Abbreviations
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ALMA |
Atacama Large Millimeter/submillimeter Array |
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Caltech |
California Institute of Technology |
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CCD |
charge-coupled device |
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CMB |
Cosmic Microwave Background radiation |
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ESA |
European Space Agency |
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LIGO |
Laser Interferometer Gravitational-Wave Observatory |
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eLISA |
Laser Interferometer Space Antenna |
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M |
Messier object – for example, M 31 (the Andromeda Galaxy) |
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NGC |
New General Catalogue object – for example, NGC 1555 (Hind’s Variable Nebula). |
Units of measurement
Astronomical unit (AU) Distance from Earth to the Sun: 150 million kilometres (93 million miles).
Gauss A unit of magnetic field.
Joules A unit of energy.
Kelvin A unit of temperature, in size the same as a degree Celsius, but with the start of the scale at a temperature of absolute zero, about -273 degrees Celsius (-460 degrees Fahrenheit).
Light second The distance that light travels in 1 second: 300,000 kilometres (186,000 miles).
Light year The distance that light travels in 1 year: 9.5 million million kilometres (5.9 million million miles).
Mega-Hertz (MHz) A unit of the frequency of radio waves, equal to 1 million oscillations per second.
Micron One millionth of a metre, equivalent to one thousandth of a millimetre.
Solar mass The mass of the Sun: 2.0 million million million million million million kilograms (2.2 thousand million million million million short tons).
Terms
Accretion/accretion disc The process by which material falls onto a celestial body, pulled in by its force of gravity. Any swirling motion of the infalling material causes the material to orbit the celestial body in a flat plane called an accretion disc, prior to completing the infall.
Acoustic oscillations Oscillations of matter in periodic waves, like sound; in astronomy, such oscillations influenced the position of material flowing in the expansion of the Universe before the formation of galaxies and caused vestigial regularities in their present positions relative to one another.
Antiparticle Every type of fundamental particle is associated with an antiparticle having the same mass and opposite electric charge. Collections of antiparticles are called antimatter.
Asteroid A small, dark body or minor planet orbiting in a planetary system (in the solar system, usually orbiting between Mars and Jupiter). In origin, an asteroid may be a planet whose development has been arrested, or a fragment of a planet or another asteroid broken off by a collision. Small asteroids are termed meteoroids.
Atom The smallest unit of ordinary matter, the basic unit of the chemical elements; a nucleus orbited by electrons.
Aurora A luminous glow in the sky, usually over the polar regions, generated by the collision of solar cosmic rays with air.
Axion A conjectural elementary particle, possibly what dark matter is made of.
Big Bang The explosive event at the start of the Universe.
Binary star system Two stars in orbit around one another. Triple and quadruple star systems have three and four members, and so on.
Black hole A body of such high mass and small size that the force of gravity at its surface stops anything, even light, from leaving. Supermassive black holes are found in the centres of galaxies; stellar black holes are formed by supernova explosions; small black holes may have been formed in the Big Bang.
Comet A small body in the solar system like an asteroid but made principally of ice, which, warmed by the Sun, releases material in a tail.
Continent A large land mass, one of seven identified on present-day Earth.
Core The central, denser volume of material within a planet, star or galaxy.
Corona An outer halo of material that surrounds a star like the Sun; the Sun’s atmosphere.
Coronal mass ejection A cloud of plasma thrown out of the solar corona into the solar system.
Cosmic Microwave Background Microwave (and infrared) radiation created in the Big Bang, pervading the whole of space as a uniform background seen in every direction.
Cosmic rays High-energy ions, originating in stars and elsewhere, and travelling at high speeds.
Crust The outer, solid, rocky layer of a terrestrial planet.
Dark energy A conjectured form of energy released from space that causes acceleration of the expansion of the Universe.
Dark matter A conjectured form of matter that emits no light, or any other radiation, but which exerts a gravitational force like the familiar, ordinary matter.
Dwarf planet A small planet. In the solar system, a planet sufficiently massive that it has taken up a spherical form due to the force of its own gravity, but not massive enough that it has attracted other material and cleared its orbital zone entirely.
Earth The third, rocky planet from the Sun.
Eclipse The occultation of a star (such as the Sun) by a planet or moon, or the shadowing of one planet or moon by another from the star that is the source of its illumination.
Electron An elementary particle, a constituent of atoms, negatively charged.
Element A pure chemical substance consisting only of atoms of the same type, all having the same number of protons in their nuclei. The larger the number of protons and neutrons together, the heavier the element.
Elliptical galaxy A galaxy with an overall elliptical shape, comprised principally of older, red stars and little or no gas.
Exoplanet A planet that lies outside the solar system, most typically in an exoplanetary system.
Exoplanetary system A group of planets in orbit around a star other than the Sun, together with associated smaller bodies like asteroids.
Fundamental particle A subatomic particle.
Galaxy A large collection of stars, interstellar gas, dust and dark matter, gravitationally bound together and isolated from other similar collections.
Gas giant planet A planet like Jupiter that is made, primarily, of gas; also known as a jupiter. Contrast with terrestrial planet.
Globular cluster A cluster of many stars having overall a spherical shape.
Gravitational wave A disturbance in spacetime caused by changes in motion of a celestial body that propagates with the same speed as light; a change of gravity in space that causes bodies to shift in position.
Half-life A measure of the time that it takes for a radioactive element to decay – specifically, the time that it takes for half of it to change to something else.
Hypernova A very energetic supernova in which the core of a massive star collapses to form a black hole with the ejection of the envelope of the star at high speed.
Inflation The conjectural period at the very beginning of the Universe during which space expands very rapidly by a very large factor.
Interferometer/interferometry An instrument with multiple detectors that respond to a passing optical, radio or gravitational wave etc., with the output of all the detectors brought together and combined into a single response. Interferometry is the technique of using an interferometer.
Intergalactic medium The material that is distributed in intergalactic space (the space between galaxies).
Interstellar medium The material that is distributed in interstellar space.
Ion An atom with one or more electrons missing, or added, so that it is positively or negatively electrically charged.
Jupiter The largest planet in the solar system, the fifth planet outwards from the Sun and a gas giant. When spelt with lower case J, ‘jupiter’ means a planet (most often an exoplanet) that is massive like Jupiter.
Kilonova An explosion during which two neutron stars, or a neutron star and a black hole orbiting each other in a binary star system, merge into each other, producing gravitational waves.
Kuiper Belt The zone beyond Neptune where Trans-Neptunian Planets orbit, including Pluto.
Lithophile The chemical elements that bond with chemical elements in rocky material in a terrestrial planet’s mantle. Contrast with siderophile.
Magellanic Cloud One of two galaxies in orbit near our own Galaxy.
Magma Molten rock.
Magnetosphere The volume of space around a magnetic planet or star, permeated by a magnetic field.
Mantle The rocky zone lying between the core of a terrestrial planet and its crust.
Messier/NGC objects Catalogued objects in space.
Meteor A meteoroid that is falling onto a planet – for example, through Earth’s atmosphere, where it heats up and is visible as a luminous streak in the sky.
Meteorite A meteoroid that has fallen as a rock onto Earth.
Meteoroid A small asteroid, a rock or piece of dust orbiting in the solar system prior to falling onto Earth.
Milky Way Massed stars that create the impression of a band of light around the celestial sphere; our Galaxy, the shape of which causes this effect.
Molecule A combination of atoms that have bonded into a single chemical structure; the basic unit of a chemical compound.
Moon A satellite of a planet; the Moon is the satellite of our Earth.
Neutrino An elementary particle, electrically neutral with a very small, almost zero mass, and having a weak interaction with other particles. There are three flavours of neutrino, each associated with one of the following particles: electrons, muons and tau particles.
Neutrino oscillation The transition in flight of one flavour of neutrino to another.
Neutron An elementary particle found with protons in the nucleus of an atom, having almost the same mass as a proton but electrically neutral.
Neutron star A small, dense star that is made of neutrons.
Nova The explosion of a star so that a bright and apparently new star appears where no star was noticed before. See supernova, kilonova, hypernova for varieties.
Nuclear fusion The combination and amalgamation of atomic nuclei such as occurs in hot, dense matter in stars.
Nuclear reaction An interaction between nuclei that results in changes in the nuclei, releasing (or absorbing) energy.
Nucleus The central, heavy part of an atom that is usually orbited by the atom’s quota of electrons. A nucleus is made up of approximately equal numbers of protons and neutrons.
Planet A body that orbits a star (or occasionally, is freely floating in space) and is too small to sustain nuclear reactions. See terrestrial planet, gas giant planet, asteroid.
Planetary nebula A nebula formed as a red giant star makes the transition to become a white dwarf. These nebulae do not have anything to do with planets, but look like planets when viewed through a small telescope, as was the case when they were first discovered and named.
Planetesimal A small, primitive planet; the stage in the growth of a planet when it lies somewhere in size between a rock and an asteroid. A comet.
Plasma The fourth state of matter (additional to solids, liquids and gas) and consisting of ions and electrons. The state of matter in hot bodies such as stars.
Proton A fundamental particle, positively charged with one unit of electricity and heavy, found in atomic nuclei. The nucleus of an ordinary hydrogen atom.
Protoplanet The last stage in the formation of a planet.
Protostar The last stage in the formation of a star.
Pulsar A rotating, magnetized neutron star, and a source of repetitive radio pulses.
Quark One of the six types of the most fundamental particles currently known, carrying an electric charge equal to either one-third or two-thirds of a unit.
Quasar An energetic supermassive black hole at the centre of a galaxy.
Quasi-Stellar Object See quasar.
Radiation Energy propagating through space.
Radioactivity The decay of the nucleus of an atom by the emission of radiation, changing the nucleus into another type.
Red giant/red supergiant A large/very large low-temperature star; a star at an advanced stage of development.
Redshift A shift in the colour of a star or galaxy caused by its motion as it recedes. Typically used of a galaxy as it partakes in the expansion of the Universe. The shift is the amount by which the wavelength of a spectral line of a particular colour is altered. If the line originates as a colour in the middle of a rainbow-like spectrum, the recessional speed of a galaxy shifts the colour away from blue and towards red, hence the origin of the term, which is used to describe an expansion speed, even for spectral lines that are not actually colours, but that are, for example, radio waves.
Resonance A condition of two orbiting celestial bodies – for example, planets whose orbital periods are in a whole number ratio, so that their configuration accurately repeats.
Siderophile An element that amalgamates with iron and sinks into the core of a terrestrial planet.
Solar system The Sun’s planetary system.
Spectrograph/spectroscopy A device that splits radiation into a spectrum of radiation of a progression of energy and records it in some way. Spectroscopy is the technique of using a spectrograph.
Spectrum Radiation arranged progressively in energy – for example, the rainbow of light that progresses from red, through orange, yellow and green, to blue.
Spiral galaxy A galaxy of stars and gas, with the bright stars and the gas laid out in a spiral pattern.
Star A large celestial body that generates and radiates energy (by nuclear reactions), which is held together by its own gravity and supported by an internal pressure.
Starburst A sudden surge of star formation in a galaxy.
Stellar black hole A black hole the mass of a star, produced by a supernova explosion.
String The underlying constituent of a fundamental particle that vibrates and interacts with other strings so as to make up the properties of the particle.
Sun The star at the centre of our solar system.
Supermassive black hole A black hole of mass in excess of, say, a million times the mass of our Sun.
Supernova An explosion of a star in which the outer layers of the star re-ejected and the core collapses to a neutron star or black hole, or entirely disintegrates.
Tectonics Geological processes in the Earth’s crust, particularly pertaining to the motion of continents.
Terrestrial planet A rocky planet like the Earth.
Universe The collection of everything.
Variable star (e.g., T Tauri, Cepheid) A star that varies in brightness, either because it actually does or because something passes in front of it. The various sorts of variable star are named after a typical member.
White dwarf A compact star produced as the exposed core of a relatively low-mass star.
WIMP A conjectural ‘weakly interacting massive particle’, possibly the particle that dark matter is made of.