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AAO Anglo-Australian Observatory, based in New South Wales, Australia. The observatory is at a height of 1150m and has two telescopes: the 3.9m Anglo-Australian Telescope (AAT) and the 1.2m United Kingdom Schmidt telescope (UKST).
Aldrin, Edwin "Buzz" Member of the Apollo 11 crew and the second man to walk on the surface of the Moon. He was born in Montclair, New Jersey in 1930 and was chosen for the astronaut programme in 1963. He set a new record for an EVA on the Gemini XII mission in 1966, before he and Neil Armstrong became the first men to walk on another world on 20th July 1969. The command module, which stayed in orbit around the moon, was flown by Col. Michael Collins.
Alpha Centauri The closest star to the Sun. Alpha Centauri is actually three stars: a bright double star and a faint red dwarf called Proxima Centauri. Proxima Centauri is the closest to us.
Amplify To increase or make bigger.
Andromeda A constellation in the northern sky named after a princess in Greek mythology. According to the myth, Andromeda was chained to a rock by her father to be eaten by the sea serpent, Cetus, which was terrorising his lands. Fortunately for Andromeda, she was saved by Perseus who swooped down on the back of his flying horse, Pegasus, killed Cetus and carried Andromeda off to be his wife.
Andromeda Galaxy The closest spiral galaxy to our galaxy the Milky Way and the largest member of the Local Group. The Andromeda Galaxy is the most distant object that can be seen with the human eye. It is an Sb spiral galaxy, and is slightly bigger than our own galaxy. It is likely to hit the Milky Way in about 2 billion years.
Aphelion The most distant point that a planet reaches from the Sun during its orbit.
Apollo 11 The first mission to land men on the moon. Neil Armstrong and Buzz Aldrin touched down the Eagle lander at the Sea of Tranquility on 20 July 1969. The Lunar Module stayed on the surface for 21 hours, and the astronauts spent 2 and a half hours outside. The command module, flown by Colonel Michael Collins, stayed in orbit around the planet.
Argon (Ar) A gas that doesn't react very much.
Armstrong, Neil The first man on the Moon. Armstrong and Buzz Aldrin became the first men to walk on the surface of another world on the 20th July 1969. The command module, which stayed in orbit, was flown by Col. Michael Collins. Armstrong was born in Wapakoneta, Ohio, in 1930. He was chosen for the astronaut programme in September 1962 and was command pilot on Gemini 8 before becoming commander of Apollo 11. He was one of the first two astronauts not from the military.
Asteroid A small rocky object in the Solar System. Asteroids usually lie in the asteroid belt between Mars and Jupiter. More
Astronomer Someone who studies space and the objects in it
Astronomical Unit A unit of length equal to the average distance of the Earth from the Sun. 1AU = 149,597,870 km.
Atmosphere A layer of gas surrounding an object in space.
Atom The smallest particle of a chemical element that can take part in a reaction. It has a nucleus, containing protons and neutrons, surrounded by a cloud of electrons at different energy levels. The number of protons in an atom gives the atomic number, and this is different for each element.
Aurora Lights seen in the upper atmosphere. Particles from the solar wind are directed down to the Earth’s poles by a magnetic field. The particles react with the gas in the atmosphere and cause it to give out light. The aurora are also called the Northern and Southern lights.
Aurora Australis The Southern Lights
Aurora Borealis The Northern Lights


Bacteria Tiny living things, too small to see with the human eye. There are bacteria everywhere, even inside us.
Bands Dark areas seen on Jupiter, where gas is falling into the planet.
Barred Spiral Galaxy A type of galaxy with spiral arms coming out from a bar of stars across the centre. More
Barringer Crater See Meteor Crater.
Beagle 2 A lander designed to search for signs of life on Mars. Beagle 2 will travel on the ESA Mars Express Spacecraft, which will be launched in 2003. Much of the lander is being designed and built in the UK.
Betelgeuse A supergiant star also called Alpha Orionis. It is hundreds of times larger than the Sun and is the tenth-brightest star in the night sky. It can be found in the constellation of Orion and it looks red because of its temperature.
Big Bang The most likely idea for the beginning of the Universe, in which the Universe has expanded from a single point. More
Big Crunch The end of a universe that will stop expanding and go back to a single point again.More
Binoculars Two small telescopes put next to each other so that both eyes can look at an object at the same time.
Black Dwarf A white dwarf that has cooled until it can no longer be seen.
Black Hole An object with such strong gravity that not even light can travel fast enough to escape from it. It is thought that they are made when very massive stars collapse.
Blueshift The Doppler shift of light towards the blue end of the spectrum. This happens when the source of this light is approaching, and the wavelength of the light is shortened. A shorter wavelength means that the light looks bluer, so it is called blueshift. More
BOOMERanG A balloon experiment to look at the Cosmic Microwave Background (CMB).
Brown Dwarf An object smaller than 0.08 times the mass of the sun that never becomes hot enough to start burning hydrogen in the centre. Brown dwarfs are very faint and hard to find and so may make up some of the "dark matter" in the Universe.


c The speed of light. c = 300,000,000 m/s, which is the same as 300 000 km/s or 10,800,000,000 km/h. The speed of light is thought to be maximum speed possible. Nothing can move faster.
Caldera A large volcanic collapse crater that may be a few km across.
Callisto The second largest moon of Jupiter and the faintest and furthest out Galilean moon. It has an icy surface with a lot of impact craters.
Caloris Basin An impact basin 1300km across, the largest on Mercury. It was probably created about 4 billion years ago. The inside of the basin has many smooth plains called Caloris Planitia.
Canyon A large, long split in the ground.
Carbon Dioxide (CO2) A gas with no colour or smell that doesn't burn and is formed when we breath.
Cartwheel Galaxy A ring-shaped galaxy in the constellation of Sculptor. Cartwheel is an irregular galaxy that was formed when a large spiral galaxy was messed up by a dwarf galaxy passing through it.
Cassini-Huygens A joint NASA, ESA and Italian Space Agency mission to Saturn. It was launched in 1997 and the probe should reach Saturn in 2004. The Cassini orbiter will look at the atmosphere, rings and moons of Saturn, whilst the Huygens probe will be dropped into the atmosphere of the largest moon, Titan, to study the atmosphere and the surface.
CCD See Charge-Coupled Device
Cephied A type of yellow giant or supergiant, variable star used to measure distances, e.g. to far-off galaxies.
Cernan, Eugene A. The last man to walk on the moon on December 14, 1972. Gene Cernan was the commander of Apollo 17, a mission carrying geologists to study the lunar surface and bring back rock and soil samples.
Charge-Coupled Device (CCD) A type of detector used on telescopes. More
Charon The only moon of Pluto, Charon has a mass of one tenth that of Pluto, and so the two are often thought of as a double planet. Charon is probably made of rock and ice.
Classification Arranging into groups
Clementine A space probe built by the US Naval Research Laboratory to track missiles. It was launched in 1994 and used four different cameras to map the surface of the moon.
Cluster of Galaxies A group of galaxies. The Milky Way is in a small cluster called the Local Group.
Collins, Michael Collins piloted the command module of the Apollo 11 mission that landed the first men on the moon. Neil Armstrong and Buzz Aldrin landed at the Sea of Tranquility, in the Eagle Lander, on July 20 1969. Collins stayed in orbit.
Coma The gas and dust around the centre of a comet.
Comet A small (tens of km across) object made of ice and dust that goes around the Sun in a very oval orbit. A comet has a "nucleus" of ice, rock and gas that does nothing when it is a long way from the Sun. As it gets closer to the Sun, the gases heat up and burst out. More
Composition What an object is made of.
Constellation A collection of stars that make up a picture in the night sky.
Copernicus, Nicolaus (1473-1543) Polish astronomer and man of the church. Copernicus was the first person to suggest that the Earth goes around the Sun and is not at the centre of the universe. More
Core The central part of a star where all of the nuclear reactions take place. Also the part in the middle of a planet.
Corona Hot gas around the Sun and some other stars. It is so thin that it can only be seen during total solar eclipses.
Corrosive Causing constant damage.
Corvus A small southern constellation that looks like a crow
Cosmic Ray A very fast moving particle travelling through space.
Cosmology The study of the Universe.
Crater A bowl-shaped hole on the surface of a planet.
Crust The rocky surface layer of a planet. Back to Top


Dark Current The charge that builds up in a CCD detector even when no light is falling on it.More
Dark Frame A picture taken with the shutter of a CCD detector closed to get rid of dark current. More
Dark Matter “Missing” mass in the Universe that cannot be seen, but we know that it is there because of the way that stars and galaxies move. More
Dark Spot A large spot on Neptune caused by a huge storm in the atmosphere.
Data Reduction Making information from a telescope better so that it can be studied. More
Day The time the Earth takes to turn once.
Declination A coordinate used to find objects in space. Declination is measured in degrees, arc minutes and arc seconds. More
Deimos The farthest out, odd shaped moon of Mars. It may be an asteroid that has been caught.
Detector Something that collects light from a telescope over a long time to make a better picture. More
Develop To treat a photographic plate with special chemicals so that the picture on it can be seen.
Diameter The distance from one side of a circle or sphere to the point opposite, passing through the centre.
Dithering A way of getting better pictures from telescopes.More
Dobsonian Telescope A type of Newtonian telescope. More
Dog Star See Sirius
Doppler Shift A change in the colour of light from an object that is moving. If the object is coming towards us it will seem bluer, this is blueshift. If it is going away it will look more red, called redshift.
Dust Lane A long, wide band of dust across a galaxy. They are most often seen in spirals when they block out light from the stars behind them. Back to Top


Earth Our planet. Earth is the third planet from the Sun and is the only place in the Universe known to support life.
Eagle Lander The Lander, carried on NASA's Apollo 11 mission, that put astronauts Neil Armstrong and Buzz Aldrin on the surface of the moon for the first time, on July 20, 1969. The command module, which stayed in orbit, was flown by Col. Michael Collins
Eclipse See Solar Eclipse; Lunar Eclipse.
Efficiency How well something works.
Einstein, Albert (1879-1955) The German-born American physicist who came up with the special and general theories of relativity. These changed the way scientists thought about space and time.
Electron A particle with a negative charge and a tiny mass. Electrons go around the centre of atoms.
Element Something that is made up of atoms with the same number of protons in them. Elements are the simplest substances.
Ellipse A flattened circle, or oval.
Elliptical In the shape of an ellipse
Elliptical Galaxy A circle or oval shaped galaxy with no spiral arms and not muchgas and dust. More
Emulsion A coating on photographic plates that reacts to light.
Energy The ability to do work.
Equator The line that divides the northern and southern hemispheres.
Eros The first asteroid known to have an orbit that passes closer to the Sun than Mars. It was discovered in 1898 and is also called asteroid 433.
Erosion The wearing away of the surface of a planet. Usually caused by running water.
ESA European Space Agency. A group from many European countries that work together to study space. ESA was formed in 1975 and is based in Paris.
Europa Fourth largest moon of Jupiter. It has a cracked, icy surface with a few impact craters on it. Europa is thought to be one of the few places in the Solar System where life could exist.
Evolve To change gradually.
Extra-Solar Planet A planet going around a star other than the Sun. More
Eyepiece Lenses used to look at the image from a telescope. Back to Top


Faulkes Telescope what
Flat Field A picture taken of an even area to get rid of differences in an image from a CCD. More
Focus The point in a telescope where rays of light meet. Causing rays of light to meet at one point is called focussing.
Force A push or pull two or more objects. Back to Top


Galactic Bar The oblong or cigar shaped area of stars across some spiral galaxies.
Galactic Bulge The central area of spiral galaxies.
Galactic Disc The much thinner area, containing stars, dust and gas, that goes around the centre of a galaxy.
Galactic Halo Any of the material around a galaxy.
Galactic Nucleus The very centre of a galaxy, often containing a group of close together stars.
Galactic Ring A ring seen around some spiral galaxies.
Galaxy A group of stars, dust and gas held together by gravity. More
Galilean Moons The four largest moons of Jupiter, discovered by Galileo in 1610. They are Io, Europa, Ganymede and Callisto.
Galileo Galilei (1564-1642) An Italian astronomer, mathematician and physicist who made the first propermeasurements using telescopes. Galileo found mountains on the moon, Jupiter’s four largest moons and the many stars in the Milky Way. Galileo believed that the Sun was the centre of the Universe, like Copernicus.
Galileo Spacecraft A NASA probe, launched to Jupiter in 1989.
Gamma Ray Burst A strong burst of energy from somewhere in space. More
Ganymede The largest of Jupiter’s moons, larger than any other moon in the Solar System. Ganymede is also the brightest of the Galilean satellites, with an icy surface and dark and light patches.
Gas Giant A planet made mainly of gas, which is much larger than the Earth. In our Solar System the gas giants are Jupiter, Saturn, Uranus and Neptune.
Giotto The first ESA probe, launched to Halley’s comet in 1985.
Gravity A force that pulls two or more things together.
Great Red Spot A huge storm in the atmosphere of Jupiter. It is so large that over 4 Earths would fit inside it.
Greenhouse Effect An effect that causes the temperature of a planet to rise because of gases in the atmosphere. Back to Top


Hale-Bopp A long-period comet that was discovered on July 23rd 1995. It has a period of about 2400 years, and was last seen in April 1997.
Halley's Comet A short-period comet that comes back about once every 76 years. Last seen in 1986, it is due to return in 2061.
Helium A light gas.
Helium Flash An explosion when helium starts to burnin the middle of small stars.
Hellas Crater A huge impact crater on Mars. It is about 2500km across and 5km deep.
Hemisphere Half of a sphere. The Earth is usually seperated into the northern and southern hemispheres, divided by the equator.
Herschel, (Frederick) William (1738-1922) German-born English astronomer and musician and private astronomer to George III. He discovered Uranus in 1781 and some of the moons of Saturn and Uranus. He also made many measurements of the sky and found that the Sun is in a flat group of stars, the Milky Way.
Herschel, John Frederick William (1792-1871) English astronomer and scientist, he carried on with the measurements made by his father William and he published these in the "General Catalogue of Nebulae and Clusters".
Hertzsprung-Russell Diagram A diagram that shows the brightness of stars and their colour or type. This can be used to classify stars. More. It can also show the life of a star. More
Hipparchus of Nicaea (c.190-c.120 BC) A Greek astronomer, mathematician and geographer who came up with the first way to classify the brightness of stars. He carefully measured 850 stars and put them into six groups.
H-R Diagram See Hertzprung-Russell Diagram
HST See Hubble Space Telescope
Hubble Deep Field A picture taken by the Hubble Space Telescope, of hundreds of very faint, far-off galaxies in a small patch of sky.
Hubble, Edwin Powell (1889-1953) American astronomer who first found other galaxies outside the Milky Way. Hubble classified these in his tuning-fork diagram. He also found that these galaxies were moving away from us faster the further away they were.
Hubble Space Telescope (HST) A joint NASA and ESA, 2.4m reflecting telescope, launched in 1990. It orbits at a height of about 600km and has taken some of the best pictures of far-off objects.
Hubble Tuning Fork Diagram A diagram that shows how Hubble arranged galaxies into groups. More
Huygens An ESA probe that will be dropped into the atmosphere of Titan, Saturn’s largest moon, in 2004, to study its surface, clouds and atmosphere. Part of the Cassini-Huygens spacecraft.
Hydrogen The lightest a element in the Universe.It is usually a colourless gas.


Ice Cap See Polar Cap
Image Picture
Inflation A very short amount of time soon after the Big Bang when the Universe grew very quickly. More
Interstellar Between the stars
Io The third largest moon of Jupiter and the closest of the Galilean satellites to the planet. Io has a lot of volcanoes because of the gravity of Jupiter. It has many mountains and plains, but no impact craters.
Ion An atom that has lost one or more of its electrons and therefore has a positive charge.
Ion Tail One of the tails of a comet carried away by the solar wind. Also called gas tails, they are often blue or green in colour and can be over 100,000,000 km long.
Iron A metal that causes rust.
Iron Oxide Rust formed by the reaction of iron with oxygen, responsible for the red colour of Mars.
Irregular Galaxy A type of galaxy with little structure. Irregular galaxies are divided into type I and type II irregulars. More
Isotropic The same in all directions Back to Top


Japanese Space Agency See NASDA.
Jovian Jupiter-like or of Jupiter.
Jovian Planet See Gas Giant Back to Top


Kepler, Johannes (1571-1630) German astronomer and mathematician who established his three laws of planetary motion after calculations of the orbit of Mars. More
Kepler's Laws Three laws that govern the movement of the planets around the Sun. More
Kuiper Belt A belt of millions of small, icy objects in the outer Solar System, lying in almost the same plain as the planets, beyond the orbit of Neptune. It is thought, by some astronomers, that Pluto and its moon Charon may, in fact, be Kuiper Belt objects. Back to Top


Large Magellenic Cloud One of two irregular galaxies that neighbour the Milky Way. The LMC is seen as a long, hazy patch of light in the constellations of Mensa and Dorado.
Laser Light Amplification by Stimulated Emission of Radiation. A device which produces a parallel beam of strong radiation. Lasers are now used as artificial standards in adaptive optics telescopes.
Lava Molten rock that issues from volcanoes
Lens A see-through object, usually made of glass, that can bend light.
Lenticular Galaxy A galaxy with a lens-like appearance, classified as an S0, which has a disc and central bulge, but no spiral arms.
Lepton A type of elementary particle, such as an electron, muon or neutrino, which is not effected by the strong nuclear force.
Light Curve A plot showing the variations in the brightness of an object with time
Light Year The distance travelled by light, through space, in one year. Equal to 9.4607x1012 km, 63,240 AU or 0.3066 parsecs.
Local Group The cluster of galaxies of which the Milky Way is a member. Containing 31 known galaxies, in a diameter of 3 million light years, the group is thought to have a mass of 3 to 5 x 1012 times that of the Sun.
Logarithmic Increasing in powers of ten.
Long Period Comet A comet with a period of over 200 years.
Luminosity The amount of radiation given out by a star.
Lunar Of the Moon.
Lunar Prospector A NASA probe, launched in 1997, to look for frozen water at the poles of the moon and to its gravitational field and volcanic emissions. Back to Top


Magellanic Clouds The two Irregular galaxies that accompany the Milky Way.
Magma Molten rock within a planet or moon.
Magnetic Field An area around a magnet where a magnetic force is felt. This is what makes magnets pull towards each other or push each other apart when they get close together.
magnify To increase, make bigger
Magnitude A measure of a star’s brightness. More
Main Sequence A strip from top left to bottom right on a Hertzprung-Russell diagram. Stars are on the Main Sequence if they are in their hydrogen-burning phase, with their position depending on their mass.
Major Planets Our Solar System has nine major planets. These are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.
Mantle The layer between the core and the crust of a planetary body.
MAP Microwave Anisotropy Probe. A NASA mission to measure fluctuations in the Cosmic Microwave Background much more accurately than ever before, to give a better picture of the early Universe.
Maria Dark, young, lowland plains on the moon, consisting of basaltic lava with few large craters.
Mariner 10 A NASA space probe, launched in November 1973, which was the first to visit two planets, Venus and Mercury.
Mars Express A planned ESA mission to Mars that will carry the Beagle 2 lander to search for sub-surface water and signs of life on the planet.
Mass The amount of matter in an object.
Matter Anything that has mass
Matter-Dominated Era The era in which the expansion of the Universe is dominated by the gravitational effect of matter over the effect of radiation pressure.
Maunder Minimum A period of low solar activity, from 1645 to 1715, in which hardly any sunspots were seen. There was also a mini ice age on Earth at around this time.
MAXIMA A balloon mission to study the Cosmic Microwave Background.
Messier Catalogue A list of astronomical objects, including nebulae, star clusters and galaxies, first put together by Charles Messier in 1771. The catalogue has now been extended beyond Messier’s 103 objects, which are still referred to by their Messier number.
Meteor A short streak of light caused by a meteoroid entering the Earth’s upper atmosphere. More
Meteor Crater A large, well-preserved impact crater in Arizona. The crater has a diameter of 1.2km and a depth of 175m. It was formed by a nickel-iron meteorite, about 50m across, which struck the ground about 50,000 years ago.
Meteorite A natural object from space that hits the surface of a planetary body. More
Meteoroid A small part from an asteroid or comet that orbits the Sun. It becomes a meteor if it enters the Earth’s atmosphere. More
Methane CH4. A colourless and odourless flammable gas.
Milky Way The spiral galaxy we live in. More
Miranda The fifth-largest moon of Uranus. It is thought to have been broken up by a huge impact, and then reformed again from the debris.
Moon The only natural satellite of Earth. More
Mount Wilson Observatory An observatory at an altitude of 1740m, in the San Gabriel Mountains near Los Angeles. It houses the 2.5m Hooker Telescope, as well as a 1.5m reflector and two tower telescopes for observing the Sun. Back to Top


NASA National Aeronautics and Space Administration, the US space agency.
NASDA National Space Development Agency. The Japanese space agency established in October 1969 to develop and promote the peaceful use of space.
NEAR Near-Earth Asteroid Rendezvous, a NASA space probe launched in 1996 to study the asteroid Eros.
Nebula A large cloud of gas and dust in space. There are three main types, these are emission nebulae, which give out light, reflection nebulae, seen by reflected light, and dark nebulae, which show up against a brighter background.
Neutral Atom An atom with an equal number of electrons to protons so that it has no net charge, i.e. it is not ionised.
Neutrino An elementary particle with no charge and only a tiny mass, but a very high velocity, close to the speed of light. There are three types of neutrinos known, these are the electron neutrino, the muon neutrino and the tau neutrino. Neutrinos are leptons and are the most likely candidate for hot dark matter, making up some of the missing mass in the Universe.
Neutron An elementary particle found in atomic nuclei. Neutrons have no charge and a mass slightly higher than that of a proton.
Neutron Star A very small, dense star that is thought to be formed during a type II supernova explosion.
Newton, Isaac (1642-1727) English mathematician and physicist who made the world's first working reflecting telescope. Newton also wrote the “Principia” dealing with the celestial mechanics of the Solar System, and one of the most important scientific books ever written.
Newton’s Law of Gravitation Two bodies attract each other with a force that depends on the product of their masses divided by the square of the distance between them. More
Newtonian Telescope A reflecting telescope that uses mirrors to collect and focus light. More
NGC New General Catalogue. A catalogue of 7840 galaxies, nebulae and star clusters, published by J. L. E. Dreyer in 1888, an extension of Herschel’s General Catalogue of Nebulae and Clusters of Stars.
Nitrogen (N) A non-metallic element, usually found in the form of an odourless, colourless and unreactive gas, N2, which makes up about 78% of the Earth's atmosphere.
Northern Hemisphere The upper half of our planet where the UK and Hawaii can be found.
Nuclear Burning Nuclear reactions that create the energy given out by stars. More
Nuclear Fusion The fusing together of two low mass atomic nuclei to form one of higher mass with the release of energy. This is the process that provides the energy to make stars shine. It does, however, need a temperature of about 108 K to overcome the initial repulsion between the positively charged nuclei.
Nuclear Reaction A reaction that involves a change to one or more nuclei, such as nuclear fusion or radioactive decay.
Nucleosynthesis The creation of elements by nuclear reactions.
Nucleus, Atomic The central part of an atom that contains positively charged protons and uncharged neutrons and makes up the majority of an atoms mass. The number of protons in the nucleus defines the atomic number and therefore the type of element.
Nucleus, Cometary The small, central, solid body, made of ice, gas and dust, in the head of a comet. This contains virtually all the mass of the comet and is also the source of any activity. Back to Top


Observer A person who looks at an object.
Offset Not in quite the same place.
Olbers, Heinrich Wilhelm Matthäus (1758-1840) German physician and amateur astronomer who pointed out what has come to be known as Olbers’ paradox. Olbers developed a method to calculate the orbit of a comet and discovered Pallas and Vesta. He also suggested that the tails of comets are expelled from their heads by the Sun.
Olbers' Paradox A paradox that states that if the Universe was infinite and with an infinite age then why is the sky dark at night? This can be resolved when the finite age of the Universe is taken into account. More
Olympus Mons A volcano on Mars, the largest in the Solar System, thought to be active within the last billion years. Olympus Mons is 27km high, with a caldera 80km across. If it were placed on Earth, Olympus Mons would collapse under its own weight due to the higher gravity.
Oort Cloud A roughly spherical cloud of comet nuclei that surrounds the Sun, some of which are occasionally affected by passing stars and sent into orbits which pass through the inner Solar System. The cloud is thought to contain around 1012 objects.
Open Universe A universe that has an infinite lifetime and will keep on expanding forever.
Orbit The path of one body around another. The planets, for example, move in elliptical orbits around the Sun.
Orbital Period The time taken to go around one orbit.
Orion A constellation, on the celestial equator, which contains the bright stars Betelgeuse and Rigel, as well as the Orion Nebula, M42.
Overdensity An area in the Universe with a slightly higher density than average, which may condense to form structure.
Oxygen (O) A gas which makes up 21% of Earth's atmosphere, usually found in the form of O2. Oxygen is needed for animal and plant life.
Ozone Layer A layer of ozone (O3) in the stratosphere of Earth’s atmosphere that absorbs dangerous ultraviolet radiation from the Sun. Back to Top


Parabola A curve that extends to infinity as its arms become parallel. It can be thought of as an ellipse with an infinite distance between the two foci.
Parallax A system of measuring distances to stars. More
Parallel Two lines are parallel if they remain at a constant distance away from each other along their lengths.
Parsec Parallax-second. A unit of distance defined as the distance at which one A.U. makes an angle of one arc second.
Pathfinder A NASA space probe launched to Mars in 1996. It carried the Sojourner rover that roamed the surface of Mars and analysed its composition.
Perihelion The closest point in an orbit around the Sun.
Period The time taken for one orbit, or the time between repeated events.
Permafrost Permanently frozen ground.
Phobos The irregularly shaped inner moon of Mars. It is covered in impact craters, the largest of these is Stickney crater with a diameter of 11.5 km. Phobos is thought to be a captured asteroid.
Photino A theoretical fundamental particle that may help explain some of the missing mass in the Universe. Photinos are fermions.
Photographic Plate A type of detector used in telescopes to collect light from distant objects. More
Photometry The measurement of light.
Photon An electromagnetic radiation particle, with zero mass and charge, which travels at the speed of light. The energy of a photon is proportional to its frequency.
Photosphere The layer of a star from which most of its energy is released as visible or infrared radiation, this is the layer we see.
Pixel One of the elements that makes up the array in a CCD camera.
Planck An ESA mission to study the Cosmic Microwave Background to help answer questions about the early Universe. Planck is due for launch in 2007.
Planet A roughly spherical body, seen by reflected light, orbiting the Sun or another star. A body with a mass greater than ten Jupiter masses becomes a brown dwarf. More
Planetary Nebula A bright cloud of dust and gas around an evolved star.
Pleiades An open cluster (M45) in Taurus known as the Seven Sisters.
Polar Cap A bright layer of ice around the poles of a planetary body, which varies in size during the year.
Pope, Alexander (1688-1744) English writer and poet.
Population I Stars Stars that lie in the disc of the galaxy, like the Sun, and have a relatively high content of heavy elements. They are thought to have formed continuously through the lifetime of the disc, containing the debris from population II supernovae.
Population II Stars Stars found in the halo and nuclear bulge of the galaxy. They have much lower concentrations of heavy elements and were probably formed early in the life of the galaxy.
P-P Chain See Proton-Proton Chain
Principia The common name for Newton’s ‘Philosophae naturalis principia mathematica’, which deals with the mechanics of the Solar System, including his law of gravitation, laws of motion and derivations of Kepler’s laws.
Prograde The movement of a body in an anticlockwise direction around the Sun or anticlockwise rotation on its axis as seen from above the Suns north pole. This is the usual direction of rotation or orbit for bodies in the Solar system and can also be called direct motion.
Proton An elementary particle found in all atomic nuclei. It has a positive charge equal in magnitude to that of an electron, and a rest mass of 1.673x10-27 kg.
Proton-Proton Chain The set of reactions that converts hydrogen into helium in stars of less than two solar masses. More
Protostar A star in its early life, before nuclear burning has begun, as it condenses from a cloud of dust and gas.
Pulsar A rapidly spinning neutron star that gives out regular radio pulses. Most pulsars are thought to have been formed in supernova explosions, however, some may come from white dwarfs which have collapsed to neutron stars.
Pulsating Star See Variable Star Back to Top


Quantum Gravity A theory that gravitational interaction is by the exchange of small packets of gravity called gravitons, similar to photons of light. These have not yet been observed.
Quark A fundamental particle that makes up hadrons, and has a charge of either 2/3 or -1/3. Baryons, such as protons and neutrons, are made up of three quarks. Back to Top


Radar Astronomy A method that uses reflected radio pulses to study bodies in the Solar System.
Radiation-Dominated Era The time up to 30,000 years after the big bang when the expansion of the Universe was dominated by radiation and high-speed particles.
Radiative Zone The area in the core of low-mass stars or the envelope of high-mass stars, where the energy is transferred mainly by radiation, rather than convection.
Radius The straight line from the centre of a sphere or circle to any point on its surface.
Red Giant A large, cool, bright star that has left the main sequence after burning all the hydrogen in its core. Red giants have diameters of over 25 times that of the Sun.
Red Giant Branch A branch on a Hertzprung –Russell diagram where red giants are located.
Redshift The amount of shift in wavelength of light to the red end of the spectrum due to Doppler shift or the expansion of the Universe. More
Reflect If light is reflected by an object it bounces back from the object instead of going through it or being absorbed. Planets are seen by the light they reflect.
Reflecting Telescope A type of telescope that uses mirrors to collect and focus light from astronomical objects. More
Refracting Telescope A type of telescope that uses lenses to collect and focus light from astronomical objects. More
Refractive Index A measure of how much that light is bent by a material.
Regolith A layer of dust and broken rock on the surface of a planetary body, similar to soil but with no organic material.
Resolution The smallest separation between two objects by which a telescope can distinguish them apart.
Retrograde The movement of a body in a clockwise direction around the Sun or clockwise rotation on its axis as seen from above the Suns north pole. Also, motion that is backwards compared to the observers own motion.
Right Ascension (RA) A coordinate used to position astronomical objects. Right ascension (symbol α) is measured clockwise around the celestial equator in hours, minutes and seconds.
Rotational Axis An imaginary line, through the centre of a body, about which it rotates.
Rotational Period The time taken for a body to rotate once about it's axis Back to Top


Satellite Any smaller object that orbits a larger one.
Schmidt-Cassegrain Telescope A compact telescope that uses two curved mirrors to collect and focus light from astronomical objects. More
Season A particular time of year which has a certain type of weather. The four seasons are spring, summer, autumn and winter.
SEDS Students for the Exploration and Development of Space. A worldwide student organisation promoting the peaceful exploration of space.
Semi-Major Axis Half of the longest diameter of an ellipse.
Semi-Minor Axis Half of the shortest diameter of an ellipse.
Seyfert Galaxy A type of galaxy, usually a spiral or barred spiral, which has a small, bright nucleus with strong, broad emission lines in its spectrum.
Shakespeare, William (1564-1616) English playright and poet whose works are considered the greatest in English Literature. The moons of Uranus are named after characters of Shakespeare and Alexander Pope.
Shell Burning The nuclear burning that continues in a shell around the core of a star when the fuel within that core has been depleted. The burning shell moves outwards as more and more fuel is used.
Shoemaker-Levy 9 A comet discovered in 1993, in a two year orbit around Jupiter. The nucleus had been disrupted by a close approach to the planet in 1992, and at least 21 fragments hit Jupiter, in July 1994, producing dark spots and then a belt, which could be seen for 18 months.
Short-Period Comet A comet with an orbital period of less than 200 years. The most famous of these is Halley’s comet, which returns once every 76 years.
Shutter The cover on a camera or telescope that opens to let an image be taken.
Silicon (Si) A non-metallic element often found in the Earth's crust.
Silicon Chip A small crystal of silicon semiconducter used in electronics.
Simulation A computer program that works out what something should do or look like without actually having to see it.
Singularity A single point of infinite temperature and pressure. The Universe is thought to have expanded from a singularity in the Big Bang.
Sirius Also know as the Dog Star, the brightest star in the night sky. Lies in the constellation of Canis Major.
Sloan Digital Sky Survey A project to study the 3-D, large scale structure of the Universe by mapping the positions of galaxies and quasars around the north pole of the galaxy.
Small Magellenic Cloud The smaller of two irregular galaxies that neighbour the Milky Way. The SMC can be seen as a hazy patch in the constellation of Tucana.
Solar Sun-like or of the Sun.
Solar Eclipse The movement of the Moon between the Sun and an observer on Earth.
Solar Mass A unit of mass equal to that of the Sun, 1.989 x 1030 kg.
Solar Maximum The period when the activity of the Sun and the number of sunspots are highest.
Solar Minimum The period when the activity of the Sun and the number of sunspots are lowest.
Solar Prominence A loop of plasma extending from the Sun, following a magnetic field line.
Solar System Our Sun and everything that orbits around it.
Solar Wind The outflow of particles, such as protons, electrons and some light nuclei, from the corona of the Sun.
Southern Hemisphere The lower half of our planet where Australia can be found.
Soviet From the former Soviet Union, now Russia.
Spectral Type The classification of a star due to its spectrum. This depends on the surface temperature and therefore the colour of the star. The order of classification is O – B – A – F – G – K – M. More
Spectroscopy The use of spectra to find information about astronomical objects.
Spectrum The full range of electromagnetic radiation, from long wavelength radio waves (105 – 10-3m) all the way to gamma rays (10-11 – 10-14m).
Sphere Anything in the shape of a ball
Spherical Aberration An effect caused by curved mirrors and lenses, in which light hitting the edge of the mirror is focussed at a slightly different point to that hitting the centre.
Spiral Arms The curved areas, extending through the discs of spiral galaxies where young stars, clusters, dust and nebulae are concentrated.
Spiral Galaxy A type of galaxy with spiral arms extending from a central bulge. More
Standard Candle A star with a known luminosity dependence that can be used to calculate cosmological distances.
Standard Star A star with accurately known magnitude and colour that can be used to calibrate images of new objects.
Star A shining ball of gas that produces energy through nuclear fusion. More
Steady State Theory A model of an expanding universe in which matter is constantly created so that the density remains constant. More
Stellar Nursery A large cloud of dust and gas where new stars are being formed.
Stellar Wind See Solar Wind
Stickney Crater A large impact crater on the surface of the Martian moon Phobos. The crater measures 11.5 km in diameter.
Strong Nuclear Force A fundamental force that binds protons and neutrons together in atomic nuclei
Sulphur Dioxide (SO2) A colourless gas, with a strong odour, produced by burning sulphur. Used to make sulphuric acid.
Sun The average yellow star about which the Earth and other planets orbit. More
Sunspot A dark spot on the Sun’s photosphere. Sunspots usually occur in pairs that are connected by strong magnetic field lines. They are more common when the Sun is at its most active, i.e. at solar maximum.
Supercluster A group of clusters of galaxies.
Supergiant A huge star at least 10,000 times brighter than the Sun and a diameter between 20 and several hundred times greater. Supergiants are stars of at least 10 solar masses that have swollen up and left the main sequence. They are the largest and brightest type of star.
Supernova The explosion of a massive star at the end of its life that can outshine the galaxy in which it lies. Supernovae are classified as either type I or type II, where type II supernovae show hydrogen lines in their spectra. See Type I Supernova and Type II Supernova.
Synchronous Lock The rotation of a satellite so that it always keeps the same face towards the body it is orbiting, or rotates whole number of times in a whole number of orbits. Back to Top


T Tauri Star A very young star, of equal or lower mass than the Sun, which is still contracting. T Tauri stars are less than about 10 million years old and lie on the Hayashi track on a Hertzprung–Russell diagram. The names comes from the first of this type of star to be identified.
Tectonics The movement of plates of planetary crust that causes continental drift on Earth, as well as volcanoes and earthquakes.
Telescope A device that uses lenses or mirrors to collect, focus and magnify light from distant objects. More
Terrestrial Of Earth, or Earth-like.
Tesla The unit of magnetic flux density i.e. the strength of magnetic field passing through unit area in unit time.
Tharsis Mons A huge upland region on Mars, including three large volcanoes, each reaching a height of 27 km.
Tidal Force The gravitational force, caused by a massive body, which creates tides on another body.
Time The property that stops all events happening at the same point from coinciding.
Titan The largest moon of Saturn and the second largest in the solar system. Titan is made of rock and ice and has a substantial atmosphere of mainly nitrogen, with some methane and hydrogen and traces of other elements.
Transit The movement of one object across another larger object, such as an extrasolar planet across the star it orbits.
Tuning Fork Diagram A diagram, in the shape of a tuning fork, which shows Hubble’s classification of galaxies. More
Type I Supernova A supernova explosion showing no sign of hydrogen in its spectrum. SNe I are divided into three classes: Ia, Ib and Ic. Type Ia are thought to be due to the explosion of a neutron star that has gained mass from another star and are usually seen among old stars. Types Ib and Ic probably come from the explosion of massive stars that have already lost their hydrogen envelope and are seen among new star populations.
Type II Supernova A violent explosion of a star greater than 8 solar masses leaving behind a neutron star, or possibly a black hole. SNe II can be distinguished from SNe I due to the presence of hydrogen in their spectra. They usually occur in young star populations. Back to Top


Ultraviolet Radiation Radiation at the far blue end of the spectrum with wavelengths between about 90 and 350 nm.
Universe Everything that exists including matter, space and time. More
USSR The former Soviet Union. Back to Top


Vacuum Empty space where no matter is present.
Valles Marineris A large system of canyons on Mars, the largest in the solar system. It stretches for over 4000 km and can be up to 500 km wide and 4 km deep. Valles Marineris can be seen from Earth as the dark streak across the red planet. It was named after the Mariner 9 spacecraft that discovered it in 1971.
Variable Star A star that varies in brightness. More
Vela Satellites A series of US military satellites designed to monitor Soviet nuclear testing by looking for gamma rays. They actually found gamma ray bursts coming from beyond the Earth.
Venera A series of Soviet space probes to Venus, which took the first pictures of the planets surface and the first analysis of the atmosphere and soil.
Virgo Cluster A large, irregular cluster of galaxies, the closest to our own. The Virgo Cluster contains about 2000 galaxies and is roughly elliptical in shape. It lies at a distance of about 50 million light years away.
Visible Band The part of the electromagnetic spectrum that can be seen by the human eye.
Visible Universe The extent of the Universe that can be seen because it is close enough that its light has had time to reach us in the age of the Universe. More
Volume The amount of space that an object takes up. Back to Top


Wavelength The distance between peaks of a wave.
Weak Nuclear Force A fundamental force between elementary particles that can be responsible for radioactive decay.
Whirlpool Galaxy An Sc-type spiral galaxy, about 25 million light years away in Canes Venatici, which is interacting with a small irregular galaxy. The Whirlpool Galaxy, also known as M51, is seen face on, and was the first galaxy in which the spiral structure could be clearly seen.
White Dwarf A small dense star left over from the collapse of the cores of less massive stars. Back to Top


X-Rays Electromagnetic radiation to the far blue end of the spectrum, with wavelengths of about 0.01-10nm, and energies of 0.1-100 keV. Back to Top


Year The time the Earth takes to orbit the Sun, approximately 365.25 days. Back to Top


Zenith The point directly above an observer on the celestial sphere, so that a line to it is at right angles to the plane of the horizon.
Zones Bright areas seen on Jupiter, where the atmospheric gas is rising. Back to Top

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Return to intermediate index Beginners Intermediate Advanced

Authors: Carolyn Brinkworth and Claire Thomas

Last updated: July 2001