Download a to z of astronomy

A TO Z OF ASTRONOMY
A
ABERRATION IN OPTICAL SYSTEMS.
The name for a number of faults that occur in telescopes including the colour defects of refractors
(chromatic aberration), spherical aberration, coma, distortion, astigmatism and field curvature.
ABSOLUTE MAGNITUDE.
A measure of the intrinsic brightness of objects. The apparent magnitude of the object [fit were at a
distance of ten parsecs. The absolute magnitude of Sirius is +1.4.
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 centers.
ALBEDO.
Another name for the reflectivity of a planet, satellite or asteroid, i.e. the ratio between the amount of light
that the object receives from the Sun, and the amount that is reflected directly back into space.
ALTITUDE.
The angular distance of an object above (or below) the horizon. It forms a positional coordinate system
for objects in the sky along with azimuth.
ANGSTROM (Å).
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widely used in astronomy, it is gradually being superseded by the standard SI unit of a nanometer (1Nm
= 10Å).
APERTURE.
The diameter of the objective of a telescope.
APPARENT MAGNITUDE.
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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.
ASHEN LIGHT.
The apparent faint illumination of the dark side of the Moon.
ASTRONOMICAL UNIT.
A unit of distance equal to the mean distance between the Earth and the Sun. 1AU = 1.496 x 1011m. Atype star A star with a temperature in the region 8000 to 10000K.
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.
A measure of the position of an object. It is the angular distance in degrees from the north direction
towards the east. Combined with the altitude it provides the instantaneous position of an object in the
sky.
B
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
The series of spectral lines due to hydrogen running from Ha in the red at 656nm, through Hb at 486nm
and 434nm, down to the series limit at 365nm. Similar series of lines occur for hydrogen in the ultraviolet
(the Lyman series), and the infrared (the Paschen, Pfund; and Brackett series).
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
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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
An imaginary object which absorbs with 100% efficiency at all wavelengths. It is quite well approximated
by a hole in box. The energy emitted by a black body has a characteristic shape (the black body
spectrum), which may be predicted theoretically and whose shape is given by Planck's equation. The
overall spectra of stars and many other astronomical objects are quite close in shape to those of black
bodies of various temperatures. The temperature of a black body which emits the same total energy as
an object is the effective temperature of that object.
BLACK HOLE
An object with so much mass compressed into such a small volume that the escape velocity equals or
exceeds the speed of light. The centres of many galaxies and quasars etc. are thought to contain black
holes with masses many millions of times that of the Sun. Smaller black holes may be formed when a
massive star ends its life
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.
B-TYPE STARS
A star with a temperature in the region 10,000 to 25,000K.
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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
A very bright supergiant variable star expanding and contracting because of lonisation and recombination
of helium in its atmosphere. The cepheids are subdivided into the Classical Cepheids, the W Virginis
stars and RR Lyrae stars.
CHANDMSEKHAR 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.
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CHROMOSPHERE
The layer in the Sun's surface immediately above the photosphere, and below the corona. It is about
5,000km thick, and its temperature ranges from 4,000K to 10,000K.
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
Aberration in optics and the head of a comet.
COMET
A minor body of the Solar System, usually in a highly elliptical orbit.
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.
CONSTELLATION
A group of stars that has, for convenience, been given a name.
CORONA
The outer atmosphere of the Sun. It extends from the top of the chromosphere outwards until it merges
with the interstellar medium some tens of astronomical units out from the Sun. The corona is a very
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rarefied plasma (mixture of ions and electrons) whose temperature can reach two or three million
degrees.
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
Material in the Universe which has so far not been directly observed (also called missing mass).
DECLINATION
One of the measures used to determine position in the sky. Together with right ascension it forms the
most widely used coordinate system in astronomy. Declination is the angular distance up or down from
the equator on the celestial sphere.
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 across 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
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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 Altazimuth mounting.
DOPPLER SHIFT
The change in wavelength of a wave motion arising from the motion of the emitting object and/or The
observer along the line of sight. The wavelength is increased when the relative motion is away, and
decreased when the relative motion is towards each other.
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 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
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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
The path of the Sun across the sky throughout the year. Since the Sun's apparent motion is
actually due to the Earth's orbital motion the ecliptic is also the plane of the Earth's orbit.
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
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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
to
rcury).
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.
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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).
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.
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F
FACULAE
Small regions of the solar photosphere that are a few hundred degrees hotter than average, and
which therefore appear as slightly brighter regions.
F CORONA
A component of radiation from the corona which is light from the solar photosphere scattered by
interplanetary dust.
FIELD STARS (galaxies etc.)
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.
silhouetted against the photosphere.
FINDER
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They are solar prominences seen
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
Sudden brightening of a region of the Sun's surface, almost invariably within or near complex
sunspot groups.
FLARE STAR
A star which suddenly brightens by about half a magnitude. The brightening is attributed to
flares on the surface of the star.
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.
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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.
F-TYPE STAR
A star with a surface temperature between about 6,000 and 7,500K.
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.
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G
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
A large group of stars, nebulae, etc. bound together by gravity.
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 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
A spherical collection of about 1,000,000 stars tightly bound together gravitationally, and orbiting as a
satellite of a galaxy.
GRANULATION
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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 LENS
Light passing near a massive object has its path bent by the local distortion of the space-time continuum.
The massive object therefore can act like a lens, and focus light from more distant objects behind it.
GRAVITATIONAL RADIATION
When an object which has mass is accelerated or otherwise disturbed, it is predicted to radiate
gravitational waves.
GREAT RED SPOT
When an object which has mass is accelerated or otherwise disturbed.It is predicted to radiate
gravitational waves.
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.
G TYPE STAR
A star with a surface temperature of 5000 to 6000K. The Sun is a Type G Star.
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
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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
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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.
HORSEHEAD NEBULA
A cool gaseous nebula in Orion about 350pc away, also known as NG02024
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.
HUBBLE CONSTANT
The constant which determines the relationship between the distance of a galaxy and its
cosmological recessional velocity. Its value is important because it determines the length of time
that has elapsed since the big bang, i.e. the age of the Universe.
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HUBBLE LAW
The linear relationship between the distance of a galaxy (D) and its cosmological recessional
velocity (V); V = HxD, where H is Hubble’s constant.
I
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.
INFARED
The part of the electromagnetic radiation spectrum with wavelengths somewhat longer than those
in the visible, the radiation we know as heat.
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
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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.
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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.
JUPITER
The fifth planet from the Sun in our Solar System. It is a gas giant and the largest of the Planets
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.
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.
KELVIN CONTRACTION
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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.
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.
KEPLER'S LAWS
The three laws of planetary motion and of any objects following Keplerian orbits.
1. Each planet moves in an ellipse with the Sun at one focus.
2. The line from the Sun to the planet sweeps out area at a constant rate.
3. The square of the orbital period is proportional to the cube of the semi-major axis of the orbit
KERR BLACK HOLE
A black hole that is rotating.
KIRKWOOD GAPS
Regions of the asteroid belt which are largely devoid of asteroids.
KRUSKAL DIAGRAM
A diagram that enables the properties of space and time to be correctly represented near a black
hole.
K-TYPE STAR
A star with a temperature in the region 3600K to 5000K.
L
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LAGRANGIAN POINTS Points near two objects which are in orbit around each other where a
much smaller object can remain in equilibrium.
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 phenomenon allowing about 59 percent of the surface of the Moon to be seen from the Earth.
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
The edge of an object as seen against the sky.
LIMB DARKENING
The reduction in surface brightness of the limbs of the Sun or other stars compared with the centres of
their visible discs.
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.
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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.
LUMINOSITY
The total amount of energy radiated by a star or any other object. For the Sun it is 4 x 1026 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
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).
MAGNETIC STARS
Stars with unusually intense magnetic fields. They are often Ap stars. The fields can reach strengths of
1T and are often highly variable. White dwarfs and neutron stars have much more intense magnetic
fields, sometimes exceeding lO8 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.
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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
A measure of brightness. Astronomers measure stars in units called magnitudes but this is not a unit like
a meter or a kilogram. Each magnitude is two and a half times brighter than the previous magnitude
which is in turn is two and a half times brighter than the previous magnitude to that. The larger the
magnitude number the dimmer the star will appear. Very bright stars have negative (minus) numbers.
There are two kinds of magnitude measurements used :APPARENT MAGNITUDE
This is how bright a star appears to be in our sky.
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.
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
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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.
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
The streak of light produced high in the Earth's atmosphere by the impact of a meteorite. Sporadic
meteors may be seen at a rate of about five to ten per hour from an average site. Meteor showers occur
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when the Earth passes through debris left by a disintegrating comet. Very large meteors are called
fireballs or bolides if they explode.
METEORITE
The fragment of a meteoroid which has survived passage through a planet's atmosphere to reach the
surface. Most meteorites are of rocky composition but about six percent are almost pure Nickel and Iron.
About two percent are formed from mixtures of rock and iron. Two subgroups are the carbonaceous
chondrites which contain some simple organic molecules and are thought to pre-date the formation of the
solar system and the SNO meteorites which may have come from Mars.
METEOROID
A small body independently orbiting the Sun. The meteoroids merge into the asteroids at the larger end
and into the inter-planetary dust at the smaller end of the scale.
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 TempleTuttle.
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.
M-TYPE STAR
A star with a temperature of about 3500K.
MUON
A sub-atomic particle which is similar to the electron but with a mass 207 times greater
N
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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
Clouds of rarefied gas in space such as emission nebulae, giant molecular clouds, HII regions, planetary
nebulae and supernova remnants.
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
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.
NEUTRON STAR
A star composed largely of neutrons. The neutrons form from the combination of protons and electrons
as the density of the material rises above 4 x 1014kg m-3. Such conditions can occur during the later
stages of a star's life when its internal pressure is no longer sufficient to support the weight of the star's
outer layers leading to a catastrophic collapse. all novae are recurrent on time scales of 10,000 years or
more.
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.
NODE
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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
A star which brightens by 12 to 15 magnitudes in a few days, fading back to its pre-outburst condition
over the following year or two. Novae occur close to binary stars where one component is a white dwarf
and the other is a star just evolving off the main sequence. Mass is exchanged between the two stars
and is accumulated on the surface of the white dwarf. Eventually the layer of material from the main
sequence star becomes hot enough to undergo a runaway nuclear fusion reaction which is seen as the
nova outburst. Some nova have been observed to explode two or more times at intervals of several
decades and are known as recurrent novae. It is likely that all novae are recurrent on time scales of
10,000 years or more.
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.
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
A small cyclic variation in precession arising from the gravitational effect of the Moon. Its amplitude varies
but is typically about 9" and its period is 18.6 years
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O
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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
The main light gathering optical component(s) of a telescope.
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.
OBLIQUITY
The angle between the plane of a planet's orbit and its equator. The obliquity of the Earth is currently
about 23.50.
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.
OORT CLOUD
The outermost part of the Solar System thought to contain large numbers of nuclei of comets.
OORT'S CONSTANTS
Two numbers which appear in the formulae which describe the rotational motions of the stars around the
Milky Way galaxy in the region of the Sun.
OPACITY
The ability of a medium to absorb radiation.
OPEN CLUSTER
An alternative name for a galactic cluster.
OPPOSITION
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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 an object moving in a gravitational field. If there are no perturbations, the shape of an orbit is
one of the conic sections, usually an ellipse.
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.
O-Type Star
A star with a surface temperature of 30000K or more.
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
PARALLAX
The angular displacement of an object when viewed from two different points in space. The parallax
motion of nearby stars caused by the Earth motion around the Sun is the basis for measuring the
distances of these stars.
P CYGNI STAR
A hot variable star with peculiar emission and absorption lines in its spectrum.
PENROSE PROCESS
A mechanism for extracting rotational energy from a black hole.
PERIOD-LUMINOSITY RELATION
The relationship between the mean luminosity of a Cepheid variable star and the period of its changes.
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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
One of the outer layers of the Sun or similar stars within which the bulk of the radiation from the star
originates. The solar photosphere is about 500km thick and ranges from a temperature of about 9,000K
at its base to 4,400K at the top, with the bulk of the radiation coming from a 100km thick region at a
temperature of about 6,000K.
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.
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
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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
The phenomenon whereby the rotational axis of the Earth itself rotates through space. The effect arises
from an imbalance between the gravitational forces from the Sun acting on the near and far sides of the
Earth. The axis rotates in a period of 25,700 years. The position of the north and south poles in the sky
therefore changes over the same time. Precession also causes the intersection of the equator and the
ecliptic known as the first point of Aries, to move completely around the ecliptic over the same period.
Since the positions of stars (right ascension and declination) are based upon the positions of the equator
and the first point of Aries, these also change with time by up to 50" per year. Star catalogues and
atlases are therefore only correct at a specific time, known as the epoch, and for accurate work, the
effects of precession have to be corrected if observing at a different date.
PRESSURE BROADENING
An increase in the widths of spectrum lines as the pressure in the gas from which they originate
increases.
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.
PROGRADE MOTION
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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 feature of the lower part of the solar corona which when observed in the red line of hydrogen at 653nm
(Ha), look like flames leaping up from the surface of the Sun. In fact they are cool (10,000ºK) dense
condensations in the bottom layer of the corona. They are the same phenomenon as solar filaments but
seen at the edge of the solar disc where they appear bright against the darker sky. Their sizes can range
from a few thousand kilometres to millions of kilometres, and their lifetimes from a few hours to a year or
more.
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
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.
PROTON-PROTON CHAIN
The main series of nucleosynthesis reactions whereby the Sun and other low mass stars generate their
energy.
PROTOSTAR
A star in the process of being born from an interstellar gas cloud.
PULSAR
A rapidly pulsating radio source. Pulsars are thought to be rotating neutron stars with the radio emission
being beamed out along their magnetic axes. The periods range from a millisecond to a few seconds and
are very highly stable. A few pulsars may be observed at visible and shorter wavelengths.
Q
QUADRATURE
The position of an outer planet when the Planet -Sun - Earth angle is 90°
QUANTUM
Appertaining to the behaviour of sub-atomic particles as described by quantum theory. Also used as an
alternative name for the photon.
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QUANTUM EFFICIENCY
The ratio between the number of photons picked up by a radiation detector to the number arriving at that
detector.
QUASAR
The most extreme form of active galaxy. The most widely accepted model for a quasar has a massive
(1000-solar mass) black hole surrounded by an accretion disc at its centre. Material from the accretion
disc spirals into the black hole releasing up to 40 percent of its rest mass energy in the process.
R
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
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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
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The 'unusual' direction of motion of Solar System objects. In space this is clockwise around the Sun as
seen from above the north pole of the Sun. In the sky it is from east to west.
RIGHT ASCENSION
One of the measures used to determine position in the sky. Together with declination it forms the most
widely used coordinate system in astronomy. Right ascension is the angular distance around the equator
to the meridian through the object, measured from the first point of Aries in an easterly direction.
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
The closest point which a satellite held together only by gravity can approach its primary without being
disrupted by the tidal effect of the primary. Its value is about 2.5 to 3 times the radius of the primary
depending upon the densities of the two objects. Smaller real satellites can approach closer to their
primaries than the Roche limit because of the tensile strength of the material from which they are formed.
Larger satellites (greater than a hundred kilometres or so), where the tensile strength is negligible, would
break up at about 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.
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.
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.
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.
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S
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.
SCHWANSCHILD 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.
SCLNWARZSCHILD RADIUS
The radius of the event horizon of a Schwarzschild black hole.
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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.
SEMI-MAJOR/SEMI-MINOR AXES
The longest and shortest radii of an ellipse. Usually used in respect of the orbits of planets and other
objects.
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 PERIOD
The period of something with respect to The stars (or to the rest of the Universe). Most often used for the
orbital motion and rotation of the planets and other Solar System objects, hence sidereal orbital periods
and sidereal rotation periods. The sidereal orbital period of the Earth is one year. The sidereal rotation
period of the Earth is about 23 hours 56 minutes. The Earth's orbital motion over a day means that an
extra four minutes is required for the Sun to return to the same position in the sky.
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SIDEREAL TIME
A measure of time based upon the motion of the stars and other fixed objects in the sky. not upon the
motion of the Sun. Sidereal time enables the hour angle of an object to be found from its right ascension
(hour angle sidereal time - right ascension).
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.
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 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 stream of particles such as protons, electrons and ions moving radially outwards from the Sun.
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 December22.
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
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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.
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
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.
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.
SPHERICAL ABERRATION
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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.
STEFAN'S LAW
A law giving the energy emitted by a black body as a function of its temperature, also known as the
Stefan Boltzmann law.
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.
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.
SUB GIANT
A star found between the main sequence and the giant region of the Hertzsprung-Russell diagram. They
are evolving towards becoming giants.
SUPERNOVA
A star brightening to between 10,000,000 and 1,000,000,000 times the luminosity of the Sun over a few
weeks. Type I supernovae are the brightest and originate from close binary stars where one component
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is a white dwarf which is accreting material from its companion. The supernova is caused by the collapse
of the white dwarf to a neutron star when its mass exceeds the Chandrasekhar limit. Type II supernovae
are the end points in the evolution of stars with masses seven more times that of the Sun.
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.
SYNCHROTRON RADIATION
Radiation, usually but not always at long wavelengths, which originates from very fast moving( electrons
spiralling around magnetic fields.
SYNODIC PERIOD
The period (orbital or rotational) as observed from the Earth. Since the Earth is moving around the Sun,
the synodic period differs from the true (or sidereal) period. Thus the synodic period, of the Moon (i.e. the
lunar month) is 29.5 days whereas its orbital period is 27.3 days.
T
TEKTLTE
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
Any device which gathers radiation and improves angular resolution.
TELESCOPE MOUNTING
Any device to hold, point and move a telescope. For terrestrial telescopes most mountings are either
equatorial or alt-azimuth.
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
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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 an angularly small object in the sky in front of an angularly larger object. Also the
passage, during its daily motion, of any object in the sky across the prime meridian.
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.
TWIAN 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 600 ahead and behind Jupiter.
TROPCSPHERE
The lowest layer in the atmospheres of the Earth and other planets. For the Earth it extends to a height of
about 15km.
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.
U
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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.
V
VAN ALIEN BELTS
Two regions within the Earth's magnetosphere where energetic charged particles are trapped. The
regions are shaped like ring doughnuts with the holes aligned with the Earth's magnetic poles.
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
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the spectrum and/or to the polarisation of the light from the star, resulting in spectroscopic and
polarimetric variables etc.
VERNAL EQUINOX
A synonym for first point of Aries.
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.
WIISON-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.
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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
Electromagnetic radiation with a wavelength between about 0.0lnm to 10nm.
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.
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
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The splitting of spectrum lines into two, three or more components when the atoms or ions emitting or
absorbing the radiation are in the presence of a magnetic field. The effect can be used to measure the
strengths of magnetic fields in sunspots and magnetic stars.
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.
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