Astronomy dictionary – A

Browse through our astronomy dictionary to find definitions for some of the most common terms used in practical astronomy and space science.

Click on one of the letters below to search for a term.


Abell cluster

A galaxy cluster that belongs to the Abell catalogue. This is a listing of more than 4,000 galaxy clusters that meet certain criteria, one of which is having at least 30 galaxies within a set magnitude range. The catalogue is divided into five groups of richness according to how many galaxies the cluster contains. Class 0 clusters contain between 30 and 49 galaxies, and class 5 clusters contain more than 299 galaxies.

Absolute magnitude

The apparent magnitude (the brightness perceived by an observer) a celestial body would have if it was 10 parsecs (2 million AU) from Earth. It’s a way of directly comparing the brightnesses of stars that are at different distances from Earth.

Absolute zero

This is the coldest temperature theoretically possible (-273.15 degrees Celsius), where the motion of atoms in a material would stop completely, leaving them only with a small amount of quantum mechanically energy.


The process by which a celestial object increases its mass by collecting matter from surrounding gas and objects, due to the attraction of its gravity.

Accretion disc

A disc of interstellar material around a celestial object, such as a star or a black hole, that has formed by matter being attracted by its gravitational pull.


An achromatic lens is one that has been designed to reduce chromatic aberration of light passing through it.

Active galactic nucleus

The centre of a galaxy, which emits large amounts of energy as electromagnetic radiation. Such objects are thought to be powered by matter falling on to a supermassive black hole.


An advanced material used by the Stardust spacecraft to capture small particles of cometary dust. It is 99.8 per cent air and was effective at slowing down the cometary particles gently so that they weren’t damaged.

Afocal coupling

The technique of imaging through a camera lens held up to the eyepiece of a telescope. It is used for cameras with non-removable lenses.


This is a measure of how reflective a surface is – the ratio of the amount of electromagnetic radiation (like visible light and infrared) reflected by a surface, to the amount that falls on it.

Albedo features

These are regions on the surfaces of planets that contrast in brightness with nearby areas.

Alfven waves

These are transverse waves that travel through electrically conducting fluids or gases in which a magnetic field is present, like the Sun’s plasma.

All-sky camera

This has a lens (or element of its design) that enables it to image the majority, if not all, of the sky.

Altazimuth Mount

A type of telescope mount that is simpler to construct than an equatorial mount, but requires simultaneous movement about the vertical (altitude) and horizontal (azimuth) axes to track a celestial object.

Amp glow

This is the (usually pink/red) glow seen on the edges of some DSLR images caused by infrared radiation from the camera’s amplifier.


Observed every day from the same location at the same time, the Sun follows a figure-of-eight path through the sky. Known as an analemma, this pattern is due to the tilt of the Earth’s axis as it orbits the Sun.


A unit of measurement equal to just 10^-10m (1 nanometre), commonly used to express the length of light waves in the visible spectrum.


This describes the type of telescope which has been made so that its optical systems show no signs of aberrations from spherical aberration and coma.


A telescope that is almost completely free from chromatic aberrations. These are caused in some telescopes by different wavelengths being brought to focus at different distances.

Apochromatic refractor

A telescope that uses three or more lenses to bring red, green and blue light to focus at the same point.


The period during which a planet is best placed for observation.


Bodies in an elliptical orbit all reach a point when they are furthest or closest to their parent object, eg a planet around a star. Apsides is a collective term for these points. Earth’s apsides are its perihelion and aphelion.


A unit of measurement in astronomy that is equivalent to 1/1800th of the angular diameter of the Full Moon.


A small unit of angular measurement, spanning one-sixtieth of a degree; an arcsecond is one-sixtieth of an arcminute. Astronomers measure the separation between stars in the night sky in terms of degrees.

Artificial star

This is simply an artificial point of light used to test the optics of a telescope in the absence of, or instead of, a suitable real star for collimation and other adjustments.


A pattern formed by stars that aren’t necessarily in the same constellation, for example the Plough and the Summer Triangle.


An irregular rocky and or metallic body left over from the formation of the Solar System. Their size ranges from tens of metres up to at least 1000km. Their composition varies generally based on their location in the Solar System. Most of the asteroids in our Solar System lie in a ‘belt’ between Mars and Jupiter. Nevertheless some of the gas giants shepherd asteroids and even hold them in their own orbits (as in the case of Jupiter).


The study of the internal structure of stars by analysis of the way they pulsate.


A relatively new branch of science that investigates the conditions necessary for life elsewhere in the Universe, and how we can detect it if it exists.


A photographic telescope. An astrograph was used by Clyde Tombaugh to discover Pluto in 1930.


The study of the precise position and movement of stars, which has led to numerous discoveries of extrasolar planets. The gravitational attraction of a planet causes its star to ‘wobble’, which can then be detected.

Astronomical Unit (AU)

One AU is a unit of distance equivalent to roughly 150,000,000 km.

Auroral corona

An auroral corona usually appears during energetic auroral displays. They are seen as rays of auroral emission coming straight at you, where the perspective often makes it look as if they are emanating from a single point in the sky.

Averted vision

A technique for observing faint objects through a telescope by viewing slightly to the side, allowing light from the object to fall on an area of the eye more sensitive to faint light.


Azimuth is a horizontal measurement used to locate the position of an object in the sky. Azimuth is measured clockwise from north, and spans 360° around you.