A TO Z OF ASTRONOMY
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A B C
D E F G
A
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 centres.
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Although still widely used in astronomy, it is gradually being superseded by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 nanometre (1Nm = 10Å).
APERTURE.
The diameter of the
objective of a telescope.
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.
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.
A-type 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.
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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
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.
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 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.
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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
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.
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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
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
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.
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. 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
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.
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
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.
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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.
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.
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.
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.
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.
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
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 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 aan 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
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
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
O
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
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