Glossary of Radio Astronomy Terms

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Absolute magnitude:

The apparent magnitude that a star would possess it if were placed at a distance of 10 parsecs from Earth. In this way, absolute magnitude provides a direct comparison of the brightness of stars.

Absolute zero:

The lowest possible temperature, from a thermodynamic, or heat/motion, perspective. A particle at absolute zero would experience almost no motion whatsoever, even subatomic motion, though some motion due to quantum effects is still possible. It is defined as 0 on the Kelvin scale (-273.15 degrees Celsius)

Absorption line:

A dark line or band at a particular wavelength on a spectrum, formed when a substance between a radiating source and an observer absorbs electromagnetic radiation of that wavelength.

Absorption line


The gathering of small particles as a result of collisions, which will eventually form celestial objects as they grow. This process is important in the evolution of stars, planets, and comets.

Accretion disk:

A disk of gas that accumulates around a center of gravitational attraction, such as a white dwarf, a neutron star, or a black hole. As the gas spirals in, it becomes hot and emits energy at a variety of wavelengths, including X-ray and radio waves.

Accretion disk

Achromatic lens (achromat):

Two different-types of glass lens designed to reduce chromatic aberration by cutting down on the amount of false color in an image.

Active Galactic Nuclei (AGN):

A galaxy with an unusually bright central region thought to contain a supermassive black hole actively pulling in tremendous amounts of matter from a swirling disk of gas, stars, and dust.

See Supermassive black hole for more

Active Galactic Nuclei (AGN)

Adaptive optics:

A technique used in astronomy to counteract the blurring effect of Earth’s atmosphere when viewing astronomical objects. Advanced ground-based optical/infrared telescopes use adaptive optics to match the performance of telescopes in space.

Adaptive optics


A device equipped with simple or telescopic sites used for determining a direction. This device works together with an astrolabe.



The angular distance of an object above the horizon.


The size or strength of an electromagnetic wave that is measured in just one wavelength.


Amplitude Modulation:

Amplitude Modulation (AM): Changing the size or strength (amplitude) of a radio wave – or any carrier wave – to transmit information.


A unit of length used to measure the wavelength of light. One angstrom is equal to one ten-billionth of a meter (about four-billionth of an inch). Visible light ranges from about 7,700 (red) to 3,900 (violet) angstroms.

Angular momentum:

The rotary inertia, or resistance of a change in speed or direction, of an object or systems of objects in motion on an axis. For example, the Earth’s angular momentum operates on an annual rotation around the sun.


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 a dish in a radio telescope.


The farthest point in a planet’s (or other body in the solar system) orbit around the Sun.



The point in a body’s orbit (e.g. the moon’s) around Earth where the orbiting body is the farthest from the Earth.


Apparent magnitude:

The measurement of how bright a star is from Earth. A very distant, very bright star can have the same apparent magnitude as a nearby, dim star.

Arc minute:

A measurement that tells you how big something in space appears from Earth. One arc minute = one sixtieth of a degree. For example, if a circle around Earth were divided equally into 360 degrees, the full moon would be approximately 30 arc minutes across.

Arc second:

A measure of angular separation, One arc second = one sixtieth of an arc minute. (1/3600th of a degree.)


An arrangement of two or more radio antennas working together by combining the data they each collect.

See ALMA, VLA, or VBLA for more



An ancient device that was used to determine the location of stars and planets as well as the time of day, based on the sun’s position. It was developed in classical Greece before it was perfected by Islamic astronomers in the 12th century



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


Astronomical unit:

Astronomical unit (AU): A unit of distance equal to the average distance between the Earth and the Sun, or 149.6 million kilometers (93 million miles).

Astronomical unit

Atacama Large Millimeter/submillimeter Array (ALMA):

Funded by the U.S. National Science Foundation and its international partners (NRAO/ESO/NAOJ), ALMA is among the most complex and powerful astronomical observatories on Earth or in space. The telescope is an array of 66 high-precision dish antennas in northern Chile.

See more here

Atacama Large Millimeter/submillimeter Array (ALMA)


Theorized low-mass particle of dark matter that may be responsible for holding galaxies together.



The angular location of a celestial objects from the observer’s perspective. The azimuth is also a specific part on a radio telescope that allows it to rotate and follow a celestial object across the sky.


Balmer lines:

Emission lines in the spectrum of hydrogen that arise from transitions between the second and higher energy states of the hydrogen atom. They were discovered by Swiss physicist J. J. Balmer.

Balmer lines

Balmer, J.J.:

A Swiss mathematician and mathematical physicist known for creating the equation that computes the wavelength of a hydrogen atom’s visible spectral lines.

Balmer, J.J.


The range of frequencies, or wavelengths, that can be detected by a radio telescope or other radio receiver. The greater the bandwidth, the more information astronomers can discover about a distant cosmic object.

Barred Spiral Galaxy:

A variety of a classic spiral galaxy in which the sweeping spiral arms extend from a “bar” that runs through the center of the galaxy. Recent observations suggest that the Milky Way galaxy may be a barred spiral.

Barred Spiral Galaxy


A subatomic particle made up of quarks, such as a proton or a neutron.


Baryonic matter:

Matter that is composed mostly of Baryons. All atoms are considered baryonic matter. Other types of matter include ghostly neutrino particles, free electrons, and – perhaps – the elusive particles that make up dark matter.

Baryonic matter


The distance between two antennas that work together to make astronomical observations. The longer the baseline, the greater the resolution – or ability to detect fine details. For example, in the Very Long Baseline Array the baseline between its easternmost antenna in St. Croix and westernmost antenna in Hawaii is nearly 5,000 miles. The maximum current baseline of the ALMA telescope is approximately 15 kilometers (10 miles).


Beam width:

The size of the point on the sky – measured as an angle – that a radio telescope can see at any given moment. A smaller the beam width, the finer the detail the telescope is observing.

Big Bang:

The well-supported theory that some 13.8 billion years ago, the entire universe was staggeringly small, hot, and dense. In a fraction of an instant, the universe expanded and continues to expand to this day.

Big Bang


Two stars in the same system that orbit around a common center of mass due to their mutual gravity. Binary stars are twins in the sense that they formed together out of the same interstellar cloud. Most stars in our galaxy have a binary companion.


Black dwarf:

One possible final stage in the evolution of a star, in which all the energy is exhausted and it no longer emits radiation. The timescale to create such a star, however, is greater than the current age of the universe.

Black Hole:

An object – typically a collapsed star – whose gravity is so strong that its escape velocity exceeds the speed of light.

See more here

Black Hole

Blazar or BL Lac Object:

Objects that resemble quasars; thought to be the cores of highly luminous galaxies aligned so they are viewed directly down into the heart of the system.

Blazar or BL Lac Object

Blue shift:

The apparent shorting in wavelength of light emitted by an object as seen by an observer when the source and observer are moving closer together. See Doppler shift.

Blue shift

Brown Dwarf:

A semi-stellar object made up mostly of hydrogen that straddles the line between a massive Jupiter-like planet and a low-mass red dwarf star. A brown dwarf, however, has insufficient mass to sustain nuclear fusion.

Brown Dwarf

Celestial Equator:

An imagined circle in the Earth’s atmosphere that is directly above its equator.

Celestial Equator

Celestial Sphere:

A non-existent sphere that surrounds Earth and is used to describe the position of objects in the sky.

Celestial Sphere

Cepheid Variable Star(s):

A luminous star whose brightness varies periodically: growing very bright quickly, and then dimming slowly. It is possible to determine the true luminosity of a Cepheid Variable star by observing the period of these changes. The true luminosity can then be compared to how bright the star appears from Earth. This makes Cepheids useful for calculating approximate astronomical distances.

Cepheid Variable Star(s)


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.

Collecting Area:

The amount of area a telescope has that is capable of collecting electromagnetic radiation. This is important for a telescope’s sensitivity because the more radiation it can collect (that is, the larger its collecting area), the more likely it is to detect dim objects.

Collecting Area

Continuous Spectrum:

A spectrum in which the energy of an object is emitted across all wavelengths.

Continuous Spectrum


A specialized supercomputer that multiplies the data from two antennas and averages the result over time. Essentially the correlator only selects the data that is spotted and analyzed by both antennas, which means the data that isn’t spotted by both is dropped.


Coronal Mass Ejection:

The mass ejected from the sun due to a solar flare.

Coronal Mass Ejection

Cosmic Microwave Background Radiation:

Radiation left over from the Big Bang. Because of the expansion of the Universe, the radiation is detected in the microwave portion of the spectrum (300 MHz [100 cm] to 300 GHz [0.1 cm]), and has a temperature of only 2.7 K (Kelvin) (or -270.45°C).

Cosmic Microwave Background Radiation

Cosmic Rays:

Cosmic rays are very high energy atomic nuclei (mostly protons) that travel through space at close to the speed of light and strike the Earth’s atmosphere.

Cosmic Rays


The study of the large-scale structure and evolution of the universe.



This is another word for the universe and derived from the Greek, meaning ‘everything’.


Cyclotron radiation:

Electromagnetic radiation emitted when charged particles are moved within a magnetic field at non-relativistic speeds (i.e., not close to the speed of light).

Circumstellar disk:

The disk of dust, gas, or debris surrounding a star.

Circumstellar disk

Dark matter:

A type of matter that does not interact with or emit visible light or other forms of electromagnetic radiation, as far as we know. Its existence is inferred by its obvious gravitational effects.

Dark matter

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.

Debris Disk


One of two coordinates used to locate the position of an object in Earth’s celestial sphere. Similar to the latitude and longitude of Earth’s surface, Declination runs between the north and south poles of the celestial equator. (See Right Ascension)


An ion with the chemical formula N2H+ that is commonly found in interstellar environments and stellar nebulas.


Diffraction Fringe:

Blurred fringe surrounding an image caused by the wave properties of light. No detail smaller than the fringe can be seen. (see fringe for more)

Diffuse ionized gas:

Also referred to as the “Reynolds Layer”, Diffused Ionized Gas is nearly fully ionized gas in the Milky Way.

Diffuse ionized gas

Double-lobed Radio Source:

A galaxy that emits radio energy from two regions located on opposite sides of the galaxy.

Double-lobed Radio Source


Doppler shift or Doppler effect: The change in wavelength due to the relative motion of source and receiver. Things moving toward you have their wavelengths shortened. Things moving away have their wavelengths lengthened.



Tiny grains of material that block and scatter visible light and shorter wavelengths. Longer wavelengths are able to pass through dust in space, which has allowed astronomers to image previously hidden objects, such as the center of the Milky Way Galaxy.


Dwarf Galaxy:

A galaxy that is significantly smaller in size compared to a normal galaxy. Dwarf galaxies are low in luminosity, and often irregular or elliptical in shape.

Dwarf Galaxy

Dwarf star:

Making up most of the main-sequence stars, dwarf stars are low in luminosity and one of the most common type of stars found in the universe. Our sun is a yellow dwarf star.

Dwarf star

Eagle Nebula:

A bright nebula surrounding a cluster of new-born stars that has been designated as M16 or NGC 6611.

Eagle Nebula

Early Type Star:

A hot star of spectral type O, B or A.

Early Type Star


Measurement of how much an orbital path deviates from a perfect circle.



A partial or total obstruction of light from a celestial body caused by an object passing between it and another body. (Example: When Earth’s Moon casts a shadow between the light coming from the Sun and Earth.



The shell-shaped region of space surrounding a star that holds a suitable temperature range in order to sustain life.


Einstein, Albert:

A German, Swiss and American theoretical physicist whose work has had a tremendous impact on science from the 20th century to present day. He is famously known for his special and general theories of relativity that relate space, time, and gravity.

Einstein, Albert

Einstein Ring:

A ring- or arc-like image created when light emitted from a distant source is bent by the gravitational influence of a massive foreground object. This effect is called gravitational lensing; it allows astronomers to observe objects that would otherwise be too distant to study.

Einstein Ring

Einstein’s Postulates of Special Relativity:

The two assumptions Einstein took to be true when developing his theory of special relativity. The first, the laws of physics are the same for all inertial reference frames. The second, the speed of light in a vacuum is constant regardless of the motion of the source or observer.


Material that is thrown out during an event, such as two objects colliding or a stellar explosion.


Electromagnetic Radiation:

A collective term for radiation from oscillating electric and magnetic fields. Examples of electromagnetic radiation include gamma rays, X-rays, ultraviolet, visible light, infrared, microwave, and radio waves.

Electromagnetic Radiation


A negatively charged particle commonly found in the outer layers of atoms.


A substance that cannot be chemically broken down into simpler materials. They are arranged on the Periodic Table based on the number of protons a single atom of a certain type would contain.



A geometric shape made up of a closed curve that contain two focal points. For any point on the curve the sum of the distances to the two focal points must be constant. In general, the orbits of celestial objects are elliptical.


Elliptical Galaxy:

A galaxy that appears round or elliptical in shape, typically containing little gas and dust, no spiral arms or disk, mostly older stars, which range in shape from spherical to “football” shaped, and few hot, bright stars.

Elliptical Galaxy


The reappearance of a celestial body from the shadow of another after it has been eclipsed or occulted.

Emission Line:

A bright line in a spectrum caused by the emission of photons from atoms. Astronomers use radio telescopes to identify specific emission lines to detect specific chemical and molecules throughout the cosmos.

Emission Line

Emission Nebula:

A nebula formed by a cloud of gas that emits different colors through the ultraviolet radiation of hot stars within it.

Emission Nebula


A table giving the predicted positions of a celestial object such as a planet or comet, at given intervals.


A coordinate used to pinpoint an object on the celestial sphere during a fixed point in time (Ex: The location of Venus during early March)


Either of two points where the Sun crosses the celestial equator.


The equation for mass-energy equivalence created by Albert Einstein. It is a consequence of his Theory of Special Relativity and states that anything that has mass has some intrinsic energy.


an irregularity in a moon’s motion caused by deviations in the sun or its planet’s motion.

Event Horizon:

The boundary surface of a black hole where nothing can escape, not even light.

Event Horizon

European Space Agency:

An organization of European nations to promote space research and technology for peaceful purposes.

European Space Agency

European Southern Observatory (ESO):

An institute of ten European countries, including the UK, that oversee several South American telescopes and observatories. These observatories include La Silla, Paranal, ALMA, APEX and the ELT.

European Southern Observatory (ESO)


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


Exobiology (Astrobiology):

The study of possible existence of life beyond Earth, also known as astrobiology.


Any planet (rocky or gas giant) orbiting a star other than the Sun. More than 3700 exoplanets have been detected to date.


False color:

A technique scientists use to help them see the details of objects within an image. The colors the scientist picks for his or her image may have nothing to do with the color of the object or they could represent properties like its density, temperature, or composition. The viewer of a false color image must always be told what the significance of each color is.

False color

Faraday rotation:

The interaction between a magnetic field and light, where radio waves passing through the magnetic field rotate the plane of polarization. This allows astronomers to measure the direction and strength of the magnetic field.

Faraday rotation

Fast Fourier Transform (FFT):

The mathematical operation that takes measurements made with a radio interferometer and transforms them into an image of the radio sky. The Fast Fourier Transform is a technique used by computer programs that allows the Fourier Transform to be computed very quickly.

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.

Flare Star


The rate of transfer of fluid, particles, or energy across a given surface.

Free-free Emission:

The emission of radio waves from interstellar clouds as electrons momentarily bind with ionized atoms, and then move on to other atoms.


A measure of wave vibrations per unit time. Typically measured in hertz, or cycles per second. In radio astronomy, high frequency corresponds to shorter wavelengths, like submillimeter waves detected by ALMA. Lower frequency refer to longer waves, like centimeter waves detected by the VLA.


Frequency Modulation (FM):

An imposed signal on transmitted energy that is varied by the frequency of the wave.


When at least two radio waves being received by an antenna, meet and combine or meet and cease to exist.


A process where nuclei collide so fast they combine, and overcome the natural repulsion of positively charged protons. In the center of most stars, hydrogen fuses together to form helium. Fusion is so powerful it supports the star’s enormous mass from collapsing in on itself, and heats the star so high it glows as the bright object we see today.


Fast Radio Burst:

A Fast Radio Burst, or FRB, is a powerful, but fleetingly short-lived, burst of radio light. Recent observations of one repeating FRB — the only such FRB currently known to exist — suggest that at least some may originate in the highly magnetized regions near supermassive black holes. FRBs may be caused by various cosmic phenomena.

Fast Radio Burst

Galactic Center:

The central region of a galaxy characterized by high densities of stars. The center might contain a supermassive black hole. In the case of the Milky Way, there is a supermassive black hole at the center of the galaxy called Sagittarius A*.

Galactic Center

Galactic Halo:

The spherical distribution of old stars and globular clusters that surround a galaxy. (see globular clusters for more)

Galactic Halo

Galactic Nucleus:

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

Galactic Nucleus


A large body of gas, dust, stars, and their companions (Planets, asteroids, moons, etc.) held together by their mutual gravitational attraction. They are grouped into three main categories: spiral galaxies, elliptical galaxies, and irregular galaxies. (see below) Another class of galaxies is peculiar galaxies, which are thought to be distorted versions of normal galaxies.


Gamma rays:

Gamma-rays have some of the smallest wavelengths and highest amount of energy compared to all other waves on the electromagnetic spectrum.

Gamma rays

Gamma-ray Burst:

A powerful event where the powerful energy of a black hole is discharged. The gamma-ray emission can last anywhere from a few minutes to several hours, yet the afterglow from the burst can last for more than a year, which makes long-term observations possible.

Gamma-ray Burst

General theory of relativity:

The theory of gravitation developed by Albert Einstein. The theory discusses the nature of black holes, consequences for the bending of light by massive objects, and the fabric of space and time.

General theory of relativity


A technique used to study the motion of the Earth’s tectonic plates through the use of an interferometer. (see below for definition of an interferometer)

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.

Giant Molecular Cloud (GMC)

Giant Star:

A star with a significantly larger radius and higher luminosity than a dwarf star. (See dwarf star for more)

Giant Star

Gigahertz (GHz):

A unit of measurement representing a frequency of one billion cycles per second.

Globular Cluster:

A spherical star cluster containing 50,000 to 1 million stars. Some of the older, metal-poor global clusters are thought to be the left over building blocks of a galaxy’s formation.

Globular Cluster


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


Gravitational Lens:

The effect when light from a distant object, such as a galaxy, is bent by the gravity of a massive object, such as another galaxy, before it reaches the Earth. If the two objects are perfectly aligned, the light from the distant object appears as a ring, to observers on Earth. This phenomenon is called an Einstein Ring, since its existence was predicted by Einstein in his theory of General Relativity. (see definition above)

Gravitational Lens

Green Bank Observatory:

The first facility in the United States dedicated to radio astronomy. Because of this observatory, the National Radio Astronomy Observatory was created. This facility was also the first national laboratory open to scientists from around the world.

Green Bank Observatory

Galaxy cluster:

A galaxy cluster, or cluster of galaxies, is a structure that contains 100s to 1,000s of galaxies all bound together by gravity. They are among the largest objects in the known universe.

Galaxy cluster

H I Region:

A region in space, seen as cloud-like features, containing mostly neutral atomic hydrogen gas. If it is of sufficient density, this region may become the birthplace of stars and planets.

H I Region

H II Region:

A region in space around one or more very hot, bright stars where most of the hydrogen gas is ionized.


The spherical region of a spiral galaxy composed of diffuse gas (diluted interstellar material and contains few stars and star clusters.



The region around the Sun and Solar System that is maintained by the solar wind given off by the Sun. The solar wind creates a bubble like effect in the interstellar medium in which our Solar System resides.


H-R Diagram (Hertzsprung-Russell Diagram):

The diagram is used by astronomers to classify stars by their their luminosity, spectral type, color, temperature and evolutionary stage. Most stars fall into the Main Sequence part of the diagram, which runs from upper left, where stars are hot and luminous, to the lower right, where stars are cool and faintly luminous. (See below for more on the Main Sequence)

H-R Diagram (Hertzsprung-Russell Diagram)


A unit of measurement of a wave’s frequency. Hertz are measured by the number of oscillations that occur per second. 1 Hertz (Hz) = 1 cycle or oscillation/second.

Hubble Constant (H):

A measure of the rate of expansion of the Universe. Current estimates say that our universe is expanding at about 70 km/s/megaparsec (Kilometers per second per megaparsec).

21-cm Hydrogen Line:

Radio emission by a neutral hydrogen atom when its electron flips its spin. This causes the electron to emit a single photon with a wavelength of 21 centimeters.

21-cm Hydrogen Line


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


For radio telescopes, this typically means unwanted signals, noise, or static. It also describes the result of combining the signals that two telescopes receive when observing the same source, which results in a pattern of oscillating values or “fringes” that depends on the separation of the two telescopes.


A radio telescope consisting of two or more antennas at some distance from one another. It uses the phenomenon of interference, which combines the signals of each antenna that is part of the interferometer, to increase the effectiveness of the antennas’ ability to sharpen the details of an image taken of an object in the cosmos.


Interplanetary magnetic field (IMF)::

The interplanetary magnetic field is a part of the Sun’s magnetic field that is carried from the Sun’s corona into the rest of the Solar System by solar winds.

Interstellar matter:

The gas, dust, and cosmic rays that fill the Interstellar Medium.

Interstellar matter

Interstellar medium:

The matter contained in the regions between star systems in a galaxy. This matter is typically made up of gas, dust, and cosmic rays.

Interstellar medium


An atom that has a net charge. The charge is the result of the atom having an unequal amount of protons and electrons.



The conversion of an atom or molecule into an ion. This conversion occurs when one or more electrons are either removed or added to the atom or molecule.



The outer layers of the Earth’s atmosphere (at least 50 miles above the Earth’s surface), where many of the gas atoms are ionized by high-energy extraterrestrial radiation.

Irregular Galaxy:

An irregular galaxy is typically smaller in size and identified by the fact that it does not have a distinct shape or spiraling arms.

Irregular Galaxy

Irregular variable:

A pulsating variable star whose variation in brightness does not follow any regular or predictable pattern.

Ionized gas:

Gas either between the stars or among galaxies that has been superheated so that electrons are stripped away from their atoms or molecules.

Ionized gas


An atom or molecule that has had one or more of electrons stripped away or added to it. Large, energetic stars are particularly efficient at ionizing atoms and molecules.



Isotopologues are molecules that differ only in their isotopic composition. Simply, the isotopologue of a molecule has at least one atom with a different number of neutrons than the parent atom.



An element — helium, carbon, oxygen, etc. — that varies in the number of neutrons in its nucleus. An isotope of hydrogen with two neutrons in its nucleus is called deuterium.


Jansky (Jy):

A unit of measurement that looks at the flux density of electromagnetic radiation. It is most often used when the object being observed can be considered a point source. This measurement was named after radio astronomy pioneer Karl G. Jansky.


A pair of beams of extremely energetic particles, usually coming from an active galactic nucleus, an exploding star, or a pulsar. (See active galactic nucleus or pulsar for more)

Jets can also come from a protostar. (See protostar)


Jansky, Karl G.:

An American physicist and radio engineer who is known as one of the founders of radio astronomy. He was the first astronomer to discover radio waves emitting from the Milky Way Galaxy.

Jansky, Karl G.

Kelvins (Kelvin):

A scale that measures an object’s temperature over absolute zero, which is the lower limit of thermodynamic temperature. A particle at this temperature would experience almost no motion whatsoever, only experiencing motion due to quantum effects.


A unit of measurement that looks at the distance between earth and objects outside of our solar system. One kiloparsec equals a distance of 1000 parsecs (3260 light-years about 19.56 quadrillion miles).

Kuiper belt:

A region of our solar system beyond the orbit of Neptune. It contains many comets, asteroids, and other icy bodies. Pluto is often considered a large resident of this region. Other star systems likely sport similar regions.

Kuiper belt

Keplerian rotation:

The orderly motion of objects around a central mass, like planets and asteroids in our solar system. The speed — or orbital periods — of these objects is determined by their distance from the central mass (or Sun in the case of our solar system). The closer in the object the faster it orbits.

Keplerian rotation

Light Waves:

Waves in the electromagnetic field. Examples include Gamma, X-Ray, Ultraviolet, Visible, Infrared, Microwave, and Radio.

Light Waves


The distance that light travels in one year in a vacuum. One light year is equivalent to about six trillion miles.


The amount of energy that a star emits per unit time. It is independent of distance or how bright or dim the star appears from Earth. It is measured in Watts (Joules/seconds) and is often given in terms relative to our Sun, L⊙ (about 3.846×10^26 W).



In astronomy, luminosity is an actual unit. It’s the amount of energy given off by a star or galaxy, or any other object — over a specific period of time. “Brightness,” in astronomy, is how apparently bright an object appears to an observer. Brightness goes down with distance. Luminosity does not.


Magellanic Clouds:

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

Magellanic Clouds


A neutron star with an intense magnetic field.


Magnetic axis:

A line that connects an objects northern and southern magnetic poles.

Magnetic axis


Region around a planet in which its magnetic field dominates the effective magnetic field.



The degree of brightness of a star or other object in the sky according to a scale of logarithms.

Main Sequence:

Main sequence stars fuse hydrogen atoms to form helium atoms in their cores. About 90 percent of the stars in the universe, including the sun, are main sequence stars. These stars can range from about a tenth of the mass of the sun to up to 200 times as massive. (See H-R diagram for more)

Main Sequence


An acronym which stands for “Microwave-amplified stimulated emission of radiation.” A maser can be used as an amplifier of radio waves (similar to a laser, which amplifies visible light). This can be a naturally occurring feature or created using special properties of certain crystals, like rubies, at temperatures near absolute zero and in strong magnetic fields. Water molecules in space can form masers that help astronomers study radio emission from objects that would normally be too faint to detect.



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.

Megahertz (MHz):

Like hertz, megahertz measures the frequency of an electromagnetic wave. The frequency of one megahertz is equivalent to one million cycles per second. (See hertz for more)


A unit of measurement, also known as a micrometer, that is equal one millionth of a meter.

Milky Way:

The galaxy that our solar system is a part of. It can be seen by the naked eye as a luminous band across the sky.

Milky Way

Millisecond Pulsar:

A pulsar that “flickers” every one thousandth of a second. (See Pulsar for more)

Millisecond Pulsar

Molecular Cloud:

An interstellar gas cloud where molecular formation occurs. Over 125 different molecules from molecular clouds have now been discovered in interstellar space through radio wavelength observations.

Molecular Cloud

Magnetic Field:

The medium for magnetic interactions. The field is created by magnetic dipoles and moving electric charges. It exerts a force on other electric charges and magnetic dipoles that are in the vicinity of the field. Combined with the electric field it creates the medium for the electromagnetic force, one of the four fundamental forces of nature.

Magnetic Field

Millimeter wavelength:

Millimeter-wavelength light is a sliver of the electromagnetic spectrum. The waves vary in length from about 1 to 10 millimeters (between the infrared and radio portions of the spectrum). ALMA was specifically designed to study this and shorter submillimeter-wavelength light.

Millimeter wavelength

Molecular gas:

Molecular gas is the raw material of star formation. It is made up mostly of hydrogen gas (H2) yet contains various other molecules, such as carbon monoxide.

Molecular gas


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


A nebula is a massive cloud composed of dust, gas and are often illuminated by near by hot stars.



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 small compressed core of a star that has gone through supernova (star explosion). These stars are almost completely made up of only neutrons and have a strong gravitational field.

Neutron Star


The effects of random electrical fluctuations in radio receivers that conceal or distort the effects of radio receivers.

Non-relativistic speeds:

The speed of particles/objects that do not travel with a speed comparable to the speed of light.

Non-thermal emission:

Electromagnetic radiation that is not dependent on the temperature of its source. Three types of non-thermal radiation includes synchrotron radiation, maser line emissions from atoms and molecules, or the mechanisms not related to temperature.

Non-thermal emission


An event that occurs in a binary star system when hydrogen-rich material from a larger star is transferred to the surface of its white dwarf companion. This causes the white dwarf to form a thin atmosphere of hydrogen. The hydrogen is then heated by the white dwarf until it reaches the point where it can undergo nuclear fusion. The resulting fusion causes the atmosphere to be expelled.


Nuclear Bulge:

The sphere of stars and other material at the heart of our galaxy.

Nuclear Bulge

Nuclear fusion:

The power that fuels the sun and stars through nuclear energy released through the fusion of two light elements (elements with low atomic numbers) into a new heavier one.

Nuclear fusion

Next Generation VLA:

A proposal to build the next great leap in radio observatories — expanding on the legacy of the VLA. Astronomers are imagining the next generation Very Large Array (ngVLA) with 244, 18-meter antennas spread over 8,860 kilometers.

Next Generation VLA


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.


OH Line:

Hydroxyl molecules (OH) appear as dark absorption lines on an electromagnetic spectrum.

Oort Cloud:

A theoretical shell of comets and icy planetesimals 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.

Oort Cloud

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.

Open Cluster


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.


protoplanetary disk:

The swirling disk of dust and gas that collapsed from a much larger cloud of material that will eventually evolve into a fully fledged planetary system. Features in the disk may already herald the presence of young planets.

ALMA Observatory image of the young star HL Tau and its protoplanetary disk. This best image ever of planet formation reveals multiple rings and gaps that herald the presence of emerging planets as they sweep their orbits clear of dust and gas. Credit: ALMA(ESO/NAOJ/NRAO); C. Brogan, B. Saxton (NRAO/AUI/NSF)


The apparent change in an object’s location due to the change in position of the observer. Astronomical parallax is measured in seconds of arc.



A unit of length that looks at the distance between earth and objects outside of our solar system. One parsec is equivalent to 3.26 light-years, or nineteen trillion miles.

Peculiar Galaxy:

A galaxy that has distorted features and does not fit completely into a normal galaxy category.

Peculiar Galaxy


An apparently small (at least to observers on Earth) yet immensely powerful cosmic object. Some quasars (quasi-stellar objects, or QSOs) are strong radio sources. Radio-emitting quasars were the first to be discovered. These are some of the most distant objects in the Universe, and are believed to be fueled by supermassive black holes residing in ancient galaxies.


Radio Astronomy:

The science that deals with the study of the universe by means of radio waves.

Radio Astronomy

Radio Galaxy:

A galaxy that emits radio waves from its central core. The energy to produce these emissions is generated by a supermassive black hole, which sends out massive jets of radio energy many millions of light-years into interstellar space.

Radio Galaxy

Radio Source:

A point or small portion of the sky giving stronger radio emission than other parts of the sky surrounding it.

Radio Telescope:

Like an optical telescope, a radio telescopes receives, focuses, and analyzes light to form an image or study a celestial object. The longer wavelengths of radio waves mean that these telescopes use dish antennas to reflect and focus radio light, a receiving system to amplify and measure the signal, and a computer to process the data and develop an image.

Radio Telescope

Radio Waves:

Also known was electromagnetic waves or electromagnetic radiation, radio waves are the basic building blocks of radio communication and radio astronomy. They are created by transmitters which send the waves into space and detected by receivers which detect distant celestial objects.

Radio Waves

Radio window:

The property of Earth’s atmosphere that allows certain wavelengths of electromagnetic radiation in the radio range to pass through.


An electronic device that amplifies, detects, and gives a measure of the intensity of radio signals.


Red Dwarf:

A cool, low mass star on the lower main sequence. (see main sequence for more)

Red Dwarf


The shift of all the spectral lines toward longer wavelengths due to the object moving further away as seen from the Earth. This recession, at great distances, is due to the overall expansion of the universe. (See spectral lines for more)



The ability of a telescope to show detail. One common way to describe the resolution of a telescope is to state the minimum angular separation at which a double star can be distinguished as two separate stars. (see double star for more)


Right ascension:

The equatorial coordinate that specifies the angle (usually specified in hours, minutes and seconds), which is measured eastward along the celestial equator from the vernal equinox to the hour circle passing through an object in the sky.

Red Giant:

A massive star with high luminosity and low surface temperature. Red giants are thought to be in a late stage of evolution when no hydrogen remains in the core to fuel nuclear fusion.

Red Giant

Resolving Power:

A telescope’s ability to sharpen images taken of different places in the universe.


Solar Mass:

A solar mass, the calculated mass of our sun (332,946 times the mass of Earth). It is a standard unit of measure for extremely massive cosmic objects.

Solar Mass

Sagittarius A*:

A supermassive black hole that is located at the center of the Milky Way Galaxy. This powerful radio source lies just 26,000 light years from Earth and was discovered in 1974 by Bruce Balick and Robert L. Brown at Green Bank Observatory. (See supermassive black holes for more)

Sagittarius A*


A natural or artificial body in orbit around a planet.


Seyfert galaxy:

A galaxy that appears to be a normal spiral galaxy, but whose core fluctuates in brightness. It is believed that these fluctuations are caused by powerful eruptions in the core of the galaxy powered by material falling toward a supermassive black hole at the center of the galaxy.

Seyfert galaxy

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.

Shell Star

Sidereal Day:

The time required for Earth to revolve 360 degrees with respect to a celestial object outside the solar system. This equals about 23 hours 56 minutes duration in terms of solar time.

Sidereal Day


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


Solar Eclipse:

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

Solar Eclipse

Solar Flare:

A brilliant outbreak in the Sun’s outer atmosphere, usually associated with active groups of sunspots.

Solar Flare

Solar System:

A star and the non-luminous objects associated with it, which may include brown dwarfs, planets, asteroids, and comets.

Solar System

Special Relativity:

A theory developed by Albert Einstein that relates space and time. It was based on the ideas that the laws of physics are the same in all inertial reference frames and that the speed of light in a vacuum is constant. Some of the predictions and consequences that come out of it are: time dilation, mass-energy equivalence, and relativity of simultaneity.


The plural term of spectrum, which refers to the range of electromagnetic radiation wavelengths.


Spectral Line:

Light given off at a specific frequency by an atom or molecule. Every different type of atom or molecule gives off light at its own set of frequencies. Therefore, astronomers can look for gas containing a particular atom or molecule by tuning the radio telescope to one of the gas’s frequencies. One example would be tuning the radio telescope to 115 gigahertz to find carbon monoxide (CO) which has a spectral line at 115 gigahertz (or a wavelength of 2.7 mm).

Spectral Line

Spectral Sequence:

The temperature classification system for stars ranging from hot (O and B) to cool (K and M). The Spectral Sequence is arranged in the following order: O, B, A, F, G, K, and M.

Spectral Sequence


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



The unique spikes in electromagnetic radiation that correspond to specific molecules and elements.



A plot or distribution of light intensity at different frequencies and wavelengths.


Speed of light (c):

The speed which light travels through a vacuum, which is measured as 299,792,458 meters per second. Einstein’s Theory of Special Relativity implies that nothing can travel faster than the speed of light. (See Einstein’s Theory of Special Relativity for more)

Speed of light (c)

Spiral Galaxy:

A galaxy that is shaped like a flattened rotating disk full of young stars, a central bulge of generally older stars, and a surrounding halo of older stars and dense clusters of old stars called globular clusters. The disk is prominent due to the presence of young, hot stars in a spiral pattern.

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.

Star Cluster

Starburst Galaxy:

A bright galaxy in which many new stars are rapidly forming, up to 1000 times faster than star formation in the Milky Way. Often triggered by galaxy mergers and interactions.

Starburst Galaxy

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.


The star that the Earth, as well as the other plants in our solar system, orbits around.



A very luminous star that is 10 to 1000 times more massive than the Sun.


Superluminal Velocity:

The apparent motion of an object at greater than light speed; this appearance is caused by a “projection effect” or by the object’s motion toward Earth.

Supermassive Black Hole:

A black hole that has a million or as much as a billion solar masses. These large black holes lurk at the centers of most galaxies.

Supermassive Black Hole


The extremely violent explosion of a star many times more massive than our Sun after the nuclear furnace at its core can no longer balance out the force of gravity. During this explosion, these stars may become as bright as all the other stars in a galaxy combined, and in which a great deal of matter is thrown off into space at high velocity and high energy. The remnant of these massive stars collapse into either a neutron star or a black hole.


Supernova Remnant:

The expanding shell of gas from a supernova explosion.

Supernova Remnant

Synchrotron Radiation:

Radiation emitted by charged particles being accelerated in magnetic fields and moving at near the speed of light.

Synchrotron Radiation

Starbursting galaxies:

Bright galaxies in which many new stars are rapidly forming, up to 1000 times faster than star formation in the Milky Way. Often triggered by galaxy mergers and interactions.

Starbursting galaxies


The two VLA transporters are special-purpose vehicles designed specifically to pick up and carry the VLA’s 230-ton dish antennas. The transporters run on two parallel sets of U.S. gauge railroad tracks — four rails in total. Weighing 90 tons themselves, the transporters are powered by 380- and 400-horsepower (respectively) diesel engines that in turn power a hydraulic system. That system drives the wheels using radial hydraulic motors and also powers the jacks that lift the antennas off their concrete-pier mountings. The wheels are mounted on assemblies at each corner of the vehicle that can rotate to allow “turning” the 90-degree rail intersections that connect each antenna mounting station with the main rail line for each arm of the VLA’s “Y” layout.



T Tauri Stars:

Young stars surrounded by gas and dust; believed to be contracting to main sequence stars.

T Tauri Stars

Thermal emission:

Radiation emitted due to an object’s temperature or ionized gas

Thermal emission

Tidal disruption event:

When a star being destroyed by a supermassive black hole is pulled apart and emits powerful jets of material along with light.

Tidal disruption event


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.


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



The donut-shaped feature of dust and gas that surrounds some supermassive black holes.



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.



The inner part of the shadow cast by an illuminated object behind a non-illuminated celestial object. (Example: During a solar eclipse, the moon moves in front of the sun, causing the moon to cast a visible shadow.)



Similar to the cosmos, this term that refers to all of space and everything that is contained within it.


Vernal Equinox:

The spring equinox that occurs on March 20

Very Large Array:

The Very Large Array is a customizable interferometer that varies between 2/3 of a mile to 22 miles long depending on the time of year. It is considered to be one of the most advanced radio telescope arrays on Earth with 28 antennas in Socorro, New Mexico.

Learn More Here

Very Large Array

Variable Star:

A star whose brightness varies with time.

Very Long Baseline Array (VLBA):

An array of 10, 25 meter radio telescopes that stretches 8000km (5,000 miles) across North America.

Very Long Baseline Array (VLBA)

Visible Light:

The wavelengths of electromagnetic radiation that are visible to the naked eye.

Visible Light

Visible spectrum:

The range of wavelengths in the electromagnetic spectrum perceptible by the human eye.

Visible spectrum

Visual binary:

A binary system visible to the human eye through a telescope

Visual magnitude:

The apparent magnitude of a celestial object in the color region where the human eye is most sensitive.



The distance between two adjacent crests of a wave.


Weakly Interacting Massive Particle (WIMP):

Short for “Weakly Interactive Massive Particle,” a WIMP is a hypothetical “dark matter” particle, which does not interact at all or very rarely with other particles via the electromagnetic force. It does, however, interact via gravitational effects.

Whirlpool galaxy:

A well-defined spiral galaxy with very prominent arms.

Whirlpool galaxy

White dwarf:

A dying star that has collapsed to approximately the size of the Earth and is slowly cooling.

White dwarf

X-ray Astronomy:

The study of electromagnetic radiation of wavelengths shorter than ultraviolet radiation but longer than gamma-rays. The Earth’s atmosphere makes viewing celestial objects difficult from the ground, so objects observed through X-ray must be observed through satellites and rockets attached with X-ray antennas.

X-ray Astronomy

X-ray Binary:

A binary system consisting of a normal star and collapsed star of either a neutron star, black hole or white dwarf.

X-ray Binary

X-ray Telescope:

An antenna often outside of Earth’s atmosphere used to observe short wavelengths on an x-ray level.

X-ray Telescope


The time taken by the Earth to complete one revolution around the sun. This is equivalent to 365.2425 days

Yellow Dwarf:

A star such as the Sun that sits at a stable point in its evolution.

Yellow Dwarf

Years ago:

Astronomers often refer to the date of an astronomical event as the date it was witnessed on Earth, even though the object is many thousands or millions of light-years away, taking that long for the light to reach us. This is concept called “Look Back Time.”

Years ago


The point on a celestial object that is vertically above the observer. (90 degrees above the observer)


Zenith distance:

the angular distance of a celestial body from a zenith

Zero-age main sequence (ZAMS):

The main sequence on the Hertzsprung-Russell diagram as defined by the stars that have yet to go through substantial evolution. (See Hertzsprung-Russell Diagram for more)