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.

 

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.
 
 

 

Angstrom

unit of measurement for microscopical distances, such as the wavelength of the radiation. 1 Angstrom equals 10 -10 m, that is the ten thousandth part of a micron.
 

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.

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 heighth 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

 

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

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 centigrades.

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, litium, 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.
 

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, neutrini. 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.
 
 

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.
 

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 major semiaxis. 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.
 
 
 

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.
 

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.
 
 

Gyroscope

solid body in rapid rotation on an axis, that remains parallel to itself during the motion of the system.
 
 

H

 
 
 

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.
 

Interferometry

a technique of observation which consists in the superimposion of two sheaves of electromagnetic waves that come from the same source, but are detected by two or more separate instruments. Their superimposion 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.
 

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

 

Light Year

the distance travelled by light in one year, that is 9,460 billion Km.
 
 
 

 

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.
 

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 coordinates.

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 meterorites, and of many compositions.
 

Microwaves

radiation with a wavelength between approximately 1 mm and 30 cm.
 
 

 

N

 
 
 
 

 

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.

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.
 
 
 

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.
 

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.
 

Photosphere

the visible surface of a star: that is the layer from which the observed radiation comes.
 
 
 

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.
 

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.
 
 

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

 

Radiative Zone

the region of a star in which the energy produced by nuclear fusion in the core is carried outward by radiation. Viceversa, 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.
 
 

 
 
 
 
 

 

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 (spheric 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.
 

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

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.
 
 

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 aswell, 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.
 

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

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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