hubble’s arrangement of types of elliptical, spiral, and barred spiral galaxies.
21-cm line: The 1420-MHz line from neutral hydrogen’s spin-flip.
twinkle: A scintillation—rapid changing in brightness—and slight changing in position of stars as their light passes through the Earth’s atmosphere.
Type la supernova: A supernova whose distribution in all types of galaxies, and the lack of hydrogen in its spectrum, make us think that it is an event in low-mass stars, probably resulting from the incineration of a white dwarf in a binary system.
Type II supernova: A supernova associated with spiral arms, and that has hydrogen in its spectrum, making us think that it is the explosion of a massive star.
ultraviolet: The region of the spectrum 100-4000 Å, also used in the restricted sense of ultraviolet radiation that reaches the ground, namely, 2000-4000 Å.
umbra (plural: umbrae): (a) of a sunspot, the dark central region; (b) of an eclipse shadow, the part from which the Sun cannot be seen at all.
uncertainty principle: Heisenberg’s statement that the product of uncertainties of position and momentum is greater than or equal to Planck’s constant. Consequently, both position and momentum cannot be known to infinite accuracy.
universal gravitation constant: The constant G of Newton’s law of gravity: force = Gm1m2/r2.