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Radio Astronomy Glossary

Radio Astronomy Glossary

The following glossary relates to radio and general astronomy and may be useful for people not familiar with some of the terms on this website.

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Absolute Magnitude

A scale for measuring the actual brightness of a celestial object without accounting for the distance of the object. Absolute magnitude measures how bright an object would appear if it were exactly 10 parsecs (about 33ly) away from Earth.

Absolute Zero

The temperature at which the motion of all atoms and molecules stops and no heat is given off. Absolute zero is reached at 0 degrees K or -273.16 degrees Celsius.


The process by where dust and gas accumulated into larger bodies.

Accretion Disk

A disk of gas that accumulates around a centre of gravitational attraction, such as a dwarf star, or black hole. Formed when matter is dragged from another nearby star usually


The reflective property of a non-luminous object. A comet for example has an albedo of around half that of a lump of coal.


The angular distance of an object above the horizon.


Matter consisting of particles with charges opposite that of ordinary matter.  In antimatter, protons have a negative charge while electrons have a positive charge.


The size of the opening through which light passes in a telescope or the size of dish in a radio telescope.


The point in the orbit of an object like a comet or planet where it is farthest from the Sun.

Apparent Magnitude

The apparent brightness of an object in the sky as it appears to an observer on Earth. Bright objects have a low apparent magnitude while dim objects will have a higher apparent magnitude.


A small planetary body in orbit around the Sun. Most asteroids can be found in a belt between the orbits of Mars and Jupiter. These can be mapped with radio telescopes using radar.


The branch of science that explores the chemical interactions between dust and gas interspersed between stars

Astronomical Unit (AU)

A unit of measure equal to the average distance between the Earth and the Sun, approximately 93 million miles or 150 million km


A layer of gases surrounding a planet, star or moon.


A glow in a planet’s ionosphere caused by the interaction between the magnetic field and charged particles from the Sun.


Also known as the poles, this is an imaginary line through the centre of rotation of an object.


The angular distance of an object around or parallel to the horizon from a predefined zero point.

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Big Bang

The theory that suggests that the universe was formed from a single point in space during a cataclysmic explosion about 13.7 billion years ago. This is the current accepted theory for the origin of the universe and is supported by measurements of background radiation and the observed expansion of space.


A system of two stars in orbit around each other.

Black Hole

The collapsed core of a massive star. The collapse carries on until a singularity is formed and the gravitational force exerted on everything around it is is so strong that not even light can escape.


A shift in the lines of an object’s spectrum toward the blue end. Objects like the Andromeda Galaxy which is coming towards us exhibit this.

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The time length of an exposure taken by a telescope

Celestial Equator

An imaginary line that divides the celestial sphere into a northern and southern hemisphere.

Celestial Poles

The North and South poles of the aforementioned celestial sphere

Celestial Sphere

An imaginary sphere around the Earth on which the stars and planets appear to be positioned.

Cepheid Variable

This is a variable whose light pulsates in a regular cycle. The period of fluctuation is linked to the brightness of the star. Brighter Cepheids will have a longer period. Used to help determine the Hubble constant by Edwin Hubble.


The part of the Sun’s atmosphere just above the surface.

Circumpolar Star

A star that never sets but always stays above the horizon. This depends on the location of the observer, and how far above or below (for North and South poles) the equator they are viewing from.

Circumstellar Disk

A torus or ring-shaped accumulation of gas, dust, or other debris in orbit around a star in different phases of its life cycle.


An area of dust or gas surrounding the nucleus of a cometary body. Usually shrouding the main nucleus which is significantly smaller.


A gigantic ball of ice and rock that orbit the Sun in a highly eccentric orbit. Some comets have an orbit that brings them close to the Sun where they form a long tail of gas and dust as they are heated by the Sun’s rays.


An event that occurs when two or more celestial objects appear close close together in the sky.


A grouping of stars that make an imaginary picture in the sky.


The outer part of the Sun’s astmosphere usually invisible except during an eclipse or from spacecraft fitted with special occulting discs.

Cosmic Ray

Atomic nuclei (mostly protons) that are observed to strike the Earth’s atmosphere with extremely high amounts of energy. Can result in marks on CCD digital cameras when taking astronomical images.



A branch of science that deals with studying the origin, structure, and nature of the universe.

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Dark Matter

A term used to describe a theory which predicts that alarge amount of the matter in our universe cannot be seen, but can be detected by its gravitational effects on other bodies.

Debris Disk

A ring-shaped circumstellar disk of dust and debris in orbit around a star. Seen in the earliest period of the formation of solar systems around stars such as our Sun.


The angular distance of an object in the sky from the celestial equator (Abbreviated DEC)


The amount of matter contained within a given volume.


The surface of the Sun or other celestial body projected against the sky.

Doppler Effect

The apparent change in wavelength of sound or light emitted by an object in relation to an observer’s position. As easy analogy for this is the sound of a police siren as it approaches and then goes away from a stationary listener, and the change in pitch.

Double Star

A grouping of two stars. This grouping can be apparent, where the stars seem close together, or physical, such as a binary system.

Dwarf Planet

A celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite. Definition clarified by the IAU several years ago when the de-classified the former planet Pluto.

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The measure of how an object’s orbit differs from a perfect circle.


The total or partial blocking of one celestial body by another.

Eclipsing Binary

A binary system where one object passes in front of the other, cutting off some or all of its radiation. (e.g. light)


An imaginary line in the sky traced by the Sun as it moves in its yearly path through the sky.

Electromagnetic Radiation

The radiation emitted across the entire electromagnetic spectrum from radio to gamma rays.

Electromagnetic Spectrum

The full range of frequencies, from radio waves to gamma waves. Long wavelength to short respectively


An ellipse is an oval shape. Johannes Kepler discovered that the orbits of planets were elliptical in shape rather than circular.

Elliptical Galaxy

A galaxy whose structure shaped like an ellipse and is smooth and lacks complex structures such as spiral arms.


The angular distance of a planetary body from the Sun as seen from Earth.


A table of data arranged by date. Ephemeris tables are typically to list the positions of the Sun, Moon, planets and other solar system objects.


The two points at which the Sun crosses the celestial  in its yearly path in the sky.

Escape Velocity

The speed required for an object to escape the gravitational pull of an astronomical  body.

Event Horizon

The invisible boundary around a black hole past which nothing can escape the gravitational pull – not even light.


A term that means outside of or beyond our own milky way galaxy.


A term used to describe anything that does not originate on Earth. SETI is the search for intelligent life outside of our planet.

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In physics and chemistry, the Faraday constant (named after Michael Faraday) is the magnitude ofelectric charge per mole of electrons


Bright patches that are visible on the Sun’s surface, or photosphere in optical wavelengths.

Flare Star

A faint red star that appears to change in brightness due to explosions on its surface. These can be rapid and sometimes periodic.

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Galactic Halo

The name given to the spherical region surrounding the centre, or nucleus of a galaxy

Galactic Nucleus

A tight concentration of stars , and in some cases a supermassive black hole, found at the innermost regions of a galaxy.


A large grouping of stars, of which our Milky Way is one type.


The highest energy, shortest wavelength form of electromagnetic radiation.

Geosynchronous Orbit

An orbit in which a satellite’s orbital velocity is matched to the rotational velocity of the planet. A spacecraft in geosynchronous orbit appears to hang motionless above one position of a planet’s surface.

Giant Molecular Cloud (GMC)

Massive clouds of gas in interstellar space composed primarily of hydrogen molecules. These clouds have enough mass to produce thousands of stars and are frequently the sites of new star formation.

Globular Cluster

A tight, spherical grouping of hundreds of thousands of stars. Globular clusters are composed of older stars, and are usually found around the central regions of a galaxy.

Gravitational Lens

A concentration of matter such as a galaxy or cluster of galaxies that bends light rays from a background object. Gravitational lensing results in duplicate images of distant objects.


A mutual physical force of nature that causes two bodies to attract each other.

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The point in space at which the solar wind meets the interstellar medium or solar wind from other stars.


The space within the boundary of the heliopause containing the Sun and the Solar System.


An element consisting of one electron and one proton. Hydrogen is the lightest of the elements and is the building block of the universe. Stars form from massive clouds of hydrogen gas.

Hubble’s Law

The law of physics that states that the farther a galaxy is from us, the faster it is moving away from us.

Hydrostatic equilibrium

A state that occurs when compression due to gravity is balanced by a pressure gradient which creates a pressure gradient force in the opposite direction. Hydrostatic equillibrium is responsible for keeping stars from imploding and for giving planets their spherical shape.


A system consisting of a spiral galaxy surrounded by several dwarf white galaxies, often ellipticals. Our galaxy and the Andromeda galaxy are examples of hypergalaxies.

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A measure of the tilt of a planet’s orbital plane in relation to that of the Earth.

Inferior Conjunction

A conjunction of an inferior planet that occurs when the planet is lined up directly between the Earth and the Sun.

International Astronomical Union (IAU)

The internationally recognized authority for assigning designations to celestial bodies and their surface features.

Interplanetary Magnetic Field

The magnetic field carried along with the solar wind.

Interstellar Medium

The gas and dust that exists in open space between the stars.


A region of charged particles in a planet’s upper atmosphere. In Earth’s atmosphere, the ionosphere begins at an altitude of about 25 miles and extends outward about 250.

Irregular Galaxy

A galaxy with no spiral structure and no symmetric shape. Irregular galaxies are usually filamentary or very clumpy in shape.

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A unit used in radio astronomy to indicate the flux density (the rate of flow of radio waves) of electromagnetic radiation received from outer space. A typical radio source has a spectral flux density of roughly 1 Jy. The jansky was named to honor Karl Gothe Jansky who developed radio astronomy in 1932.


A narrow stream of gas or particles ejected from an accretion disk surrounding a star or black hole.

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A temperature scale used in sciences such as astronomy to measure extremely cold temperatures. The Kelvin temperature scale is just like the Celsius scale except that the freezing point of water, zero degrees Celsius, is equal to 273 degrees Kelvin. Absolute zero, the coldest known temperature, is reached at 0 degrees Kelvin or -273.16 degrees Celsius.

Kepler’s Laws

Three laws that define the motion of bodies around s star or in orbit, formulated by Johannes Kepler.


A distance equal to 1000 parsecs.

Kuiper Belt

A large ring of icy, primitive objects beyond the orbit of Neptune. Kuiper Belt objects are believed to be remnants of the original material that formed the Solar System. Some astronomers believe Pluto and Charon are Kuiper Belt objects.

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Lagrange Point

A series of stable orbital positions proven by French mathematician and astronomer Joseph Louis Lagrange.

Lenticular Galaxy

A disk-shaped galaxy that contains no conspicuous structure within the disk. Lenticular galaxies tend to look more like elliptical galaxies than spiral galaxies.


An effect caused by the apparent wobble of the Moon as it orbits the Earth. The Moon always keeps the same side toward the Earth, but due to libration, 59% of the Moon’s surface can be seen over a period of time.

Light Year

An astronomical unit of measure equal to the distance light travels in a year, approximately 5.8 trillion miles.


The outer edge or border of a planet or other celestial body.

Local Group

A small group of about two dozen galaxies of which our own Milky Way galaxy is a member.


The amount of light emitted by a star.

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Magellanic Clouds

Two small, irregular galaxies found just outside our own Milky Way galaxy. The Magellanic Clouds are visible in the skies of the southern hemisphere.

Magnetic Field

A condition found in the region around a magnet or an electric current, characterized by the existence of a detectable magnetic force at every point in the region and by the existence of magnetic poles.

Magnetic Pole

Either of two limited regions in a magnet at which the magnet’s field is most intense.


The area around a planet most affected by its magnetic field. The boundary of this field is set by the solar wind.


The degree of brightness of a star or other object in the sky according to a logarithmic scale. Our Sun has a magnitude of -26.74.


A measure of the total amount of material in a body, defined either by the inertial properties of the body or by its gravitational influence on other bodies.


Typically used to describe all everything with mass that can be seen/measured in the context of astronomy.


An imaginary circle drawn through the North and South poles of the celestial equator.

Minor Planet

A term used to describe objects, such as asteroids, that are in orbit around the Sun but are not planets or comets.

Molecular Cloud

An interstellar cloud of molecular hydrogen containing trace amounts of other molecules such as carbon monoxide and ammonia.

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A term used to describe a point directly underneath an object or body.


A cloud of dust and gas in space, usually illuminated by one or more stars. Nebulae represent the raw material the stars are made of.


A fundamental particle produced by the nuclear reactions in stars. Neutrinos are very hard to detect because the vast majority of them pass completely through the Earth without interacting.

Neutron Star

A compressed core of an exploded star made up almost entirely of neutrons. Neutron stars have a strong gravitational field and some emit pulses of energy along their axis. These are known as pulsars.

Newton’s laws of motion

A series of laws used to describe forces on bodies developed by Sir Isaac Newton.


A star that flares up to several times its original brightness for some time before returning to its original state.

Nuclear Fusion

The nuclear process whereby several small nuclei are combined to make a larger one whose mass is slightly smaller than the sum of the small ones. Nuclear fusion is the reaction that fuels the Sun, where hydrogen nuclei are fused to form helium.

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The angle between a body’s equatorial plane and orbital plane.


An event that occurs when one celestial body conceals or obscures another. For example, a solar eclipse is an occultation of the Sun by the Moon.

Oort Cloud

A theoretical shell of comets that is believed to exist at the outermost regions of our solar system. The Oort cloud was named after the Dutch astronomer who first proposed it.

Open Cluster

A collection of young stars that formed together. They may or may not be still bound by gravity. Some of the youngest open clusters are still embedded in the gas and dust from which they formed.


The position of a planet when it is exactly opposite the Sun in the sky as seen from Earth. A planet at opposition is at its closest approach to the Earth and is best suitable for observing.


The path of a celestial body as it moves through space.

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The apparent change in position of two objects viewed from different locations.


A large distance often used in astronomy.  A parsec is equal to 3.26 light-years.


The point in the orbit of the Moon or other satellite at which it is closest to the Earth.


The point in the orbit of a planet or other body where it is closest to the Sun.


To cause a planet or satellite to deviate from a theoretically regular orbital motion.


A particle of light composed of a minute quantity of electromagnetic energy.


The bright visible surface of the Sun


A celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its local region of planetesimals.

Planetary Nebula

A shell of gas surrounding a small, white star. The gas is usually illuminated by the star, producing a variety of colors and shapes.


A form of ionized gas in which the temperature is too high for atoms to exist in their natural state. Plasma is composed of free electrons and free atomic nuclei.


The apparent shift of the celestial poles caused by a gradual wobble of the Earth’s axis.


An explosion of hot gas that erupts from the Sun’s surface. Solar prominences are usually associated with sunspot activity and can cause interference with communications on Earth due to their electromagnetic effects on the atmosphere.

Prograde Orbit

In reference to a satellite, a prograde orbit means that the satellite orbits the planet in the same direction as the planet’s rotation. A planet is said to have a prograde orbit if the direction of its orbit is the same as that of the majority of other planets in the system.

Proper Motion

The apparent angular motion across the sky of an object relative to the Solar System.

Protoplanetary Disk

A rotating circumstellar disk of dense gas surrounding a young newly formed star.


Dense regions of molecular clouds where stars are forming.


A spinning neutron star that emits energy along its gravitational axis. This energy is received as pulses as the star rotates. Discovered by Dame Jocelyn Bell and Anthony Hewish in the late 1960s

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An highly luminous object found in the remote areas of the universe. Quasars release incredible amounts of energy and are among the oldest and farthest objects in the known universe.

Quasi-Stellar Object

Sometimes also called quasi-stellar source, this is a star-like object with a large redshift that gives off a strong source of radio waves. They are highly luminous and presumed to be extragalactic.

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Radial Velocity

The movement of an object either towards or away from a stationary observer.


Energy radiated from an object in the form of waves or particles.

Radio Galaxy

A galaxy that gives off large amounts of energy in the form of radio waves.

Radio Telescope

A Telescope used to detect radio Waves


A shift in the lines of an object’s spectrum toward the red end. Redshift indicates that an object is moving away from the observer. The larger the redshift, the faster the object is moving. Can be denoted by the letter “z”


A state in which an orbiting object is subject to periodic gravitational perturbations by another.

Retrograde Motion

The phenomenon where a celestial body appears to slow down, stop, them move in the opposite direction. This motion is caused when the Earth overtakes the body in its orbit.

Retrograde Orbit

The orbit of a satellite where the satellite travels in a direction opposite to that direction of the planet’s rotation.

Right Ascension

The amount of time that passes between the rising of Aries and another celestial object. Right ascension is one unit of measure for locating an object in the sky. Abbreviated RA

Roche Limit

The smallest distance from a planet or other body at which purely gravitational forces can hold together a satellite or secondary body of the same mean density as the primary. At a lesser distance the tidal forces of the primary would break up the secondary.


The spin of a body about its axis.

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A natural or artificial body in orbit around a planet.

Seyfert Galaxy

A main-sequence star that rotates rapidly, causing a loss of matter to an ever-expanding shell.

Shell Star

A type of star which is believed to be surrounded by a thin envelope of gas, which is often indicated by bright emission lines in its spectrum.


Of, relating to, or concerned with the stars. Sidereal rotation is that measured with respect to the stars rather than with respect to the Sun or the primary of a satellite.


The centre of a black hole, where the curvature of space time is maximal. At the singularity, the gravitational tides diverge.

Solar Cycle

The approximately 11-year quasi-periodic variation in frequency or number of solar active events.

Solar Eclipse

A phenomenon that occurs when the Earth passes into the shadow of the Moon.

Solar Flare

A bright eruption of hot gas in the Sun’s photosphere.  Solar prominences are usually only detectable by specialized instruments but can be visible during a total solar eclipse.

Solar Wind

A flow of charged particles that travels from the Sun out into the Solar System.


An detector that can be connected to a telescope that can separates the signals into different frequencies, producing a spectrum.

Spiral Galaxy

A galaxy that contains a prominent central bulge and luminous arms of gas, dust, and young stars that wind out from the central nucleus in a spiral formation. Our galaxy, the Milky Way, is a spiral galaxy.


A giant ball of hot gas that creates and emits its own radiation through nuclear fusion.

Star Cluster

A large grouping of stars, from a few dozen to a few hundred thousand, that are bound together by their mutual gravitational attraction.

Steady State Theory

The theory that suggests the universe is expanding but exists in a constant, unchanging state.

Stellar Wind

The ejection of gas from the surface of a star. Many different types of stars, including our Sun, have stellar winds. The stellar wind of our Sun is also known as the Solar wind. A star’s stellar wind is strongest near the end of its life when it has consumed most of its fuel.

Supergiant Star

The stage in a star’s evolution where the core contracts and the star swells to about five hundreds times its original size. The star’s temperature drops, giving it a red colour.


A supernova is a vast explosion caused when a star above a certain mass exhausts its fuel and then gradually wosk its way through higher elements, buring them until a point where fusion can no longer occur. The core of the star can then collapse and if the star is large enough cause a supernova explosion.

Supernova Remnant

An expanding shell of gas ejected at high speeds by a supernova explosion. Supernova remnants are often visible as diffuse gaseous nebulae usually with a shell-like structure. Many resemble “bubbles” in space.

Synchrotron radiation

The electromagnetic radiation emitted when charged particles are accelerated radially.

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An instrument used to collect electromagnetic radiation (e.g. radio waves) from  distant objects and enable direct observation.


The boundary between the light side and the dark side of a planet or other body.


The passage of a celestial body across an observer’s meridian; also the passage of a celestial body across the disk of a larger one.

Trans-Neptunian Object (TNO)

Any one of a number of celestial objects that orbit the Sun at a distance beyond the orbit of the planet Neptune.

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Electromagnetic radiation at wavelengths shorter than the violet end of visible light. The atmosphere of the Earth effectively blocks the transmission of most ultraviolet light, which can be deadly to many forms of life.

Universal Time (UT)

Also known as Greenwich Mean Time, this is local time on the Greenwich meridian. Universal time is used by astronomers as a standard measure of time.

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Van Allen Belts

Radiation zones of charged particles that surround the Earth. The shape of the Van Allen belts is determined by the Earth’s magnetic field.

Variable Star

A star that fluctuates in brightness. These include eclipsing binaries.

Visible Light

Wavelengths of electromagnetic radiation that are visible to the human eye.

Visual Magnitude

A scale used by astronomers to measure the brightness of a star or other celestial object. Visual magnitude measures only the visible light from the object. A logarithmic scale is used.

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The distance between consecutive crests of a wave. This serves as a unit of measure of electromagnetic radiation.

White Dwarf

A very small, white star formed when an average sized star uses up its fuel supply and collapses. This process often produces a planetary nebula, with the white dwarf star at its centre.

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Electromagnetic radiation of a very short wavelength and very high-energy. X-rays have shorter wavelengths than ultraviolet light but longer wavelengths than cosmic rays.

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Yellow Dwarf

An ordinary star such as the Sun at a stable point in its evolution.

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A term used by astronomers to sumbolise the redshift of an object.

Zeeman Effect

Named after the Dutch physicist Pieter Zeeman, is the effect of splitting a spectral line into several components in the presence of a static magnetic field.


A point directly overhead from an observer.