A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

 

A

Absolute Brightness (Absolute Magnitude)--A measure of the true brightness of an object. The absolute brightness or magnitude of an object is the apparent brightness or magnitude it would have if it were located exactly 32.6 light-years (10 parsecs) away. For example, the apparent brightness of our Sun is much greater than that of the star Rigel in the constellation Orion because it is so close to us. However, if both objects were placed at the same distance from us, Rigel would appear much brighter than our Sun because its absolute brightness is much larger.

Absolute Zero--The coldest possible temperature, at which all molecular motion stops. On the Kelvin temperature scale, this temperature is the zero point (0 K), which is equivalent to –273° C and –460° F.

Absorption--The process by which light transfers its energy to matter. For example, a gas cloud can absorb starlight that passes through it. After the starlight passes through the cloud, dark lines called absorption lines appear in the star’s continuous spectrum at wavelengths corresponding to the light-absorbing elements.

Absorption Line--A dark line in a continuous spectrum caused by absorption of light. Each chemical element emits and absorbs radiated energy at specific wavelengths, making it possible to identify the elements present in the atmosphere of a star or other celestial body by analyzing which absorption lines are present.

Accelerating Universe--A model for the universe in which a repulsive force counteracts the attractive force of gravity, driving all the matter in the universe apart at speeds that increase with time. Recent observations of distant supernova explosions suggest that we may live in an accelerating universe.

Accretion Disk--A relatively flat, rapidly rotating disk of gas surrounding a black hole, a newborn star, or any massive object that attracts and swallows matter. Accretion disks around stars are expected to contain dust particles and may show evidence of active planet formation. Beta Pictoris is an example of a star known to have an accretion disk.

Active Galactic Nucleus (AGN)--A very bright, compact region found at the center of certain galaxies. The brightness of an active galactic nucleus is thought to come from an accretion disk around a supermassive black hole. The black hole devours matter from the accretion disk, and this infall of matter provides the firepower for quasars, the most luminous type of active galactic nucleus.

Active Galaxy--A galaxy possessing an active galactic nucleus at its center.

Advanced Camera For Surveys (ACS)--An optical camera aboard the Hubble Space Telescope that covers twice the area, has twice the sharpness, and is up to ten times more efficient than the telescope’s Wide Field and Planetary Camera 2. The camera’s wavelength range spans from ultraviolet to near-infrared light. The camera’s sharp eye and broader viewing area enable astronomers to study the life cycles of galaxies in the remotest regions of the cosmos. Astronauts installed the camera aboard the telescope in March 2002.

Afterglow--The fading fireball of a gamma-ray burst — a sudden burst of gamma rays from deep space — that is observable in less energetic wavelengths, such as X-ray, optical, and radio. After an initial explosion, an expanding gamma-ray burst slows and sweeps up surrounding material, generating the afterglow, which is visible for several weeks or months. The afterglow is usually extremely faint, making it difficult to locate and study.

Alpha Process--A process by which lighter elements capture helium nuclei (alpha particles) to form heavier elements. For example, when a carbon nucleus captures an alpha particle, a heavier oxygen nucleus is formed.

Amplitude--The size of a wave from the top of a wave crest to its midpoint.

Angular Momentum--A property that an object, such as a planet revolving around the Sun, possesses by virtue of its rotation or circular motion. An object’s angular momentum cannot change unless some force acts to speed up or slow down its circular motion. This principle, known as conservation of angular momentum, is why an object can indefinitely maintain a circular motion around an axis of revolution or rotation.

Angular Resolution--The ability of an instrument, such as a telescope, to distinguish objects that are very close to each other. The angular resolution of an instrument is the smallest angular separation at which the instrument can observe two neighboring objects as two separate objects. The angular resolution of the human eye is about a minute of arc. As car headlights approach from a far-off point, they appear as a single light until the separation between the lights increases to a point where they can be resolved as two separate lights.

Angular Size--The apparent size of an object as seen by an observer; expressed in units of degrees (of arc), arc minutes, or arc seconds. The moon, as viewed from the Earth, has an angular diameter of one-half a degree.

Antimatter--Matter made up of elementary particles whose masses are identical to their normal-matter counterparts but whose other properties, such as electric charge, are reversed. The positron is the antimatter counterpart of an electron, with a positive charge instead of a negative charge. When an antimatter particle collides with its normal-matter counterpart, both particles are annihilated and energy is released.

Apparent Brightness (Apparent Magnitude)--A measure of the brightness of a celestial object as it appears from Earth. The Sun is the brightest object in Earth's sky and has the greatest apparent magnitude, with the moon second. Apparent brightness does not take into account how far away the object is from Earth.

Arc Minute--One arc minute is 1/60 of a degree of arc. The angular diameter of the full moon or the Sun as seen from Earth is about 30 arc minutes.

Arc Second--One arc second is 1/60 of an arc minute and 1/3600 of an arc degree. The apparent size of a dime about 3.7 kilometers (2.3 miles) away would be an arc second. The angular diameter of Jupiter varies from about 30 to 50 arc seconds, depending on its distance from Earth.

Association of Universities for Research in Astronomy, Inc. (AURA)--A consortium of educational and other non-profit institutions that operates world-class astronomical observatories. Members include five international affiliates and 29 U.S. institutions, including the Space Telescope Science Institute in Baltimore, Maryland, the science operations center for NASA’s Hubble Space Telescope.

Asteroid--A small, rocky object revolving around the Sun, sometimes called a minor planet or planetoid. The vast majority of asteroids is found in the asteroid belt, a region between the orbits of Mars and Jupiter. The largest known asteroid, Ceres, has a diameter of 926 kilometers (579 miles).

Asteroid Belt--A region of space between Mars and Jupiter where the great majority of asteroids is found.

Astronomer--A scientist who studies the universe and the celestial bodies that reside in it, including their composition, history, location, and motion. Many of the scientists at the Space Telescope Science Institute are astronomers. Astronomers from all over the world use the Hubble Space Telescope.

Astronomical Unit (AU)--The average distance between the Earth and the Sun, which is about 150 million kilometers (93 million miles). This unit of length is commonly used for measuring the distances between objects within the solar system.

Atmosphere--The layer of gases surrounding the surface of a planet, moon, or star.

Atom--The smallest unit of matter that possesses chemical properties. All atoms have the same basic structure: a nucleus containing positively charged protons with an equal number of negatively charged electrons orbiting around it. In addition to protons, most nuclei contain neutral neutrons whose mass is similar to that of protons. Each atom corresponds to a unique chemical element determined by the number of protons in its nucleus.

Atomic Nucleus--The positively charged core of an atom consisting of protons and (except for hydrogen) neutrons, and around which electrons orbit.

Aurora--A phenomenon produced when the solar wind (made up of energized electrons and protons) disturbs the atoms and molecules in a planet’s upper atmosphere. Some of the energy produced by these disturbances is converted into colorful visible light, which shimmers and dances. Auroras have been seen on several planets in our solar system. On Earth, auroras are also known as the “Northern Lights” (aurora borealis) or “Southern Lights” (aurora australis), depending on in which polar region they appear.

B

Barred Spiral Galaxy--A galaxy with a “bar” of stars and interstellar matter, such as dust and gas, slicing across its center. The Milky Way is thought to be a barred spiral galaxy.

Baseline--The distance between two or more telescopes that are working together as a single instrument to observe celestial objects. The wider the baseline, the greater the resolving power.

BATSE (Burst and Transient Source Experiment)--A high-energy astrophysics “experiment” used to investigate gamma-ray bursts (GRBs). BATSE consisted of eight detectors that were mounted on the corners of NASA’s Earth-orbiting Compton Gamma-Ray Observatory, whose mission ended in 2000.

BeppoSAX--A space-based X-ray observatory built and operated by the Italian Space Agency and the Netherlands Agency for Aerospace Programs. BeppoSAX has been instrumental in identifying and locating gamma-ray bursts.

Big Bang--A broadly accepted theory for the origin and evolution of our universe. The theory says that the observable universe started roughly 15 billion years ago from an extremely dense and incredibly hot initial state.

Binary Star System--A system of two stars orbiting around a common center of mass that are bound together by their mutual gravitational attraction.

Black Hole--A region of space containing a huge amount of mass compacted into an extremely small volume. A black hole’s gravitational influence is so strong that nothing, not even light, can escape its grasp. Swirling disks of material — called accretion disks — may surround black holes, and jets of matter may arise from their vicinity.

Blue Star--A massive, hot star that appears blue in color. Spica in the constellation Virgo is an example of a blue star.

Blueshift--The shortening of a light wave from an object moving toward an observer. For example, when a star is traveling toward Earth, its light appears bluer.

Bolide--Large, brilliant meteors that enter the Earth’s atmosphere. Friction between a fast-moving meteor and Earth’s air molecules generates tremendous heat, which causes the meteor to heat up, glow, and perhaps disintegrate. In some cases, the meteor literally explodes, leaving a visible cloud that dissipates slowly.

Brown Dwarf--An object too small to be an ordinary star because it cannot produce enough energy by fusion in its core to compensate for the radiative energy it loses from its surface. A brown dwarf has a mass less than 0.08 times that of the Sun.

Bulge--The spherical structure at the center of a spiral galaxy that is made up primarily of old stars, gas, and dust. The Milky Way’s bulge is roughly 15,000 light-years across.

C

Carbonaceous Chondrite--A meteorite with embedded pebble-sized granules that contain significant quantities of organic (complex carbon-rich) matter.

Celestial Sphere--An imaginary sphere encompassing the Earth that represents the sky. Astronomers chart the sky using the celestial coordinates of the sphere to locate objects in the cosmos. This sphere is divided into 88 sections called constellations. Objects are sometimes named for the major constellation in which they appear.

Celsius (Centigrade) Temperature Scale--A temperature scale on which the freezing point of water is 0° C and the boiling point is 100° C.

Cepheid Variable--A type of pulsating star whose light and energy output vary noticeably over a set period of time. The time period over which the star varies is directly related to its light output or luminosity, making these stars useful standard candles for measuring intergalactic distances.

Chandra X-Ray Observatory--A space-based X-ray observatory; also known as the Advanced X-ray Astrophysics Facility (AXAF). Chandra is designed to observe X-rays from high-energy regions of the universe, such as hot gas in the remnants of exploded stars. The satellite was launched and deployed in July 1999.

Charge-Coupled Device (CCD)--An electronic detector that records visible light from stars and galaxies to make photographs. These detectors are very sensitive to the extremely faint light of distant galaxies. They can see objects that are 1,000 million times fainter than the eye can see. CCDs are electronic circuits composed of light-sensitive picture elements (pixels), tiny cells that, placed together, resemble mesh on a screen door. The same CCD technology is used in digital cameras. The Hubble Space Telescope’s Wide Field and Planetary Camera 2 has four CCDs; each contains 640,000 pixels. The light collected by each pixel is translated into a number. These numbers (all 2,560,000 of them) are sent to ground-based computers, which convert them into an image.

Chemical Compound--A pure substance consisting of atoms or ions of two or more different elements. The elements are in definite proportions. A chemical compound usually possesses properties unlike those of its constituent elements. For example, table salt (the common name for sodium chloride) is a chemical compound made up of the elements chlorine and sodium.

Chemical Evolution--The chemical (i.e., pre-biological) changes that transformed simple atoms and molecules into the more complex chemicals needed for the origin of life. For example, hydrogen atoms in the cores of stars combine through nuclear fusion to form the heavier element helium.

Chromosphere--The middle layer of the solar atmosphere between the photosphere and the corona. The chromosphere is roughly 10,000 kilometers (6,200 miles) thick and is composed primarily of hydrogen. It varies in temperature from below 10,000 Kelvin (18,000° F) to over 100,000 Kelvin (180,000° F).

Closed Universe--A geometric model of the universe in which the overall structure of the universe closes upon itself like the surface of a sphere. The rules of geometry in a closed universe are like those that would apply on the surface of a sphere.

Collecting Area--The area of a telescope’s primary light-collecting mirror. A telescope’s light-gathering power rises with an increase in its collecting area.

Colliding Galaxies--A galactic “car wreck” in which two galaxies pass close enough to gravitationally disrupt each other’s shape. The collision rips streamers of stars from the galaxies, fuels an explosion of star birth, and can ultimately result in both galaxies merging into one.

Collisional Process--An event involving a collision of objects; for example, the excitation of a hydrogen atom when it is hit by an electron.

Color--The visual perception of light that enables human eyes to differentiate between wavelengths of the visible spectrum, with the longest wavelengths appearing red and the shortest appearing blue or violet.

Comet--A ball of rock and ice, often referred to as a “dirty snowball.” Typically a few kilometers in diameter, comets orbit the Sun in paths that either allow them to pass by the Sun only once or that repeatedly bring them through the solar system (as in the 76-year orbit of Halley's Comet). A comet’s “signature” long, glowing tail is formed when the Sun’s heat warms the coma or nucleus, which releases vapors into space.

Comet Shoemaker-Levy 9 (SL-9)--A comet that became gravitationally bound to Jupiter, colliding with the planet in July 1994. Prior to entering the planet’s atmosphere, the comet broke into several distinct pieces, each with a separate coma and tail.

Compton Gamma-Ray Observatory (CGRO)--A space-based observatory that collected high-energy gamma-ray light from celestial objects. The Compton satellite consisted of the BATSE, COMPTEL, EGRET, and OSSE instruments. Astronauts aboard the space shuttle Atlantis deployed the CGRO into low-Earth orbit in April 1991. The satellite plunged into the Pacific Ocean in June 2000.

Conservation of Energy And Mass--A fundamental law of physics, which states that the total amount of mass and energy in the universe remains unchanged. However, mass can be converted to energy, and vice versa.

Constellation--A geometric pattern of bright stars that appears grouped in the sky. Ancient observers named many constellations after gods, heroes, animals, and mythological beings. Leo (the Lion) is one example of the 88 constellations.

Convection--The transfer of heat through a liquid or gas caused by the physical upwelling of hot matter. The heat transfer results in the circulation of currents from lower, hotter regions to higher, cooler regions. An everyday example of this process is boiling water. Convection occurs in the Sun and other stars.

Convection Zone--The region below a star's surface where energy flows outward by the rising of hot gas known as convection.

Core--The central region of a planet, star, or galaxy.

Corona--The outermost layer of the atmosphere of a star, including the Sun. The corona is visible during a solar eclipse or when special adapters or filters are attached to a telescope to block the light from the star’s central region. The gaseous corona extends millions of kilometers from the star’s surface and has a temperature in the millions of degrees.

Coronal Hole--Regions in the corona from which the high-speed solar wind is known to originate. Coronal holes, usually found near the Sun's poles, are large regions in the corona that are less dense and cooler than the surrounding region.

Cosmic Abundances--The relative proportions of chemical elements in the Sun, the solar system, and the local region of the Milky Way galaxy. These proportions are determined by studies of the spectral lines in astronomical objects and are averaged for many stars in our cosmic neighborhood. For example, for every million hydrogen atoms in an average star like our Sun, there are 98,000 helium atoms, 360 carbon atoms, 110 nitrogen atoms, 850 oxygen atoms, and so on.

Cosmic Microwave Background--Radiative energy filling the universe that is believed to be the radiation remaining from the Big Bang. It is sometimes called the “primal glow.” This radiation is strongest in the microwave part of the spectrum but has also been detected at radio and infrared wavelengths. The intensity of the cosmic microwave background from every part of the sky is almost exactly the same.

Cosmic Rays--High-energy atomic particles that travel through space at speeds close to the speed of light; also known as cosmic-ray particles.

Cosmological Principle--This principle states that the distribution of matter across very large distances is the same everywhere in the universe and that the universe looks the same in all directions. According to this principle, our view of the universe is like the view from a boat on an ocean, which is essentially the same for any other person on any other boat on any other ocean. Measurements of matter and energy in the universe on the largest observable scales support the cosmological principle.

Cosmology--The investigation of the origin, structure, and development of the universe, including how energy, forces, and matter interact on a cosmic scale.

Crater--A bowl-shaped depression caused by a comet or meteorite colliding with the surface of a planet, moon, or asteroid. On geologically active moons and planets (like Earth), craters can result from volcanic activity.

Critical Density--The minimum average density that matter in the universe would need in order for its gravitational pull to slow the universe’s expansion to a halt.

D

Dark Dust Cloud--A region of interstellar space that contains a rich concentration of gas and dust. Such a cloud is often irregular in shape but sometimes has a well-defined edge. Visible light cannot pass through these clouds, so they obscure the light from stars beyond them.

Dark Matter--Matter that is too dim to be detected by telescopes. Astronomers infer its existence by measuring its gravitational influence. Dark matter makes up most of the total mass of the universe.

Declination (DEC)--One of two celestial coordinates required to locate an astronomical object, such as a star, on the celestial sphere. Declination is the measure of angular distance of a celestial object above or below the celestial equator and is comparable to latitude. To familiarize yourself with declination, hold out your arm in the direction of the North Star (Polaris). You are now pointing at plus 90 degrees declination. Move your arm downward by 90 degrees. You are now pointing at 0 degrees declination.

Degree of Arc--One degree of arc is 1/360 of a full circle. The apparent sizes of objects as seen from Earth can be measured in degrees of arc. The angular diameter of the full moon or the Sun as seen from Earth is one-half of a degree.

Density--The ratio of the mass of an object to its volume. For example, water has a density of one gram of mass for every milliliter of volume.

Deuterium--A special form of hydrogen (an isotope called “heavy hydrogen”) that has a neutron as well as a proton in its nucleus.

Differentiation--The separation of heavy matter from light matter, thus causing a variation in density and composition. Differentiation occurs in an object like a planet as gravity draws heavier material toward the planet’s center and lighter material rises to the surface.

Diffraction Grating--A device that splits light into its component parts or spectrum. A diffraction grating often consists of a mirror with thousands of closely spaced parallel lines, which spread out the light into parallel bands of colors or distinct fine lines or bars.

Digital Image--A visible image that is recorded by an electronic detector and subdivided into small picture elements (pixels). Each element is assigned a number that corresponds to the brightness recorded at its physical location on the detector. Computer software converts the numerical information into a visual image. The Hubble Space Telescope records digital images.

Doppler Effect--The change in the wavelength of sound or light waves caused when the object emitting the waves moves toward or away from the observer; also called Doppler Shift. In sound, the Doppler Effect causes a shift in sound frequency or pitch (for example, the change in pitch noted as an ambulance passes). In light, an object’s visible color is altered and its spectrum is shifted toward the blue region of the spectrum for objects moving toward the observer and toward the red for objects moving away.

Dwarf Galaxy--A relatively small galaxy. The Large and Small Magellanic Clouds, visible in the Southern Hemisphere, are two dwarf irregular galaxies that are neighbors of the Milky Way.

E

Earth--The third planet from the Sun and one of four terrestrial planets in the inner solar system. Earth, the only planet where water exists in large quantities, has an atmosphere capable of supporting myriad life forms. The planet is 150 million kilometers (93 million miles) away from the Sun. Earth has one satellite — the Moon.

Electromagnetic Force--A fundamental force that governs all interactions among electrical charges and magnetism. Essentially, all charged particles attract oppositely charged particles and repel identically charged particles. Similarly, opposite poles of magnets attract and like magnetic poles repel.

Electromagnetic Radiation--A form of energy that propagates through space as vibrations of electric and magnetic fields; also called radiation or light. All electromagnetic radiation is a form of light.

Electromagnetic Spectrum--The entire range of wavelengths of electromagnetic radiation, including radio waves, microwaves, infrared light, visible light, ultraviolet light, X-rays, and gamma rays.

Electromagnetism--The science dealing with the physical relationship between electricity and magnetism. The principle of an electromagnet, a magnet generated by electrical current flow, is based on this phenomenon.

Electron--A negatively charge elementary particle that typically resides outside the nucleus of an atom but is bound to it by electromagnetic forces. An electron’s mass is tiny: 1,836 electrons equals the mass of one proton.

Electron Volt (eV)--A unit of energy that is equal to the energy that an electron gains as it moves through a potential difference of one volt. This very small amount of energy is equal to 1.602 * 10^–19 joules. Because an electron volt is so small, engineers and scientists sometimes use the terms MeV (mega-million) and GeV (giga-billion) electron volts.

Element--A substance composed of a particular kind of atom. All atoms with the same number of protons (atomic numbers) in the nucleus are examples of the same element and have identical chemical properties. For example, gold (with 79 protons) and iron (with 26 protons) are both elements, but table salt is not because it is made from two different elements: sodium and chlorine. The atoms of a particular element have the same number of protons in the nucleus and exhibit a unique set of chemical properties. There are about 90 naturally occurring elements on Earth.

Elementary Particles--Particles smaller than atoms that are the basic building blocks of the universe. The most prominent examples are photons, electrons, and quarks.

Ellipse--A special kind of elongated circle. The orbits of the solar system planets form ellipses.

Elliptical Galaxy--A galaxy that appears spherical or football-shaped. Elliptical galaxies are comprised mostly of old stars and contain very little dust and “cool” gas that can form stars.

Emission Line--A bright line in a spectrum caused by emission of light. Each chemical element emits and absorbs radiated energy at specific wavelengths. The collection of emission lines in a spectrum corresponds to the chemical elements contained in a celestial object.

Erosion--Natural processes that wear or grind away the surface of an object. On Earth, the major agents of erosion are water and wind.

Escape Velocity--The minimum velocity required for an object to escape the gravity of a massive object.

European Space Agency (ESA)--A fifteen-member consortium of European countries for the design, development, and deployment of satellites. The Space Telescope — European Coordinating Facility (ST-ECF) supports the European astronomical community in exploiting the research opportunities provided by the Earth-orbiting Hubble Space Telescope. The ESA members are Austria, Belgium, Denmark, Finland, France, Germany, Ireland, Italy, the Netherlands, Norway, Portugal, Spain, Sweden, Switzerland, and the United Kingdom, with Canada as a cooperating state.

Event Horizon--The spherical outer boundary of a black hole. Once matter crosses this threshold, the speed required for it to escape the black hole’s gravitational grip is greater than the speed of light.

Excited State--A greater-than-minimum energy state of any atom that is achieved when at least one of its electrons resides at a greater-than-normal distance from its parent nucleus.

Extraterrestrial--An adjective that means “beyond the Earth.” The phrase “extraterrestrial life” refers to possible life on other planets.

F

Fahrenheit Temperature Scale--A temperature scale on which the freezing point of water is 32° F and the boiling point is 212° F.

Faint Object Camera (FOC)--An instrument aboard the Hubble Space Telescope that recorded high-resolution images of faint celestial objects in deep space. Built by the European Space Agency, the camera collected ultraviolet and visible light from celestial objects. The camera served as Hubble’s “telephoto lens” — recording the most detailed images over a small field of view. The FOC’s resolution allowed Hubble to single out individual stars in distant star clusters. The instrument was replaced in March 2002 during Servicing Mission 3B.

Faint Object Spectrograph (FOS)--An instrument aboard the Hubble Space Telescope that acted like a prism to separate light from the cosmos into its component colors, providing a wavelength “fingerprint” of the object being observed. Such information yields clues about an object’s temperature, chemical composition, density, and motion. Spectrographic observations also reveal changes in celestial objects as the universe evolves. The instrument was replaced in February 1997 during the Second Servicing Mission.

Far-Infrared Spectrum--The region of the infrared spectrum that exhibits the longest wavelengths and the lowest frequencies and energies.

Fault--A geological term that refers to a fracture or a break in a hard surface like the Earth’s crust. This area is a zone of weakness and may be the site of earthquakes or volcanoes. All planets or moons with a hard crust are candidates for faults or breaks on their surfaces.

Field of View (FOV)--A telescope’s viewing area, measured in degrees, arc minutes, or arc seconds. A telescope that can just fit the full moon into its complete viewing area has a field of view of roughly 30 arc minutes.

Filter--A type of window that absorbs certain colors of light while allowing others to pass through. Astronomers use filters to observe how celestial objects appear in certain colors of light or to reduce the light of exceptionally bright objects. For example, a pair of sunglasses acts as a type of filter, reducing the amount of incoming light while still allowing some light to pass through to the eyes.

Filter Wheels--Rotating wheels in a telescope instrument that allow specific colors of light from a celestial object to pass through and form an image on the detector. The Wide Field and Planetary Camera 2 aboard the Hubble Space Telescope has 12 filter wheels, each of which holds four filters.

Fine Guidance Sensors (FGS)--Targeting devices aboard the Hubble Space Telescope that lock onto “guide stars” and measure their positions relative to the object being viewed. Adjustments based on these precise readings keep Hubble pointed in the right direction. The sensors also are used to perform celestial measurements.

Fission--A nuclear process that releases energy when heavyweight atomic nuclei break down into lighter nuclei. Fission is the basis of the atomic bomb.

Fixed Head Star Trackers (FHST)--Small telescopes with wide fields of view that are aboard the Hubble Space Telescope and used in conjunction with the Fine Guidance Sensors. The star trackers locate the bright stars that are used to orient the telescope for scientific observations.

Flare--A sudden and violent outburst of solar energy that is often observed in the vicinity of a sunspot or solar prominence; also known as a solar flare.

Flat Universe--A geometric model of the universe in which the laws of geometry are like those that would apply on a flat surface such as a table top.

Flux--The flow of fluid, particles, or energy through a given area within a certain time. In astronomy, this term is often used to describe the rate at which light flows. For example, the amount of light (photons) striking a single square centimeter of a detector in one second is its flux.

Frequency--Describes the number of wave crests passing by a fixed point in a given time period (usually one second). Frequency is measured in Hertz (Hz).

Fusion--A nuclear process that releases energy when light atomic nuclei combine to form heavier nuclei. Fusion is the energy source for stars like our Sun.

G

Galactic Center--The central hub or nucleus of a galaxy. The Milky Way’s galactic center is about 28,000 light-years from Earth.

Galactic Disk--A flattened disk of gas and young stars in a galaxy. Some galactic disks have material concentrated in spiral arms (as in a spiral galaxy) or bars (as in barred spirals).

Galactic Halo--Spherical regions around spiral galaxies that contain dim stars and globular clusters. The radius of the halo surrounding the Milky Way extends some 50,000 light-years from the galactic center.

Galactic Nucleus--The central concentration of matter (stars, gas, dust, and perhaps a black hole) in a galaxy, typically spanning no more than a few light-years in diameter.

Galactic Plane--The imaginary projection of the Milky Way’s disk on the sky. Most of the galaxy’s stars and interstellar matter reside in this disk. Objects in the galaxy are often referred to as being above, below, or in the galactic plane.

Galaxy--A collection of stars, gas, and dust bound together by gravity. The smallest galaxies may contain only a few hundred thousand stars, while the largest galaxies have thousands of billions of stars. The Milky Way galaxy contains our solar system.

Galaxy Cluster--A collection of dozens to thousands of galaxies bound together by gravity.

Galaxy Evolution--The study of the birth of galaxies and how they change and develop over time.

Galaxy Supercluster--A vast collection of galaxy clusters that may contain tens of thousands of galaxies spanning over a hundred million light-years of space. Galaxy superclusters are the largest structures in the universe.

Gamma Rays--Light with the shortest wavelengths and the highest energies and frequencies in the electromagnetic spectrum; also called gamma radiation.

Gamma-Ray Burst (GRB)--A brief, intense, and powerful burst of gamma rays, the highest-energy, shortest-wavelength radiation in the electromagnetic spectrum. These bursts emanate from distant sources outside our galaxy and last only a few seconds. They are the brightest and most energetic explosions known.

Ganymede--One of Jupiter’s largest moons. Ganymede, the largest satellite in our solar system, is about 5300 kilometers (3300 miles) wide and larger than the planet Mercury.

Gaseous Nebula--A glowing cloud of gas in interstellar space. The cloud of gas may be either an emission nebula, which absorbs ultraviolet light from nearby stars and re-radiates visible light, or a reflection nebula, which reflects light off of its dust particles.

Geocentric--An adjective meaning “centered on the Earth.” Most early civilizations had a geocentric view of the universe.

Geosynchronous Orbit--Also known as geostationary. An orbit in which an object circles the Earth once every 24 hours, moving at the same speed and direction as the planet’s rotation. The object remains nearly stationary above a particular point, as observed from Earth. The International Ultraviolet Explorer (IUE) and some weather satellites are examples of satellites in geosynchronous orbit.

Giant Star--A dying star that has used up the hydrogen fuel in its core and has begun to expand. Giant stars are generally larger than our Sun.

Globular Cluster--A collection of hundreds of thousands of old stars held together by gravity. Globular clusters are usually spherically shaped and are often found in the halos of galaxies. Each star belonging to a cluster revolves around the cluster’s common center of mass.

Goddard High Resolution Spectrograph (GHRS)-A science instrument aboard the Hubble Space Telescope that made finely detailed spectroscopic observations of ultraviolet sources. The GHRS was removed from Hubble in February 1997 and replaced with the Space Telescope Imaging Spectrograph.

Goddard Space Flight Center (GSFC)--NASA’s flight control center in Greenbelt, Maryland, which receives data from orbiting observatories such as the Hubble Space Telescope (HST). HST digital data are then relayed to the Space Telescope Science Institute in Baltimore, Maryland, where they are interpreted into pictures. Goddard also conducts scientific investigations, develops and operates space systems, and works toward the advancement of space science technologies.

Grand Unified Theory (GUT)--A theory stating that that strong and weak nuclear forces and electromagnetic forces are varying aspects of the same fundamental force.

Gravitational Clustering--The process by which a large-scale structure grows as its gravity attracts smaller building blocks. Astronomers believe that all the large-scale structures (such as galaxies, galaxy clusters, and galaxy superclusters) that we see in the universe today formed through gravitational clustering.

Gravitational Constant (G)--A value used in the calculation of the gravitational force between objects. In the equation describing the force of gravity, “G” represents the gravitational constant and is equal to 6.672 * 10^–11 Nm²/kg².

Gravitational Instability--A condition that occurs when an object’s inward-pulling gravitational forces exceed the outward-pushing pressure forces, thus causing the object to collapse on itself. For example, when the pressure forces within an interstellar gas cloud cannot resist the gravitational forces that act to compress the cloud, then the cloud collapses upon itself to form a star.

Gravitational Lens--A massive object that magnifies or distorts the light of objects lying behind it. For example, the powerful gravitational field of a massive cluster of galaxies can bend the light rays from more distant galaxies, just as a camera lens bends light to form a picture.

Gravitational Redshift--The reddening of light from a very massive object caused by photons escaping and traveling away from the object’s strong gravitational field. An example of gravitational redshift is light escaping from the surface of a neutron star.

Gravity (Gravitational Force)--The attractive force between all masses in the universe. All objects that have mass possess a gravitational force that attracts all other masses. The more massive the object, the stronger the gravitational force. The closer objects are to each other, the stronger the gravitational attraction.

Gravity Assist--An effect through which an orbiting object, such as a spacecraft or a comet, gains or loses speed by virtue of the gravitational might of a planet or other celestial object that it passes. For example, the Cassini spacecraft in its journey to Saturn used a gravity assist from Earth to increase its velocity by about 36,000 kilometers per hour (22,300 miles per hour).

GRB990123--One of the most energetic gamma-ray bursts (GRBs) ever detected, occurring at 4:47 a.m. EST, January 23, 1999. The “burst” equaled the power of nearly 10 million billion suns. It became the first GRB to be viewed simultaneously in both gamma-ray and optical wavelengths.

Great Red Spot--A circulating storm located in Jupiter’s upper atmosphere. The storm, which rotates around the planet in six days, is the width of two to three Earths. Galileo first observed the spot in the 17th century.

Greenhouse Effect--The result of a planet’s atmosphere trapping infrared heat, rather than allowing it to escape into space. This effect increases the planet’s surface temperature, a phenomenon known as global warming.

Ground State--The minimum energy state of an atom that is achieved when all of its electrons have the lowest possible energy and therefore are as close to the nucleus as possible.

Group of Galaxies--A small collection of galaxies bound together by gravity. The number of galaxies in a group can range from a few to dozens. The Milky Way is a member of the Local Group, a collection of more than 30 galaxies.

Guide Star--A star that a telescope’s guidance system locks onto to ensure that a celestial object is followed and observed as the telescope moves, owing either to the Earth’s rotation or the telescope’s orbital trajectory. The Hubble Space Telescope uses two of its three Fine Guidance Sensors to detect and lock onto guide stars. The telescope’s science operations center has more than 15 million guide stars in its database — the Guide Star Catalogue.

Gyroscope--A spinning wheel mounted on a non-stationary frame that stabilizes and points a space-based observatory. This spinning wheel resists applied external forces and tends to retain its original orientation in space. For example, balancing on a moving bicycle is easier than balancing on a stationary one because of this tendency. Gyroscopes are used in navigational instruments for aircraft, satellites, and ships. The Hubble Space Telescope has six gyroscopes on board for navigation and sighting purposes.

H

Habitable Zone--A region around a star where planets with liquid water may be present. A planet on the near edge of the habitable zone would have a surface temperature slightly lower than the boiling point of water. A planet on the distant edge of the habitable zone would have a surface temperature slightly higher than the freezing point of water.

HDF-N--Hubble Deep Field North (HDF-N) is a tiny region of the northern sky near the Big Dipper toward which the Hubble Space Telescope was pointed for ten straight days in 1995. Because this observation was designed to detect very faint light from the most distant galaxies Hubble can observe, the field contains few bright celestial objects. Seemingly devoid of light, this small area provided a “keyhole” view of the universe’s past, reaching across space and time to see infant galaxies. By probing these remote regions of space, astronomers are gaining more information on galaxy development.

HDF-S--Hubble Deep Field South (HDF-S) is a tiny region of the southern sky near the Southern Cross toward which the Hubble Space Telescope was pointed for ten straight days in 1998. Because this observation was designed to detect very faint light from the most distant galaxies Hubble can observe, the field contains few bright celestial objects. Seemingly devoid of light, this small area provided a “keyhole” view of the universe’s past, reaching across space and time to see infant galaxies. By probing these remote regions of space, astronomers are gaining more information on galaxy development.

Heliocentric--An adjective meaning “centered on the Sun.”

Hertzsprung-Russell Diagram--A plot showing the relationship between the brightness (luminosity) and the surface temperatures of many stars. Often the spectral class, which is based on the temperature of the star, is used as a label.

High Speed Photometer (HSP)--An original science instrument aboard the Hubble Space Telescope that made very rapid photometric observations of celestial objects in near-ultraviolet to visible light. The instrument was removed in December 1993 during the First Servicing Mission.

Host Galaxy--A galaxy in which a cosmic phenomenon, such as a supernova explosion or a gamma-ray burst, has occurred.

Hubble Constant (H_o)--A number that expresses the rate at which the universe expands with time. H_o appears to be between 60 and 75 kilometers per second per megaparsec.

Hubble Space Telescope (HST)--An orbiting telescope that collects light from celestial objects in visible, near-ultraviolet, and near-infrared wavelengths. The telescope was launched April 24, 1990 aboard the NASA Space Shuttle Discovery. The 12.5-ton (11,110-kg), tube-shaped telescope is 13.1 m (43 ft) long and 4.3 m (14 ft) wide. It orbits the Earth every 96 minutes and is mainly powered by the sunlight collected by its two solar arrays. The telescope’s primary mirror is 2.4 m (8 ft) wide. The telescope is operated jointly by the National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA). HST is one of the many NASA Origins Missions, which include current satellites such as the Far Ultraviolet Space Explorer (FUSE) and future space observatories such as the Next Generation Space Telescope (NGST).

Hubble’s Law--Mathematically expresses the idea that the recessional velocities of faraway galaxies are directly proportional to their distance from us. Hubble’s Law describes the relationship of velocity and distance by the equation V=H_o * d, where V is the object’s recessional velocity, d is the distance to the object, and H_o is the Hubble constant. Essentially, the more distant two galaxies are from each other, the faster they are traveling away from each other. American astronomer Edwin Hubble discovered this relationship in 1929 when he observed that galaxies and clusters of galaxies were generally moving away from each other.

I

Image Intensifier--A device capable of intensifying light from a faint source so that it may be more easily detected.

Impact--When one body strikes another with great force. Some examples include a meteor colliding with the Moon or a comet, such as Shoemaker-Levy 9, slamming into Jupiter.

Impact Event--A collision between two solar system bodies that releases exceptionally large amounts of energy. Some examples are the 1908 Siberian Tunguska impact by a comet or an asteroid and the asteroid that struck Earth 65 million years ago, which may have led to the extinction of the dinosaurs and other species of the Cretaceous-Tertiary era.

Inflation-The theory that the universe expanded very rapidly shortly after the Big Bang.

Infrared--Radiation that has longer wavelengths and lower frequencies and energies than visible light.

Infrared Telescope--An instrument that collects the infrared radiation emitted by celestial objects. There are several Earth- and space-based infrared observatories. The Infrared Telescope Facility, an Earth-bound infrared telescope, is the U.S. national infrared observing facility at the summit of Mauna Kea, Hawaii. A planned space-based infrared observatory is the Space Infrared Telescope Facility (SIRTF).

Intensity--The amount, degree, or quantity of energy passing through a point per unit time. For example, the intensity of light that Earth receives from the Sun is far greater than that from any other star because the Sun is the closest star to us.

Interferometer--An instrument that combines the signal from two or more telescopes to produce a sharper image than the telescopes could achieve separately.

International Ultraviolet Explorer (IUE)--The longest operating (1978—1996) and most productive ultraviolet space observatory launched into a high geosynchronous orbit.

Interplanetary Matter--Dust, gas, and other debris found within the solar system.

Interplanetary Space--The region of space surrounding our Sun. Asteroids, comets, Earth, and the solar wind are examples of things occupying interplanetary space.

Interstellar Dust--Small particles of solid matter, similar to smoke, in the space between stars.

Interstellar Medium (ISM)--The sparse gas and dust located between the stars of a galaxy.

Interstellar Space--The dark regions of space located between the stars.

Inverse Square Law--A law that describes any quantity, such as gravitational force, that decreases with the square of the distance between two objects. For example, if the distance between two objects is doubled, then the gravitational force exerted between them is one-fourth as strong. Likewise, if the distance to a star is doubled, then its apparent brightness is only one-fourth as great.

Invisible Radiation--Radiation that the eye cannot detect, such as gamma rays, radio waves, ultraviolet light, and X-rays.

Io--The innermost of Jupiter’s four large moons. Due to Jupiter's gravitational might, Io is geologically active; its surface is peppered with volcanoes that send sulfurous eruptions into its thin atmosphere. Io appears to have the most active volcanoes in the solar system.

Ion--An atom with one or more electrons removed (or added), giving the atom a positive (or negative) charge.

Ionization--The process by which ions are produced, typically by collisions with other atoms or electrons, or by absorption of electromagnetic radiation.

Ionosphere--A region of the Earth’s upper atmosphere where solar radiation ionizes the air molecules. This region affects the transmission of radio waves and extends from 50 to 400 kilometers (30 to 250 miles) above the Earth's surface.

Io Plasma Torus--A bagel-shaped region of trapped sulfur ions around Jupiter that originates from the surface of Io, one of Jupiter’s moons. Gravitational tidal forces between Jupiter, other Galilean moons, and Io cause tidal friction in Io’s interior, producing geysers that spew sulfur at tremendous speeds. Some of the sulfur ions leave Io’s surface and become trapped around Jupiter.

Irregular Galaxy--A galaxy that appears disorganized and disordered, without a distinct spiral or elliptical shape. Irregular galaxies are usually rich in interstellar matter, such as dust and gas. The Large and Small Magellanic Clouds are examples of nearby irregular galaxies.

Isotope--An atom of a given element having a particular number of neutrons in the nucleus. Isotopes of a given element differ in the numbers of neutrons within the nucleus. Adding or subtracting a neutron from the nucleus changes an atom’s mass but does not affect its basic chemical properties.

J

Jets--Narrow, high-energy streams of gas and other particles generally ejected in two opposite directions from some central source. Jets appear to originate in the vicinity of an extremely dense object, such as a black hole, pulsar, or protostar, with a surrounding accretion disk. These jets are thought to be perpendicular to the plane of the accretion disk.

Jovian Atmosphere--The atmosphere surrounding the giant, massive planet Jupiter. The Jovian atmosphere is composed primarily of hydrogen (90 percent) and helium (10 percent). Other minor ingredients include water, hydrogen sulfide, methane, and ammonia.

Jovian Planets--The planets Jupiter, Saturn, Uranus, and Neptune. They are called Jovian planets because of similarities in their composition and location. This group is also known as the “giant planets,” the “gas planets” and, when grouped with the planet Pluto, the “outer planets.”

Jovian Winds--The hurricane-force, high-velocity motion of gas molecules in Jupiter’s atmosphere. The wind speed increases as one travels deeper into Jupiter’s atmosphere. The various patterns of atmospheric winds are easily identified in Jupiter’s upper cloud layer.

Jupiter--The fifth planet from the Sun and the largest planet in our solar system, twice as massive as all the other planets combined. Jupiter is a gaseous planet with a very faint ring system. Four large moons and numerous smaller moons orbit the planet. Jupiter is more than five times the Earth’s distance from the Sun. It completes an orbit around the Sun in about 12 Earth years.

K

Keck Observatory--Two telescopes known as the world's largest optical and infrared telescopes, jointly operated by the California Institute of Technology and the University of California. The telescopes comprise the W.M. Keck Observatory and are located on the summit of Hawaii's dormant Mauna Kea volcano.

Kelvin Scale--The temperature scale most commonly used in science, on which absolute zero is the lowest possible value. On this scale, water freezes at 273 K and boils at 373 K.

Kepler’s Laws--Three laws, derived by 17th century German astronomer Johannes Kepler, that describe planetary motion.

1. Kepler’s first law: The orbits of planets are ellipses, with the Sun at one focus. Therefore, each planet moves in an elliptical orbit around the Sun.

2.Kepler’s second law: An imaginary line connecting any planet to the Sun sweeps over equal areas in equal intervals of time.

3.Kepler’s third law: The square of any planet’s orbital period is proportional to the cube of its mean distance from the Sun.

Kilometer (km)--A measure of distance in the metric system equal to 1000 meters or about 0.6 of a mile.

Kinetic Energy--The energy that an object has by virtue of its motion.

Kitt Peak Observatory--The world’s largest collection of telescopes, located high above the Sonora Desert in Arizona. Eight astronomical research institutions share the 22 optical and two radio telescopes at Kitt Peak. The National Optical Astronomy Observatories oversee site operations at the observatory.

Kuiper Belt--A region in our outer solar system where many short-period comets (possessing orbits of less than 200 years) originate. This region begins beyond Neptune’s orbit and encompasses an estimated distance of between 30 and 100 astronomical units. There may be as many as 100 million Kuiper belt comets.

L

Light Curve--A plot showing how the light output of a star (or other variable astronomical object) changes with time.

Light-Year--The distance that a particle of light (photon) will travel in a year — about 10 trillion kilometers (6 trillion miles). It is a useful unit for measuring distances between stars.

Lithosphere--The solid part of a planet’s surface, composed of the crust and upper mantle. On Earth, it includes the continents and the sea floor.

Local Group--A small cluster of more than 30 galaxies, including the Andromeda galaxy, the Magellanic Clouds, and the Milky Way galaxy.

Long-Period Comet--A comet having an orbital period greater than 200 years and usually moving in a highly elliptical, eccentric orbit. Comets have orbits that take them great distances from the Sun. Most long-period comets pass through the inner solar system only once. Hale-Bopp is an example of a long-period comet.

Luminosity--The amount of energy radiated into space every second by a celestial object, such as a star. It is closely related to the absolute brightness of a celestial object.

Lunar Eclipse--A darkening of the Moon, as viewed from Earth, caused when our planet passes between the Sun and the Moon.

Lyman Limit--A specific wavelength (91.2 nm) that corresponds to the energy needed to ionize a hydrogen atom (13.6 eV). Galactic space is opaque at wavelengths shorter than the Lyman limit. Subsequently, light from cosmic objects at wavelengths less than the Lyman limit is exceedingly difficult to detect.

M

Magellanic Clouds--The Magellanic Clouds are two dwarf irregular galaxies. Known as the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC), the galaxies are in the Local Group. The closer LMC is 168,000 light-years from Earth. Both galaxies can be observed with the naked eye in the southern night sky.

Magnetic Field--A region of space in which magnetic forces may be detected or may affect the motion of an electrically charged particle. As with gravity, magnetism has a long-range effect and magnetic fields are associated with many astronomical objects.

Magnetic-Field Lines--Imaginary lines used to visualize a magnetic field. Magnetic field lines are related to the strength of the magnetic object’s influence and point in the same direction as a compass needle would.

Magnetopshere--A region of space above the Earth’s (or other planet’s) atmosphere where magnetic fields influence the motions of charged particles. The magnetosphere magnetically deflects or traps charged particles from space that would otherwise bombard the planet’s surface.

Mantle--The interior region of a terrestrial (rocky) planet or other solid body that is below the crust and above the core.

Maria--A dark, flat, large region on the surface of the Moon. The term is also applied to the less well-defined areas on Mars. Although maria literally means “seas,” watery regions do not exist on the Moon or Mars. Marias on the Moon may be evidence of past volcanic lava flows.

Mars--The fourth planet in the solar system and the last member of the hard, rocky planets (the inner or terrestrial planets) that orbit close to the Sun. The planet has a thin atmosphere, volcanoes, and numerous valleys. Mars has two moons: Deimos and Phobos.

Marshall Space Flight Center (MSFC)--NASA center overseeing the research, development, and implementation of three primary areas essential to space flight: reusable space transportation systems, generation and communication of new scientific knowledge, and management of all space lab activities. Located in Huntsville, Alabama, the center aided in the design, development, and construction of the Hubble Space Telescope.

Mass--A measure of the total amount of matter contained within an object.

Matter-Antimatter Annihilation--A highly efficient energy-generation process in which equal amounts of matter and antimatter collide and destroy each other, thus producing a burst of energy.

Megaparsec (MPC)--Equals one million parsecs (3.26 million light-years) and is the unit of distance commonly used to measure the distance between galaxies.

Mercury--The closest planet to the Sun. The temperature range on Mercury’s surface is the most extreme in the solar system, ranging from about 400° C (750° F) during the day to about –200° C (–300° F) at night. Mercury, which looks like Earth’s moon, has virtually no atmosphere, no moons, and no water.

Meteor--A bright streak of light in the sky caused when a meteoroid enters the Earth’s atmosphere. The streak of light is produced from heat generated by the meteoroid traveling into the Earth’s atmosphere.

Meteorite--The remains of a meteoroid that plunges to the Earth’s surface. A meteorite is a stony or metallic mass of matter that did not completely vaporize when it entered the Earth’s atmosphere.

Meteoroid--A small, solid object moving through space. A meteoroid produces a meteor when it enters the Earth’s atmosphere.

Micrometeoroid--A very small meteoroid with a diameter of less than a millimeter. Micrometeoroids form the bulk of the interplanetary solid matter scattered throughout the solar system.

Milky Way Galaxy--The Milky Way, a spiral galaxy, is the home of Earth, the Sun, and the rest of our solar system. The Milky Way contains more than 100 billion stars, has a diameter of 100,000 light-years, and belongs to the Local Group of galaxies. The Sun is about 28,000 light-years from the Milky Way’s center in the Orion spiral arm.

Molecular Cloud--A relatively dense, cold region of interstellar matter where hydrogen gas is primarily in molecular form. Stars generally form in molecular clouds. Molecular clouds appear as dark blotches in the sky because they block all the light behind them.

Molecular Velocity--The average speed of the molecules in a gas of a given temperature.

Molecule--A tightly knit group of two or more atoms bound together by electromagnetic forces among the atoms’ electrons and nuclei. For example, water (H₂O) is two hydrogen atoms bound with one oxygen atom. Identical molecules have identical chemical properties.

Moon--A large body orbiting a planet. On Earth’s only moon, scientists have not detected life, water, or oxygen on this heavily cratered body. The Moon orbits our planet in about 28 days.

Multi-Layer Insulation (MLI)--A “skin” or blanket of insulation covering the Hubble Space Telescope, which protects the observatory from temperature extremes. This insulation protects the telescope from the cold of outer space and also reflects sunlight so that the telescope does not become too warm. The MLI on Hubble is made up of many layers of aluminized Kapton, with an outer layer of aluminized Teflon.

N

National Aeronautics And Space Administration (NASA)--A Federal agency created on July 29, 1958 after President Dwight Eisenhower signed the National Aeronautics and Space Act of 1958. NASA coordinates space exploration efforts as well as traditional aeronautical research functions.

Near Infrared Camera And Multi-Object Spectrometer (NICMOS)--An instrument that sees objects in near-infrared wavelengths, which are slightly longer than the wavelengths of visible light. (Human eyes cannot see infrared light.) NICMOS is actually three cameras in one, each with different fields of view. Many secrets about the birth of stars, solar systems, and galaxies are revealed in infrared light, which can penetrate the interstellar gas and dust that blocks visible light. In addition, light from the most distant objects in the universe “shifts” into infrared wavelengths due to the universe’s expansion. By studying objects and phenomena in this spectral region, astronomers probe our universe’s past, present, and future; and learn how galaxies, stars, and planetary systems form. Astronauts installed NICMOS aboard the Hubble Space Telescope in February 1997 during the Second Servicing Mission.

Near-Infrared--The region of the infrared spectrum that is closest to visible light. Near-infrared light has slightly longer wavelengths and slightly lower frequencies and energies than visible light.

Nebular Theory--The idea that our solar system originated in a contracting, rotating cloud of gas that flattened to form a disk as it contracted. According to this theory, the Sun formed at the center of the disk and the planets formed in concentric bands of the disk.

Neptune-The eighth planet and the most distant giant gaseous planet in our solar system. The planet is 30 times the Earth’s distance from the Sun, and each orbit takes 165 Earth years. Neptune is the fourth largest planet and has at least eight moons, the largest of which is Triton. Neptune has a ring system, just like all the giant gaseous outer planets.

Neutrino--A neutral, weakly interacting elementary particle having a very tiny mass. Stars like the Sun produce more than 200 trillion trillion trillion neutrinos every second. Neutrinos from the Sun interact so weakly with other matter that they pass straight through the Earth as if it weren’t there.

Neutrino Detector--A device designed to detect neutrinos.

Neutron--A neutral (no electric charge) elementary particle having slightly more mass than a proton and residing in the nucleus of all atoms other than hydrogen.

Neutron Star--An extremely compact ball of neutrons created from the central core of a star that collapsed under gravity during a supernova explosion. Neutron stars are extremely dense: they are only 10 kilometers or so in size, but have the mass of an average star (usually about 1.5 times more massive than our Sun). A neutron star that regularly emits pulses of radiation is known as a pulsar.

Non-Thermal Radiation--Radiation that is not produced from heat energy — for example, radiation released when a very fast-moving charged particle (such as an electron) interacts with a magnetic force field. Because the electron’s velocity in this case is not related to the gas temperature, this process has nothing to do with heat.

North Celestial Pole (NCP)--A direction determined by the projection of the Earth’s North Pole onto the celestial sphere. It corresponds to a declination of +90 degrees. The North Star, Polaris, sits roughly at the NCP.

Nova--A binary star system (consisting of a white dwarf and a companion star) that rapidly brightens, then slowly fades back to normal.

Nuclear Transformation--The process by which an atomic nucleus is transformed into another type of atomic nucleus. For example, by removing an alpha particle from the nucleus, the element radium is transformed into the element radon.

O

Observable Universe--The portion of the entire universe that can be seen from Earth.

Oort Cloud--A vast spherical region in the outer reaches of our solar system where a trillion long-period comets (those with orbital periods greater than 200 years) reside. Comets from the Oort Cloud come from all directions, often from as far away as 50,000 astronomical units.

Opacity--The degree to which light is prevented from passing through an object or a substance. Opacity is the opposite of transparency. As an object’s opacity increases, the amount of light passing through it decreases. Glass, for example, is transparent and most clouds are opaque.

Open Cluster--Also known as a galactic cluster, an open cluster consists of numerous young stars that formed at the same time within a large cloud of interstellar dust and gas. Open clusters are located in the spiral arms or the disks of galaxies. The Pleiades is an example of an open cluster.

Open Universe--A geometrical model of the universe in which the overall structure of the universe extends infinitely in all directions. The rules of geometry in an open universe are like those that would apply on a saddle-shaped surface.

Optical Telescope--A telescope that gathers and magnifies visible light. The two basic types of optical telescopes are refracting (using lenses) and reflecting (using mirrors). The Hubble Space Telescope is an example of a reflecting telescope.

Ozone Layer--A region in the upper atmosphere that has high concentrations of ozone (triatomic oxygen, 0₃). The ozone layer protects the Earth by absorbing the Sun’s high-energy ultraviolet radiation.

P

Parallax--The apparent shift of an object’s position when viewed from different locations. Parallax, also called trigonometric parallax, is used to determine the distance to nearby stars. As the Earth’s position changes during its yearly orbit around the Sun, the apparent locations of nearby stars slightly shift. The stars’ distances can be calculated from those slight shifts with basic trigonometric methods.

Parsec (PC)--A useful unit for measuring the distances between astronomical objects, equal to 3.26 light-years and 3.085678 * 10¹³ kilometers, or approximately 18 trillion miles. A parsec is also equivalent to 103,132 trips to the Sun and back.

Periodic Comet--A comet in a closed, elliptical orbit within our solar system. These comets typically have orbital periods of less than 200 years. Many comets have orbits that keep them in the inner solar system and allow their trajectories to be calculated with great accuracy and precision. Perhaps the best-known periodic comet is Halley’s comet, whose orbital period is 76 years.

Periodic Table (of the Elements)--A chart of all the known chemical elements arranged according to the number of protons in the nucleus (also known as the atomic number). Elements with similar properties are grouped together in the same column.

Period-Luminosity Law--A relationship that describes how the luminosity or absolute brightness of a Cepheid variable star depends on the period of time over which that brightness varies.

Photoelectric Effect--The release of electrons from a solid material when it is struck by radiant energy, such as visible or ultraviolet light, X-rays, or gamma rays.

Photometer-An instrument that measures the intensity of light. Astronomers use photometers to measure the brightness of celestial objects.

Photometry--A technique for measuring the brightness of celestial objects.

Photon--A packet of electromagnetic energy, such as light. A photon is regarded as a charge-less, mass-less particle having an indefinitely long lifetime.

Photosphere--The extremely thin, visible surface layer of the Sun or a star. The average temperature of the Sun’s photosphere is about 5800 Kelvin (about 10,000° F). Although the Sun is completely made up of gas, its gas is so dense that we cannot see through it. When we look at the Sun, we are seeing the photosphere.

Pickoff Mirror--One of four flat mirrors inside the Hubble Space Telescope. Each mirror is tilted at a 45-degree angle to the incoming light, diverting a small portion of it to the optical detectors or to one of the fine guidance sensors.

Pixel--A light-sensitive picture element on a charge-coupled device (CCD) or some other kind of digital camera. A pixel is a tiny cell that, placed together with other pixels, resembles the mesh on a screen door. The Hubble Space Telescope’s Wide Field and Planetary Camera 2 has four CCDs, each containing 640,000 pixels. Each pixel collects light from a celestial object and converts it into a number. The numbers (all 2,560,000 of them) are sent to ground-based computers, which convert them into an image. The greater number of pixels, the sharper the image.

Planck Curve--The graphical representation of the mathematical relationship between the frequency (or wavelength) and intensity of radiation emitted from an object by virtue of its heat energy.

Planet--An object that orbits a star. Although smaller than stars, planets are relatively large and shine only by reflected light. Planets are made up mostly of rock or gas, with a small, solid core. In our solar system, the inner planets — Mercury, Venus, Earth, and Mars — are the rocky objects, and most of the outer planets — Jupiter, Saturn, Uranus, and Neptune — are the gaseous ones. Because Pluto is made largely of ice, like a comet, some astronomers do not consider it a true planet.

Planetary Nebula--An expanding shell of glowing gas expelled by a star late in its life. Our Sun will create a planetary nebula at the end of its life.

Planetesimal--A small body of rock and/or ice — under 10 kilometers (6 miles) across — formed during the early stages of the solar system. Planetesimals are the building blocks of planets, but many never combined to form large bodies. Asteroids are one example of planetesimals.

Plasma--A substance composed of charged particles, like ions and electrons, and possibly some neutral particles. Our Sun is made of plasma. Overall, the charge of a plasma is electrically neutral. Plasma is regarded as an additional state of matter because its properties are different from those of solids, liquids, and normal gases.

Pluto--The outermost planet in our solar system. Pluto is, on average, about 40 times the Earth’s distance from the Sun. Pluto orbits the Sun in 248 Earth years.

Potential Energy--The energy of an object owing to its position in a force field or its internal condition, as opposed to kinetic energy, which depends on its motion. Examples of objects with potential energy include a diver on a diving board and a coiled spring.

Primary Mirror--A large mirror in a reflecting telescope that captures light from celestial objects and focuses it toward a smaller secondary mirror. The primary mirror in the Hubble Space Telescope measures 94.5 inches (2.4 meters) in diameter.

Prime Focus--The location where light reflected from the primary mirror of a reflecting telescope comes into focus. Placing a secondary mirror in the light path allows the light to be focused elsewhere, in a more convenient location for the science instruments.

Primordial Nucleosynthesis--Element building that occurred in the early universe when the nuclei of primordial matter collided and fused with one another. Most of the helium in the universe was created by this process.

Prominence--An eruption of gas from the chromosphere of a star. Solar prominences are visible as part of the corona during a total solar eclipse. These eruptions occur above the Sun’s surface (photosphere), where gases are suspended in a loop, apparently by magnetic forces that arch upward into the solar corona and then return to the surface.

Proper Motion--The apparent motion of a star across the sky (not including a star’s parallax), arising from the star’s velocity through space with respect to the Sun.

Protogalaxy--Matter that is beginning to come together to form a galaxy. It is the precursor of a present-day galaxy and is sometimes called a “baby galaxy.”

Proton--A positively charged elementary particle that resides in the nucleus of every atom.

Proton-Proton Chain--A series of nuclear events occurring in the core of a star whereby hydrogen nuclei (protons) are converted into helium nuclei. This process releases energy.

Protoplanet--A small body that attracts gas and dust as it orbits a young star. Eventually, it may form a planetary body.

Protostar--A collection of interstellar gas and dust whose gravitational pull is causing it to collapse on itself and form a star.

Pulsar--A neutron star that emits rapid and periodic pulses of radiation.

Q

Quark--A basic building block of protons, neutrons, and other elementary particles.

Quasar--The brightest type of active galactic nucleus, believed to be powered by a supermassive black hole. The word “quasar” is derived from quasi-stellar radio source, because this type of object was first identified as a kind of radio source. Quasars also are called quasi-stellar objects (QSOs). Thousands of quasars have been observed, all at extreme distances from our galaxy.

R

RADAR (Radio Detection and Ranging)--A method of detecting, locating, or tracking an object by using beamed, reflected, and timed radio waves. RADAR also refers to the electronic equipment that uses radio waves to detect, locate, and track objects.

Radial Motion--The component of an object’s velocity (speed and direction) as measured along an observer’s line of sight.

Radiation--The process by which electromagnetic energy moves through space as vibrations in electric and magnetic fields. This term also refers to radiant energy and other forms of electromagnetic radiation, such as gamma rays and X-rays.

Radiative Process--An event involving the emission or absorption of radiation. For example, a hydrogen atom that absorbs a photon of light converts the energy of that radiation into electrical potential energy.

Radio Waves--Radiation with the longest wavelengths and smallest frequencies and energies in the electromagnetic spectrum.

Radioactivity--The spontaneous decay of certain rare, unstable, atomic nuclei into more stable atomic nuclei. A natural by-product of this process is the release of energy.

Reaction Wheel--One of four spinning flywheels aboard the Hubble Space Telescope. The flywheels work together to make the observatory rotate either more rapidly or less rapidly toward a new target.

Recessional Velocity--The velocity at which an object moves away from an observer. The recessional velocity of a distant galaxy is proportional to its distance from Earth. Therefore, the greater the recessional velocity, the more distant the object.

Red Giant Star--An old, bright star, much larger and cooler than the Sun. Betelgeuse (alpha Orionis) is an example of a red giant.

Redshift--The lengthening of a light wave from an object that is moving away from an observer. For example, when a galaxy is traveling away from Earth, its light shifts to the red end of the electromagnetic spectrum.

Reflector--A type of telescope, also known as a reflecting telescope, that uses one or more polished, curved mirrors to gather light and reflect it to a focal point.

Refractor--A telescope, also known as a refracting telescope, that uses a transparent lens to gather light and bend it to a focus.

Regolith--The layer of loose rock resting on bedrock (sometimes called mantle rock), found on the Earth, the Moon, or a planet. Regolith is made up of soils, sediments, weathered rock, and hard, near-surface crusts. On the surface of the Moon, regolith is a fine rocky layer of fragmentary debris (or dust) produced mainly by meteoroid collisions.

Relativity--A theory of physics that describes the dynamical behavior of matter and energy. The consequences of relativity can be quite strange at very high velocities and very high densities. A direct result of the theory of relativity is the equation E = mc², which expresses a relationship between mass (m), energy (E), and the speed of light (c).

Revolution--The orbital motion of one object around another. The Earth revolves around the Sun in one year. The moon revolves around the Earth in approximately 28 days.

Right Ascension (RA)--A coordinate used by astronomers to locate stars and other celestial objects in the sky. Right ascension is comparable to longitude, but it is measured in hours, minutes, and seconds because the entire sky appears to pass overhead over a period of 24 hours. The zero hour corresponds to the apparent location of the Sun with respect to the stars on the day of the vernal (spring) equinox (approximately March 21).

Robotic Optical Transient Search Experiment (ROTSE)--A terrestrial telescope that searches for the optical counterparts of gamma-ray bursts. When orbiting satellites detect a gamma-ray burst, ROTSE begins searching for its visible-light afterglow. ROTSE-I (an array of four electronic telephoto cameras) and ROTSE-II (a set of identical telescopes) are located in Los Alamos, New Mexico.

Roche Limit--The smallest distance at which two celestial bodies can remain in a stable orbit around each other without one of them being torn apart by tidal forces. The distance depends on the densities of the two bodies and their orbit around each other.

Rotation--The spin of an object around its central axis. Earth rotates about its axis every 24 hours. A spinning top rotates about its center shaft.

S

Saturn--The sixth planet in the solar system, noted for its obvious ring structure. Saturn is almost ten times the Earth’s distance from the Sun. The planet completes a circuit around the Sun in about 30 Earth years. Saturn is the second largest and the least dense planet in our solar system. The planet has more than 21 moons, including Titan, the second largest known moon in our solar system.

Schwarzchild Radius--The distance from the “center” of a black hole to its “edge” (called an event horizon). If the Earth became a black hole, all of its mass would be squeezed into a sphere with a Schwarzschild radius of 0.03 cm, about the size of a bacterium.

Secondary Atmosphere--A gas or gases, such as helium, that a planet discharges from its interior after having lost its primary or primordial atmosphere.

Secondary Mirror--A small mirror in a reflecting telescope that redirects light from the larger primary mirror toward the light-sensitive scientific instruments. In a Cassegrain-type telescope like the Hubble Space Telescope, the secondary mirror is slightly convex and directs light from the primary mirror back through a hole in the center of the primary mirror. Hubble’s secondary mirror measures 12.2 inches (0.3 meters) in diameter.

Seismic Wave--The transfer of energy throughout a celestial object, such as a planet, resulting from an external impact or an internal event. On Earth, seismic waves are generated primarily by earthquakes.

Seyfert Galaxy--A galaxy characterized by a moderately bright, compact active galactic nucleus, presumably powered by a black hole.

Shock Wave--A high-pressure wave that travels at supersonic speeds. Shock waves are usually produced by an explosion.

Short-Period Comet--Comets that orbit mainly in the inner solar system. Usually these objects orbit the Sun in less than 200 years. Halley’s comet is an example of a short-period comet.

Singularity--A black hole’s center, where the matter is thought to be infinitely dense, the volume is infinitely small, and the force of gravity is infinitely large.

Solar Arrays--Two rigid, wing-like arrays of solar panels that convert sunlight directly into electricity to operate the Hubble Space Telescope’s scientific instruments, computers, and radio transmitters. Some of the energy generated is stored in onboard batteries so the telescope can operate while in Earth’s shadow (which is about 36 minutes out of each 97-minute orbit). The solar arrays are designed for replacement by visiting astronauts during servicing missions.

Solar Constant--The average amount of solar radiation reaching a planet; usually expressed in watts (energy per unit time) per square meter. For Earth, the solar constant equals 1,372 W/m². Each planet has a unique solar constant depending on its distance from the Sun.

Solar Cycle--The periodic changing of the Sun’s magnetic field, which determines the number of sunspots and the amount of particles emitted in the solar wind. The period of the cycle is about 11 years.

Solar Eclipse--A phenomenon in which the Moon’s disk passes in front of the Sun, blocking sunlight. A total eclipse occurs when the Moon completely obscures the Sun’s disk, leaving only the solar corona visible. A solar eclipse can only occur during a new phase of the Moon.

Solar Maximum--The midpoint in the solar cycle where the amount of sunspot activity and the output of cosmic particles and solar radiation is highest.

Solar Minimum--The beginning and the end of a sunspot cycle when only a few sunspots are usually observed, and the output of particles and radiation is normal.

Solar System--The Sun and its surrounding matter, including asteroids, comets, planets and moons, held together by the Sun’s gravitational influence.

Solar Wind--Streams of charged particles flowing from the Sun at millions of kilometers per hour. This high-speed solar wind varies in composition, always streams away from the Sun, and interacts with other regions of matter in the solar system.

South Celestial Pole (SCP)--A direction determined by the projection of the Earth's South Pole onto the celestial sphere. The SCP is exactly 180 degrees from the North Celestial Pole and corresponds to a declination of –90 degrees.

Space Infrared Telescope Facility (SIRTF)--A space-borne infrared telescope that will study planets, comets, stars, galaxies, and other celestial objects. NASA plans to launch SIRTF in December 2002 on a Delta rocket. SIRTF represents the fourth and final satellite in NASA’s Great Observatories program, which includes the Hubble Space Telescope and the Chandra X-Ray Observatory.

Space Shuttle--A reusable U.S. spacecraft operated by astronauts and used to transport cargo, such as satellites, into space. The spacecraft uses rockets to launch into space, but it lands like an airplane. A space shuttle carried the Hubble Space Telescope into space in 1990. Astronauts aboard subsequent space shuttles have visited the telescope to service it.

Space Telescope Imaging Spectrograph (STIS)--An instrument that acts like a prism to separate light from the cosmos into its component colors, providing a wavelength “fingerprint” of the object being observed. The information yields clues about an object’s temperature, chemical composition, density, and motion. Spectrographic observations also reveal changes in celestial objects as the universe evolves. Astronauts installed STIS aboard the Hubble Space Telescope in February 1997 during the Second Servicing Mission. STIS spans ultraviolet, visible, and near-infrared wavelengths. The spectrograph can sample 500 points along a celestial object simultaneously.

Space Telescope Science Institute (STScI)--The astronomical research center responsible for operating the Hubble Space Telescope as an international scientific observatory. Located in Baltimore, Maryland, STScI is managed by AURA (Association of Universities for Research in Astronomy) under contract to the National Aeronautics and Space Administration (NASA) and the National Science Foundation (NSF).

Spacetime--The four-dimensional coordinate system (three dimensions of space and one of time) in which physical events are located.

Spectral Class (Spectral Type)--A classification scheme that groups stars according to their surface temperatures and spectral features.

Spectral Line--In a spectrum, an emission (bright) or absorption (dark) at a specific frequency or wavelength.

Spectrograph (Spectrometer)--An instrument that spreads electromagnetic radiation into its component frequencies and wavelengths for detailed study. A spectrograph is similar to a prism, which spreads white light into a continuous rainbow.

Spectroscopy--The study and interpretation of a celestial object’s electromagnetic spectrum. A spectrograph or spectrometer is used to analyze an object’s electromagnetic spectrum.

Speed Of Light (c)--The speed at which light (photons) travels through empty space is roughly 3 * 10⁸ meters per second or 300 million meters per second.

Spiral Arms--A pinwheel structure, composed of dust, gas, and young stars, that winds its way out from the core of a normal spiral galaxy and from the ends of the bar in a barred spiral galaxy.

Spiral Galaxy--A spiral-shaped system of stars, dust, and gas clouds. A typical spiral galaxy has a spherical central bulge of older stars surrounded by a flattened galactic disk that contains a spiral pattern of young, hot stars, as well as interstellar matter.

Sprites--Gamma-ray flashes produced in Earth’s atmosphere by severe lightning storms and upper atmospheric events.

Standard Candle--An object whose properties allow us to measure large distances through space. The absolute brightness of a standard candle can be determined without a measurement of its apparent brightness. Comparing the absolute brightness of a standard candle to its apparent brightness therefore allows us to measure its distance. For example, the distinct variations of Cepheid variable stars in other galaxies tell us their absolute brightness. By accurately measuring the apparent brightness of these stars, astronomers can precisely determine the distance to the galaxy in which they reside.

Star--A gaseous, self-luminous object held together by its own gravity. The core of a star is extremely hot and releases energy by fusing lighter atomic nuclei into heavier nuclei. Our Sun, the center of our solar system, is a star of average temperature and size.

Star Cluster--A group of stars born at almost the same time and place, capable of remaining together for billions of years because of their mutual gravitational attraction.

Starburst Galaxy--A galaxy undergoing an extremely high rate of star formation. Starburst galaxies contain massive, deeply embedded stars that are among the youngest stars observed.

Stellar Black Hole--A black hole formed from the death of a massive star during a supernova explosion. A stellar black hole, much like a supermassive black hole, feeds off of nearby material — in this case, the dead star. As it gains mass, its gravitational field increases.

Stellar Evolution--The process of change that occurs during a star’s lifetime from its birth to its death.

Stellar Parallax--The apparent change in the position of a nearby star when observed from Earth due to our planet’s yearly orbit around the Sun. This method allows astronomers to calculate distances to stars that are less than 100 parsecs from Earth.

Strong Force--The force that binds protons and neutrons within atomic nuclei and is effective only at distances less than 10^—13 centimeters.

Sun--The star at the center of our solar system. An average star in terms of size and mass, the Sun is a yellow dwarf of spectral type G2. It is about 5 billion years old, contains 2 * 10³⁰ kilograms of material, and has a diameter more than 100 times that of Earth.

Sunspot--A region on the Sun’s photosphere that is cooler and darker than the surrounding material. Sunspots often appear in pairs or groups with specific magnetic polarities that indicate electromagnetic origins.

Sunspot Cycle--The change in strength of the Sun’s magnetic field, which determines the number of sunspots and the amount of particles emitted in the solar wind. The period of the cycle is about 11 years.

Supermassive Black Hole--A black hole possessing as much mass as a million or a billion stars. Supermassive black holes reside in the centers of galaxies and are the engines that power active galactic nuclei and quasars.

Supernova-The explosive death of a massive star whose energy output causes its expanding gases to glow brightly for weeks or months.

Supernova Remnant--The glowing, expanding gaseous remains of a supernova explosion.

T

Telescope--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.

Temperature--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.

Terrestrial--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.

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.

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.

Triton--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.

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

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

U

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

Universe--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.

Uranus--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.

V

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.

Velocity--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.

Venus--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--Electromagnetic radiation that human eyes can detect; also known as the visible spectrum. The visible colors are red, orange, yellow, green, blue, indigo, and violet, along with various combinations and shades of these colors. Within the visible part of the spectrum, red light has a longer wavelength than blue light.

Volcano--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.

W

Wave--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.

Wavelength--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.

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 / 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 snaps high-resolution images of faraway objects. Its 48 filters allow scientists to study precise wavelengths of light and to sense a range of wavelengths from ultraviolet to near-infrared light. The instrument has four CCDs (charge-coupled devices), which collect information from stars and galaxies to make photographs. WFPC2 was installed aboard the Hubble telescope during the December 1993 servicing mission.

X

X-Rays--Electromagnetic radiation with very short wavelengths and very high energies and frequencies. X-rays fall between gamma rays and ultraviolet radiation; also called X-radiation or Roentgen ray.

Z

Zenith--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.