Astronomical Glossary

Compiled by Brian Timmins

[ A ] [ B ] [ C ] [ D ] [ E ] [ F ] [ G ] [ H ] [ I ] [ J ] [ K ] [ L ] [ M ]
[ N ] [ O ] [ P ] [ Q ] [ R ] [ S ] [ T ] [ U ] [ V ] [ W ] [ X ] [ Y ] [ Z ]

A

• ABBERATION: In astronomy, the apparent angular displacement of the position of a celestial body in the direction of motion of the observer, caused by the combination of the velocity of the observer and the velocity of light.
• ABSOLUTE MAGNITUDE: This is how bright stars would appear if they were all the same distance away from us. The standard distance for measuring absolute magnitude is 10 parsecs or 32.6 Light Years. It can be seen that a star two magnitudes brighter than another star will be 2.5 x 2.5 = 6.25 times brighter. Three magnitudes will be 2.5 x 2.5 x 2.5 = 15.6 times brighter. So a star with a magnitude of 13 will be 156250 times fainter than a star of magnitude 0. Very bright stars have a magnitude less than 0 and therefore have negative magnitudes for example Sirius in Canis Major which is the brightest star visible from Britain, has an apparent magnitude of –1.47. Venus has a maximum apparent magnitude of –4.5 and the Sun is -27.
• ABSORPTION LINES: Dark lines interrupting a continuous color spectrum, caused by a cool gas between the light source and the observer. Cool gas absorbs light in the same frequencies as it emits when hot, e.g. double yellow line of sodium. Such dark lines in the Sun's spectrum were discovered by Joseph Frauenhofer.
• ACHROMATIC LENS: Two lenses made from different glasses, reducing, but not eliminating, the problem of chromatic aberration. The colour defect can be further reduced by using more lenses to produce an apochromatic lens.
• ACTIVE GALAXY: A galaxy emitting intensely at optical and/or radio and/or X-ray wavelengths from a small central core. Examples are Seyfert galaxies, EL Lac objects, and quasars. Most explanations of active galaxies involve material falling into a gigantic black hole at their centres.
• ALBEDO: That percentage of light which having fallen on the surface of an object is reflected from it. Albedo is measured between 0 and 1so a low albedo planet absorbs most of the radiation that falls on it. It does not send much of the radiation back into space, and so it does not shine brightly. The surface of a low albedo planet is dark and rough. A high albedo planet or moon reflects a lot of light. A moon with a mirror surface would reflect back everything, and so have an albedo of one, but it seems unlikely that there would be any moons or planets like that. The surface of a high albedo planet is light and smooth.
• ANGSTROM UNIT: The unit of length often used for giving the wavelengths of light. It has a value of 10-10m. Although still widely used in astronomy, it is gradually being superseded by the standard SI unit of a nanometre (1Nm = 10Å). Named after Anders Jonas Ångström.
• APERTURE: The diameter of the objective of a telescope.
• APHELION: That distance where a planet is at its furthest distance from the sun.
• APOGEE: That distance where a satellite is at its furthest distance from it's primary.
• APPARENT MAGNITUDE: The measure used for the brightness of astronomical objects as seen in the sky. The smaller the value of the magnitude, the brighter the object. The faintest stars visible to the naked eye from a good site are around magnitude +6. Sirius is magnitude -1.45.
• APPARENT MOTION: The observed motion of a heavenly body across the celestial sphere, assuming the Earth is at the sphere's center and is standing still.
• ASHEN LIGHT: The apparent faint illumination of the dark side of the Moon.
• ASTEROIDS: See "What are Bolides, Meteoroids, Meteors, Meteorites, Asteroids & Comets"
• ASTEROID BELT: Asteroids are primoridal objects left over from the formation of the Solar System. While some have suggested that they are the remains of a protoplanet that was destroyed in a massive collision long ago, the prevailing view is that asteroids are leftover rocky matter that never successfully coalesced into a planet. Most planetary astronomers still believe that the planets of the Solar System formed from a nebula of gas and dust and ices that coalesced into a dusty disk around the developing Sun. Within the disk, tiny dust grains (and ices in the colder environs beginning around two AUs inside of Jupiter's orbit) coagulated into larger and larger bodies called planetesimals, many of which eventually accreted into planets over a period as long as a 100 million years.
• AURORA: Also known as the northern or southern lights (Aurora Borealis and Aurora Australis). Caused by the influx of charged particles from the Sun into the Earth's upper atmosphere.
• AZIMUTH AND ELEVATION: Two angles which give the direction of a surveyor's telescope (theodolite). Azimuth is the rotation angle of the telescope around a vertical axis, measured from due north, clockwise from above; that is, the directions (north, east, south, west) have azimuth (0°, 90°, 180°, 270°). Elevation is the angle the telescope is lifted above the horizontal plane.
  [In 3-dimensional polar coordinates centered on the instrument, azimuth is 360°-φ (since φ is measured counterclockwise), elevation is 90°-θ, the direction of straight up has elevation 90° and θ = 0].
  
  

B

• B-TYPE STARS: A star with a temperature in the region 10,000 to 25,000K.
• BAILY'S BEADS: Parts of the intensely bright photosphere of the Sun shining through valleys at the edge of the Moon at the start and end of totality in a solar eclipse. When only one part of the photosphere shines through, it is known as the 'diamond ring'.
• BALMER SERIES: A series of 4 visible colors emitted by glowing hydrogen, appearing as a series of 4 "lines" in the spectrogram of hydrogen light (additional lines can be observed by instruments, beyond the range of the human eye). Johann Balmer, a high-school teacher, discovered in 1885 a simple but accurate formula connecting the wave frequencies of these colors, which provided the first clues to processes inside atoms, starting a process which ultimately led to quantum physics.
• BARNARD'S STAR: The star with the largest known proper motion across the sky. It moves at a rate of about 10 arc seconds per year.
• BARRED SPIRAL: A spiral galaxy in which the arms originate from the ends of linear extensions to the nucleus, rather than from the nucleus itself.
• BAYER NAMES: Names for the stars derived from the system used in the Uranometria star catalogue (published In 1603). letters of the Greek alphabet are used, with a usually a for the brightest star in a constellation, b for the second brightest etc- For example, Sirius is 'a' Canis Majoris.
• BIG BANG THEORY: The most widely accepted group of models for the way in which the Universe came into being.
• BINARY STAR: Two stars which are physically close together in space, held together gravitationally, and are orbiting their common centre of mass. They are to be distinguished from double stars which are two stars seen close together in the sky, but may be physically very distant from each other.
• BLACK BODY RADIATION: Light or other electromagnetic radiation emitted due to heat by a solid, liquid or dense gas, with no color of its own (hence "black"). Distinguished by a continuous distribution of spectral color, with its peak of emission shifting towards shorter wavelengths as the temperature increases: e.g. infra-red for a warm hand, red for a hot iron bar, yellow for the glowing filament in a lightbulb.
  The spectrum of black body radiation created serious theoretical difficulties at the turn of the 20th century. They were only resolved in 1900, after Max Planck proposed the spectrum could be explained, if the energy of emitted radiation was limited to "quanta" of energy whose size increased with the frequency of the radiation. His theory marked the beginning of quantum physics.
• BLACK HOLE: See "What are Black Holes?"
• BOLIDE: See "What are Bolides, Meteoroids, Meteors, Meteorites, Asteroids & Comets"
• BODE'S LAW: A mathematical relationship giving the distances in astronomical units of the planets (plus the asteroids) out to Uranus from the Sun. At one time thought to have physical significance, it is now regarded as a mnemonic only. The law is (0.3n + 0.4), with n = 0, 1, 2,4,8, etc.
• BRADLEY'S ABERRATION: An apparent motion observed in all stars, shifting their celestial position around a small ellipse with a 1-year period. Discovered by the British Royal Astronomer James Bradley, it can be viewed as the result of the vectorial addition of the velocity v due to Earth's orbital motion and the velocity c of light. This is a "first order effect" involving v/c (not its square) and as such does not contradict relativity.
  
  

C

• CARBON CYCLE: One of the two main routes whereby hydrogen is converted into helium inside stars.. The carbon cycle is the main source of energy in stars more massive than the Sun.
• CARBON STAR: A group of cooler stars in which carbon is over-abundant.
• CASSEGRAIN TELESCOPE: One of the most widely used designs for telescopes. It is a reflecting telescope with a concave parabolic primary mirror and a convex hyperbolic secondary mirror. The secondary mirror is placed before the focus of the primary and reflects the light out through a hole in the centre of the primary to the Cassegrain focus at the back of the telescope.
• CASSINI DIVISION: A narrow gap in the rings of Saturn caused by the gravitational perturbations from Saturn's satellites.
• CATACLYSMIC VARIABLE: A variable star in which the change in brightness is very rapid and of large amplitude. The class includes novae, dwarf novae and supernovae.
• CELESTIAL SPHERE: An imaginary sphere, centred on the Earth. Positions of objects in the sky are obtained from their projected positions on the celestial sphere.
• CEPHEID AND RR LYRAE VARIABLES: Cepheids are a type of variable star that pulsates regularly. The properties of variable stars will be discussed later, but because these stars have a particular brightness to variation ratio, they server as good rulers for determining distances. RR Lyrae variables can also be used as rulers, but they pulsate faster and have about a 1 magnitude variation in brightness. The benefit is they seem to reside mostly in globular clusters.
• CHANDRESEKHAR LIMIT: An upper limit to the mass of a white dwarf. It has a value of about 1.4 times the mass of the Sun. If a white dwarf exceeds this mass, then it will collapse to a neutron star.
• CHERENKOV RADIATION: is emitted whenever charged particles pass through matter with a velocity exceeding the velocity of light in that medium.
• CHROMOSPHERE: a reddish layer in the Sun´s atmosphere, the transition between the photosphere and the corona
• CIRCUMPOLAR STAR: A star which is high enough in the sky never to set.
• COLOUR INDEX: The difference between two measurements of the magnitude of an object obtained at two different wavelengths. The most widely encountered colour index is obtained by measurements in the B band (centred on 440nm in the blue) and the V band (centred on 550nm in the yellow-green), and is known as the B-V colour index.
• COLOUR TEMPERATURE: The temperature obtained by assuming that an object radiates like a black body and then measuring its intensity at two different wavelengths.
• COMA: 1)Aberration in optics and 2)The head of a comet.
• COMET NUCLEUS: The solid core - usually small (a few kilometres across) in comparison with the comet's head and tail, but contains almost all the mass of the comet. It is thought to be composed of small particles of dust and pebbles cemented together by frozen gases such as water, carbon dioxide and ammonia.
• COMET: See "What are Bolides, Meteoroids, Meteors, Meteorites, Asteroids & Comets"
• CONSTELLATION: A group of stars that has, for convenience of easy ecognition, been given a name.
• CORONA: the outermost layer of the Sun´s atmosphere, visible to the eye during a total solar eclipse; it can also be observed through special filters and best of all, by X-ray cameras aboard satellites. The corona is very hot, up to 1-1.5 million degrees centigrade, and is the source of the solar wind
• CORONAL HOLE: an area in the Sun's corona that appears dark when viewed in the far UV or in the long-wavelength end of the x-ray range. Coronal holes seem associated with sources of fast solar wind, probably because their field lines do not curve back to the Sun. Over most of the Sun their shapes are changeable and irregular, but the Sun's polar regions seem to contain "permanent" coronal holes.
• CORONAL MASS EJECTION (CME): a huge cloud of hot plasma, occasionally expelled from the Sun. It may accelerate ions and electrons and may travel through interplanetary space as far as the Earth´s orbit and beyond it, often preceded by a shock front. When the shock reaches Earth, a magnetic storm may result.
• COSMIC BACKGROUND RADIATION: Radiation mostly in the microwave region (and hence also known as the microwave background) which pervades the whole of space. It is thought to be the remnant of the radiation from the big bang.
• COSMIC RAYS: Very high energy particles which pervade at least the whole of the galaxy, and possibly the whole of space. Most of the particles are protons and helium nuclei, with small numbers of nuclei of heavier atoms, and a few electrons.
  
  

D

• DARK CLOUD: A relatively dense cloud of interstellar material containing dust particles. The dust particles absorb light from the more distant stars etc, so that the region appears dark compared with its surroundings. The clouds are often of low temperature and contain many molecules.
• DARK MATTER: See What is Dark Matter?"
• DECLINATION: One of the two angles uses to specify location on the celestial sphere. Declination is like latitude, but unlike latitude, it is measured from the north pole. The pole has declination 0, the equator 90 degrees, the southern celestial pole 180 degrees. See right ascension and declination
• DIFFRACTION GRATING: A device used to produce the spectrum in astronomical spectroscopes consisting of many narrow parallel apertures or mirrors.
• DIFFUSE NEBULA: A general name for any concentration of gas and dust in the interstellar medium.
• DIRECT MOTION: The movement acro:ss the sky, around an orbit, or the rotation of an object which follows the normal pattern of motion within the Solar System. In the sky, the movement is from west to east.
• DISTANCE MODULUS: The difference between the absolute and apparent magnitudes of an object used to calculate its distance in the absence of any interstellar absorption.
• DOBSONIAN: The design of telescope developed by the American astronomer John Dobson and used by many amateur astronomers. It comprises a Newtonian tube design mounted on a simple Alt-Azimuth mounting.
• DOPPLER SHIFT: Have you ever wondered what is happening when a Police car speed past and the pitch of it's siren changed as the vehicle moved fistly towards, then away from you? First the pitch becomes higher, then lower. First analysed properly by the Austrian mathematician and physicist, Christian Doppler (1803-53), this change in pitch results from a shift in the frequency of the sound waves. On approach, the sound waves from the siren are compressed and on ecession, the sound waves are stretched, relative to the observer, thus causing the siren's pitch to increase and decrease. If you measured the rate of change of pitch, you would be able to calculate the ambulance's speed. In astronomy, the Doppler effect was originally studied in the visible part of the electromagnetic spectrum. Today, the Doppler shift, as it is also known, applies to electromagnetic waves in all portions of the spectrum. Also, because of the inverse relationship between frequency and wavelength, we can describe the Doppler shift in terms of wavelength. Radiation is redshifted when its wavelength increases, and is blueshifted when its wavelength decreases. Astronomers use Doppler shifts to calculate precisely how fast stars and other astronomical objects move toward or away from Earth.
• DOUBLE STAR: Two stars seen close together in the sky.
• DWARF NOVA: A close binary star containing a white dwarf and a main sequence or post-main sequence star, in which regular explosions occur on the surface of the white dwarf.
• DWARF PLANET: See Dwarf Planets Versus Planets"
• DWARF STAR: A star like our Sun which is on the main sequence portion of the Hertzsprung Russell (HR) Diagram. This is a stable star burning hydrogen in the normal way which it will do for the vast majority of its life. It will remain a dwarf until it has used most of its Hydrogen fuel then it will expand and become a Red Giant.
  
  

E

• EARTHSHINE: The reflected light from the Earth which may be seen as a faint illumination of the dark side of the Moon.
• ECCENTRICITY: The degree to which an ellipse deviates from circularity. It is usually used in connection with orbits,. and is denoted by e.
• ECLIPSE: When one object passes in front of another as seen from the Earth. The term is usually used when the two objects are of roughly the same angular size, as in an eclipse of the Sun by the Moon. When the angular sizes are very different the phenomenon is called an occultation or a transit.
• ECLIPSING BINARY: A binary star system with the orbital plane close to the line of sight from the Earth. The stars therefore alternately pass in front of each other. The binary is usually detected from the periodic reductions in brightness arising from the eclipses.
• ECLIPTIC: A line around the middle of the celestial sphere, connecting the points occupied by the Sun over the year. The moon and the visible planets also appear to move very close to that line, which cuts the celestial equator at an angle of about 23.5^o . See plane of the ecliptic:
• EFFECTIVE TEMPERATURE: The temperature of a black body that would radiate the same amount of energy per unit area as the object in question.
• ELECTRON: One of the subatomic particles which, along with protons and neutrons, make up atoms. It has a negative charge of 1.6 x 1O-19C, equal and opposite to the charge on the proton. Its mass of 9.1 x 10-31kg is only about 1/2000 of the mass of the proton.
• ELLIPTICAL GALAXY: One of the major classes of galaxy: Elliptical in shape and generally containing old and relatively cool (and therefore reddish) stars, with little interstellar gas and dust.
• ELONGATION: The angle between the Sun and a planet in the sky. For an outer planet the elongation can range from 0° (conjunction} to 180° (opposition). For Venus and Mercury it can range from 0° (superior or inferior conjunction) to a maximum of 47° (Venus) or 28° for (Mercury).
• EMISSION NEBULA: A hot mass of thin gas in interstellar space. The nebula is usually heated by stars embedded in it.
• ENCKE'S DIVISION: A narrow gap in the outer (or A) ring of Saturn. It is about 900km wide.
• ENERGY LEVEL: The energy of an electron within an atom. Movement of electrons between energy levels results in the emission or absorption of photons, and produces spectral lines
• EPHEMERIS: A listing of the successive positions in the sky of a moving object.
• EPICYCLE: A circle around a point which (in the simplest form of Ptolemy's system) moved steadily around the celestial sphere. Greek astronomers proposed that planets moved along epicycles around the Sun or around other points which circled around the sky; later additional corrections were added. The theory of epicycles was the earliest explanation for the irregular apparent motion of the planets: prograde (forward), then retrograde
  
  Copernicus' Assumptions:

  1. There is no one center of all the celestial circles or spheres
  2. The center of the earth is not the center of the universe, but only of gravity and of the lunar sphere
  3. All the spheres revolve about the sun as their mid-point, and therefore the sun is the center of the universe
  4. The ratio of the earth's distance from the sun to the height of the firmament is so much smaller than the ratio of the earth's radius to its distance from the sun that the distance from the earth to the sun is imperceptible in comparison with the height of the firmament
  5. Whatever motion appears in the firmament arises not from any motion of the firmament, but from the earth's motion. The earth together with its circumjacent elements performs a complete rotation on its fixed poles in a daily motion, while the firmament and highest heaven abide unchanged
  6. What appear to us as motions of the sun arise not from its motion but from the motion of the earth and our sphere, with which we revolve about the sun like any other planet. The earth has, then, more than one motion
  7. The apparent retrograde and direct motion of the planets arises not from their motion but from the earth's. The motion of the earth alone, therefore, suffices to explain so many apparent inequalities in the heavens
• EQUATION OF TIME: The difference between the true solar time (as given by a sundial) and civil or clock time (ignoring any summer time adjustments).
• EQUATORIAL AXIS: Among the two mutually perpendicular axes of a telescope, the one that points at the celestial pole. To keep a star in view, the telescope must be rotated around this axis at the same rate as the Earth turns.
• EQUINOX: The two times of the year when the Sun is on the equator, or the two positions in the sky where the equator and ecliptic intersect. The vernal equinox occurs on or around March 21 each year and the autumnal equinox on or about September 21. The position in the sky of the vernal equinox is also known as the first point of Aries, and is the zero point for right ascension measurements. Despite its name, it is actually to be found in Pisces, having moved due to precession since it was originally identified some two and a half thousand years ago.
• ESCAPE VELOCITY: The minimum velocity needed to escape completely from the surface of an object. The escape velocity of Earth is 11.2km/s
• EVENT HORIZON: The boundary of a region surrounding a black hole from where the escape velocity equals the speed of light. It is usually regarded as the surface of the black hole though it is not a solid surface in any way.
• EYEPIECE: An optical device used to produce images visible to the eye.
  
  

F

• F CORONA: A component of radiation from the corona which is light from the solar photosphere scattered by interplanetary dust.
• F-TYPE STAR: A star with a surface temperature between about 6,000 and 7,500K.
• FACULAE: Small regions of the solar photosphere that are a few hundred degrees hotter than average, and which therefore appear as slightly brighter regions.
• FIELD STARS: Stars (or galaxies etc.) which are in the same field of view as the object of interest, but which are not physically associated.
• FILAMENT: An elongated dark region on the surface of the Sun. They are solar prominences seen silhouetted against the photosphere.
• FINDER: A smaller telescope attached to an astronomical telescope, used to locate an object which is to be observed. Large telescopes have a very small field of view so the wider field of the finder allows more of the sky to be seen.
• FLARE STAR: A star which suddenly brightens by about half a magnitude. The brightening is attributed to flares on the surface of the star.
• FLARE: Sudden brightening of a region of the Sun's surface, almost invariably within or near complex sunspot groups.
• FLUX: The total amount of a quantity (usually radiation) passing through a surface.
• FOCAL LENGTH: The distance from a lens or mirror to its focal point when the object being imaged is at a large (infinite) distance.
• FOCAL RATIO: (f-ratio) The ratio of the focal length of a lens or mirror to its diameter.
• FORBIDDEN LINE: A spectrum line which normally has a very low probability of occurrence.
• FRAUNHOFER LINES: Strong spectral lines in the solar spectrum labelled with the letters A to K by Joseph Fraunhofer in early nineteenth century. The sodium D lines and the calcium H and K lines are the most commonly encountered examples of this usage today.
• FREQUENCY: The number of cycles per second of a wave, measured in hertz (Hz). Optical radiation has a frequency of around 5 x 10>Hz.
• FUSION: An atomic reaction in which two or more lighter elements combine to form a heavier element, for example the formation of helium from hydrogen.
  
  

G

• G TYPE STAR: A star with a surface temperature of 5000 to 6000K. The Sun is a Type G Star.
• GALACTIC (OR OPEN) CLUSTER: A collection of stars physically close together and bound into a stable group by gravity.
• GALACTIC COORDINATE: A system of coordinates for the positions of objects in the sky based upon the plane of the Milky Way Galaxy and the direction to the centre of the Galaxy in Sagittarius. Galactic latitude is the angle up or down from the plane of the galaxy, galactic longitude. the angle eastward from the galactic centre.
• GALAXY: See What are Galaxies & Clusters?"
• GALILEAN SATELLITES: The four largest satellites of Jupiter, discovered by Galileo. They are named after the mythical companions of Jove and are, in order of increasing distance from Jupiter; lo, Europa, Ganymede and Callisto.
• GAMMA RAY BURSTS: Gamma-ray bursts (GRBs) are the most luminous electromagnetic events occurring in the universe since the Big Bang. They are flashes of gamma rays emanating from seemingly random places in deep space at random times. The duration of a gamma-ray burst is typically a few seconds, but can range from a few milliseconds to minutes, and the initial burst is usually followed by a longer-lived "afterglow" emitting at longer wavelengths (X-ray, ultraviolet, optical, infrared, and radio). Gamma-ray bursts are detected by orbiting satellites about two to three times per week.
Most observed GRBs appear to be collimated emission caused by the collapse of the core of a rapidly rotating, high-mass star into a black hole.
• GAMMA RAYS: Photons at the Flare stars A faint cool dwarf high frequency end of the spectrum with wavelengths of 0.0lnm or less
• GENERAL RELATIVITY: The very powerful theory developed by Einstein in 1915 of how things behave when accelerations are involved. In it, the three dimensions of space, plus time, are combined into the space-time continuum.
• GIANT MOLECULAR CLOUD (GMC): A gaseous nebula containing some hundreds of thousands of solar masses of cold gas, and occupying a volume of space some tens of parsecs across. The gas is predominantly molecular hydrogen.
• GIBBOUS: A phase of the Moon, Mercury, Venus, etc. between half and full.
• GLOBULAR CLUSTER: See What are Galaxies & Clusters?"
• GNOMON:The gnomon is that part of a sundial that casts the shadow. It is an ancient Greek word meaning "indicator", "one who discerns," or "that which reveals". When used in a sundial it subtends an angle to the plate concomitant with the latitude on which it stands, and is set parallel to the equator. Ancient astronomers used a vertical version for observing the altitude of the Sun, especially when on the meridian.
• GRANULATION: The second-of-arc scale mottled pattern in the solar photosphere. Individual granules last for a few minutes and are thought to be the tops of convection cells.
• GRAVITATIONAL COLLAPSE: The collapse of an object when its internal forces are no longer able to support it against the force of gravity.
• GRAVITATIONAL LENSING: See What is Gravitational Lensing?"
• GRAVITATIONAL RADIATION: When an object which has mass is accelerated or otherwise disturbed, it is predicted to radiate gravitational waves.
• GREAT RED SPOT: The Great Red Spot is a giant, persistent anticyclonic storm in the atmosphee of the planet Jupiter, just south of the equator, which has lasted for at least 178 years and possibly as long as 343 years or more. The storm is easily large enough to be visible through Earth-based telescopes.
• GREENHOUSE EFFECT: The increased temperatures at the surface of planets because of the presence of their atmospheres. Some of the constituents of the atmosphere, especially carbon dioxide and methane allow the solar energy in, but then blanket the long wave radiation back from the surface.
  
  

H

• HALLEY'S COMET: A well known short period comet, named after Edmund Halley who first determined its orbit. Its period is just over 76 years, and it last came into the inner solar system in 1986.
• HALO: The outer regions of a galaxy extending well beyond the normally visible galaxy (galactic halo); roughly spherical in shape and containing isolated stars and globular clusters. A luminous ring occasionally to be observed around the moon in the sky (lunar halo) due to ice crystals high in the Earth's atmosphere.
• HAYASHI TRACK: A part of the path of a protostar on the Hertzsprung-Russell diagram.
• HELIUM FLASH: The explosive start of nuclear reactions converting helium into carbon in the core of an aging star.
• HELIUM PROBLEM: The problem of trying to explain why the basic form of matter throughout the Universe is about three-quarters hydrogen and one quarter helium. Although helium is produced during nucleosynthesis in stars, there has not been enough time to have converted 25% of the hydrogen into helium. The problem is solved in big bang cosmologies because of the formation of helium early in the big bang itself.
• HENYEY TRACK: A part of the path of a protostar on the Hertzsprung-Russell diagram.
• HERBIG-HARO OBJECTS: Small, molecular clouds often occurring in pairs. They are thought to be where jets from young stars or protostars are colliding with the surrounding interstellar material.
• HERTSPRUNG-RUSSELL (HR) DIAGRAM: A plot of the luminosity or absolute magnitude of a star against its temperature or spectral type.
• HI REGION: A cool gaseous nebula containing mostly atomic hydrogen.
• HII REGION: A hot gaseous nebula heated by recently formed stars embedded within it: most of it is ionised.
• HOHMANN ORBIT: Also known as the "Hohmann transfer orbit," it represents the most energy-efficient orbit for transferring a spacecraft from one circular orbit to another in the same plane. This orbit is a Keplerian ellipse touching the larger orbit at its greatest distance and the smaller orbit at its smallest distance.
• HORSEHEAD NEBULA: A cool gaseous nebula in Orion about 350pc away, also known as NGC2024
• HOUR ANGLE: The angle to an object measured westward from the prime meridian (the great circle for a particular observer which goes through the celestial poles and the zenith). It is measured in hours, minutes and seconds, and may be calculated for a particular object from its right ascension and the sidereal time: (HA = ST - RA)
• HUBBLE CLASSIFICATION OF GALAXIES: A classification of the elliptical and spiral galaxies based on their appearance.
• HYDROGEN SPECTRUM: The simplest line spectrum of any element, consisting of a number of separate sequences of lines, the lines of each getting closer and closer together as they approach a limit. Four lines are in the visible spectrum and belong to the Balmer sequence, but other sequences also satisfy formulas like Balmer's.
  
  

I

• INFARED: The part of the electromagnetic radiation spectrum with wavelengths somewhat longer than those in the visible, the radiation we know as heat.
• INFERIOR CONJUCTION: An alignment of the Earth, Sun, and either Mercury or Venus. The planet is at its closest to the Earth, but is also at its worst for observations as it shows only its un-illuminated side
• INFLATION: A period very early on in the big bang origin of the Universe when the expansion rate was many times the speed of light.
• INSOLATION: The amount of energy per unit area received from the Sun. At the top of the Earth's atmosphere it is about 1.4kWm-2.
• INSTABILITY STRIP: A region of the Hertzsprung-Russell diagram wherein a large number of variable stars are to be found.
• INTENSITY: The amount of radiant energy received per unit time, per unit solid angle, per unit area of the receiver oriented perpendicularly to the line of sight to the source.
• INTERFEROMETER: One of a number of different devices which achieve higher sensitivity and/or resolution than any of their constituent telescopes through interference between the outputs to two or more telescopes.
• INTERSTELLAR ABSORPTION: The absorption of light from distant stars and galaxies by the material in the interstellar medium.
• INTERSTELLAR DUST: Particles ranging from a few tens of nanometres to a few hundreds of nanometres in size, present in the interstellar medium to the extent of about one percent or two percent by mass. The particles are thought to have cores of graphite and/or of silicates, and to be covered in a layer of solidified gases frozen from the interstellar medium. The cores may originate in the outer atmospheres of cool red giants. The dust particles produce interstellar absorption and, when aligned by the interstellar magnetic field, polarisation of the light from distant stars. the long wave radiation back from the surface.
• INTERSTELLAR MOLECULES: About a hundred different molecules have been found existing in the interstellar medium, mostly in the giant molecular clouds. They range from simple diatomic molecules like ON and OH, to complex organic molecules with a dozen or more atoms.
• IONISATION: The loss, or less frequently the gain, of an electron by an atom or molecule to give it a net electric charge. Atoms may be Ionised as many times as they have electrons. Thus hydrogen can be Ionised only once, but iron has twenty six different stages of lonisation. Ionised atoms are often symbolised by their chemical symbols and a roman numeral which is one larger than the number of electrons that have been lost. Thus neutral hydrogen is HI, and Ionised hydrogen, HII. An alternative notation uses superscript pluses (pr minuses when the atom gains an electron) as in N-+, H- etc.
• IONOSPHERE: A layer in a planet's atmosphere with a higher than average level of lonisation. On the Earth, the ionosphere is at a height between 50 and 400km, and is produced by lonisation of the Earth's atmospheric gases by ultraviolet radiation from the Sun and by solar cosmic rays.
• IRON PEAK: A peak in the cosmic abundance of elements near to iron. It arises from the high stability of the iron nucleus which makes it the end point for normal nucleosynthesis reactions.
• IRREGULAR GALAXY: A galaxy which has no obvious shape or structure.
• ISOTOPE: The identity of an element is determined by the number of protons in its nucleus. The number of neutrons in the nucleus can vary without changing the element, producing different isotopes of that element.
  
  

J

• JEANS' LENGTH: The size of a condensation within an interstellar or intergalactic gas cloud at which it will start to collapse under its own gravitational forces. For a typical interstellar nebula it has a value of a few parsecs.
• JEANS' MASS: The mass of a condensation whose size is given by the Jeans' length.
• JET: Material expelled from an object in the form of a collimated stream (like water from a hose pipe). Often two jets are emitted in opposite directions leading to bi-polar outflows.
• JULIAN DAY: A calendar based upon counting the days elapsed since January 1, 4713BC on the Julian calendar. The Julian day (JD) starts at midday. Thus midday on January 1, 2000 is the start of JD2451545.
  
  

K

• K CORONA: A component in the spectrum of the solar corona characterised by a continuum. It is due to solar radiation that has been scattered by the electrons in the corona.
• K-TYPE STAR: A star with a temperature in the region 3600K to 5000K.
• KELVIN CONTRACTION: The contraction of a star or other object as a result of energy being lost by radiation and not being replaced from sources such as nucleosynthesis reactions. Kelvin contraction probably occurs during the formation of a star and during the collapse of a star to a white dwarf.
• KELVIN TIME: The time taken for a star or other object to collapse from an infinite size to its present size by Kelvin contraction. It is roughly the lifetime for any object which is shining because of the release of gravitational energy. For the Sun its value is about twenty five million years.
• KELVIN: Unit of the measurement of temperature. The unit is the same as Centigrade but zero it taken as the temperature at absolute zero minus 273°C. Centigrade has its zero at the freezing point of water at sea level.
• KEPLERLAN ORBIT: The orbit of one mass around another when the sizes of the objects are small compared with their separation, and there are no forces other than gravity involved nor any other objects around to produce perturbations.
• KERR BLACK HOLE: A black hole that is rotating.
• KINETIC ENERGY: Kinetic energy is the energy that an object has because of its motion. An object's kinetic energy is equal to 0.5 times its mass times its velocity squared.
• KIRKWOOD GAPS: Regions of the asteroid belt which are largely devoid of asteroids.
• KLEMPERER ROSETTE: is a gravitational system of a number of heavier and lighter bodies, set out in a regular repeating pattern around a common barycenter, around which they all orbit.
• KRUSKAL DIAGRAM: A diagram that enables the properties of space and time to be correctly represented near a black hole.
• KUIPER BELT: (Rhymes with "viper") is an area of the solar system extending from the orbit of Neptune (at 30 AU) to 50 AU from the Sun. The objects within the Kuiper Belt, together with the members of the scattered disk extending beyond, are collectively referred to as trans-Neptunian, along with any hypothetical Hills cloud and Oort cloud objects.
  
  

L

• LAGRANGIAN POINTS: In a system of two large bodies (Sun-Earth or Earth-Moon), these are the points where a small third body will keep a fixed position relative to the other two. Named for French astronomer Louis Lagrange (1736-1813) who first studied them and who showed there existed 5 such points. In the Sun-Earth system only two are important, both on the Earth-Sun line: the L1 point 236 Earth radii sunward of Earth, and the L2 point at a similar distance on the night side. The L1 point is a good "early warning" outpost intercepting shocks and particles emitted by the Sun and its vicinity has been occupied by several spacecraft. Altogether five Lagrangian points exist in the Earth-Sun or Earth-Moon system.
• LATE TYPE STARS: The cooler stars (spectral types K and M). The term dates from the late nineteenth century when it was thought that hot stars evolved into cooler stars.
• LAUNCH WINDOW: The period when a rocket or spacecraft has to be launched in order to achieve the desired orbit.
• LEPTON: A family of sub-atomic particles which includes electrons, neutrinos and muons.
• LIBRATION: A periodic back-and-forth swing of the elongated axis of the Moon across the Moon-Earth line, apparent to a viewer on Earth. So named because it resembles the swings of two-pan scales (in Latin, "Libra") around their points of equilibrium.
  This is almost entirely an apparent motion: actually, the Moon rotates at an almost constant rate, but by Kepler's laws, its orbital motion around the Earth is not constant. If it were, we would always see the same face of the Moon, but actually the Moon sometimes falls behind or advances ahead, allowing more to be viewed.
  Because of its libration, the Moon shows Earth (now and then) areas beyond the 50% of the surface facing Earth. Other (small) effects which also expand the viewable area are included under "libration" as well.
• LIGHT CONE: The volume of space time through which a light signal can travel towards or away from an event.
• LIGHT GRASP: The increase in the amount of light received from point sources like stars when they are viewed through a telescope compared with looking at them with unaided eye. Light grasp is given by 20,000 D2, where D is the diameter of the telescope in metres.
• LIMB DARKENING: The reduction in surface brightness of the limbs of the Sun or other stars compared with the centres of their visible discs.
• LIMB: The edge of an object as seen against the sky.
• LINE PROFILE: A graphical plot of the variation of intensity across a spectrum line.
• LITHIUM STARS: Stars with an over abundance of lithium compared with normal. They include peculiar cool giants known as carbon stars, and I Tauri stars. The significance of the excess of lithium is that this element is very quickly destroyed during nucleosynthesis reactions, so its presence indicates either a very young star, or one with unusual processes occurring within it.
• LOCAL GROUP: The small cluster of galaxies which includes the Milky Way Galaxy, the galaxy in Andromeda (M31), the Magellanic clouds and about another 25 small nearby galaxies.
• LONG PERIOD VARIABLES: Variable stars with periods ranging from several months to a few years. The change in optical brightness can be up to ten magnitudes (a factor of x 10,000). They are cool red giants or supergiants. Mira (o Cet) is one example.
• LUMINANCE: A body's quality of being luminous - emitting or reflecting light.
• LUMINOSITY: The luminosity of stars is measured in two forms: apparent (counting visible light only) and bolometric (total radiant energy). A bolometer is an instrument that measures radiant energy over a wide band by absorption and measurement of heating. When not specified, luminosity refers to bolometric luminosity, which is measured in the SI units watts, or in terms of solar luminosities, L_{\odot} ; that is, how many times as much energy the object radiates than the Sun, whose luminosity is 3.846x10^26 W.
• LUNATION: The synodic period of the Moon. It has a value of 29.53 days and is the interval over which a complete cycle of the lunar phase occurs.
• LYMAN LINES: A regular series of lines in the ultraviolet part of the hydrogen spectrum.
  
  

M

• M-TYPE STAR: A star with a temperature of about 3500K.
• MACH'S FORMULATION: Newton's second and third laws reduce to "When two small bodies act on each other, they accelerate in opposite directions and the ratio of their accelerations is always the same."
• MAGNETARS: A magnetar is a neutron star with an extremely powerful magnetic field, the decay of which results in the emission of huge amounts of high-energy electromagnetic radiation, particularly X-rays and gamma-rays. Not much is known about the physical structure of a magnetar, because there are none close to Earth. They are close to 20 kilometres in diameter and substantially more massive than our Sun. Most of them recorded so far rotate very rapidly, at least several times per second.
• MAGELLANIC CLOUDS: Two small satellite galaxies of the Milky Way, visible from the southern hemisphere and named to commemorate Ferdinand Magellan's expedition which first circumnavigated the Earth (1519 -1522).
• MAGNETIC FIELD: The region around a permanent magnet Pr flowing electrical current throughout which another magnet will experience a measurable force. The Earth's magnetic field has a strength of about 0.00001 tesla (T).
• MAGNETOPAUSE T: he interface between a region containing a magnetic field and the outside. Most frequently used in connection with the Earth's and other planet's magnetic fields. The solar wind compresses the field on the sunward side and drags it out into the magnetotail on the opposite side.
• MAGNETOSPHERE: The region within the magnetopause where the magnetic field of the object is dominant.
• MAGNIFICATION: The increase in linear or angular size of the image of an object when compared with the original.
• MAGNITUDE: In astronomy, measure of the brightness of a star or other celestial object. The larger the number denoting the magnitude, the fainter the object. Zero or first magnitude indicates some of the brightest stars. Still brighter are those of negative magnitude, such as Sirius, whose magnitude is -1.46. Apparent magnitude is the brightness of an object as seen from the Earth; absolute magnitude is the brightness at a standard distance of 10 parsecs (32.616 light years). Each magnitude step is equal to a brightness difference of 2.512 times. Thus a star of magnitude 1 is (2.512)5 or 100 times brighter than a sixth-magnitude star just visible to the naked eye. The apparent magnitude of the Sun is -26.8, its absolute magnitude +4.8.
• MAIN SEQUENCE: The spectral class which contains the majority of the stars. Stars spend most of their lives on the main sequence and change very little during that time. The Sun has a main sequence lifetime of about 1010 years. Whilst on the main sequence, stars obtain their energy by conversion of hydrogen to helium in their cores.
• MAKSUTOV TELESCOPE: A telescope which uses both a mirror and a lens as its main light gathering optics. The mirror is spherical and the Pens a meniscus with -spherical surfaces. The secondary mirror is aluminised onto the rear surface of the lens. It gives very high quality images- but is Limited to small sizes because of the thick lens required.
• MARE: A large area on a satellite or planet which is distinctly smoother in appearance than the rest of the surface- The name derives from the Latin for "sea" The circular maria, such as Mare Imbrium on the Moon are the largest forms of impact crater subsequently flooded by lava flows resulting from the impact. The irregular maria are low-lying areas also flooded by Lava, but from some other source.
• MASCON: A region of increased gravitational attraction on the Moon. Most mascons are associated with circular maria and are due to increased densities of the subsurface rocks.
• MASER: A highly intense source of microwave radiation occurring when metastable states in atoms or molecules become over-populated. Naturally occurring masers are found in some giant molecular clouds and around red giants.
• MASS EXCHANGE: The exchange of material between two objects. This usually occurs in close binary stars, when one component evolves and expands to fill its Roche lobe. Material then flows through the inner Lagrangian pointtowards the second star. Usually the material orbits the accreting star as an accretion disc before turbulence and viscosity cause it to fall to the star's surface.
• MAUNDER MINIMUM: A period of about seventy years from 1645 to 1715 when the sunspot cycle ceased and there were almost no sunspots visible on the Sun. It coincided with a period of lower than average temperatures on Earth, but the causal link is not certain.
• MEAN ANOMALY: An angle used in calculating orbital motion obeying Kepler's laws, increasing by 360 degrees each orbit. The polar angle of an orbiting object around the center of attraction: the "true anomaly": also increases by 360 degrees each orbit. However, while the true anomaly changes unevenly: faster during closest approach: the mean anomaly increases steadily, in proportion to time. The mean anomaly is one of 6 orbital elements defining Keplerian motion.
• MERIDIAN: A great circle on the Earth or the celestial sphere passing through the north and south poles. On the Earth it is a line of constant longitude, on the celestial sphere a line of constant hour angle or right ascension. The meridian passing through the zenith for a particular observer is called the prime meridian and from it is measured the hour angle of an object.
• MESOSPHERE: A layer in the Earth's atmosphere from about 50 to 90km in height.
• MESSIER CATALOGUE: A catalogue of just over one hundred fuzzy objects to avoid (if you are a comet hunter) compiled by Charles Messier in 1784.
• METEOR SHOWER: A series of meteors lasting from a few hours to several days which have parallel paths through space. Perspective means that the meteor tracks appear to diverge from a point in the sky called the radiant. The position of the radiant is often used to give the shower a name: thus the Leonids have their radiant in Leo. The particles producing the meteors are thought to be debris from a comet the Leonids for example originate from comet Temple-Tuttle.
• METEOR, METEORITE and METEOROID: See "What are Bolides, Meteoroids, Meteors, Meteorites, Asteroids & Comets"
• METONIC CYCLE: Period of 19 years when the Moon's phases repeat themselves on the same days of the month.
• MILKY WAY: The faint irregular glowing band which circles the sky. It is a small part of our own galaxy and comprises tens of millions of stars, each too faint to be seen with the naked eye individually, but clearly seen in aggregate. It gives its name to our galaxy.
• MIRA VARIABLE: Long period variable stars.
• MULTIPLET: A closely spaced group of spectral "lines" (wavelengths), suggesting that a single energy level was split into several closely-lying ones, by some additional factor such as electron spin.
• MUON: A sub-atomic particle which is similar to the electron but with a mass 207 times greater
  
  

N

• N TYPE GALAXY: An active galaxy somewhere between a quasar and a Seyfert galaxy in its properties. The nucleus is very small and bright, and sometimes variable in its intensity. The remainder of the galaxy is very faint.
• N-TYPE STAR: A star with a similar temperature to the M-type stars, but with very strong features in its spectrum due to carbon-based molecules such as 02, OH and ON. Also known as a carbon star.
• NADIR: The direction directly underneath the observer, the opposite of the zenith.
• NASMYTH FOCUS: One of two focal points available for telescopes mounted on alt-azimuth mountings which are fixed as the telescope moves in altitude. The light is reflected down the hollow altitude axis to emerge at the side of the telescope.
• NEBULA: See "What are Nebulæ"
• NEUTRINO: A sub-atomic particle which is produced in huge numbers during supernovae and is one of the products of nuoleosynthesis The neutrino has a rest mass of zero or very close to it and so moves at or near the speed of light. Neutrinos interact very weakly with ordinary matter and so can escape directly from the centre of the Sun.
• NEUTRON STAR: See "What are Neutron Stars"
• NEUTRON: One of the constituents of atomic nuclei. It is a subatomic particle with zero electric charge and a mass of 1.67 x 10-27kg.
• NEW GENERAL CATALOGUE (NGC): A catalogue containing some 7840 nebulae, star clusters and galaxies. The catalogue number is frequently used as the name for an object.
• NEWTONIAN TELESCOPE: A design for a reflecting telescope invented by Sir Isaac Newton in 1668 which uses a parabolic mirror as the telescope's objective, and a secondary flat mirror, set at 450 to the optical axis and placed just before the focus of the primary mirror, to reflect the light out through the side of the instrument.
• NODAL MONTH: The nodical month, 27.212 days in duration, is the average interval between successive northward passages of the Moon across the ecliptic, points known as nodes. Since eclipses can occur only when the Sun and Moon are near such nodes, this period is also known as a draconic month, after the Chinese mythical dragon that supposedly ate the Sun to cause a solar eclipse.
• NODAL: successive northward passages of the Moon across the ecliptic, points known as nodes.
• NODE: The points in space where the orbit of a Solar System object intersects the plane of the Earth's orbit (the ecliptic). Solar and lunar eclipses can only occur when the Moon is at or close to one of the nodes of its orbit around the Earth.
• NOISE: Variations in any form of signal which are not due to the originating object of that signal.
• NONTHERMAL RADIATION: Radiation originating through processes other than the heat of the source.
• NOVA: See "What are Novæ, Supernovæ & Hypernovaæ"
• NUCLEON: One of the primary subatomic particles making up an atomic nucleus. The two types of nucleon are protons and neutrons.
• NUCLEOSYNTHESIS: The processes whereby elements heavier than hydrogen are built up from hydrogen. Most nucleosynthesis occurs inside stars, but some occurred during the early stages of the big bang, converting about 24 percent of the hydrogen to helium and some occurs during supernova explosions producing the elements heavier than Iron.
• NUTATION CYCLE: an extra deviation from the wobble of the earth axis around the pole star.
• NUTATION: A slight wobbling motion of the earth's axis. The causes of nutation are similar to those of the precession of the equinoxes, involving the varying attraction of the moon on the earth's equatorial bulge. However, the period of the motion is only 18.6 years, the same as that of the precession of the moon's nodes, as opposed to the nearly 26,000-year period of the precession of the equinoxes.
  
  

O

• O-TYPE STAR: A star with a surface temperature of 30000K or more.
• OB ASSOCIATION: A group of hot stars (spectral types 0 and B) in a region from a few to a few hundred parsecs across. The stars are generally not gravitationally bound together and so the associations are dispersing. They are the remnant of recent star formation in a large H II region.
• OBJECTIVE PRISM: A large thin prism placed before the objective of a telescope. Each star or other object is then seen as a short spectrum at the focus.
• OBJECTIVE: The main light gathering optical component(s) of a telescope.
• OBLATENESS: the property possessed by a rounded shape that is flattened at the poles; "the oblateness of the planet".
• OBLIQUITY: A deviation from a vertical or horizontal line, plane, position, or direction.
• OBSERVATORY: Any type of permanent or semi-permanent shelter for a telescope, or a group of telescopes.
• OCCULTATION: When an angularly large celestial object passes in front of an angularly small object.
• OLBER'S PARADOX: This asks why, if the universe contains an infinite number of stars distributed approximately evenly, is the night sky not lit up by starlight coming from all directions. Astronomers have grappled with this issue for some time. The two chief explanations seem to be (a) that light from galaxies is limited by their finite life and the finite speed of light and (b) that the expansion of the universe shifted much of the light coming toward us out to longer wavelengths. Paul S. Wesson of the University of Waterloo in Ontario (519-885-1211) has performed new studies which favor the finite-age argument. Basically the universe is not old enough for light from certain distant stars to have reached us yet.
• OORT CLOUD: is an immense spherical cloud surrounding the planetary system and extending approximately 3 light years, about 30 trillion kilometers from the Sun.
• OPACITY: The ability of a medium to absorb radiation.
• OPEN CLUSTER: An alternative name for a galactic cluster.
• OPPOSITION: A straight line alignment of the Sun, Earth and an outer planet. The planet is then usually at or near its closest approach to the Earth, and so best placed for observing.
• OPTICAL PAIR: A double star in which there is no physical connection between the stars.
• ORBIT: The path of a body in space, generally under the influence of gravity.
• ORBITAL ELEMENTS: Variables which characterize the motion of an orbiting body. For a planet or satellite in an elliptic orbit, 6 orbital elements exist: the semi-major axis gives its size, eccentricity its shape and mean anomaly its position along the orbit, at the given time. The three other elements are three angles which give the orientation in space of its orbital plane, e.g. that plane's inclination (to the plane of the Earth's equator or the ecliptic,depending on choice of coordinates).
• ORBITAL INCLINATION: The angle between the orbital plane of a satellite or planet and some reference plane, usually linked to the center of attraction (e.g. Earth equatorial plane, or plane of the ecliptic). The angle between two planes is defined as the angle between their perpendiculars, at any point of their intersection. Orbital inclination is one of 6 orbital elements.
• ORBITAL MOTION ANOMALY: One of the angles which gauges the motion of a planet or satellite around its orbit, increasing by 360^o every revolution. The true anomaly f equals the polar angle f in polar coordinates with origin at the center of the motion (e.g. Sun or Earth). The mean anomaly is a related angle which increases in direct proportion to the time elapsed (the true anomaly does not: the motion is faster near the center). The eccentric anomaly is an auxiliary angle used in relating true anomaly (which is observed) and mean anomaly (which is calculated).
• ORBITAL PERIOD: The length of time required for a body to complete one full (closed) orbit.
• ORRERY: Originally a clockwork mechanical model of the Solar System showing the planets moving around the Sun and sometimes the satellites around their parent planets.
• OZONE LAYER: A layer of the Earth's atmosphere at a height of about 20 to 50km containing small amounts of ozone (a molecule composed of three oxygen atoms). The ozone absorbs the solar ultraviolet radiation at wavelengths shorter than 330nm protecting life from its harmful effects.
  
  

P

• P CYGNI STAR: A hot variable star with peculiar emission and absorption lines in its spectrum.
• PARALLAX: Half the angle which a star appears to move as the earth moves from one side of the sun to the other.
• PENROSE PROCESS: A mechanism for extracting rotational energy from a black hole.
• PERHELION: That distance where a planet is at its nearest distance to the sun.
• PERIGEE: That distance where a satellite is at its nearest distance to the earth.
• PERIOD-LUMINOSITY RELATION: The relationship between the mean luminosity of a Cepheid variable star and the period of its changes.
• PERTURBATION: Departure of a celestial body from the trajectory it would follow if moving only under the action of a single central force. Perturbations may be caused by either gravitational or nongravitational forces.
• PHA: This stands for Potentially Hazardous Asteroid, which is a space rock larger than approximately 100m, that can come closer to Earth than 0.05 AU. None of the known PHAs is on a collision course with our planet. As at 19 May 2007, 863 are known; although astronomers are finding new ones all the time.
• PHASE: Most widely used in astronomy in connection with the fraction of a disc of the Moon or a planet which is illuminated by the Sun. Phase is also used in connection with lunar and solar eclipses, where it is the fraction or percentage of the disc that is in umbral shadow (lunar eclipse) or that is obscured (solar eclipse).
• PHOTOMETRY: The science, craft and practice of measuring the intensity of radiation from celestial objects.
• PHOTOMULTIPLER: A widely used detector working in the optical region.
• PHOTON: Light has a dual nature, sometimes behaving like a wave, at other times behaving like a particle. The photon, also known as the quantum, is the 'particle' of light.
• PHOTOSPHERE: The layer of the Sun from which all visible light reaches us. The Sun is too hot to have a solid surface and the photosphere consists of a plasma at about 5500 degrees centigrade.
• PIXEL: A term derived from 'picture element'. It refers to images obtained by array type detectors, such as CCD's and to the detectors and is one element of the image or detector.
• PLAGE: A brightening of the solar chromosphere usually to be found near a sunspot region.
• PLANE OF THE ECLIPTIC: (also called "the ecliptic" for short) The orbital plane of the Earth around the Sun. The line of the ecliptic on the celestial sphere is formed by the intersection of the plane of the ecliptic with that sphere. The reason the major planets and Moon appear in the sky close to the ecliptic is that the solar system is flat, and its orbital planes are very close to each other. We observe their motion (very nearly) edge-on.
• PLANETARY NEBULAE: An emission nebula which results from the loss of surface material from a star near the end of its life. The ultraviolet radiation from thematerial, and recombination of the ions and electrons results in the visible light emission by which The nebulae are seen.
• PLANISPHERE: A device for showing the constellations to be seen at a given time of night and time of year.
• PLASMA: A gas in which the atoms have been completely ionised so that it is composed of bare atomic nuclei and free electrons only.
• POLARISATION: The property of a beam of light (or any other electro-magnetic radiation) whereby the direction of vibration of the waves is not random.
• POPULATIONS I AND II: A division of the stars on basis of age. Population I stars are younger and generally hotter and bluer than population II stars. Population I stars predominate in the spiral arms of galaxies, while population II stars are found in the nuclei of spiral galaxies and in elliptical galaxies
• POSITION ANGLE: The angle on the sky between two objects, such as the components of a double star. It is measured north - east and south - west, from 0 to 360º.
• POSITRON: The anti-particle of the electron. It has a positive charge equal in magnitude to the negative charge of an electron.
• PRECESSION CYCLE: The earth is wobbling about it's axis of rotation like a spinning top To make one complete cycle takes about 23,000 years.
• PRECESSION: In general, the term precession is used for the slow, periodic conical motion of the rotation axis of a spinning body. It is the uniformly progressing motion of the pole rotation of a free rotating body undergoing torque from external gravitational forces. In the case of Earth, it is the slow gyration of the Earth's axis.
• PRESSURE BROADENING: An increase in the widths of spectrum lines as the pressure in the gas from which they originate increases.
• PRIMARY: The body at the focus of any orbital system. (Usually in reference to a star and its planets, but also appropriate to a planet and its moons)
• PRIME MERIDIAN: The meridian passing through the north and south points on the horizon and the zenith. It divides the sky into the eastern and western halves, and is reference point from where hour angle is measured.
• PRIMORDIAL MICROWAVES: A diffuse microwave radiation apparently filling the entire universe, a remnant of gamma rays emitted by the "primordial fireball" in the early universe, following the "Big Bang." As the universe expanded, the frequency dropped and is now in the microwave range, but it still follows a black-body spectrum.
• PROGRADE MOTION: The 'normal' direction of motion of Solar System objects. In space this is anti-clockwise around the Sun as seen from above the north pole of the Sun. In the sky it is from west to east.
• PROMINENCE: A cloud of cooler plasma extending high above the Sun"s visible surface, rising above the photosphere into the corona.
• PROPER MOTION: The movement of a celestial object across the sky due to its actual motion through space. For stars proper motions range downwards from a maximum of a few seconds of arc per year.
• PROTON-PROTON CHAIN: The main series of nucleosynthesis reactions whereby the Sun and other low mass stars generate their energy.
• PROTON: One of the constituents of atomic nuclei. It is a subatomic particle with unit positive electric charge and a mass of 1.67 x 10-27kg.
• PROTOSTAR: A star in the process of being born from an interstellar gas cloud.
• PULSAR: See What is a Pulsar?
  
  

Q

• QUADRATURE: The position of an outer planet when the Planet -Sun - Earth angle is 90°
• QUANTUM EFFICIENCY: The ratio between the number of photons picked up by a radiation detector to the number arriving at that detector.
• QUANTUM: Appertaining to the behaviour of sub-atomic particles as described by quantum theory. Also used as an alternative name for the photon.
• QUARK STAR: See What is a Quark Star?
• QUASAR: See What is a Quasar?
  
  

R

• R TYPE STARS: These stars are similar to those of spectral types G and K, but with an apparent over abundance of carbon. Their spectra therefore contain intense bands due to carbon-rich molecules such as C2, CH and CN.
• R-PROCESS: A set of reactions in nucleosynthesis where neutrons are added to nuclei more rapidly than those nuclei can undergo radioactive decay. The process is thought to occur during supernova explosions and to produce many of the heavier elements.
• RADIAL VELOCITY: Normally the component of the velocity of a celestial object along the line of sight from the Earth. It is positive when the object is moving away from the Earth, and negative when it is moving towards us. The term is also used for the velocity of material towards or away from some other object, such as the surface layers of an oscillating star like a Cepheid or the expanding nebula around a nova or supernova.
• RADIAN: A unit for measuring angles. A complete circle (360º) has 2o radians (6.283r), so one radian is about 57.296º.
• RADIANT: The point In the sky from which meteors in a meteor shower appear to diverge. It is the direction in space of the relative velocity of the meteors with respect to the Earth.
• RADIATION PRESSURE: The pressure exerted by light or other forms of electromagnetic radiation.
• RADIO GALAXY: A Galaxy emitting much more than the normal amount of radio energy. The optically visible galaxies are often giant elliptical galaxies with the radio emission coming from pairs of regions on either side and well outside the visible part of the galaxy. The radio emission can reach a million times that of a normal galaxy. They am classed as active galaxies and their peculiarities may be due to super-massive central black holes as with Seyfert galaxies and quasars.
• RADIO TELESCOPE: A telescope designed for receiving long wave radiation. Many radio telescopes operate on similar principles to optical telescopes and use a parabolic mirror to focus the radio waves. The minors of such radio telescopes however have to be huge: up to 300m in diameter, in order to gather sufficient energy and to resolve close sources. Greater resolution and sometimes sensitivity is obtained by using two or more such basic radio telescopes in an interferometer. Large versions of such systems can provide the resolution equivalent to a telescope thousands of kilometres across, though not the sensitivity of such an instrument, via Very Long Baseline Interferometry (VLBI).
• RAYLEIGH LIMIT: The measure conventionally used for the angular resolution of a telescope or other instrument.Recombination The recapture of an electron by an ionised atom.
• RED GIANT: A cool star of large physical size. They are stars in the late stages of their lives, having consumed the hydrogen in their cores and evolved off the main sequence.
• REDSHIFT OF THE GALAXIES: The general shift of lines in the spectra of galaxies towards longer wavelengths. The shift is greater the further away the galaxy is from us, and it is generally taken to be a Doppler shift due to the motion of the galaxy. The redshift, or rather the underlying velocities, are remnants of the explosive origin of the universe in the big bang.
• REFLECTING TELESCOPE: A telescope which uses a mirror as its objective. The main designs currently in use are the Cassegrain, its variant the Ritchey - Chrétien, and the Newtonian telescopes.
• REFLECTION NEBULA: An interstellar nebula whose presence is revealed by reflected (scattered) light from one or more nearby stars.
• REFRACTING TELESCOPE: A telescope which uses a lens as its objective. The lens is usually achromatically corrected.REGOLITH The layer of soil on the surface of an airless planet or satellite. Mostly composed of rock and meteorite fragments.
• RESOLUTION: The ability of an instrument to separate two close features.
• REST MASS: The mass of an object when it is at rest with respect to the observer. Special relativity tells us that the mass of an object increases as its velocity increases with respect to the observer, becoming infinite as the object reaches the speed of light.
• RETROGRADE MOTION: Temporary reversal of the apparent motion of a planet along the ecliptic. Caused because (by Kepler's 3rd law) a planet moves faster the closer it is to the Sun, so that (for instance) Jupiter appears to move backward when the faster-moving Earth overtakes it.
• RIGHT ASCENSION AND DECLINATION: Two angles marking the position of a star on the celestial sphere. Imagine a line from the observer to the star, and draw its projection (like a shadow) onto the celestial equator. Declination d is the angle between the line and its projection (d = 90^o - q, where q is the angle to the direction to the celestial pole); it is negative south of the equator. RA is the angle between the projection and the direction to the vernal equinox or first point in Aries:
• RITCHEY - CHRÉTIEN TELESCOPE: A variation on the Cassegrain telescope which has improved images over a relatively wide field of view. The parabolic primary mirror of the Cassegrain design is deepened to an hyperbola in the Ritchey - Chrétien, and the secondary mirror is a stronger hyperbola than that of the equivalent Cassegrain. The design is now widely used for modern large telescopes.
• ROCHE LIMIT: See "What Is The Roche Limit"
• ROCHE LOBE: One of two volumes in the space around a pair of mutually orbiting bodies wherein the gravitational field of one of the bodies predominates. Within the Roche lobe another smaller object will be gravitationally bound to the body at the centre of the lobe. Outside the lobes, a small particle may swap between the bodies, or even be lost entirely to the system.
• RR LYRAE STARS: Variable stars similar to the cepheids. They are blue giants with periods of about ten to fifteen hours and they change in brightness by about one magnitude.
  
  

S

• S-TYPE STARS: A giant star with a surface temperature of about 3,500K. This is the same temperature range as for the M type stars, and S type stars are differentiated by the presence of zirconium band in their spectra in place of titanium oxide bands.
• SAGITTARIUS A: A complex radio source at the centre of the Milky Way Galaxy. At least a part of the energy is thought to originate from interactions in an accretion disc around a black hole with a mass a few million times that of the Sun. SAROS A period of about 18 years after which a sequence of similar solar or lunar eclipses is repeated. Since the saros is not an exact number of days, the new set of eclipses occurs about 1200 west of the preceding set.
• SATURN'S RINGS: The spectacular aggregation of countless billions of small rocky and icy particles that surround and orbit Saturn in its equatorial plane. Three main rings can be seen even in small telescopes from Earth. Despite their enormous width, the rings are very thin, perhaps less than a kilometre thick.
• SCATTERING: The interaction of radiation with matter in which the photon's direction is changed. but its energy (or wavelength or frequency) remains the same as before the interaction, or is changed by only a very small proportion. The blue light from the daytime sky is due to sunlight scattered in the Earth's atmosphere. The colour occurs not because the white light from the Sun is changed in wavelength, but because the scattering process involved here (known as Rayleigh scattering) is much more effective at the shorter wavelengths. Red light from the Sun is thus scattered to a much lesser extent than the blue light.
• SCHMIDT CAMERA: An astronomical camera with a relatively wide field of view, designed by Bernhard Schmidt in 1930.
• SCHMIDT-CASSEGRAIN TELESCOPE: The Schmidt camera cannot be used visually. An adaptation of the design however can be used to look through and now forms one of the most popular designs for small telescopes. The Schmidt Cassegrain telescope is similar to the Cassegrain telescope in having a pierced primary mirror and secondary mirror and in having the light beam coming to a focus at the back of the telescope through the hole in the primary- It differs in using spherical mirrors and through the addition of a complex thin correcting lens placed close to the secondary mirror.
• SCHWARTSCHILD BLACK HOLE: A non-rotating, electrically neutral black hole. Since most real black holes are expected to be rotating, a pure Schwarzschild black hole is unlikely to be found, Schwarrschild black holes are relatively easy to deal with mathematically, and so are still studied theoretically.
• SCINTILLATION: The twinkling of stars caused by inhomogeneities in the Earth's atmosphere.
• SECONDARY COSMIC RAYS: High energy photons and sub atomic particles produced 30 to 60km up in the Earth's atmosphere by the impact of a primary cosmic ray particle. The main particles in secondary cosmic rays am nucleons, and pions which decay to produce gamma rays, muons and electrons.
• SEEING: A component of scintillation which arises from low altitude turbulence in the Earth's atmosphere. The effects can often be reduced by careful design of the observatory building, by equating the observatory and telescope temperatures to the ambient temperature and by planting low growing shrubs around the observatory.
• SEMIMAJOR: Half the distance across the widest part of an ellipse.
• SERN-REGULAR VARIABLE: A variable star whose changes are more-or-less repetitive but where the intervals between the changes can vary irregularly. The variables are usually medium to cool giants or supergiants, and their periods can average from 20 days to five or more years. The brightness changes can be by up to four magnitudes, and they arise from pulsations of the star.
• SEYFERT GALAXIES: A class of spiral and barred spiral galaxies with small but very bight nuclei whose spectra show emission lines. The Seyfert galaxies are subdivided into two types depending on their emission lines. The phenomena in Seyfert galaxies are widely thought to be due to interactions in an accretion disc surrounding a massive black hole at the centre of the galaxy. It is possible that Seyfert galaxies are less energetic versions of quasars.
• SHELL STAR: A star which is surrounded by an extensive shell of gas. The majority of shell stars are of spectral class B, and may be at the stage of just evolving away from the main sequence.
• SIDEREAL DAY: The true period of rotation of the Earth, about 4 minutes shorter than 24 hours. The mean solar day is the average time from noon to noon, between two southward passages of the Sun in the sky, but during that time the Sun's position relative to the stars changes, too. If we were to measure a day between two southward passages of some star ("sidus"=star), we would get a sidereal day.
• SIDEREAL MONTH: The average period of revolution of the moon around the earth in reference to a fixed star, equal to 27 days, 7 hours, 43 minutes in units of mean solar time.
• SIDEREAL TIME: time measured relative to the fixed stars; thus, the sidereal day is the period during which the earth completes one rotation on its axis so that some chosen star appears twice on the observer's celestial meridian.
• SIGNAL TO NOISE RATIO: The ratio of the intensity of the desired signal to the intensity of the background noise. A minimum S/N ratio pf one is normally needed in order to have detected the object being measured, but S/N ratios of five or better are needed for reliable measurements.
• SINGULARITY: A point in space, such as at the centre of a black hole, where the density of matter is theoretically infinite according to the current laws of physics.
• SODIUM SPECTRUM: The sodium atom is somewhat similar to that of hydrogen. Its spectrum has similarity to that of hydrogen, but with a more general scheme of energy levels.
• SOLAR ACTIVITY: A general term for those processes and changes on the Sun that rise and fall with the sunspot cycle, e.g. flares.
• SOLAR APEX: The direction in space towards which the Sun, and the rest of the Solar System, is moving. It is in the constellation of Hercules, about 12 southwest of Vega (a Lyr).
• SOLAR CONSTANT: The amount of energy per square metre received from the Sun at the top of the Earth's atmosphere. Its value is about 1.37kw m.
• SOLAR CYCLE: (or sunspot cycle): an irregular cycle, averaging about 11 years in length, during which the number of sunspots (and of their associated outbursts) rises and then drops again. Like the sunspots, the cycle is probably magnetic in nature, and the polar magnetic field of the Sun also reverses each solar cycle.
• SOLAR ENERGETIC PARTICLES: high energy particles occasionally emitted from active areas on the Sun, associated with solar flares and coronal mass ejections. The Earth's magnetic field keeps them out of regions close to Earth (except for the polar caps) but they can pose a hazard to space travelers far from Earth.
• SOLAR FLARE: a rapid outburst on the Sun, usually in the vicinity of active sunspots. A sudden brightening (only rarely seen without special filters, isolating the red light of hydrogen) may be followed by the signatures of particle acceleration to high energies: x-rays, radio noise and often, a bit later, the arrival of high-energy ions from the Sun. Flares appear to be associated with rapid energy releases high above the photosphere, apparently from the magnetic fields of sunspots. Their link to coronal mass ejections, which may also be powered by magnetic energy, is still unclear.
• SOLAR NEUTRINO PROBLEM: The difference, by a factor of three, between the observed intensity of neutrinos from the Sun and their theoretically predicted intensity.
• SOLAR WIND: A fast outflow of hot gas in all directions from the upper atmosphere of the Sun ("solar corona"), which is too hot to allow the Sun's gravity to hold on to its gas. Its composition matches that of the Sun's atmosphere (mostly hydrogen) and its typical velocity is 400 km/sec, covering the distance from Sun to Earth in 4-5 days. The solar wind confines the Earth's magnetic field inside a cavity known as the magnetosphere and supplies energy to phenomena in the magnetosphere such as polar aurora ("northern lights") and magnetic storms.
• SOLSTICE: The points on the ecliptic with the most northerly and southerly declinations, also the times of year when the Sun is at those points. The summer solstice, when the Sun is highest in the sky for northern observers, occurs on or about June 21 and the winter solstice on or about December 22.
• SPACE-TIME: The combination of the three normal dimensions of physical space with time as a fourth dimension that is used to describe the properties of the Universe in the special and general relativity theories.
• SPECIAL RELATIVITY: The theory which describes the laws of physics applying to observers and systems that are in relative motion with respect to each other at constant velocities
• SPECTRAL CLASSIFICATION: A classification of stars based upon the appearance of their spectra. The spectrum lines present in a spectrum depend upon the surface temperature of the star, so spectral classification is also a temperature classification of the stars. The classes are labelled with upper case letters, in the order: 0, B, A, F, O,K, M.
• SPECTRAL INDEX: For astronomical radio sources the intensity often varies with frequency in an exponential fashion. The power of the frequency, 0, is the spectral index. It takes values around +1 for thermal radio sources and around -1 for synchrotron and other non-thermal origins for the radiation.
• SPECTRAL LINE: A narrow range of spectral color, emitted (or absorbed) by a specific atom (or molecule).The energy of its photon corresponds to the difference between two energy levels of the atom, and such photons are emitted when the atom "falls" from the higher level to the lower one.
• SPECTROHELIOGRAM: A narrow band image of the Sun obtained using a spectrohelioscope. The spectrohelioscope is a spectroscope in which a second slit is used at its focus to isolate a small part of the spectrum, usually centred on the hydrogen Ha line or the calcium H or K lines. Spectroheliograms show a layer of the Sun in the chromosphere some 2,000 to 4,000km above the photosphere. Features such as flares, filaments, prominences and plages. which are very difficult or impossible to observe in white light, are clearly revealed on spectroheliograms.
• SPECTROPHOTOMETRY: The study of brightness variations over a spectrum.
• SPECTROSCOPE: A device for splitting light up into its component wavelengths (colours). The terms spectrograph and spectrometer may also be encountered.
• SPECTRUM LINES: The dark, or occasionally bright, very narrow features to be seen in a spectrum - They arise from absorption or emission of photons by atoms, ions and molecules in the object.
• SPECTRUM: The electromagnetic spectrum is the full range of electromagnetic radiation. This comprises, going from the longest observed wavelengths to the shortest: radio waves, microwaves, infrared, visible, ultraviolet, X-rays and y rays.
• SPHERICAL ABERRATION: A fault in the image produced by an optical instrument in which the light rays towards the edge of a light beam passing through the instrument come to a focus at a different point from those near the centre of the beam.
• SPICULE: A needle-like feature of the solar chromosphere projecting upwards into the corona. They are typically 1,000km wide and 10,000km long, and last for 10 to 20 minutes They may be seen projected against the solar disc on spectroheliograms where they have a netlike distribution, which probably arises from the pattern of convection some distance below the photosphere.
• SPIRAL GALAXIES: A galaxy with a prominent spiral shape - In ordinary spirals two or three spiral arms emerge directly from the nucleus, in barred spirals there is a linear extension to the nucleus (the bar) and the arms extend out from the ends of the bar.
• STEADY STATE THEORY: A theory of cosmology due to Herman Rondi, Tommy Cold and Fred Hoyle which they proposed in 1948. The theory was unable to give credible explanations for modern discoveries such as the microwave background radiation, and so has now generally been superseded by the big bang theories.
• STELLAR EVOLUTION: (stellar=of a star). The different phases in the lifetime of a star, from its formation out of gas and dust, to the time after its nuclear fuel is exhausted. Based on observations of stars at various stages of their evolution, astronomers have developed a general theory of stellar evolution, by which the Sun is a typical "main sequance" star, in the middle of its evolutionary lifespan. See also "Stellar Classes" Page
• SUB GIANT: A star found between the main sequence and the giant region of the Hertzsprung-Russell diagram. They are evolving towards becoming giants.
• SUB-DWARF: A star that has about 20 to 40 percent of the luminosity of a main sequence (dwarf) star of the same temperature. They are old stars (population II), with low abundances of the heavier elements and the latter is the cause of their low luminosities.
• SUNSPOT: See What is a Sunspot?
• SUPERNOVA REMNANT: The nebulous remains of a supernova explosion, abbreviated as SNR. The nebula originates as the outer layers of the star and is blasted out into space during the explosion.
• SUPERNOVA: See "Novæ, Supernovæ & Hypernovaæ"
• SYNCHROTRON RADIATION: This occurs when electrons move in large spirals around magnetic field lines of force. In the cosmos particles such as electrons can be accelerated to high energies, near the speed of light, by electric and magnetic fields. These high-energy particles can produce synchrotron photons with wavelengths ranging from radio up through X-ray and gamma-ray energies. Synchrotron radiation from cosmic sources has a distinctive spectrum, or distribution of photons with energy. The radiation falls off with energy less rapidly than does the spectrum of radiation from a hot gas. When synchrotron radiation is observed in supernova remnants, cosmic jets, or other sources, it reveals information about the high-energy electrons and magnetic fields that are present.
• SYNODIC MONTH: The average time between successive new or full moons, equal to 29 days, 12 hours, 44 minutes. Also called lunar month.
• SYNODIC PERIOD: The time it takes for a planet until it returns to the same position relative to the Earth.
• SYNODIC: Relating to the conjunction of celestial bodies, especially the interval between two successive conjunctions of a planet or the moon with the sun.
  
  

T

• T TAURI STARS: Cool, young, irregularly variable stars, often associated with gaseous nebulae such as the Orion nebula (M42). The stars have formed relatively recently from within the nebulae and are evolving towards the zero age main sequence.
• TEKTITE: A glassy pebble which often shows signs of atmospheric ablation and of having been molten whilst travelling through space or the Earth's atmosphere. They are almost certainly debris from meteorite impacts with the Earth.
• TELESCOPE MOUNTING: Any device to hold, point and move a telescope. For terrestrial telescopes most mountings are either equatorial or alt-azimuth. For many decades the equatorial mount was the more popular as it makes it simpler to "track" a celestial object. However with modern day computer control the alt-azimuth mount is coming into its own as it is considerably more stable and therefor much less likely to be subject to vibation errors.
• TELESCOPE: Any device which gathers radiation and improves angular resolution.
• TERMINATOR: The line dividing The light and dark halves of a planet or satelife. The term is per)culark used with reference to the Earth's Moon.
• TERRESTRIAL PLANET: A small rocky planet like the Earth. The terrestrial planets are Mercury, Venus, Earth and Mars.
• THERMAL ENERGY: The energy of a substance due to the random thermal motions of its constituent atoms, ions or molecules.
• THERMAL RADIATION: Electromagnetic radiation originating from any substance whose temperature is above absolute zero, by reason of its temperature. The spectrum of thermal radiation is often very close to that of black body radiation.
• TIDE: An effect arising from the gravitational differences across an object caused by a second object.
• TRANSFER ORBIT: An orbit which enables a spacecraft to move (transfer) from one object to another. The lowest energy transfer orbit, called a Hdhmann transfer orbit, is an ellipse which is tangential to the orbits of the two objects between which the spacecraft is travelling.
• TRANSIT: The passage of Venus in front of the Sun's disk. This rare astronomical event (transits occur in pairs, more than a century apart) was proposed by Edmond Halley as the basis of a method of measuring the astronomical unit.
• TRIPLE a PROCESS: The nucleosynthesis reaction in which three helium nuclei (a particles) are converted to a single carbon nucleus. The reaction powers the later stages of stars of the Sun's mass or greater. It requires temperatures in excess of 100,000,000K before it becomes a significant source of energy.
• TROJAN POINTS: See also Lagrangian points. Two points in the orbit of Jupiter (and theoretically other planets as well) where small objects such as asteroids have stable positions. The points are 60° ahead and behind Jupiter.
• TROPOSPHERE: The lowest layer in the atmospheres of the Earth and other planets. For the Earth it extends to a height of about 15km.
• TRUE ANOMALY: The polar angle of an object in a Kepler orbit, measured from in the orbital plane from the position at closest approach. See mean anomaly, also orbital elements.
  
  

U

• UBV SYSTEM: A widely used system for defining the wavelengths at which star's magnitudes are measured. The letters stand for ultraviolet, blue and visual.
• ULTRAVIOLET: The part of the electro-magnetic radiation spectrum with wavelengths just shorter than those in the visual range. It extends from about 380nm to 100nm..
• UMBRA: The central and darkest part of a shadow. Within the umbra the light source is totally obscured. Outside the umbra is the penumbra wherein the light source is partially visible. During solar eclipses, the eclipse can only be seen as total from within the umbral part of the Moon's shadow. During lunar eclipses, the eclipse is normally only detectable to the eye when the Earth's umbra shadow is on the Moon. Also the central and darkest part of the sunspot is called the umbra, and the outer region, the penumbra.
• UNCERTAINTY PRINCIPLE: The principle, due to Werner Heisenberg, that there is a limit to the precision with which two related quantities can be measured simultaneously. It becomes important at atomic and smaller scales.
• UNIVERSAL TIME (UT): Time based upon the Earth's rotation and the basis for civil time keeping. It is essentially the same as Greenwich Mean Time.
• UNIVERSE: Everything there is, both known and unknown. As well as obvious components like matter and radiation, the Universe includes the fabric of space itself.URCA PROCESS A process whereby neutrinos are produced in large numbers within supernovae.
• URCA PROCESS: In the final rapid collapse of a supernova, the energy release causes nuclear reactions. Most of the energy then does not produce heating, but is instead drained away as neutrinos. Heat would have counteracted the star's collapse, but the "Urca process" by which the energy is drained away allows it to proceed very rapidly. It is therefore responsible for the supernova "explosion."

V

• VAN ALLEN RADIATION BELT: is a torus of energetic charged particles (plasma) around Earth, held in place by Earth's magnetic field. The Van Allen belts are closely related to the polar aurora where particles strike the upper atmosphere and fluoresce.
• VARIABLE STAR: A star, one or more of whose properties changes with time. The most widespread use of the term is for photometric variables, where the brightness of the star changes. This can arise from changes to the star itself external factors such as eclipsing binary stars (extrinsic variables). Changes can also occur within the spectrum and/or to the polarisation of the light from the star, resulting in spectroscopic and polarimetric variables etc.
• VERY LONG BASE LINE INTERFEROMETRY (VLBI): Interferometry, so far only at radio wavelengths, where the component aerials of the interferometer are separated by 1000's of kilometres. The large separation enables observations to be made at resolutions of 0.001" or better, but also means that the signals cannot be mixed directly as in a conventional interferometer. Instead the signals are recorded along with time signals from an atomic clock, and the recordings then combined afterwards.VIGNETTING Shadowing of the image plane due to components within the optical system.
• VISUAL MAGNITUDE: The magnitude of an object measured in the visual part of the spectrum. This may be estimated by eye, or as in the UBV system, filters used to define the waveband.
  
  

W

• WAVELENGTH: The linear distance between two crests or troughs in a set of waves. It is most usually applied to electromagnetic radiation and sound.
• WHITE DWARF: One of a group of stars found in the bottom left of the Hertzsprung-Russell diagram; This corresponds to high surface temperatures and low luminosities, implying very small sizes for the stars. White dwarfs represent the end points of the lives of solar type stars. There is a maximum mass for a star to remain as a white dwarf of about 1.4 times the mass of the Sun. This is known as the Chandrasekhar limit. If a white dwarf should exceed that mass, then it will collapse to a neutron star.
• WIDMANSTATTEN PATTERN: A regular pattern with in some types of iron meteorite arising from the intergrowth of crystals of nickel-iron which have slightly differing compositions.
• WILSON-BAPPU EFFECT: An effect whereby the absolute magnitude of the cooler stars correlates with the strength of the emission core of the ionised calcium at 393nm. The cause of the effect is not understood, but it can be used to determine the star's distance by comparing its absolute and apparent magnitudes.
• WOLF NUMBER: A parameter used to estimate the strength of sunspot activity.
• WOLF-RAYET STAR: A very hot, large star with apparent compositional peculiarities. The stars are losing mass very rapidly in the form of a high velocity stellar wind. They may be the cores of massive stars, revealed by the loss of their outer layers to the stellar wind and where their abundance peculiarities are just.
  
  

X

• X-RAY BINARY: A binary star that is a strong X-ray source. The binaries are thought to contain a neutron star or black hole as one component, and the X-rays to originate from material accreting onto the compact object from its companion.
• X-RAY BURSTER: A strong outburst of X-rays lasting a few seconds appearing to originate from an X-ray source at the edge of the known universe.
• X-RAY: Electromagnetic radiation with a wavelength between about 0.0lnm to 10nm.
  
  

Y

• YEAR: The period of the Earth's orbit around the Sun or of the Sun around the sky. The sidereal year is taken relative to the stars and is 365.2564 days. The tropical year is the interval between two successive passages of the Sun through the Vernal Equinox. Since the latter is changing its position slowly due to precession, the tropical year is 365.2422 days long. The tropical year is related to the seasons and so is the basis of the calendar. For convenience the calendar or Gregorian year is taken to be 365.2425 days long.
  
  

Z

• ZEEMAN EFFECT: The splitting of the frequency of light emitted by atoms, when the emission occurs in a strong magnetic field, into two or more closely spaced frequencies. The Zeeman effect of light from sunspots gave in 1908 the first clue that these regions were intensely magnetized.
• ZENITH: The point in the sky directly overhead (the opposite of nadir).
• ZERO AGE MAIN SEQUENCE (ZAMS): The line in the Hertzsprung-Russell diagram occupied by stars which have just completed their formation processes. It forms the bottom edge of the main sequence strip.
• ZODIAC: The constellations through which the Sun passes during its yearly movement.
• ZODIACAL LIGHT: A faint band of light concentrated along the ecliptic and therefore running through the zodiac. It is solar light scattered back towards the Earth by small dust particles lying in the plane of the planetary system.