On this Glossary page


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.

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.

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.


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

Association of Universities for Research in Astronomy, Inc. (AURA)

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

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


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.

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.

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.

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.


Canadian Space Agency (CSA)

The Canadian Space Agency is an independent federal agency responsible for managing all of Canada's civil space-related activities.

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.

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.

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.


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.


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


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.

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.


A branch of chemistry that examines the chemical composition of, and changes in, objects in the universe.

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.

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.

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.

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.


Early Release Science Program

A competitively awarded set of telescope observations that will be taken during Webb's first five months of science operations and made available to the astronomical community immediately after initial data processing occurs.


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

Electromagnetic 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.


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.

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.

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.

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.


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.

Far-Infrared Spectrum

The region of the infrared spectrum that exhibits the longest wavelengths and the lowest frequencies and energies.

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.


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


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.


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.

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.

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 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.

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 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.

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.

Guaranteed Time Observation Program

A program designed to reward scientists who helped develop the key hardware and software components or technical and inter-disciplinary knowledge for the James Webb Space Telescope. These programs will be administered in the first three years of the telescope's science operations.


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.


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.

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.

Invisible Radiation

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


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.

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.


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.


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.

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.


L2 Orbit

L2 is the second Lagrange point, directly opposite the Sun from the Earth. There are five Lagrange points wherein three bodies can orbit each other but also stay in the same position relative to each other.


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.

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.


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.

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 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.


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.

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.


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.

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.


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


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 (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.


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


Observable Universe

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


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


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.


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.

Organic Compound (Molecule)

An organic molecule is normally found in living systems. They are made up of at least one carbon, attached to oxygen, nitrogen, hydrogen, or another element.

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.


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

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 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 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.


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


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.



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.

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.

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.

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.


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.


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.

Shock Wave (Shockwave)

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


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

Solar 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 System

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

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).

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.


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.

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.


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.

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 Wind

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


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.



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.

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.

Variable Star

A star whose luminosity (brightness) changes with time.


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.

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


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

  • Wavelength and frequency: Light is measured by its wavelength (in nanometers) or frequency (in Hertz, Hz).
  • One wavelength: Equals the distance between two successive wave crests or troughs.
  • Frequency (Hertz): Equals the number of waves that passes a given point per second.

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 Infrared Survey Telescope (WFIRST)

WFIRST 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.


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.