GLOSSARY
A B C D E F G H I L M N O P Q R S T U V W X Z
A
Aberration
defect of an optical instrument in the formation of images; the defect
occurs when the light source is not monochromatic, when it is situated
off the axis of the optical system, or when its aperture is large. It is
possible to partly correct the aberrations by using special artifices.
Accretion disk
circular and flattened cloud of gas, orbiting around a star or a black
hole, and slowly falling upon it
Active galaxy
a galaxy that emits huge amounts of energy, a fact that cannot be explained
by the mere production of radiation by its stars. This emission, which
generally comes from a small region of the galaxy, is today attributed
to phenomena such as the growth of matter on a massive black hole situated
in the centre of the galaxy.
Active Optics
a method that allows the correction, on an astronomical image, of the defects
due to the deformation of the mirror of the telescope. It is carried out
during the acquisition phase, by the actuators, which are "pistons" situated
under the mirror, that constantly correct the deformations due to the movements
of the telescope or to sudden changes of temperature.
Adaptive optics
a kind of optics that allows the correction, on an astronomical image,
of the defects due to the atmospheric turbulence, which deforms the path
of the luminous rays.
Albedo
reflecting power of a passive celestial body (that is a body not
emitting light and reflecting that of a star). It is the faction
of received light that is emitted back. An albedo equal to 1 indicates
a totally reflecting body, an albedo equal to 0 a totally dark body.
Angstrom
unit of measurement for microscopic distances, such as the wavelength of
the radiation. 1 Angstrom equals 10 -10 m, that is the ten thousandth part
of a micron.
Angular momentum
for a solid body rotating about one axis, it is the product of the rotational
angular speed by the mass of the body.
Anticyclone
rotating system of winds which diverge from a point of maximum pressure.
Antiparticle
elementary particle whose mass is equal to that of the most common particle,
while other characteristics (such as the electric charge) are opposite.
For example, the antiproton has the same mass as the proton, but opposite
electric charge. Each particle has an antiparticle: the electron, nuclear
particles such as neutrons and protons; subnuclear particles such as quarks.
The atom composed of antiparticles is somehow a mirror image of that composed
of particles, and it keeps both the appearance and the properties.
Aphastron
point of maximum distance from a star, in the orbit of a body rotating
around the star
Aphelion
point of maximum distance from the Sun, in the orbit of a body of the Solar
System.
Apogee
point of maximum distance from the Earth in the orbit of the Moon or of
an artificial satellite.
Astrolabe
ancient instrument used to measure the apparent height of the stars on
the horizon. The modern sextant derives from the astrolabe.
Astronomical Unit
Unit of astronomical unit, that corresponds to the average distance between
the Earth and the Sun, 150 million km, approximately.
B
Basalt
igneous basic rock , of dark colour, composed by minerals relatively rich
in iron and magnesium.
Big Bang
massive explosion of matter and energy, with which it is thought the history
of our Universe began. The matter was initially concentrated in an infinitesimal
space, at very high temperature and density; after the explosion, while
the Universe began to expand, the matter became more and more cold and
rarefied.
Black Body
a hypothetical body able to absorb all the radiation it receives without
emitting any, and therefore it appears completely black. However, if the
black body is heated, a radiation emission occurs, with a maximum of intensity
at a wavelength which is lower, as the temperature reaches higher levels.
Therefore, in Astrophysics, the radiation emitted by a star can be also
described by the black body temperature; that is the temperature of a black
body which emits a radiation with the same wavelength distribution as the
star.
Black Hole
the result of the gravitational collapse of a massive star. Its gravitational
attraction acts as a kind of "hole" which swallows every celestial body
that comes near to it; it is so strong that not even light can escape,
hence its name.
Booster
rocket for space vehicles.
Brown Dwarf
a celestial body that is too small to reach the temperatures that are necessary
in the centre to trigger the reactions of nuclear fusion, and to become
a star.
C
Celestial Equator
imaginary plane marked on the celestial sphere by the extension of the
terrestrial equatorial plane
Celestial Latitude
angular distance between a celestial body and the celestial Equator (or
the Ecliptic) measured along a great circle passing through the body and
the celestial poles (or the Ecliptic poles).
Celestial poles
imaginary points marked on the celestial sphere by the extension of the
Earth rotational axis.
Celestial sphere
imaginary sphere, in the centre of which the observer is located, on the
surface of which the celestial bodies are projected, as well s the reference
celestial co-ordinates.
Cerenkov Light
a luminous emission that happens when a charged particle, such as a particle
of the cosmic rays, crosses a material medium at a speed higher than that
of light in the same medium. It can find an application in some kinds of
particle counters.
Convection
modality of heat propagation in fluids, by displacement of masses of fluid
matter.
Copernican or Heliocentric system
the theory according to which the planets, including the Earth, rotate
around the Sun. A theory that opposed the ancient Geocentrical theory of
the Aristotelian school, according to which all planets and the Sun rotate
around the Earth, which is situated in the centre of the Universe. t
Cosmic Background Radiation
an electromagnetic radiation, diffused and almost even, that comes from
all directions, and that permeates the whole Universe. It has a maximum
of intensity at the wavelength of 2.6 cm and, it is thought, is what remains
of the radiation emitted during the Big Bang, the massive explosion that
originated the Universe according to the modern cosmological theories.
It is called "3 K radiation" because its wavelength corresponds to that
emitted by a black body at the temperature of 3 K, that is -270 degrees
centigrade.
Cosmic Rays
a kind of radiation constituted by corpuscles, not by actual electromagnetic
radiation. It is a flux of charged particles, mostly ions of light elements
(Hydrogen, deuterium, helium, lithium, etc..) and electrons, that move
at a very high speed, near to the speed of light. They have high energy
and fill the whole galaxy like a kind of interstellar gas. They are emitted
by the stars and during some energetic events, such as explosions of supernovae.
Crust
external layer of the Earth or other terrestrial planets, surrounding a
denser material layer, called mantle. On the Earth, it is composed by rocks
rich in silicon, aluminium and magnesium.
Cyclone
rotatory system of winds which converge towards a point where the atmospheric
pressure is minimum.
D
Dark matter
a hypothetical form of matter, according to the cosmological theories,
that does not emit light and therefore escapes observations. Scientists
think that the dark matter is diffused in the whole Universe, mixed to
the ordinary matter, and is constituted of particles such as, for example,
neutrinos. The dark matter is almost definitely present in the halos that
surround the galaxies and in the space among the galaxies of a cluster.
Its existence is suggested by indirect evidence, like the gravitational
effects that it produces on the ordinary matter.
Deuterium
chemical element called "heavy hydrogen", it is an isotope of hydrogen
whose nucleus is composed by one proton and one neutron.
Diffraction
a phenomenon by which light, due to its undulatory nature, travels following
a direction different from the straight line as expected according to geometrical
optics, when it encounters small objects or it crosses slits. Due to diffraction,
the image of a punctiform source appears as a disc, its size representing
the diffraction limit; it is not possible to discriminate between two different
objects if the distance between their images is smaller than such limit.
Dynamo Effect
generation of a magnetic field on a planet, due to the motion of charged
particles during the rotation of the planet around fluid conductive nucleus.
E
Eccentricity
measure of the oblateness of an elliptic orbit, sometimes called "ellipticity"
; it is the ratio between the distance of a focus of the ellipse from the
centre and the semimajor axis. The eccentricity of a circumference equals
zero.
Eclipse
total or partial darkening of a celestial body, due to the interposition
of an other object between the former and the observer. The eclipse of
the Sun is caused by the interposition of the Moon between the Sun and
the Earth, whereas the eclipse of the Moon is caused by the interposition
of the Earth between the Moon and the Sun; the Earth projects its shadow
on the lunar disc.
Ecliptic
imaginary circumference marked on the celestial sphere by the Earth orbital
plane. It is inclined by
23 ° and 27' on the celestial Equator.
Ellipsoid
the surface obtained rotating an ellipse around one of its axes.
Equinox Precession
motion of the terrestrial rotation axis, during which it describes a cone
with a 25,800-year period. The Equinox Precession depends on the gravitational
action of the Sun and the Moon on the equatorial swelling of the Earth,
which is responsible for the fact that the point of intersection of the
celestial equator with the terrestrial orbital plane moves in a retrograde
direction each year. This motion combines with a motion of smaller amplitude,
nutation, to act on the celestial pole so that it follows a curving trajectory
around a fixed point.
F
Fluorescence
the property by which a substance, under the effect of light, emits a different
luminous radiation.
Flyby
close passage of a space probe near a celestial body.
Focal distance
distance from the centre of a lens to the plane where the image is
formed.
Fundamental Forces
Four fundamental forces or interactions operate in the Universe:
the gravitational force, which produces the mutual attraction of bodies
that have a mass.
the electromagnetic force, which induces the mutual attraction of charged
particles of opposite sign, and the repulsion of particles of equal sign;
it is this force that maintains the electrons in their orbit around the
nuclei to form the atoms.
the weak interaction, which regulates the processes of nuclear decay
and radioactivity.
the strong interaction, which binds protons and neutrons to form the
atomic nuclei; such interaction plays a role in the process of nuclear
fusion.
Gravitational force is the less intense interaction, but the one with
the greatest range of action; on the contrary, the strong interaction is
the most intense, but has a very short range of action, its order of magnitude
is that of the atomic nucleus (10 -15 m).
G
Galaxy
a set of billions of stars, held together by the reciprocal gravitational
attraction. Galaxies are the bricks that constitute the Universe. They
can be single galaxies, or they can be gathered in groups and clusters.
On average, their diameters measure one billion billions of Km, and they
can contain from 1 to 1000 billion stars. There are many kinds of galaxies:
elliptical, spiral, irregular.
Gamma Rays
the radiation with the higher frequency and energy known; its wavelength
ranges between 10 -12 cm (10-4 Angstrom) and 10-9 cm (0.1 Angstrom).
Giant or Gaseous or Jovian Planets
the giant planets (Jupiter, Saturn, Uranus and Neptune), are characterized
by the absence of a visible solid surface, because they are covered by
a dense and thick mantle of gas. They probably have a small rocky nucleus,
buried at a very big depth.
Gravitational Lens
a phenomenon that consists in the production of multiple images of a far
source, such as a galaxy or a quasar; it is caused by the curving of the
rays of light coming from the source, by a massive object such as a large
galaxy or a cluster of galaxies, interposed between source and observer.
Greenhouse effect
heating of the surface of a planet, caused by the gases present in its
atmosphere, which hold the infrared radiation coming from the Sun. The
effect is mostly due to carbon dioxide.
Gyroscope
solid body in rapid rotation on an axis, that remains parallel to itself
during the motion of the system.
H
H alpha
narrow line of red light emitted and absorbed by the hydrogen, often used
to study the Sun and the stars.
Heliosphere
a region in space where the solar magnetic field is active. Its intensity
decreases progressively as the distance from the Sun becomes greater, up
to a region called Heliopause.
I
Inflation
a phase of violent expansion of the Universe, according to the modern cosmological
theories, during which the Universe is thought to have increased its dimensions
by a factor 1050. The inflation should have started 10-35 seconds after
the Big Bang.
Infrared Radiation
a radiation with a wavelength between 7,800 Angstrom and 1 mm, approximately.
Inner Planets
Mercury and Venus are planets internal to the Earth orbit
Interferometry
a technique of observation which consists in the superimposition of two
sheaves of electromagnetic waves that come from the same source, but are
detected by two or more separate instruments. Their superimposition can
determine a strengthening or an attenuation of the intensity of the light;
one can obtain different information about the source by studying the images
obtained, such as for example its angular dimensions.
Ion
an atom without one or more electrons, and therefore positively charged.
Ionosphere
a region of intense ionization of the high atmosphere of a planet.
Isochronism
uniformity in time
Isotope
the name of elements that have the same number of protons and equal chemical
properties, but different number of neutrons, that is different atomic
weight.
L
Libration
apparent oscillation of the Moon, caused by the fact that its revolution
motion is slower at the apogee and faster at the perigee, so that more
than half of the lunar surface can be observed during one revolution.
Light Year
the distance travelled by light in one year, that is 9,460 billion Km.
Longitude
angular distance of a celestial body from the point of intersection of
the Ecliptic with the celestial Equator, measured along the Equator (or
along the Ecliptic).
M
Magnetosphere
magnetic shell that surrounds the planets with a magnetic field. Its shape
is asymmetrical because it is delimited by the pressure of the solar wind,
in the direction of the Sun, while it forms a long tail on the opposite
side.
Magnitude
apparent or absolute luminosity of a celestial body. The former measures
the luminosity of the body as seen from the Earth, that is at its true
distance. The second one measures the luminosity it would have if located
at a standard distance from the observer, that is the intrinsic one. The
definition of magnitude is such that, when a star is brighter its magnitude
is smaller.
Mantle
zone of a planet just below the crust, which surrounds the nucleus. It
is formed by melted rocky material, but it is rigid due to the pressure
of the upper layers.
Megaparsec
unit of measurement of galactic and cosmological distances, equal to one
million parsecs.
Meridian
the maximum circle that passes through the poles, perpendicular to the
equator and that passes through a given point. Together with the parallels,
the meridians constitute the system of terrestrial and celestial co-ordinates.
Meteorite
a solid body, metallic or made of stone, of extraterrestrial origin, penetrated
in the terrestrial atmosphere and fallen to the ground. There are many
kinds of meteorites, and of many compositions.
Micron
unit of measurement of microscopic dimensions, like the wavelength of part
of the radiation spectrum. A micron corresponds to one millionth of meter,
that is a thousandth of one millimetre.
Microwaves
radiation with a wavelength between approximately 1 mm and 30 cm.
N
Nadir
point opposite to Zenith on the celestial sphere.
Nebula
the name indicates a generic mass of gas, more or less rarefied, and dust.
There are many kinds of Nebulae, with different origins. They can be dark,
or they can have a luminous source in the centre (like the stars), or they
can reflect the light coming from an external source. The Protosolar Nebula
is the cloud of primordial gas from which the Solar System originated,
by gravitational contraction.
Neutrino
subnuclear particle produced within the stars during thermonuclear fusion
reactions. Neutrinos do not have any mass or electric charge, and for this
reason they go through matter without interacting with it. They are very
difficult to detect.
Neutron Star
extremely compact and dense star, formed during the final evolution of
a massive star. The matter in a Neutron Star is not in the ordinary physical
state that we all know: the pressure of the concentrated matter is so high
that the atoms "break", and protons and electrons merge forming a sea filled
with neutrons.
Nodes
(line of) line identified by the intersection between the orbital plane
of the Earth and that of the Moon, or, in the general case, between the
orbital planes of any two celestial bodies. It is the line along which
three bodies align when an eclipse occurs.
Nova
a star that shows a sudden increase in luminosity, up to 100,000 times,
in a time that can even be as short as a few hours. This phenomenon is
caused by a violent expansion of the outer layers of the star.
Nuclear Fusion Reactions
a process in which two or more atomic nuclei combine to form a larger one,
with a mass that is slightly smaller than the sum of the two original masses.
The mass difference is converted into energy according to the famous equation
E=mc2, by Einstein.
Nutation
motion of the terrestrial rotation axis. It is due to the fact that the
gravitational attraction of the Moon and the Sun on the terrestrial equatorial
bulge varies in time according to their relative positions. Due to the
joint effect of nutation and of another perturbations of greater amplitude
(precession), the terrestrial rotation axis has a tortuous motion in the
sky, instead of maintaining a fixed direction in space.
O
Opposition
the position of two stars the longitudes of which differ by 180°; in
other words they are on opposite sides if seen from the Earth.
Orbit
elliptical path described by a celestial body rotating around another one.
The orbit is completely determined by 6 parameters: the semimajor axis,
the eccentricity, the inclination with respect to some plane, the longitude
of the ascending node, the longitude of the periastron and the revolution
period.
Outer Planets
Mars, Jupiter, Uranus, Neptune and Pluto are planets external to the terrestrial
orbit.
P
Parallax
modification of the apparent position of a star, when observed from two
different points. The Parallax allowed to measure the distance of the nearest
stars.
Parallel
a circumference parallel to the (celestial or terrestrial) equator; it
measures the latitude of a point.
Parsec
short form of "parallax second". It is the distance at which an observer
would see the mean radius of the terrestrial orbit (that is an Astronomical
Unit) under an angle of 1 arc second. It corresponds to 3.26 light years,
to 206,000 UA and to 30,640 billion km.
Periastron
in the orbit of a body around another, it represents the point of minimum
distance between the two.
Perigee
point of minimum distance from the Earth in the orbit of the Moon or of
an artificial satellite.
Perihelion
point of minimum distance from the Sun in an orbit.
Phases
different aspects that a celestial body can subsequently show, due to its
position compared to another celestial body. For example, the phases of
the Moon are due to its positions towards the Sun, which cause a different
illumination of its surface, as can be seen from the Earth.
Photons
particles that carry the electromagnetic energy of the radiation. It is
believed that electromagnetic radiation has a dual nature. In certain cases
it behaves like a wave, in other ones like a particle. For example, the
radiation is absorbed by the atoms in the form of particles, or "packets"
of energy, the photons.
Photosphere
the visible surface of a star: that is the layer from which the observed
radiation comes.
Planetesimals
rocky primordial objects, formed by aggregation of dusts within the protosolar
cloud. It is believed that from these planetesimals the asteroids and the
planets formed by mutual gravitational attraction.
Plasma
the state of matter in which the atoms are completely ionized, that is
when they have lost all their electrons. This is the ordinary state of
matter within the stars.
Polar Aurora
luminescence in the ionosphere of a planet, caused by the interaction between
the planet magnetic field and the ionized particles flux coming from the
Sun.
Polygonation
a topographical operation that is done to draw maps on a large scale. It
consists in the surveying of a series of points on the ground, and in the
determination of the mutual distances and the relative altitudes, also
using triangulation.
Protosolar Disc
primordial cloud made of gas, in the shape of a disc, from which it is
thought the Sun and the planets originated, by gravitational contraction.
Protostar
stage in the life of a star which follows the gravitational contraction
of the cloud of gas from which it originates and that precedes the start
of the nuclear fusion reactions in its centre.
Position Angle
the angle formed by a fixed straight line and by the line that joins
a celestial body and the observer
Pulsars
the contraction of the English expression pulsating radio source, that
indicates some sources that emit radio pulses at regular and very close
intervals. It is thought that Pulsars are neutron stars in rapid rotation
around their own axis. They have a very strong magnetic field, the axis
of which and the rotation axis do not coincide: at each rotation we receive
a pulse from the star, when one of the magnetic poles is directed towards
us.
Q
Quark
elementary particle, that constitutes protons and neutrons.
Quasar
the contraction of the English expression quasi-stellar radio source.
These are the farthest objects known, and they are still moving away
from us at a very high speed; due to their distance, they are also the
youngest objects we can observe, they are only a few billion years old.
Quasars emit most of the energy in the radio band, due to the strong redshift.
Maybe they are nuclei of active galaxies, much more luminous than the surrounding
galaxy, and that is why, in the distance, they appear more like "stars"
than like galaxies.
R
Radiation Belts
collection of charged particles (positive ions and electrons) emitted by
the Sun and trapped by the planets magnetosphere. The radiation belts of
the Earth are named "Van Allen" belts, after their discoverer.
Radiative Zone
the region of a star in which the energy produced by nuclear fusion in
the core is carried outward by radiation. Vice versa, in the convective
zone of a star the energy is carried outside by the bulk outward motions
of matter.
Radio Waves
a radiation with a wavelength greater than 30 cm, approximately.
Radiogalaxy
a galaxy that emits most of its radiation in the radio band.
Radiotelescope
an instrument used for research in radioastronomy, basically constituted
by a big paraboloid that detects the radio waves and sends them to an antenna
situated on its focus. Various radiotelescopes placed far from each other,
are often used in an interferometric manner; their signals are summed in
order to obtain one signal with a better resolution.
Radio waves
radiation whose wavelength is larger than about 30 cm.
Red Giant
the step in the evolution of a star, during which its outer layers expand
and cool down; therefore the star appears larger and more luminous, due
to the fact that the emitting surface increases; besides, the temperature
decrease determines the shift of the maximum of the light intensity towards
the red, that is towards greater wavelengths.
Redshift
the shift of the spectrum of the radiation coming from a source towards
the red, that is towards wavelengths higher than those at which the radiation
was emitted, due to the fact that the source moves away from the observer.
The opposite event, that is the shift of the light towards the violet end
of the spectrum when the source moves approaching the observer, is called
"blueshift".
Reflecting and Refracting Telescopes
in the reflecting telescopes, a concave mirror (spherical or parabolic)
carries the light from the source to the focus of the instrument. In the
refracting telescopes light is carried by lenses to the focal plane (where
the image is formed).
Reflectivity
the capability of a body to reflect the radiation it receives. It is measured
as the ratio between the intensity of the reflected light and that of the
received light, and its values range between 0 and 1. It is also called
"reflecting power" or "albedo"
Resolving Power
the property of an instrument to distinguish distinct objects that are
near to each other. The greater the resolving power, the smaller the minimum
distance at which two objects can be distinguished.
Revolution
the rotatory motion of a celestial body around another, along an elliptic
or circular orbit.
Roche limit
minimum distance of a stable satellite from a planet. If the satellite
is located at smaller distances, the tidal force produced by the planet
tends to disrupt it.
Rocky or Terrestrial Planets
the rocky planets (Mercury, Venus, the Earth and Mars) are almost completely
constituted by solid matter, sometimes covered by atmosphere, less thick
than the radius of the planet.
Rotation
the rotatory motion of a celestial body around its own axis.
S
Shock wave
pressure wave produced in a fluid when an immersed body, or an explosive
compression phenomenon, propagate at a speed larger than the speed of sound
in that fluid. The speed of sound is a speed at which the molecules of
the fluid move in order to transmit from one point to another a pressure
change.
Slingshot Effect
a mechanism by which when a spacecraft flies very near a planet it receives
an acceleration; thus the spacecraft gains further speed compared to when
it left the Earth.
Solar Corona
a region of rarefied and highly ionized gas, with temperatures over a million
degrees Celsius, that surrounds the Sun and spreads for millions of kilometres.
Solar (stellar) Wind
a flux of charged particles (plasma) emitted by the Sun or by a star.
Spectral Type
stars are divided into several spectral types according to the lines of
the spectrum that they emit; from these lines, information on its temperature,
on the pressure at the surface of the star, as well as on its chemical
composition can be obtained. To each spectral type corresponds a certain
surface temperature interval.
Spectrum
the distribution of the light intensities to the different wavelengths.
Actually, the spectrum of any source is a luminous strip of different colours,
obtained when the radiation of the source is made go through a prism or
another object; the prism resolves the light into the different wavelengths
that compose it. The spectrum of a stellar source has dark lines, called
absorption lines, while other sources have lines of emission as well, more
brilliant than the rest of the spectrum. The spectral lines provide indications
on the chemical composition and on the temperature of the source.
Spectrograph
instrument that allows to register the spectrum of a source, after the
resolution of it.
Spectrometer
instrument that allows to resolve the spectrum of a source, and to measure
the intensity of the light, at different wavelengths.
Star Cluster
cluster of stars held together by the mutual gravitational attraction;
an open cluster is a set of a few hundreds or thousands of stars, with
an irregular shape. A globular cluster is more compact, with a spheroidal
shape, and it can contain up to hundreds of thousands of stars.
Sun spots
dark areas on the photosphere of the Sun, that appear gathered in groups,
in connection with the solar magnetism; they are dark because they are
colder than the surrounding photosphere.
Supergiant
a star with greater dimensions and luminosity than any other known star.
There are blue supergiants, with high superficial temperature, and red
supergiants, which are colder.
Supernova
the final step in the evolution of a massive star, during which the star
explodes reaching an exceptional luminosity, even equal to that of a whole
galaxy. The outcome of the explosion can be the destruction of the star,
or a neutron star or a black hole.
T
Telescope monture
the whole of the structure and the geometry of a telescope, which allows
the rotation motion around two axes, in order to follow the movement of
a star in the sky. In the altazimuth monture, one of the axes is the vertical
line that passes through the observer, the other is parallel to the plane
of the horizon. In the equatorial monture, on the contrary, one of the
axes is parallel to the axis of terrestrial rotation, the other is parallel
to the plane of the horizon, and perpendicular to the former.
Thermonuclear fusion
process during which two or more nuclear atoms merge in order to form a
larger one, whose mass is slightly less that the sum of those of the former
ones. The difference in mass is converted into energy according to the
famous Einstein equation E=mc2
Tidal Force
this is a sort of "stretching" that a body undergoes due to the differential
gravitational attraction by another body. In fact, the gravitational attraction
varies according to the distance of the various points of one body, from
the attractor body. If two points are attracted with forces of different
intensities, they undergo a reciprocal stretching force. The Tidal Force
of the Moon is responsible for the periodic rising of the sea during the
orbit of the Moon around the Earth.
Triangulation
a surveying method consisting in the localization of the position of one
or more points by using the trigonometric calculus.
U
Ultraviolet Radiation
a radiation with higher frequency and energy than visible light. Its wavelength
ranges between 40 Angstrom and 3,500 Angstrom, approximately.
V
Visible Light
the radiation with a wavelength between approximately 3,500 and 7,800 Angstrom.
This interval is also called "optical window".
W
Wavelength
in the radiation, the wavelength indicates the distance between two subsequent
peaks of intensity of the electromagnetic field transported by it. The
frequency indicates the number of oscillations of the electromagnetic field
in one second and is proportional to the energy carried by the wave. Il
product of wavelength and frequency is a constant, therefore the greater
the wavelength, the lesser the frequency and the energy of the radiation.
White Dwarf
the final step in the evolution of a not very big star, after the consumption
of the nuclear fuel in the centre. The name is due to the fact that the
star is compact, small and not very bright, but, seeing as it is very hot,
it emits "white" light, that is light with small wavelengths.
X
X Rays
a radiation with high frequency and energy; its wavelength ranges between
10- 9 cm (0.1 Angstrom) and 4 10- 7 cm (40 A). It is very penetrating,
and is therefore used to study the internal structure of substances, such
as crystals.
Z
Zenith
point on the celestial sphere located above the vertical of an observer.
Zodiacal Light
diffused faint light visible in the celestial region of the Zodiac, before
dawn at east or after sunset in the west. It is due to the diffusion of
the solar radiation by the electrons and the dust of the interplanetary
space.
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