Space And Astronomy Related Definitions And Explanations

When you're new to a subject, and begin reading about it, sometimes there seems to be a billion new words and terms to learn that you don't see everyday - like "Tidal Deceleration" and "Quasar". Space and Astronomy is a field of science and as such can be full of difficult new words. To help you conquer the sometimes confusing and technical space-related terms, I have complied the essential terms that you might come across while reading Spaceopedia or learning about Space and Astronomy elsewhere. If you have a word you'd like to see added, feel free to contact me through the About and Contact page. For your convenience, you can jump to a letter you want by clicking on the letters below:

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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.

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.

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.

Aerobraking is a spaceflight maneuver that reduces the high point of an elliptical orbit (apoapsis) by flying the vehicle through the atmosphere at the low point of the orbit (periapsis). The resulting drag slows the spacecraft gradually circularizing an orbit into a low circular orbit. The maneuver requires less fuel than does the direct use of a rocket engine.

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 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.

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.

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.

The point at which an object orbiting the Sun is at its furthest. The opposite is Perihelion.

The point at which an object orbiting the Earth, such as the Moon or a satellite, is at its furthest. The opposite is Perigee.

The point at which an object orbiting a larger mass (or centre of mass) is at its furthest. The opposite is Periapsis. Specific terms are available for objects in orbit about the Sun, Earth and other stars.

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 as if it were a single telescope many miles across. The VLA (Very Large Array) has 27 telescope dishes arranged in a “Y” pattern.

Artificial Gravity
Artificial gravity is a force that simulates the effect of gravity in a spaceship. It is not caused by the attraction to the Earth but is instead caused by acceleration or centrifugal force. There is a need for artificial gravity in spacecraft to counter the effect of weightlessness on the astronauts. A rotating circular space station can create artificial gravity for its passengers. The rate of rotation necessary to duplicate the Earth's gravity depends on the radius of the circle.

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 nearly 1,000 kilometers (620 miles) to 10 meters (33 feet) and possibly smaller. The largest known asteroid, Ceres, has a diameter of 926 kilometers (579 miles).

Asteroid belt
A region of space between Mars and Jupiter where the great majority of asteroids are 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.

An astronaut is a term used for an American space traveller. The only difference between a Cosmonaut, astronaut and a Taikonaut is whose technology the person used to get into space.

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.

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.

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

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

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

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,  but 13 - 80 times the mass of a Jupiter-sized planet.

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.

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.

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.

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.

Chemistry for life
The building blocks that enable life to form and to sustain itself. Life as we know it requires a source of energy, organic (carbon-based) compounds, and water. Scientists believe that atmospheric detection of water, oxygen, methane, carbon dioxide, and other compounds can signal the possibility of life on a planet.

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

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

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

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

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 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).

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 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.

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

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

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

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.


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.

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.


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.

When an object moves in front of another so as to block its view. In terms of space, there are two main ones, lunar and solar. In a Lunar Eclipse, the Earth blocks out the moon and is common. In a Solar Eclipse, the moon moves in between the Earth and the Sun. For more information, read Eclipses .

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 comprised mostly of old stars and contain very little dust and “cool” gas that can form stars.

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.

Extrasolar planet (Exoplanet)
A planet that orbits a star other than the sun.

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


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)
The area of the sky visible through a telescope. The telescope’s viewing area is 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 nuclear process that releases energy when heavyweight atomic nuclei break down into lighter nuclei. Fission is the basis of the atomic bomb.

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.

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

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
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.

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
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.

Gemini Program
The Gemini missions was N.A.S.A.'s attempt at putting a two man vehicle into space. It is named after the zodiac constellation Gemini which are the twins.

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

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

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

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

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

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

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

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

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

Gravity 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.

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.

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 Earth’s 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.


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

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.

Hill Sphere
A celestial bodies Hill Sphere is the region of space in which it dominates the attraction of satellites. To be retained by a planet, a moon must have an orbit that lies within the planet's Hill sphere.

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

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

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

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 = Ho * d, where V is the object’s recessional velocity, d is the distance to the object, and Ho 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 was launched April 24, 1990 aboard the NASA Space Shuttle Discovery. The 12.5-ton (11,110-kg), tube-shaped telescope is 13.1 m (43 ft) long and 4.3 m (14 ft) wide. It orbits the Earth every 96 minutes and is mainly powered by the sunlight collected by its two solar arrays. The telescopes primary mirror is 2.4 m (8 ft) wide. The telescope is operated jointly by the National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA). HST is one of the many NASA Origins Missions, which include current satellites such as the Far Ultraviolet Space Explorer (FUSE) and future space observatories such as the James Webb Space Telescope (JWST).


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.

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.

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

Infrared (IR) 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. The Infrared Telescope Facility, an Earth-bound infrared telescope, is the U.S. national infrared observing facility at the summit of Mauna Kea, Hawaii. A planned space-based infrared observatory is the Space Infrared Telescope Facility (SIRTF).

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 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.

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.

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.


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.”


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.

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

Kuiper belt
A region in our outer solar system where many short-period comets 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.


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

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.


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.

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.

Classification devised by French Astronomer Charles Messier for far off objects. These Messier numbers have been given to galaxies, star clusters and nebulas. A large number of Messier's have proper names such as M32 is better known as the Andromeda Galaxy.

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 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.

Mercury Program
The Mercury missions were N.A.S.A's first missions to get people into space. The capsules consisted of one person which then led to the Gemini Missions which contains two people.

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.

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.

A moon is any natural satellite orbiting a planet. Earth's moon is known imaginatively as 'The Moon'. Moons can either have been created from the planet it orbits as our one was. They could be asteroids that flew too close to planet and was caught by the planets gravity such as phobos and Deimos . Finally they could be formed in the same way as planets in their own way such as Titan or Europa but orbit a planet instead of the Sun. Most moons are dead, lumps of rock with no atmosphere. Titan on the other hand has an atmosphere and Europa has a hard surface with possible liquid underneath.


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.

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.

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.

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.

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.

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.

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

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

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.

Occultation (to occult)
An occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The term is often used in astronomy, but can also refer to any situation in which an object in the foreground blocks from view (occults) an object in the background.

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.

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.

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.

Orbit Eccentricity
The orbit eccentricity is the amount at which a circle or ellipsis deviates from a perfect circle. If the value is zero, it is a circular orbit, if the value is between 0 and 1, then the orbit is an elliptical. When the value is 1, it is a Parabola, greater than 1 is a Hyperbola which may look like a ellipse at one side but the end doesn't close.


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

The point at which an object orbiting a larger mass (or centre of mass) is at its closest. The opposite is Apoapsis. Specific terms are available for objects in orbit about the Sun, Earth and other stars.

The point at which an object that is orbiting Earth is at its closest. The opposite is Apogee .

The point at which an object orbiting the Sun is at its closest. The opposite is Aphelion.

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.

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.

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.

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

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

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.

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.

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.

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.

Prograde Orbit (motion)
Orbital motion in the usual direction of celestial bodies within a given system; specifically, of a satellite, motion in the direction of rotation of the primary.

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.

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.


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.


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

Radio Galaxy
A Radio Galaxy is one that active galaxies that are very bright at radio wavelengths. An example of a Radio Galaxy is M87 .

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.

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.

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

Reflector (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.

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.

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).

Retrograde Orbit (as opposed to Prograde)
This is an orbit where the satellite travels in the opposite direction of the primary bodies rotation. Triton is a prime example of such a moon in a retrograde orbit about a planet. Due to tidal forces these orbits lead to orbital decay.

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.

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.

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.


A satellite is an object that travels round another object. A satellite may be natural ( a planet, a moon ) or artificial ( a man-made satellite )

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.

Semi-Major Axis
The semi-major axis of an object is the distance from the centre of an ellipsis to the farthest edge.

Semi-Minor Axis
The semi-major axis of an object is the distance from the centre of an ellipsis to the nearest edge.

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

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.

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 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 Sails
Solar sails (also called light sails or photon sails) are a form of spacecraft propulsion using radiation pressure exerted by sunlight on a surface or sail. A useful analogy may be a sailing boat; the light exerting a force on the surface is akin to a sail being blown by the wind.

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 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.

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

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 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.

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.

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

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

Stellar 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.

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.

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

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
The glowing, expanding gaseous remains of a supernova explosion.

Sidereal Orbital Period
The time taken by one object to complete an orbit round another object relative to the background stars.

Solar Wind
Electrons and protons moving at high speed from the Sun acting like a wind. It is hoped by some people that they can harness this power to produce spacecrafts that sail on the Solar Winds. The solar winds exist in heliosphere but when they reach the heliopause, their power becomes diminished.

Solid State Theory
A theory put forward by Sir Fred Hoyle to say the Universe has always been here and always will.

Soyuz Program
The Soyuz missions is Russia's current vehicle it uses to transport cosmonauts and astronauts into space.


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).

A Taikonaut is a term used for a Chinese person in space. The only difference between a cosmonaut, astronaut and a Taikonaut is whose technology the person used to get into space.

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.

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.

Tidal Forces
A secondary effect of the gravitational forces between two objects orbiting each other, such as the Earth and Moon, that tend to elongate each body along the axis of a line connecting their centers. Tidal forces are responsible for the fluctuation of the tides as well as for the synchronous rotation (tidally locked) of certain moons as they orbit their planets which usually results in the same side of the secondary body always  facing the primary - as with the Earth's Moon.

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


Unidentified Flying Object - UFO
A term used to describe anything in the sky that cannot be identified. People associate UFOs with crafts from outer space.

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

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

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


Van Allen belt
In 1958, American astronomer James Van Allen discovered 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.

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.

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
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.

Voskhod Program
The Voskhod missions were the Russian's attempt at putting a two man craft into space, it was a failure.


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.


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.


Yellow Star
A relatively cool star (5,000->6000 K) of which our Sun is an example. It is cool compared to Blue Stars which are over 33,000K. They are also known as G Class stars.


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.

Zodiacal Light
This is a faint, diffuse, and roughly triangular white glow visible in the night sky that appears to extend from the vicinity of the Sun along the ecliptic or zodiac - it's sunlight scattered by interplanetary dust.