Glossary of astronomy Terms
Albedo: The amount of light reflected by a body, such as a planet or part of a planet’s surface.
Aperture: The clear diameter of the objective lens or primary mirror of a telescope or other optical instrument.
Aphelion: The point on its elliptical orbit around the Earth at which a body such as a planet, asteroid or comet is at its greatest distance from the Sun.
Apogee: The point on its elliptical orbit around the Earth at which a body such as the Moon or spacecraft is at its greatest distance from the Earth.
Asterism: A pattern of stars that itself is not a constellation. An example of which is the Plough or Big Dipper, which forms part of the constellation ursa major (the Great Bear).
Asteroid: One of a vast number of small bodies that revolve around the Sun. The main concentration of these lie between the orbits of Mars and Jupiter in what is called the Main Belt.
Astronomical Unit (AU): A unit of distance measurement equal to the average distance between the Earth and the Sun (92,956,000 miles).
Big Bang: The event in which the universe was born. The origin of space, time and matter.
Binary Star: Two stars that revolve around each other under their mutual gravitational attraction.
Black Hole: A compact region of space within which gravity is so powerful no material object, light or any other kind of radiation can escape.
Blue Shift: The displacement of spectral lines to shorter wavelengths that occurs when a light source is approaching an observer.
Brown-Dwarf Star: A body that forms in the same way as a star but contains too little mass to become hot enough in order to ignite nuclear-fusion reactions.
Celestial Equator: A great circle on the celestial sphere that is a projection of the Earth’s own equator onto the celestial sphere.
Celestial Poles: the two points at which the line of the Earth’s axis, extended outwards, meets the celestial sphere around which the stars appear to revolve.
Celestial Sphere: An imaginary sphere that surrounds the Earth, the inside surface of which the stars are seemingly attached.
Cepheid Variable: A variable star that decreases and increases in brightness in a regular periodic way.
Chromosphere: A thin layer of the Sun’s atmosphere that lies between the photosphere and the corona.
Circumpolar: A term used to describe a celestial body that remains above the horizon at all times when viewed from a particular place on the Earth’s surface.
Coma: A cloud of gas and dust that surrounds the nucleus of a comet.
Comet: A small body composed mainly of dust-laden ice that revolves around the Sun, usually in a highly elongated orbit.
Conjunction: A close alignment in the sky of two celestial bodies, which occurs when both lie in the same direction as viewed from Earth.
Constellation: One of 88 regions of the night sky containing groupings of stars joined together by imaginary lines to represent a figure.
Corona: The outermost region of the atmosphere of the Sun or a star.
Cosmology: The study of the nature, structure, origin and evolution of the Universe.
Dark Matter: Matter that exerts a gravitational influence on its surroundings but does not emit detectable amounts of radiation.
Declination: The angular distance of a celestial body north or south of the celestial equator.
Doppler Effect: The observed change in wavelength or frequency of radiation caused by the motion of its source towards or away from an observer.
Double Star: Two stars that appear close together in the sky because they happen to lie in almost exactly the same direction when viewed from Earth.
Eclipse: The passage of one celestial body into the shadow cast by another.
Ecliptic: The track along which the Sun appears to travel around the celestial sphere.
Elongation: The angle between the Sun and a planet or other Solar System, when viewed from Earth.
Equatorial Mounting: One that enables a telescope to be turned around two axes in order to follow the motion of a celestial object across the sky.
Equinox: An occasion when the Sun is vertically overhead at a planet’s equator and day and night have equal duration for the whole planet.
Extrasolar Planet (Exoplanet): A planet that revolves around a star other than the sun.
Focal Length: The distance between the centre of a lens or the front surface of a concave mirror and the point at which it forms a sharp image of a distant object.
Galaxy: A large aggregation of stars and clouds of gas and dust. Galaxies may be elliptical, spiral or irregular in shape and contain from a few million to a several trillion stars.
Galilean Moon: One of the four largest natural satellites of the planet Jupiter that were discovered in 1610 by Italian astronomer Galileo Galilei.
Gas Planet (Gas Giant): A large planet that, like Jupiter or Saturn, consists predominantly of hydrogen and helium.
Globular Cluster: A near spherical cluster of between 10,000 and more than 1 million stars.
Gravity: The attractive force that acts between material bodies, particles and photons.
Heliocentric: (i) Treated as being viewed from the centre of the Sun. (ii) Having the Sun at the centre of a system.
Inferior Planet: A planet that travels round the Sun on an orbit inside the Earth. The two inferior planets are Venus and Mercury.
Kuiper Belt: A flattened distribution of ‘small’ icy objects that orbit the Sun at distances in the region of 30-100 times the earth’s distance from the Sun.
Light-Year: A unit of distance equal to the distance light travels in one year – 9,460 billion km (5,878 billion miles).
Local Group: A small cluster of more than forty member galaxies to which the Milky Way belongs.
Magnification: The increase in the apparent angular size of an object when viewed through an optical instrument, such as a telescope.
Magnitude (Apparent): The measure of the brightness of an object as seen in the night sky.
Magnitude (Absolute): The apparent magnitude a star would have it were located at a standard distance of 10 parsecs (32.6 light-years) from Earth.
Main Belt: See asteroid
Main Sequence: A term given to the point in a star’s lifecycle, such as the Sun, that shine by converting hydrogen in their cores to helium.
Meridian: A great circle on the surface of the Earth or another astronomical body that passes through the north and south poles and crosses the equator at right angles.
Messier Catalogue: A widely used catalogue of celestial objects (star clusters, nebulae and galaxies) first published by Charles Messier in 1781. For example M31 the Andromeda Galaxy.
Meteor: The short-lived streak of light seen when a meteoroid plunges into the Earth’s atmosphere and is heated by friction.
Meteor Shower: A substantial number of meteors that app[ear to radiate from a common point in the sky.
Meteorite: A rocky or metallic meteoroid that survives passage through the Earth’s atmosphere and reaches the Erath’s surface in one piece or fragments.
Meteoroid: A lump or small particle of rock, metal or ice orbiting the Sun.
Milky Way: (i) The spiral galaxy that contains the Sun. (ii) A faint misty band of light that stretches across the night sky.
Multiple Star: A system consisting of two or more stars bound together by gravity.
Nebula: A cloud of gas and dust in interstellar space.
Neutron Star: An extremely dense compact star. Typically with a diameter of around 6 miles but having the same mass as the Sun.
New General Catalogue (NGC): A catalogue of nebulae, clusters and galaxies that was published in 1888.
Nova: A star that suddenly brightens by a factor of thousands or more and then fades back to its original brightness over a period of weeks or months.
Occultation: The passage of one body in front of another, which causes the more distant object to be wholly or partially hidden.
Oort Cloud: A spherical distribution of trillions of icy planetesimals and cometary nuclei that surrounds the Solar System.
Open Cluster: A loose cluster of up to a few thousand stars that lies in or close to the plane of the Milky Way galaxy.
Opposition: The position of a planet when it is exactly on the opposite side of the Earth from the Sun.
Orbit: The path of a body that is moving within the gravitational field of another.
Parallax: The apparent shift in position of an object when it is observed from different locations.
Parsec: A distance equal to 3.26 light-years.
Perigee: The point on its orbit at which a body that is revolving around the Earth is at its closest to the Earth.
Perihelion: The point on its orbit at which a planet or other Solar System body is closest to the Sun.
Photosphere: The thin gaseous layer at the base of the solar atmosphere.
Planet: A body that is much less massive than a star, revolves around a star and shines by reflecting that star’s light.
Planetesimal: One of a large number of small bodies, composed of rock or ice from which the planets were eventually assembled.
Prominence: A flame-like plume of gas that follows magnetic field lines in the solar atmosphere.
Pulsar: A rapidly rotating neutron star from which we receive brief pulses of radiation.
Quasar: A very compact but extremely powerful source of radiation.
Radiant: The point in the sky from which the tracks of meteors that are members of a particular shower appear to radiate.
Radio Telescope: An instrument that is designed to detect radio waves from astronomical sources.
Red-Dwarf Star: A cool, red, low luminosity star.
Red-Giant Star: A large, highly luminous star with a low surface temperature and a reddish colour.
Red Shift: The displacement of spectral lines to longer wavelengths that is observed when a light source is receding from an observer.
Red Supergiant Star: A extremely large star of very high luminosity and low surface temperature.
Reflecting Telescope: A telescope that uses a concave mirror to collect light, reflect light rays to a focus and form an image of a distant object.
Refracting Telescope: A telescope that uses a lens to refract (bend) light rays in order to bring them to a focus and form an image of a distant object.
Rocky Planet: A planet (also called a terrestrial planet) that is composed mainly of rocks and has similar basic characteristics to Earth.
Satellite: A body that revolves around a planet, otherwise known as a moon. An artificial satellite is an object deliberately placed in orbit around the Earth or other Solar System body.
Solar Cycle: A cyclic variation in solar activity (for example, the production of sunspots and flares), which reaches a maximum at intervals of about 11 years.
Solar Flare: A violent release of energy - in the form of electromagnetic radiation from a site located just above the surface of the Sun.
Solar Mass: A unit of mass equal to the mass of the Sun, which provides a convenient standard for comparing the masses of stars.
Solar System: The Sun together with everything that revolves around it.
Solar Wind: A stream of fast-moving charged particles that escapes from the Sun and flows outwards through the Solar System like a wind.
Spiral Arm: A spiral-shaped structure extending outward from the central bulge of s spiral or barred spiral galaxy.
Spiral Galaxy: A galaxy that consists of a spheroidal central concentration of stars surrounded by a flattened disc composed of stars, gas and dust, within which the major visible features are clumped together into a pattern of spiral arms.
Star: A self-luminous body of hot plasma that generates energy by means of nuclear fusion reactions.
Stellar Wind: An outflow of charged particles from the atmosphere of a star.
Sunspot: A patch on the surface of the Sun that appears dark because it is cooler than its surroundings.
Supergiant: An exceptionally large star with a very large diameter.
Superior Planet: A planet that travels around the Sun on an orbit outside the orbit of the Earth.
Supernova: A catastrophic event that destroys a star and causes its brightness to increase, temporarily, by a factor of around 1 million.
Transit: The passage of a body in front of a larger one (for example the passage of the planet Venus across the face of the Sun.
Variable Star: a star that varies in brightness.
Wavelength: The distance between two successive crests or between two successive troughs in a wave motion.
White-Dwarf Star: A star of low luminosity but relatively high surface temperature, that has been compressed by gravity to a diameter comparable to that of the Earth.
Zenith: The point on the sky directly above an observer (that is 90 degrees above the observer’s horizon).
Zodiac: A band around the celestial sphere, on either side of the ecliptic, through which the Sun, Moon and naked-eye planets appear to travel.
Zodiacal Light: A faint cone-shaped glow that extends along the ecliptic from the western horizon after sunset or from the eastern horizon before sunrise.