Glossary items by letter: t — z

T-Tauri Star

A class of very young, flaring stars on the verge of becoming normal stars fueled by nuclear fusion.


A tail is made up of dust and gas from a comet’s coma. A tail forms when the solar wind separates dust and gas from the coma, pushing it outward and away from the Sun in either a slightly curved path (for dust) or a straight path (for gas).


An instrument used to observe distant objects by collecting and focusing their electromagnetic radiation. Telescopes are usually designed to collect light in a specific wavelength range. Examples include optical telescopes that observe visible light and radio telescopes that detect radio waves.


A measure of the amount of heat energy in a substance, such as air, a star, or the human body. Because heat energy corresponds to motions and vibrations of molecules, temperature provides information about the amount of molecular motion occurring in a substance.


A measure of computer data storage capacity equal to approximately a thousand billion bytes (or a thousand gigabytes). In computer language, a byte of information represents a letter or digit. So, a thousand billion bytes is equal to a thousand billion letters.


Planets whose density and chemical makeup are similar to those of Earth.

Terrestrial Planets

The four planets of the inner solar system (Mercury, Venus, Earth, and Mars) are called terrestrial planets because they are made up mostly of rock.


An accepted idea used to explain nature. Theories not only explain an observed event, they can also be used to predict what will happen. Sometimes, an idea that is really a hypothesis is incorrectly called a theory. A true scientific theory is a hypothesis that makes predictions. Those predictions have been tested and have proven to be accurate.

Thermal Radiation

Radiation released by virtue of an object’s heat, namely, the transfer of heat energy into the radiative energy of electromagnetic waves. Examples of thermal radiation are sunlight, the orange glow of an electric range, and the light from in incandescent light bulb.

Titanium Oxides

Minerals composed of oxygen and the metal titanium. Titanium oxides frequently contain other metals. One such titanium oxide is the mineral ilmenite, which contains titanium, oxygen, and iron. Ilmenite is found in both lunar rock and Earth rocks.

Tracking and Data Relay Satellite System (TDRSS)

A network of four communication satellites used to relay data and commands to and from U.S. spacecraft, including the Hubble Space Telescope. The Goddard Space Flight Center provides the day-to-day management and operations of TDRSS, the first space-based global tracking system.


The largest of Neptune’s satellites. Triton has an atmosphere and is roughly the size of Earth's moon. It has an “ice cap” of frozen nitrogen and methane with “ice volcanoes” that erupt liquid nitrogen, dust, and methane compounds from beneath its frozen surface.


Unstable and disorderly motion, as when a smooth, flowing stream becomes a churning rapid.

Ultraviolet (UV)

Electromagnetic radiation with shorter wavelengths and higher energies and frequencies than visible light. UV light is lower in frequency than X-rays.

Ultraviolet (UV) Light

The part of the electromagnetic spectrum that has slightly higher energy than visible light, but is not visible to the human eye. Just as there are high-pitched sounds that cannot be heard, there is high-energy light that cannot be seen. Too much exposure to ultraviolet light causes sunburns.


The totality of space and time, along with all the matter and energy in it. Current theories assert that the universe is expanding and that all its matter and energy was created during the Big Bang.


The third largest planet in the solar system and the seventh from the Sun. Uranus is 19 times the Earth’s distance from the Sun and completes a circuit around the Sun in about 84 Earth years. This gaseous, giant outer planet has a visible ring system and over 20 moons, the largest of which is Titania. Uranus is tipped on its side, with a rotation axis in nearly the same plane as its orbit.

Van Allen Belt

A region containing charged particles trapped in the Earth’s magnetic force field (magnetosphere). The belt’s lower boundary begins at about 800 kilometers (496 miles) above the Earth’s surface and extends thousands of kilometers into space.

Variable Star

A star whose luminosity (brightness) changes with time.

Vela Satellite

Launched by the U.S. in the 1960s to monitor the Limited Nuclear Test Ban Treaty. The satellite’s mission was to detect the gamma rays produced during nuclear blasts. Although not intended for astronomical studies, the Vela satellite provided useful celestial data, detecting an unexpected blast of cosmic gamma radiation in 1967. The satellite discovered several other gamma-rays bursts during the years of the Vela project, which ceased operation in 1979.


The speed of an object moving in a specific direction. A car traveling at 35 miles per hour is a measurement of speed. Observing that a car is traveling 35 miles per hour due north is a measurement of velocity.


An inner, terrestrial (rocky) planet that is slightly smaller than Earth. Located between the orbits of Mercury and Earth, Venus has a very thick atmosphere that is covered by a layer of clouds that produce a “greenhouse effect” on the planet. Venus’s surface temperature is roughly 480° C (900° F), making it the hottest planet in the solar system.

Very Large Array (VLA)

One of the world’s premier radio observatories, consisting of 27 antennas arranged in a huge “Y” pattern. The VLA spans up to 22 miles (36 km) across, which is roughly one and a half times the size of Washington, D.C. Each antenna is 81 feet (25 meters) in diameter. Located in Socorro, New Mexico, the telescopes work in tandem to produce a sharper image than any single telescope could record.

Visible Light

The part of the electromagnetic spectrum that human eyes can detect; also known as the visible spectrum. The colors of the rainbow make up visible light. Blue light has more energy than red light.


A break or vent in the crust of a planet or moon that can spew extremely hot ash, scorching gases, and molten rock. The term volcano also refers to the mountain formed by volcanic material.


A vibration in some media that transfers energy from one place to another. Sound waves are vibrations passing in air. Light waves are vibrations in electromagnetic fields.


The distance between two wave crests. Radio waves can have lengths of several feet; the wavelengths of X-rays are roughly the size of atoms.

Wavelength and frequency

Light is measured by its wavelength (in nanometers) or frequency (in Hertz).

One wavelength
equals the distance between two successive wave crests or troughs.

Frequency (Hertz)
equals the number of waves that passes a given point per second.

Weak Force

The force that governs the change of one kind of elementary particle into another. This force is associated with radioactive processes that involve neutrons.

White Dwarf Star

The hot, compact remains of a low-mass star like our Sun that has exhausted its sources of fuel for thermonuclear fusion. White dwarf stars are generally about the size of the Earth.

Wide Field Camera 3 (WFC3)

This new camera replaced the Wide Field and Planetary Camera 2 during Servicing Mission 4. WFC3 has the latest CCD (charge-coupled device) technology and optical coatings which provide a broader range of colors, spanning ultraviolet, visible, and near-infrared wavelengths. WFC3 will greatly enhance Hubble’s observational capabilities by studying a diverse range of objects and phenomena, from early and distant galaxy formation to nearby planetary nebulae, and finally our own backyard — the planets and other bodies of our solar system.

Wide Field / Planetary Camera (WF/PC)

A collection of eight separate, yet interconnected, cameras originally used as the main optical instrument on the Hubble Space Telescope. Four cameras were used in tandem to observe in either wide-field, low-resolution mode or narrow-field, high-resolution (“planetary”) mode. The Wide Field and Planetary Camera 2 replaced the WF/PC during the December 1993 servicing mission.

Wide Field and Planetary Camera 2 (WFPC2)

The Hubble Space Telescope’s “workhorse” instrument, WFPC2 snapped high-resolution images of faraway objects. Its 48 filters allowed scientists to study precise wavelengths of light and to sense a range of wavelengths from ultraviolet to near-infrared light. The instrument’s four CCDs (charge-coupled devices) collected information from stars and galaxies to make photographs. WFPC2 was installed aboard the Hubble telescope during the December 1993 servicing mission and was replaced by Wide Field Camera 3 in 2009 during Servicing Mission 4.


The part of the electromagnetic spectrum with energy between ultraviolet light and gamma rays. X-rays are used in medicine to detect broken bones and cavities in teeth. Astronomers can detect X-rays from exploding stars and black holes.


Identifies the magnifying power of a lens or mirror. For example, a 50-power telescope makes the image 50 times larger than it is when viewed without the telescope.

X-ray sources

Celestial objects that give off X-rays. These exotic objects are producing very energetic radiation and include black holes, neutron stars (pulsars), supernovae remnants, and the centers of galaxies.

X-ray telescope

A special telescope used to detect X-rays – high-energy electromagnetic radiation. The high energy of X-rays means they will go through rather than bounce off a “normal” telescope mirror. Instead, the mirrors are arranged so the X-rays skip across them much like a stone skips across the surface of a lake.


The point on the celestial sphere that is directly above the observer. Holding a balloon overhead places the balloon at your zenith. Although celestial objects appear to rise and set as they move across the sky, they rarely reach the zenith point.

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