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 every day - 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, here you'll find many of 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:



Absolute brightness (absolute magnitude)

Absolute zero


Absorption line

Accelerating universe

Accretion disk

Aerobraking is a spaceflight manoeuvre 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 manoeuvre requires less fuel than does the direct use of a rocket engine.





Angular momentum

Angular resolution



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 the Periapsis. Specific terms are available for objects in orbit about the Sun, Earth and other stars.

Apparent brightness (apparent magnitude)


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

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


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 kilometres (93 million miles). This unit of length is commonly used for measuring the distances between objects within the solar system.



Atmospheric distortion


An imaginary line through the centre 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 centre. The Milky Way is thought to be a barred spiral galaxy.

Big Bang

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

Black hole

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


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 centre 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


Celestial object

Celestial sphere

Cepheid variable

Chemical compound

Chemistry for life

The middle layer of the solar atmosphere between the photosphere and the corona. The chromosphere is roughly 10,000 kilometres (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

Collecting area

Colliding galaxies


A ball of rock and ice often referred to as a “dirty snowball.” Typically a few kilometres 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 vapours into space.

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

Comet Tail


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 kilometres from the star's surface and has a temperature in the millions of degrees.

Cosmic abundances

Cosmic background radiation

Cosmic rays

Cosmological Principle




Dark dust cloud

Dark energy

Dark matter

Declination (DEC)

Degree of arc

Doppler effect

Double stars
A system of two stars that are gravitationally bound to each other. They orbit each other around a common centre. 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 neighbours of the Milky Way.

Dwarf planet



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

Ellipse (elliptical)

Elliptical galaxy


Escape velocity

European Space Agency (ESA)

Event horizon

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




Field of view (FOV)
The area of the sky visible through a telescope. The telescope’s viewing area is measured in degrees, arcminutes, 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.



Flat universe

Flyby spacecraft



Galactic centre
The central hub or nucleus of a galaxy. The Milky Way’s galactic centre is about 28,000 light-years from Earth.

Galactic disk

Galactic halo

Galactic nucleus

Galactic Plane


Galaxy cluster

Galaxy evolution

Galaxy Supercluster

Gamma-ray burst (GRB)

Gamma rays

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

General Theory of Relativity

An adjective meaning “centred on the Earth.” Most early civilizations had a geocentric view of the universe.

Geosynchronous orbit

Gemini Program
The Gemini missions were NASA's attempt at putting a two-man vehicle into space. It is named after the zodiac constellation Gemini which is the twins.

Giant star

Globular cluster

Goddard Space Flight Center (GSFC)

Grand Unified Theory (GUT)

Gravitational clustering

Gravitational constant (G)

Gravitational instability

Gravitational lens

Gravitational redshift
The reddening of light from a very massive object caused by photons escaping and travelling 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

Gravity (gravitational force)

Great Red Spot

Group of 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 centre has more than 15 million guide stars in its database the Guide Star Catalogue.


Habitable zone

An adjective meaning “centred on the Sun.”


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

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

Hubble Deep Field North (HDF-N)

Hubble’s law

Hubble Space Telescope (HST)



Impact crater




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 a 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

Interplanetary space

Interstellar dust

Interstellar space
The dark regions of space located between the stars.

Inverse square law

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 kilometres (30 to 250 miles) above the Earth's surface.

Irregular galaxy


Jovian planets


Kármán line
The Kármán line, or Karman line, lies at an altitude of 100 km (62 mi; 330,000 ft) above Earth's sea level and commonly represents the boundary between Earth's atmosphere and outer space. So above this is where Space begins!

Keck Observatory
Two telescopes are 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

Kepler’s laws
Three laws, derived by 17th-century German astronomer Johannes Kepler, that describe planetary motion.


Kitt Peak Observatory

Kuiper belt


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

The distance that a particle of light (photon) will travel in a year – about 10 trillion kilometres (6 trillion miles). It is a useful unit for measuring distances between stars.

Local group

Long-period comet


Lunar eclipse


Magellanic Clouds

Magnetic field

Magnetic-field lines


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.


Marshall Space Flight Center (MSFC)
NASA centre 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 centre aided in the design, development, and construction of the Hubble Space Telescope.


Matter-antimatter annihilation

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 travelling into the Earth’s atmosphere.



A very small meteoroid with a diameter of less than a millimetre. 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 millimetre and one meter.

Milky Way galaxy

Modern physics

Molecular cloud

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 the planet and was caught by the planet's gravity such as Phobos and Deimos. Finally, they could be formed in the same way as planets are (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 water 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.



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 centre 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 kilometres 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

North Celestial Pole (NCP)

Northern Hemisphere




Observable universe

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.


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

Open cluster

Open universe


Optical telescope


The act of travelling 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 elliptical. When the value is 1, it is a Parabola, greater than 1 is a Hyperbola which may look like an 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 kilometres, 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




Planetary nebula

A small body of rock and/or ice – under 10 kilometres (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.


Primary mirror

Primordial nucleosynthesis


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

Proton-proton chain



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

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

Recessional velocity

Red giant star

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

Reflector (reflecting telescope)

Refractor (refracting telescope)



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.


Right ascension (RA)


Roche limit

Rocky planet



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

Schwarzschild radius

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

Short-period comet

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

Solar arrays

Solar constant

Solar cycle
The periodic changing of the Sun’s magnetic field, which determines the number of sunspots and the number of particles emitted in the solar wind. The period of the cycle is about 11 years.

Solar eclipse

Solar maximum

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

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

Solar telescope

Solar wind
Streams of charged particles flowing from the Sun at millions of kilometres 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)

Southern Hemisphere

Space shuttle



Speed of light (c)

Spiral arms

Spiral galaxy

Standard candle

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, colours, and temperatures. Our Sun, the centre of our solar system, is a yellow star of average temperature and size.

Starburst galaxy

Star cluster

Stellar black hole

Stellar evolution

Stellar nursery

Stellar parallax



Sunspot cycle

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


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

Sidereal Orbital Period
The time taken by an object to complete an orbit around 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 spacecraft 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 are Russia's current vehicle it uses to transport cosmonauts and astronauts into space.



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.


Terrestrial planets

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 centres. 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.

Transit (astronomical transit)
A transit is an event where a celestial body (i.e. an object in space like a planet, moon, asteroid etc) passes directly between a larger body and the observer, from a particular vantage point. This results in the smaller transiting body appears to move across the face of the larger body (i.e. the Planet Venus as it cross in front of the Sun as viewed from Earth), covering a small portion of it.

T-Tauri Star


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)

Ultraviolet (UV) light



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 kilometres (496 miles) above the Earth’s surface and extends thousands of kilometres into space.

Variable star
A star whose luminosity (brightness) changes with time.

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

Very Large Array (VLA)

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


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



X-ray sources

X-ray telescope


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.



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.