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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.


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.


In astronomy, accretion is the process of growing a massive object by gravitationally attracting less massive objects. Many objects, such as stars and black holes, grow by pulling gas and rocks from a gravitationally bound accretion disk.

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 Nuclei (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 uses CCD detectors to make images. The camera covers twice the area, has twice the sharpness, and is up to 10 times more efficient than the telescopes Wide Field and Planetary Camera 2. The ACS wavelength range spans from ultraviolet to near-infrared light. The cameras sharp eye and broader viewing area allow astronomers to study the life cycles of galaxies in the remotest regions of the universe. Astronauts installed the camera aboard the telescope in March 2002, but the camera experienced an electrical short in 2007 that shut down all but one data channel. During Servicing Mission 4 in 2009, astronauts replaced the failed circuit boards and added a new power supply box to restore power to the camera.


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.


A mixture of two or more metals. Brass (a mixture of copper and zinc) and bronze (a mixture of copper and tin) are common alloys.

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.


A type of telescope mounting that supports the weight of the telescope and allows it to move in two directions to locate a specific target. One axis of support is vertical (called the altitude) and allows the telescope to move up and down. The other axis is horizontal (called the azimuth) and allows the telescope to swing in a circle parallel to the ground. This makes it easy to position the telescope: swing it around in a circle and then lift it to the target. However, tracking an object as the Earth turns is more complicated. The telescope needs to be adjusted in both directions while tracking, which requires a computer to control the telescope.

Ammonia (ice)

This is a molecule composed of one nitrogen and three hydrogen atoms. It is an abundant molecule in the universe and can be found in either gaseous or solid phase (as ice) in the vacuum of space, depending on the temperature.


To make larger or more powerful; increase. Radio signals are amplified because they are very weak.


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.

Angstrom (Å)

A unit of length equal to one ten-billionth of one meter (0.0000000001, or 10-10 meters). An angstrom is equal to one tenth of one nanometer. 


An electrical device used to send or receive electromagnetic waves. The aerial (a long piece of metal attached to the front or rear fender) on a car is the antenna for the radio.


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.


An orderly arrangement or impressive display. For radio telescopes, an array is a group of individual radio dishes that work together. The VLA (Very Large Array) has 27 telescope dishes arranged in a “Y” pattern.

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 the NASA Hubble Space Telescope and NASA’s James Webb Space Telescope.


A small solar system object composed mostly of rock. Many of these objects orbit the Sun between Mars and Jupiter. Their sizes range anywhere from 33 feet (10 meters) in diameter to less than 620 miles (1,000 kilometers). The largest known asteroid, Ceres, has a diameter of 579 miles (926 kilometers).

Asteroid Belt

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


A scientist who studies the universe and the celestial bodies residing 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.


The study of the universe and the celestial bodies that reside in it, including their composition, history, location, and motion.


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

Atmospheric Distortion

The blurring of an image due to the layer of gases surrounding the surface of Earth. As starlight travels through the atmosphere, pockets of air act like little lenses and bend the light in unpredictable ways. This distortion causes stars to appear to twinkle.


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.


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.


An imaginary line through the center of an object. The object rotates around this line.


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.


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 NASAs Earth-orbiting Compton Gamma-Ray Observatory, whose mission ended in 2000.


Batteries provide all the electrical power to support Hubble operations during the night portion of its orbit, when the telescope is in Earth’s shadow. The telescope's orbit is approximately 97 minutes long. Roughly 61 minutes of Hubble’s orbit are in sunlight and 36 minutes are in Earth’s shadow. During Hubble’s sunlight or daytime period, the solar arrays provide power to the onboard electrical equipment. The solar arrays also charge the spacecraft’s batteries so they can power the spacecraft during the night portion of Hubble’s orbit. Hubble has six nickel-hydrogen batteries. These batteries, which had been onboard Hubble since the telescope was launched in 1990, were replaced during Servicing Mission 4.


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 13.7 billion years ago from an extremely dense and incredibly hot initial state.

Binary Star System (Binary Stars)

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


A hypothetical object that absorbs all of the electromagnetic radiation – all light of all wavelengths – that strikes it. Blackbodies emit radiation with a blackbody spectrum. In astronomy, hot, dense objects like planets and stars are modeled as black bodies.

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.


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.

Blue Star

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


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, Central (astronomical)

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.


Carbonaceous Chondrite

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

Carbon Dioxide

This organic molecule composed of one carbon and two oxygen atoms. It is an abundant molecule in the universe and can be found in either gaseous or solid phase (as ice) in the vacuum of space, depending on the temperature.

Cassegrain Telescope

A type of reflecting telescope whose eyepiece is located behind the primary mirror. The primary mirror is cast with a hole in the center. When light enters the telescope, it reflects from the primary mirror to the secondary mirror. The secondary mirror reflects the light back through the hole in the primary mirror to the eyepiece.


Of or relating to the sky or visible objects in the sky, like the Moon, Sun, planets, comets, asteroids, stars, and galaxies.

Celestial Object

An object in the sky – examples include the Moon, the Sun, planets, comets, asteroids, stars, and galaxies.

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.

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.

Chromatic Aberration

Visible light is made of different colors. When visible light passes through a glass lens or a prism, it gets dispersed, or split, into its many colors. A lens focuses each color at a different point, causing a fringe of color to appear around bright objects.


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.


A system of two moveable mirrors used in solar telescopes. The mirrors follow the Sun and keep its image in the same location as Earth rotates.

Collecting Area (mirror)

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.


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.


The cloud of gas and dust that forms around a comet’s nucleus. This cloud is created when the solar wind strikes the surface of the nucleus.


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 Nucleus

The core of a comet, made up of ice, dirt, and rock.

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.

Comet Tail

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


The constituent materials that make up an object or group of objects.

Compton Gamma-Ray Observatory (CGRO)

The constituent materials that make up an object or group of objects.

Concave vs. Convex

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.


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.


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 star.

Convection Zone

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


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


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 stars 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.

Corrective Optics Space Telescope Axial Replacement (COSTAR)

An apparatus installed during Hubble's 1993 First Servicing Mission. By placing small and carefully designed mirrors in the telescope, COSTAR successfully improved restored Hubble's vision to its original design goals. All the new instruments installed during the servicing missions have internal corrections for spherical aberration and do not require the services of COSTAR. Hubble’s last original instrument, the Faint Object Camera, was replaced by the Advanced Camera for Surveys during SM3B. COSTAR was replaced by the Cosmic Origins Spectrograph during Servicing Mission 4 and returned to Earth in the space shuttle.

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 (Cosmic Background Radiation)

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 Origins Spectrograph (COS)

A spectrograph that detects ultraviolet light. A spectrograph works by breaking up light from an object into its individual wavelengths so that its composition, temperature, motion, and other chemical and physical properties can be analyzed. COS will study the structure of the universe and how galaxies, stars and planets formed and evolved. Astronauts installed COS during SM4.

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.


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


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.

Crown Glass

Originally the main material used to make flat planes of glass for windows, it is composed of soda-lime glass. It can be used to make lenses and prisms. Crown glass bends and disperses, or spreads out, light less than flint glass.


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 Energy

A mysterious force that seems to work opposite to that of gravity and makes the universe expand at a faster pace.

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.


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.


A device used to measure the amount of electromagnetic radiation emitted by celestial objects. Frequently, detectors are used to sense light that is not visible.


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


The distance from one side of a circle to the other measured through the center. For telescopes, the diameter of a lens or mirror is measured from one side to the opposite side, passing through the center.


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.

Disk, Cosmic

Disks are common astronomical phenomena as they are the natural shape for material left over after compression of gas. Galaxies and protoplanetary systems both take on a disk shape.


Visible light is actually made up of different colors. Each color bends by a different amount when refracted by glass. That’s why visible light is split, or dispersed, into different colors when it passes through a lens or prism. Shorter wavelengths, like purple and blue light, bend the most. Longer wavelengths, like red and orange light, bend the least.

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.

Double Stars

A system of two stars that are gravitationally bound to each other. They orbit each other around a common center. They can also be called binary stars.

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.

Dwarf Planet

A celestial body within the Solar System that shares the characteristics of planets. It orbits the Sun, is not a moon, and has a spherical or nearly spherical shape. Unlike a planet, however, a dwarf planet has not cleared away any loose cosmic rubble from its orbit. Dwarf planets include Ceres, Pluto, and Eris.



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."


Traveling around Earth, in the path followed by an object moving in the gravitational field of Earth. For example, the telescope travels around, or orbits, Earth because Earth’s gravitational field keeps the telescope in its path, or orbit.

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.


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.


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, Chemical

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 (Elliptical)

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 composed 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.


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.


The process of allowing electromagnetic radiation to fall on light-sensitive materials such as photographic films or plates. An exposure is also the image created by the process. A long exposure time is needed in order to obtain an image of dim and distant celestial objects.


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

Exoplanet (Extrasolar Planet)

A planet that orbits a star other than the Sun.


The lens or lens group closest to the eye in an optical instrument such as a telescope or microscope.


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 objects 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.


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.


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.

Fine Guidance Sensors (FGS)

Cameras that help keep the Hubble Space Telescope pointed precisely in the right direction. These targeting devices aboard the telescope 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.


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.


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.

Flint Glass

The lead glass that was produced in the United States and the United Kingdom prior to the 1860s. This glass is used to make telescope lenses and prisms. Flint glass bends and disperses, or spreads out, light more than crown glass.


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.

Flyby Spacecraft

A spacecraft that travels past a celestial object. Frequently, such a spacecraft is unmanned and takes images of the object.

Focal Length

Focal length (shown in orange) is the distance between the center of a convex lens or a concave mirror and the focal point of the lens or mirror – the point where parallel rays of light meet, or converge.

Focal Point

The focal point of a lens or mirror is the point in space where parallel light rays meet after passing through the lens or bouncing off the mirror. A “perfect” lens or mirror would send all light rays through one focal point, which would result in the clearest image.


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).


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.


Galactic Center (Galaxy Core, Milky Way 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.


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. Galaxies are classified or grouped by their shape. Round or oval galaxies are elliptical galaxies and those showing a pinwheel structure are spiral galaxies. All others are called irregular because they do not resemble elliptical or spiral galaxies.

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-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.

Gamma Rays

The part of the electromagnetic spectrum with the highest energy; also called gamma radiation. Gamma rays can cause serious damage when absorbed by living cells.


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.

Gas Giant (Jovian Planet)

A large planet with a small, rocky core and a deep atmosphere composed mostly of hydrogen and helium. Our solar system contains four gas giants: Jupiter, Saturn, Uranus, and Neptune. This group is also known as Jovian planets.

General Theory of Relativity

A theory Einstein developed to explain how gravity influences space and time.


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.


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

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.

Gravitational Waves

Gravitational waves are ripples in space-time which travel at the speed of light.

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).

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.

GRB 990123

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


A spinning wheel mounted on a movable frame that assists in stabilizing and pointing a space-based observatory. Gyroscopes are important because they measure the rate of motion as the observatory moves and help ensure the telescope retains correct pointing during observations. The gyroscopes provide the general pointing of the telescope while the fine guidance sensors provide the fine tuning. Gyroscopes are used in navigational instruments for aircraft, satellites, and ships. The Hubble Space Telescope has six gyroscopes for navigation and sighting purposes.


Habitable Zone, Circumstellar

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.


An adjective meaning “centered on the Sun.”


Half of a spherical or roughly spherical body; for example, the northern and southern halves of the Earth, above and below the equator.

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.


A planet that is physically similar to Jupiter, orbiting close to its host star with an orbital period of less than 10 days.

Hubble Constant (H0)

A number that expresses the rate at which the universe expands with time. H0 appears to be between 60 and 75 kilometers per second per megaparsec.

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=H0 × d, where V is the object’s recessional velocity, d is the distance to the object, and H0 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.

Hubble Space Telescope (HST)

An orbiting telescope that collects light from celestial objects in visible, near-ultraviolet, and near-infrared wavelengths. The telescope’s primary mirror is 2.4 meters (8 feet) wide. It orbits the Earth about every 96 minutes and is powered by sunlight collected with its two solar arrays.


The lightest and most abundant chemical substance in the universe.


Image Intensifier

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


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 Crater

A large depression on a moon or a planet. An impact crater is created when an asteroid, a comet, or a meteorite strikes the moon or the planet with great force.

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.


The part of the Deep Impact spacecraft that crashed into comet 9P/Tempel 1. When launched, the impactor and the flyby spacecraft were attached to each other. The spacecraft launched the impactor a day before the crash. As the impactor punched through the comet’s crust, the flyby craft recorded the event from a safe distance away.


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

Infrared Radiation (IR)

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

Infrared Light

The part of the electromagnetic spectrum that has slightly lower energy than visible light, but is not visible to the human eye. Just as there are low-pitched sounds that cannot be heard, there is low-energy light that cannot be seen. Infrared light can be detected as the heat from warm–blooded animals.

Infrared Telescope

An instrument that collects the infrared radiation emitted by celestial objects. There are several Earth- and space-based infrared observatories.


Any device that measures and/or records energy from astronomical objects. Some astronomical instruments include spectrometers, photometers, spectroheliographs, and charge-coupled devices.


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.


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


The process used to combine the signal from two or more telescopes to produce a sharper image than each telescope 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.


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.


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


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


A region of the Earth’s upper atmosphere where solar radiation ionizes the air molecules. This region affects the transmission of radio wave 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.


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.



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.


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.


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.

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


An explosion caused when two neutron stars or a neutron star and a black hole merge. Short gamma-ray bursts are also thought to originate from such a merger.

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 (Object)

A region in our outer solar system where many "short-period" comets originate. The orbits of short-period comets are less than 200 years. This region begins near Neptune’s orbit at 30 astronomical units (AU) and extends to about 50 AU away from the Sun. An astronomical unit is the average distance between Earth and the Sun. The Kuiper Belt may have as many as 100 million comets.



A carefully ground or molded piece of glass, plastic, or other transparent material that causes light to bend and either come together or spread apart to form an image.

Lens Doublet

A set of two lenses, one concave and one convex, made from different types of glass. Together the lenses correct both spherical and chromatic aberrations. A single lens alone cannot correct these aberrations.

Light Curve

A graph showing the brightness of an object over time.


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.


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.


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.


Magellanic Clouds

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.


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.


Enlargement in the size of an optical image. For telescopes, magnification is not as important as the ability to gather light, which depends on the diameter of the primary lens or mirror.


The process of enlarging the size of an optical image.


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


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.


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.


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.


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° Celsius (750° Fahrenheit) during the day to about –200° Celsius (–300° Fahrenheit) at night. Mercury, which looks like Earth’s moon, has virtually no atmosphere, no moons, and no water.


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.


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.


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


A chemical compound consisting of five atoms: one of carbon and four of hydrogen. On Earth, methane is a colorless, odorless gas and is the principal ingredient of natural gas. In the cold vacuum of space, methane is a white solid but, when hit by sunlight, it can become a gas.


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.

Micron (µm)

A unit of length equal to one millionth of a meter (0.000001 meters, or 10-6 meters). A micron is equal to 1,000 nanometers. Also called a micrometer.


An electromagnetic wave in the region between infrared and radio wavelengths. Microwave wavelengths fall between one millimeter and one meter.

Milky Way (Galaxy)

The Milky Way, a spiral galaxy, is the home of Earth. The Milky Way contains more than 100 billion stars and has a diameter of 100,000 light-years.

Minerals / Mineralogy

The building blocks of rocks. They are naturally occurring substances formed through geological processes, and often have a crystalline form. They can be single elements (such as gold or silver) or compounds (such as quartz, marble or turquoise).

Modern Physics

A group of several theories developed in the early to mid-20th century that explains how small particles are affected by light, how measurements change when objects move very fast, and how gravity affects space and time.

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.


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.


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.


The support structure for a telescope that bears the weight of the telescope and allows it to be pointed at a target. The mounting of today’s research telescopes also allows astronomers to track the object as it appears to move across the sky.

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.


Nanometer (nm)

A unit of length equal to one billionth of one meter (0.000000001 meters, or 10-9 meters). A nanometer is equal to one thousandth of one micron.

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 Light

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.

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 universes expansion. By studying objects and phenomena in this spectral region, astronomers probe our universes 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 Hubble's Second Servicing Mission.


A cloud of gas and dust located between stars and/or surrounding stars. Nebulae are often places where stars form.

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.


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.


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.


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.

New Outer Blanket Layer (NOBL)

Covers that protect Hubbles damaged external blankets and help to maintain the telescopes normal operating temperatures. The covers are made of specially coated stainless steel foil, which is trimmed to fit each particular equipment bay door.

Newtonian Reflector

A type of reflecting telescope whose eyepiece is located along the side of the telescope. When light enters the telescope, it reflects from the primary mirror to the secondary mirror. The secondary mirror reflects the light at a right angle through the side of the telescope to the eyepiece.

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.


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.


The core of a comet, made up of ice, dirt, and rock.


Observable Universe

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


In science, an observation is a fact or occurrence that is noted and recorded. The Hubble Space Telescope is a tool astronomers use to make observations of celestial objects.


A structure designed and equipped for making astronomical observations. Observatories are located on Earth and in space.


An event in which an object is temporarily hidden from an observer's view by an object in the foreground; for example, when an asteroid passes in front of a distant star. 

One-Dimensional Spectrum 

A line graph that represents the amount of light from each wavelength over a range of wavelengths. This representation of a spectrum can be used for a specific object or location, like a star. From this spectrum, for example, we can learn about color, temperature, composition, winds, and motion. 

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.


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.


The point at which a planet appears opposite the Sun in our sky. During the Martian opposition, for example, Mars and the Sun are on opposite sides of the Earth.

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.


A person who grinds lenses and mirrors.


The science that deals with the properties of light; in this case specifically dealing with the way light changes directions when it is either refracted and dispersed by a lens or reflected from a mirror.


The act of traveling around a celestial body; or the path followed by an object moving around a celestial body. For example, the Planets travel around, or orbit, the Sun because the Sun’s gravity keeps them in their paths, or orbits.

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.


Parabola vs. Sphere

If cross-sections of a spherical surface and a parabolic surface were made by slicing each surface in half, these would be the shapes you would see.


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 from Earth to the Sun and back.

Perfect Lens

The perfect lens does not exist. Due to the nature of glass, light is dispersed when passing through glass. In the case of convex lenses, red light bends less than blue light, so the focal points are in different places, making the image blurry. A single lens cannot counter this effect.

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.


Regularly occurring changes in the appearance of the Moon or a planet. Phases of the Moon include new, full, crescent, first quarter, gibbous, and third quarter.

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.


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


The measurement, study, or interpretation of the apparent brightness of a celestial object — the amount of light (intensity or flux) from the object that reaches the observer.


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


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.


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. On a telescope instrument, each pixel collects light from a celestial object and converts it into a number. The numbers 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.


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 the outer planets (Jupiter, Saturn, Uranus, and Neptune) are the gaseous ones.

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.

Planetary Ring (Ringed Planet, Ring System)

A disk or ring orbiting an astronomical object that is composed of solid material such as dust and moonlets, and is a common component of satellite systems around giant planets.


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.


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.


A column of material that is shaped like a long feather.


A dwarf planet whose small size and composition of ice and rock resembles the comets in the Kuiper Belt, a region beyond Neptune's orbit where Pluto resides. Pluto was considered the ninth planet until August 2006, when the International Astronomical Union reclassified it as a dwarf planet. Pluto's orbit is more elliptical than those of the eight solar system planets.

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 Lens

A large convex lens in a refracting telescope that captures light from celestial objects and focuses it toward the eyepiece.

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.


Usually a triangular-shaped piece of glass used to refract, or bend, light. The shape of the glass causes the light to disperse, or spread out, as it bends, producing a rainbow of colors from the white light.


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.


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."


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.


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


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


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



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


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.


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 (Electromagnetic, EMR)

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.


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.

Radio Waves

The part of the electromagnetic spectrum with the lowest energy. Radio waves are the easiest way to communicate information through the atmosphere or outer space.

Rate Sensor Units (RSUs)

Boxes that house Hubbles gyroscopes. Each rate sensor unit contains two gyroscopes. Astronauts remove the rate sensor units when they replace gyroscopes, so gyroscopes are always replaced two at a time.

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.


The part of the radio telescope that detects long wavelength electromagnetic radiation and converts it to an electrical signal so that we can sense it.

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, Cosmological

The stretching of light to longer, redder wavelengths as it moves through space due to the expansion of the universe. Light emitted by a distant astronomical object will appear redder because of the expansion of the space between the emitter and the receiver. This is distinct from Doppler redshift, in which the wavelengths of light are lengthened by the movement of the object emitting them away from the observer, not the actual stretching of space itself by the expansion of the universe.

Redshift, Doppler

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.


Reflection occurs when light changes direction as a result of "bouncing off" a surface like a mirror.

Reflector (Reflecting Telescope)

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.


Refraction is the bending of light as it passes from one substance to another. Here, the light ray passes from air to glass and back to air. The bending is caused by the differences in density between the two substances.

Refractor (Refracting Telescope)

A type of telescope that uses a transparent convex lens to gather light and bend it to a focal point.


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.


This is the process that caused the matter in the universe to be ionized after the first atoms were formed. This change occurred once objects that were energetic enough to re-ionize neutral hydrogen started to condense. As these objects formed and radiated energy, the universe reverted from being neutral to once again being an ionized plasma. This occurred between 150 million and 1 billion years after the big bang.


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).

Resolution, Angular (Resolving Power)

A measure of the smallest separation at which a telescope can observe two neighboring objects as two separate objects.


The ability of a telescope to distinguish objects that are very close to each other as two separate objects.


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).


A long, narrow depression on the Moon’s surface. A rille can be straight, have a sweeping arc, or meander, with many curves going in random directions.

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.

Rocky Planet

A planet located in the inner solar system and made up mostly of rock. The rocky planets are Mercury, Venus, Earth, and Mars. This group is also known as terrestrial planets

Roman Space Telescope (Nancy Grace Roman Space Telescope, Roman)

The Roman Space Telescope is a 2.4-meter telescope with Hubble's sensitivity and resolution, but a field of view 100 times larger than Hubble or Webb. This mission, in formulation for launch in the mid-2020s, will revolutionize our understanding of dark energy, exoplanets, and many topics in general astrophysics by combining surveys with guest observer and archival guest investigator programs.

Originally called the Wide Field Infrared Survey Telescope (WFIRST), NASA renamed the telescope in honor of Dr. Nancy Grace Roman, NASA’s first Chief Astronomer, who paved the way for space telescopes focused on the broader universe.


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.



An object that orbits Earth, the Moon, or another celestial object. Artificial satellites are man-made objects placed into orbit. Natural satellites are smaller celestial bodies that orbit around larger celestial bodies. Two examples are moons that go around planets, and small galaxies that orbit larger galaxies.


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.


A flash of light produced when gamma rays strike a certain material. The high energy of gamma rays makes them hard to capture but they can be detected using scintillation.

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 Eclipse

The portion of a planet's orbit in which the planet is behind its star from the perspective of the observer. During a secondary eclipse, the light from the planet is blocked by the star. A secondary eclipse is a type of occultation.

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.

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.

Servicing Missions

Hubble was the first space telescope designed to be serviced in space. Scientists believed that periodic servicing missions would extend Hubble's operating life and keep the observatory up-to-date. Astronauts have visited Hubble five times. The first servicing mission was in December 1993 and the second in February 1997. The third mission was split into two visits. Part A took place in December 1999 and part B in March 2002. The final servicing mission visit occurred in May 2009.

Seyfert Galaxy

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

Shock Wave (Shockwave)

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

Short-Period Comet

A comet that orbits mainly in the inner solar system. Short-period comets usually orbit the Sun in less than 200 years. Halley’s comet is an example of a short-period comet.


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.

Soft Capture Mechanism (SCM)

When Hubble reaches the end of its mission, NASA must be able to safely return the telescope to Earth. When that time comes, the space shuttle will no longer be operating, so another means of capturing the telescope must be available. The soft capture mechanism is a compact device that, when attached to the Hubble Space Telescope, will assist in its safe de-orbit. This device has structures and targets that will allow a next generation space vehicle to more easily capture and guide the telescope into a safe, controlled re-entry.

Solar Arrays

Two rigid, wing-like arrays of solar panels that convert sunlight directly into electricity to operate the Hubble Space Telescopes scientific instruments, computers, and radio transmitters. Some of the energy generated is stored in onboard batteries so the telescope can operate while in Earths 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 1,372W/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 Mass (M)

The mass of our sun, which is 1.989 × 1030 kilograms. We frequently measure other stars, galaxies, and black holes in terms of their relative weight to our sun.

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 Panels

Two rigid, wing-like structures that convert sunlight directly into electricity to operate a 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.

Solar System

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

Solar Telescope

A special reflecting telescope designed to study our closest star, the Sun. Solar telescopes differ from normal telescopes in that they are stationary and use small tracking mirrors to direct sunlight into the primary mirror. This is necessary because the Sun appears to move across the sky due to Earth’s rotation.

Solar Wind

Streams of charged particles flowing from the Sun at millions of kilometers an hour. The composition of this high-speed solar wind may vary, but it always streams away from the Sun. The solar wind is responsible for the Northern and Southern Lights on Earth and causes the tails of comets to point away from the Sun.

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.

Southern Hemisphere

Half of a spherical or roughly spherical body; for example, the Southern Hemisphere of Earth is the half below the equator.

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 used rockets to launch into space, but it landed like an airplane. A space shuttle carried the Hubble Space Telescope into space in 1990. Astronauts aboard subsequent space shuttles had visited the telescope to service it. The space shuttle was retired in 2011.

Space Telescope Imaging Spectrograph (STIS)

The Space Telescope Imaging Spectrograph (STIS) is a general-purpose spectrograph that spans ultraviolet, visible, and near-infrared wavelengths. It was installed in February 1997 during Hubble's Second Servicing Mission. A spectrograph works by breaking up light from an object into its individual wavelengths so that its composition, temperature, motion, and other chemical and physical properties can be analyzed. STIS stopped functioning in 2004 due to a power supply failure, but astronauts replaced a low-voltage power supply board during Servicing Mission 4.

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).


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.

Spectroscopic Observations

Observations made using a spectrograph.

Spectrograph (Spectrometer, Spectroscope)

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.


An instrument used in solar telescopes to photograph the Sun in a single wavelength of light. Different wavelengths reveal different features of the Sun’s surface.


The study and interpretation of a celestial object’s electromagnetic spectrum. A spectrum breaks light into its component wavelengths and reveals clues to the object’s state, temperature, speed, quantity, distance, and composition.


The entire range of electromagnetic rays from the longest radio waves to the shortest gamma rays. Arranged from longest to shortest wavelengths, the spectrum of electromagnetic radiation includes radio waves, microwaves, infrared light, visible light, ultraviolet light, X-rays and gamma rays.

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.

Spherical Aberration

Spherical aberration is an optical defect of a lens or mirror caused by its rounded shape. Spherical lenses and mirrors produce a distorted (blurry) image.

Spherical Aberration in Lenses

The shape of a spherical lens causes a problem called spherical aberration.

In spherical aberration, parallel light rays that pass through the central region of the lens focus farther away than light rays that pass through the edges of the lens. The result is many focal points, which produce a blurry image. To get a clear image, all rays need to focus at the same point.

Spherical Aberration in Mirrors

The shape of a spherical telescope mirror causes a problem called spherical aberration.

In spherical aberration, parallel light rays that bounce off the central region of a spherical mirror focus farther away than light rays that bounce off the edges. The result is many focal points, which produce a blurry image. To get a clear image, all rays need to focus at the same point.

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.


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.


A huge ball of gas held together by gravity. The central core of a star is extremely hot and produces energy. Some of this energy is released as visible light, which makes the star glow. Stars come in different sizes, colors, and temperatures. Our sun, the center of our solar system, is a yellow star of average temperature and size.

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.

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.

Star Formation

Star formation is the process by which dense regions within molecular clouds in interstellar space, sometimes referred to as "stellar nurseries" or "star-forming regions", collapse and form stars.


Random noise in a radio receiver. It can also be heard in telephone lines and cell phones.

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 Disk/Circumstellar Disk

A  torus, pancake, or ring-shaped accumulation of matter composed of gas, dust, planetesimals, asteroids, or collision fragments in orbit around a star. Around the youngest stars, they are the reservoirs of material out of which planets may form.

Stellar Evolution (Stellar Lifecycle)

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

Stellar Nursery

A region in space where stars are forming from a cloud of gas and dust.

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.

Stellar Wind

Fast moving flows of material (protons, electrons, and atoms of heavier metals) that are ejected from stars.

Strong Force

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


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.


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.


An exoplanet with a mass higher than Earth's, but substantially below the masses of the Solar System's ice giants, Uranus and Neptune, which have masses of 15 and 17 times Earth's, respectively.

Supermassive Black Hole (SMBH, SBH)

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 (SN, SNe)

The explosive death of a massive star whose energy output causes its expanding gases to glow brightly for weeks or months. A supernova remnant is the glowing, expanding, gaseous remains of a supernova explosion.

Supernova Remnant (SNR)

The glowing, expanding, gaseous remains of a supernova explosion.



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


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


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


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


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

Terrestrial Planets

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


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

Thermal Emission

Heat energy given off by an object in the form of electromagnetic radiation (light). Thermal emission is directly related to an object's temperature. 

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.

Time-Series Observation

A set of measurements taken over a period of time, typically at regular intervals (cadence) of seconds, minutes, or hours. Time-series observations are used to detect and record changes in an object’s position, brightness, and other properties over time. Common examples of time-series observations include those of variable stars, eclipsing binary stars, variable brown dwarfs, and exoplanet transits.

Titanium Oxides

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

Tracking and Data Relay Satellite System (TDRSS)

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


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

T-Tauri Star

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


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

Two-Dimensional Spectrum

This representation of a spectrum is multiple one-dimensional spectrums. This slightly more complex representation of a spectrum is useful for examining the light across a source, like a galaxy. The intensity of light at each wavelength across different positions on the sky is represented by the intensity of color.



Ultraviolet (UV) Light

Electromagnetic radiation with shorter wavelengths and higher energies and frequencies than visible light, but is not visible to the human eye. Just as there are high-pitched sounds that cannot be heard, there is high-energy light that cannot be seen. Too much exposure to ultraviolet light causes sunburns. UV light is lower in frequency than X-rays.


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


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


Van Allen belt

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

Variable Star

A star whose luminosity (brightness) changes with time.

Vela Satellite

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


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


An inner, terrestrial (rocky) planet that is slightly smaller than Earth. Located between the orbits of Mercury and Earth, Venus has a very thick atmosphere that is covered by a layer of clouds that produces a "greenhouse effect" on the planet. Venus' surface temperature is roughly 480° Celsius (900° Fahrenheit), 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 (36km) 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 (Visible Spectrum)

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


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



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


The distance between two successive peaks on a wave. (The length of one complete wave cycle, from peak to peak, or trough to trough.)

Radio waves can have lengths of several feet; the wavelengths of X-rays are roughly the size of atoms. Wavelength is also what differentiates the various bands of light on the electromagnetic spectrum. Human eyes recognize differences in visible wavelengths of light as differences in color. On the visible part of the spectrum, shorter wavelengths look bluer and longer wavelengths look redder.

Wavelengths of ultraviolet, visible, and infrared light are typically measured in microns, nanometers, or angstroms.

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.

Webb Telescope (James Webb Space Telescope, JWST)

The world’s premier infrared space observatory of the next decade. The James Webb Space Telescope is an international project led by NASA with its partners, the European Space Agency (ESA) and the Canadian Space Agency (CSA). The Space Telescope Science Institute (STScI) will serve as its mission operations center and lead the science operations.

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 and Planetary Camera 2 (WFPC2)

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

Wide Field Camera 3 (WFC3)

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

Wide Field Infrared Survey Telescope (WFIRST)

WFIRST has been renamed in honor of NASA’s first Chief Astronomer, Dr. Nancy Grace Roman. The Roman Space Telescope is a 2.4-meter telescope with Hubble's sensitivity and resolution, but a field of view 100 times larger than Hubble or Webb. This mission, in formulation for launch in the mid-2020s, will revolutionize our understanding of dark energy, exoplanets, and many topics in general astrophysics by combining surveys with guest observer and archival guest investigator programs.

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 Camera2 replaced the WF/PC during the December 1993 servicing mission.



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


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

X-Ray Sources

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

X-Ray Telescope

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



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