Secrets of the Universe

by Paul Murdin

  degenerate matter Matter made of leptons (usually electrons) so dense that it shows quantum-mechanical effects of pressure and temperature, as found in a white dwarf.

  double star A body made up of two stars in orbit around each other; a binary star is formally synonymous but in usage refers to a closer pair.

  dwarf planet (1) A planet in the Solar System – not one of the eight major planets, and not a satellite – large enough to have become spherical under its own gravity, like Vesta or Ceres, but not so large that it has cleared its neighbourhood of (other) minor planets (see asteroid) and planetesimals. (II) An exoplanet of the same type.

  eccentric orbit An orbit that is appreciably non-circular; hence ‘eccentricity’, the amount by which the orbit of a body around its host is non-circular.

  eclipse A celestial event that occurs, either when a body obscures another behind (as in a solar eclipse, or an eclipsing variable star), or when a body obscures the source of illumination of another (as in a lunar eclipse).

  ecliptic The plane of the orbit of the Earth, and by extension the line that this plane makes when it intersects the celestial sphere.

  Edgeworth–Kuiper belt See Kuiper belt.

  ellipse A geometrical figure having the shape of an elongated circle; the closed figure described by the orbit of a lone planet round a star.

  elliptical galaxy (1) A galaxy of stars that has an elliptical shape on the sky. (II) A galaxy that has the form of a three-dimensional triaxial ellipsoid, which looks elliptical when projected onto the plane of the celestial sphere.

  epicycle In orbital theory, an imaginary circular orbit, which is itself performing a circular orbit.

  escape velocity The speed required for an object to escape from the gravitational pull of a planet, star or other body if it is projected straight upwards.

  esker A geological feature consisting of a winding ridge of sand and gravel lying on the surface of a plain; it is caused by the deposition of sediment in the bed of a river that flows along a canal cut into a glacier, the sediment being left behind when the glacier retreats.

  exoplanet A planet in a planetary system outside the Solar System.

  extra-galactic nebula A term, no longer in current use, for a galaxy outside our own Galaxy.

  Fermi Paradox The contradiction, formulated by Enrico Fermi, that intelligent life may be ubiquitous in our Galaxy, but has not yet been identified by the inhabitants of Earth.

  fireball A meteor that shows brightly in the sky.

  Fraunhofer lines Spectroscopic indications at various wavelengths in the spectrum of a star (especially the Sun) of the presence of various elements in the star’s atmosphere.

  galaxy A system of stars like our own Galaxy.

  Galaxy The system of stars to which the Sun belongs.

  gamma rays The most energetic form of radiation.

  gamma-ray burst A burst of celestial gamma rays from an astronomical object (called a ‘gamma-ray burster’).

  gas-giant planet A planet made mainly of gas, such as Jupiter or Saturn; formed and (in the Solar System, but not in many of the known exoplanetary systems) still orbiting at a great distance from the Sun, they have no solid surface. Compare terrestrial planet.

  General Relativity Einstein’s theory of gravitation.

  geocentric theory The theory that the Earth is at the centre of the Solar System of planets; see also Ptolemaic theory.

  geodesic The natural path taken by a light ray as it traverses a gravitational field.

  geothermal spring A source of water heated by energy within the Earth.

  giant molecular cloud A large cloud of gas and dust, so dense that molecules survive within it; from these molecules, stars and planetary systems can form.

  glacial moraine Earth and rocks carried by a glacier and deposited off its end; the material accumulated by this method that is left behind if the glacier retreats.

  globular cluster A cluster of stars, in the form of a sphere or globule, reckoned to be old.

  gradualism The theory that geological formations are created incrementally. Contrast catastrophism; see also uniformitarianism.

  gravitational lens(ing) A term used to characterize the effect of gravity on light rays, which produces distorted images.

  gravitational redshift The loss of energy of radiation as it climbs out of a gravitational field; as visible light loses energy it becomes redder.

  gravitational waves (gravitational radiation) The disturbances in space produced by changes in the gravitational field of an object, such as a binary star.

  gravity (gravitation) The attractive force produced by all matter, each particle attracting every other across space and tending to change its motion; the force giving ‘weight’ to ‘mass’; the dominant long-range force and so the force that is most important in astronomy.

  greenhouse effect The warming effect produced by an atmosphere that is transparent to incoming light and opaque to outgoing infrared radiation, which thus becomes trapped.

  Hadley cell, wave A large zone or wave in the atmosphere, of global proportions.

  heliocentric theory The theory that the Sun is at the centre of the Solar System of planets; see also Copernican theory.

  helioseismology The study of the oscillations of the Sun.

  Hubble Deep Fields Observations made by the Hubble Space Telescope of typical regions of cosmic space.

  hydrogen The simplest element and primary constituent of the Universe.

  infrared radiation A form of radiation, slightly less energetic than light, which is emitted by warm objects.

  interferometry A technique in which two separated detectors, each picking up radiation (such as radio waves, light, gravitational waves), are used to simulate (in some ways) the capacity of a telescope equal in size to the distance between them.

  intergalactic cloud A cloud of gas that lies between galaxies.

  interstellar dust Dust particles that lie between the stars.

  interstellar hydrogen Hydrogen gas that lies between the stars.

  interstellar space Space that separates the stars.

  inverse square law A law that intensity or force (as in radiation or gravitational pull) diminishes according to the square of the distance.

  ionosphere The spherical layer (or layers) of ionized air lying at an altitude of approximately 100 kilometres from the Earth.

  irregular galaxy A galaxy that has no particular shape.

  isothermal Of a body, having a homogeneous temperature.

  isotope (from Greek: ‘same’ ‘place’ – in the Periodic Table of elements) A nucleus with a certain number of protons and a further number of neutrons. The number of protons in the nucleus of an atom defines the chemical element that it is (and therefore its place in the Periodic Table); within a collection of atoms of a given element, the number of neutrons in the nucleus of each atom varies from atom to atom. Nuclear reactions may alter isotopes from one kind to another.

  isotropic Of an object, looking the same from all directions.

  jet A flow of material outwards in a straight line.

  jupiter A gas-giant planet in another planetary system that resembles Jupiter in the Solar System.

  Kelvin degree A degree on the centigrade (Celsius) temperature scale, but counted from a temperature of absolute zero.

  KreeP A group of elements – potassium (K), rare earth elements (ree) and phosphorus (P) – found together in some rocks in high abundance.

  Kuiper belt The belt of asteroids and/or comets, or other Trans-Neptunian Objects, which lie beyond Neptune; hence, ‘Kuiper Belt Object’, one of the bodies in the Kuiper belt.

  Late Heavy Bombardment (lunar cataclysm) In the early history of the Solar System, an event in which a rain of meteorites crashed down on the surface of the planets.

  lava flow A flow of liquid magma from a volcano or other fissure in the surface of a planet.

  lenticular galaxy A galaxy that is lens-shaped.

  light year The distance t
hat light travels in a year; hence also ‘light minute’, ‘light second’, etc.

  log N–log S In astronomy, a mathematical expression that counts and interprets the numbers of astronomical sources of a given brightness.

  lunar cataclysm See Late Heavy Bombardment.

  lunar eclipse The event that occurs when the Moon passes into the shadow cast by the Earth in the light from the Sun.

  lunar seas The magma plains that show as flat, dark areas on the Moon, once thought to be areas of liquid.

  magnetosphere The region on and around a planet influenced by its magnetic field.

  magnitude In astronomy, the brightness of a star.

  mantle In relation to the Earth or a similar planet, the outer rocky layers wrapped around the core.

  Medicean stars The name given by Galileo to the four satellites of Jupiter.

  melt The process by which a solid, as it warms, becomes first a liquid and then a gas.

  meteor The phenomenon of a streak of light and radar echoes caused by a solid body (meteoroid) dropping from space into the Earth’s or another planet’s atmosphere.

  meteor shower The appearance of many meteors over a period of hours or days, all coming from the same part of the sky.

  meteorite The surviving part of the solid body, or meteoroid, that causes a meteor.

  meteoroid A solid body orbiting in space, which, when it plunges into the Earth’s or another planet’s atmosphere, becomes a meteor.

  microwave radiation Radiation with a wavelength intermediate between infrared radiation and radio waves.

  Milankovič (Milankovitch) cycles Oscillations in the Earth’s temperature caused by variations in the planet’s orbital parameters (such as its eccentric orbit or its tilt).

  millimetre waves Radiation with a wavelength in the millimetre region: short-wavelength microwave radiation.

  minor planet Obsolescent term for asteroid.

  moon A satellite in orbit around a planet or asteroid.

  nebula (Latin: ‘cloud’) A body of gaseous material and/or dust grains in space, which emits or reflects light and other energy picked up from stars nearby.

  Nebular Hypothesis The central tenet of a number of theories of the origin of the Solar System, according to which the planets formed from a nebula in orbit around the newly born Sun.

  neutrino A particle with no electric charge, very little mass (formerly thought to have no mass at all), and spin ½, which is produced as a by-product of numerous nuclear reactions. There are three types, or ‘flavours’, of neutrino.

  neutron star A star so small that its constituent material is made primarily of neutrons, as opposed to electrons and protons or other nuclei; see pulsating radio star.

  nova (Latin: ‘new [star]’) A star that flares up because of an explosion on its surface and becomes temporarily bright where no star was noticeable before; see supernova.

  nuclear energy Energy released by nuclear processes; the source of star- and sun-light.

  nuclear fusion A nuclear process in which nuclei are fused together to form a heavier nucleus; the process that fuels the stars.

  nucleus (of an active galaxy) The central, small, luminous area of a galaxy, the source of much of its radiated energy, the location of its supermassive black hole.

  oblate The term used to describe an ellipsoid that is flattened at the poles: the three-dimensional shape produced by rotating an ellipse around its minor axis; compare prolate.

  obliquity of the ecliptic The angle (currently about 23.5 degrees) between the Earth’s Equator and the plane of the Earth’s orbit; thus, the tilt of the Earth’s polar axis relative to its orbit.

  occultation An astronomical event in which a nearby body passes in front of and hides (occults) a more distant one, particularly but not exclusively when the Moon passes in front of a planet or a star. A total solar eclipse is an occultation of the Sun by the Moon.

  Olbers’ Paradox The contradiction, popularized by Wilhelm Olbers, between the assumption that the Universe is infinite and eternal and the fact that it is dark at night.

  Oort Cloud The hypothetical region on the periphery of the Solar System from which long-period comets come.

  orbital plane The plane containing the orbit of a body that moves round another.

  parabola An open geometric figure described by the orbit of a body falling into the gravitational field of another, as in the case of a comet falling in towards the Sun from a very large distance (effectively ‘from infinity’). Akin to an ellipse, which describes the orbit of a comet that is always contained within the Solar System.

  parallax The apparent shift in position of a distant object, caused by a change in the position from which it is observed. In astronomy, a shift of 1 arc second in the angular position of a star relative to its average position as the Earth moves around the Sun in one year is described as a parallax of 1 arc second, and means that the star is 1 parsec (3.26 light years) away.

  perihelion The point in a planet’s orbit at which it is closest to the Sun.

  permafrost Ground that is perpetually (or at least for an entire season) at a temperature below the freezing point of water.

  phase The varying appearance of a planetary body, such as the Moon or Venus, due to its illumination by the Sun and the angle at which it is viewed; in different phases such a body will appear as a crescent, a gibbous shape, or a complete circle.

  planet As defined by the International Astronomical Union, a celestial body orbiting its parent star, the body being of such a mass and size that it has become rounded and has also cleared the nearby orbital region of all the planetesimals among and from which it formed. Compare dwarf planet.

  planetary nebula A nebula that resembles a planet in appearance; such nebulae were formed from material ejected by a star during its lifetime, and now illuminated by the star’s core as it evolves to its end as a white dwarf.

  planetary rings A system of meteoroids in orbit in a thin disc around a planet, as, for example, Saturn’s rings.

  planetary system A system of planets in orbit around a star, as, for example, the Solar System.

  planetesimal In a newly formed planetary system, a small planet, formed from accreted dust and probably about to merge with others to form a large planet.

  plenum A material, hypothesized by René Descartes, that fills space and transmits force (gravity) from one body to another.

  plutino A Trans-Neptunian Object, whose orbit, like Pluto’s, resonates with that of Neptune, and which makes two orbits to Neptune’s three.

  pre-biotic chemistry In organic chemistry, a process that involves complex chemicals, which, though similar to those involved in biochemistry, are not necessarily or not perhaps as complex as biological ones.

  precession The gyrating motion of a planet (notably the Earth) as its rotational axis describes a cone in space; it resembles the motion of a spinning top.

  principle of equivalence The principle that the mass of a body participating in gravitational attraction and the mass that resists acceleration are identical.

  prolate The term used to describe an ellipsoid that is pointy at the poles: the shape produced by rotating an ellipse around its major axis; compare oblate.

  proper motion The motion of a star through space across the line of sight; it is ‘proper’ in the sense of ‘belonging to the star itself’, as opposed to resulting from the way that it is viewed.

  proplyd A short form for proto-planetary disc.

  proton A fundamental particle, the nucleus of a typical hydrogen atom, with positive charge, spin ½ and of considerable mass.

  proto-planet A newly forming planet.

  proto-planetary disc (proplyd) A disc of dust and proto-planets in orbit around a new-born star.

  Ptolemaic theory The geocentric theory, attributed to Ptolemy.

  pulsar A short form for pulsating radio star.

  pulsating radio star (pulsar) A rotating neutron star, showing regular and rapid pulses of radio waves.
/>   quasar A short form for quasi-stellar radio source.

  Quasi-Stellar Radio Source (Quasar; Quasi-Stellar Object, QSO) A point-like radio source coincident with a star-like optical source, indicating a very distant active galaxy.

  radial velocity The motion of a star in the line of sight away from or towards the observer.

  radiant In a meteor shower, the vanishing point of the tracks of the individual meteors in their orbit round the Sun, as seen in perspective from the Earth.

  radiation pressure The pressure caused by light or other radiation on a solid surface.

  radio galaxy A galaxy that emits radio waves.

  radio waves Radiation with a wavelength upwards of about 1 metre.

  red giant A large star, red because it has a low surface temperature; an evolved star.

  reflection nebula A nebula of interstellar dust, which reflects the light of a very nearby or embedded star; essentially the same as a dark nebula except that there is a star to illuminate it.

  Roche limit The distance from a planet that constitutes the limit outside which a satellite will maintain its integrity, but within which it will break into pieces and form a planetary ring.

  satellite A body that is in orbit around, and subordinate to another; particularly a smaller body or moon in orbit around a larger planet or asteroid, but also a smaller galaxy in orbit around a larger body (as with the Magellanic Clouds).

  scintillation Twinkling, as of a star, due to the atmosphere, or a radio-emitting quasar, due to interplanetary plasma.

  Seyfert galaxy An active galaxy, showing spectroscopic evidence of gas in an accretion disc and a circum-nuclear disc.

  solar eclipse An occultation in which the Moon passes in front of the Sun, and may obscure it partially or totally.

  solar flare An explosive event on the Sun’s surface.

  solar nebula The nebula that surrounded the Sun at its formation.

  solar neutrino problem The question of why there is a scarcity, below original expectations, of solar neutrinos, which is answered by the theory of neutrino oscillations.

  solar wind The outward flow of material from the solar surface into the rest of the Solar System.