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3C219
A Seyfert galaxy, i.e. a
spiral galaxy
but it has an active nucleus (less energetic than a quasar). It is at a
distance of 544 Mpc from us.
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3C273
A quasar
with a powerful emission at radio wavelengths and is about 100 times
more luminous than a normal galaxy. It is at a distance of 2 billions of
light years from us
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- A -
antimatter
It is formed of antiparticles. The
particles are very common in the Universe (for example the electron and
the proton) and they define the matter.
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antiparticle
In particle physics every fermion has a
corresponding antiparticle (antifermion). A particle and its
antiparticle have identical mass but opposite electric charge and fermionic
(barionic or leptonic) quantum number. The
interactions among antiparticles are
essentially identical to those among the corresponding particles.
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atomic orbital
The region in space where the probability
of finding an electron
of an atom at a given position is higher. It is computed using the mathematical tools of
quantum mechanics.
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- B -
Big Bang (Theory)
It is the best known and more accredited theory which describes the
evolution of the Universe at the very beginning. The Universe was born
with the Big Bang.
Initially, all matter and energy were contained in a point
(singularity) which then has grown and reached the dimensions of the
present Universe.
The super powerful "explosion"
that gave birth to the Universe; immediately after the Big Bang the Universe had very
small dimensions and very high density, pressure and temperature. The Big
Bang occurred about 15 billion years ago
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black body
It is a theoretical object that absorbs all electromagnetic radiation
that falls onto it. No radiation passes through it and none is reflected
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black hole
An extreme state of collapsed matter (or antimatter).
Nothing, not even light, can escape its gravity. A massive
star can end its evolution in a black hole. The nuclei of galaxies can
harbour a supermassive black hole, containing about billions stellar
masses.
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- C -
chromosphere
It is the region of the solar atmosphere
between the photosphere and the corona and it is visible during the
total eclipse of the Sun.
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cluster
A group (from a few hundred to some
thousands) of stars, or tens to hundreds of galaxies,
gravitationally bound.
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corona
It is the Sun’s outer atmosphere, that
extends beyond the Earth with an extremely low density. During the total
eclipses of the Sun, when the photosphere is occulted, the corona
appears as a pearly white crown with a variety of features including
streamers, plumes, and loops. These features change following a cycle
similar to that of solarspots.
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cosmic radiation (cosmic rays)
It is made of protons, helium nuclei and some heavier nuclei of high energy,
which reach the Earth from outer space. Cosmic rays are the only
example of particles with mass which reach us from outside the solar
system. The mechanisms which accelerate some cosmic rays to extreme
energies are unknown. Primary cosmic rays interact with the nuclei (N, O)
of the molecules which make up the higher atmosphere of the Earth. In
these collisions many new particles are produced, most of which are
unstable. The particles which reach the surface (secondary cosmic rays)
are positive and negative muons, eletcrons
and
positrons.
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- D -
- E -
electromagnetic interaction
This interaction originates from electric charges and is mediated by
photons. The electromagnetic
interaction binds electrons and a
nucleus to make an atom. The force, which
binds atoms to make the molecules, is a "residual" electromagnetic force
shielded by the atomic electrons.
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electromagnetic radiation
Energy in transit in the form of electromagnetic
waves (photons).
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electromagnetic waves (e.m.)
Visible light, ultraviolet rays, infrared rays, radio and TV waves,
microwaves are examples of
electromagnetic waves.
Also X rays and
gamma rays or high energy
photons are electromagnetic waves.
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elliptical
galaxy
It is an ellipsoidal agglomeration of stars, which usually
do not contain much interstellar matter without much structure.
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- F -
fermion
Every particle with half integer spin (1/2, 3/2, ... in units
of
 ).
All "matter particles" (quarks and
leptons)
are fermions.
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fossil radiation
This radiation is considered the remnant of the Big Bang , which cooled down to
a temperature of 3 K (-270^o C).
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frequency
band
Range of frequencies of a signal
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fundamental particle
A particle with no internal substructure. In the Standard Model of the microcosm,
quarks and leptons are fundamental fermions,
photons, gluons,
W and Z bosons
are fundamental bosons. All other objects are made from these
particles.
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- G -
galactic
centre
The centre of our Galaxy (the Milky way) cannot be
studied at visible wavelengths because of cool interstellar dust,
which
is, however, transparent at X-ray, infrared and radio wavelengths.
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galactic
coordinates
It is an astronomical coordinate system,
which refers the stellar positions to the equator (Galactic plane) and
to the poles (perpendicular to the Galactic plane). The Milky Way’s
north galactic pole is in the Coma Berenices . The coordinates are
galactic latitude and galactic longitude, respectively.
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galactic nuclei
A significant fraction of the energy
output is not emitted by the normal components of the galaxies (i.e.
starts, dust and interstellar gas). The energy can be emitted across
most of the electromagnetic spectrum. The standard model predicts that
the energy is generated by a supermassive black hole of between
106 and 109 solar masses.
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galaxy
It is a large gravitationally bound system
of stars. Typical galaxies contain about 100 billions stars. Based upon
visual morphological type the galaxies are classified as ellipticals,
spirals, barred spirals, and irregulars.
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gamma rays
They are electromagnetic waves of very short
wavelengths. It is important to consider the corpuscolar aspect:
a gamma ray is a photon of high
energy. Gamma rays are emitted by radioactive nuclei, and in high energy collisions. Gamma
rays are penetrating radiations, like
X rays (but with shorter wavelengths).
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Gauss
It is the unit of the magnetic flux density.
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gravitational interaction
Interaction between bodies due to their mass/energy. The graviton is the
mediator of the gravitational interaction.
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- H -
Hubble telescope
It is the first large optical telescope in
orbit around the Earth. The telescope, launched in 1990, was build by
NASA with important contributions by ESA (European Space Agency). It is
equipped with instruments suitable for observations in the visible and
ultraviolet.
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- I -
- J -
- K -
Kelvin
It is the unit of temperature. Zero
kelvins is absolute zero (the lowest temperature achievable)
corresponding to -273 degrees Celsius.
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- L -
lepton
Fundamental constituent which does not participate in the strong
interaction. The lepton family includes the electron
(e-), the muon
(mu-), the tau
(tau-) and the corresponding
neutrinos (nue,
numu, nutau). The antileptons are e+,
mu+, tau+, anti(nue), anti(numu),
anti(nutau).
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light year
It is the distance that the light travels in
one year; roughly 9.460 billions of kilometres
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M -
M81
A spiral galaxy, that, together with the companion M82, was
discovered by J.E. Bode in 1744. It is at a distance of 12 millions of
light years from us.
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magnetic
storm
It is a temporary disturbance of the Earth’s
magnetic field, which can have rapid and intense variations. The
magnetic storms is connected with the sunspots and the solar flares.
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MERLIN
The Multi Element Radio Linked Interferometer Network is a network of radio telescopes located in Great Britain, with
maximum separation between two elements of 217 km. It receives radio
waves in the range between 151 MHz and 24 GHz.
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microwaves
Electromagnetic waves with frequencies of 109 -3 x 1011 Hz. (cycles per second).
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Milky Way
Our Galaxy (the Milky way) is a spiral galaxy
with about 200 billions of stars, among them the Sun and the solar
system. It has a dense nuclear core, which is visible from the Earth in
the direction of Sagittarius, and a flat part (disc) with a diameter of
about 100.000 light years and a central thickness of about 25.000 light
years. The Sun is located in the outer arm, at about 27.000 light years
from the nucleus and 20 light years above the equatorial plane. The
Milky Way appears in the night sky as a hazy band of white light
reflecting the position of the Sun.
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muon
Fundamental lepton with negative electric charge (mu-), it is
similar to the electron, but it has a mass 200
times larger. It is abundant in the secondary cosmic rays. The
corresponding antiparticle is the mu+.
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- N -
NGC1316
A massive elliptical galaxy in the Fornax cluster of galaxies.
Some features are typical of a spiral galaxy;
(disc, dust lane). This galaxy could be the final result of a merger
with another galaxy occurred billions years ago. Its distance is 70
million light years from us.
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NGC4261
A giant elliptical in the Virgo cluster
of galaxies at 45 million light years distant from us.
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NGC6251
An elliptical galaxy at a distance of
300 million light years from us
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NGC 6946
A spiral galaxy 20 million light years
distant from us.
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neutrini
Fundamental neutral leptons with mass zero or very
small. Three types exist: electron neutrino (nue), muon
neutrino (numu)
and tau neutrino(nutau).
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neutron
Neutral hadron with
spin 1/2
;
it is made of three quarks: n = udd. Neutrons are found
inside atomic nuclei. The free neutron is unstable,
with a life time of approximately 900 s.
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neutron
star
The massive stars, at the end of their
evolution, can explode forming a supernova. The internal part (typical
mass about 1.4 solar mass) collapses into a star with a few tens of km
in diameter. The gravitational collapse produces a very high density
composed mostly of densely packed neutrons. The pulsars are neutron
stars.
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nuclear hydrogen combustion
A process in which light nuclei fuse and form a heavier nucleus
with release of large amounts of energy. To start this combustion, high
density and high temperature are necessary; in the stellar nuclei
thermonuclear reactions take place
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- P -
photon (quantum of electromagnetic field)
An elementary particle, with no net
electric charge and spin equal 1. Photons mediate the electromagnetic
interaction. Depending on their energy they are indicated as radio
waves, infrared radiation, light, ultraviolet radiation, X-rays, and
gamma rays
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photosphere
The visible surface of the Sun, from which most of the emission comes.
The solarspots form in the photosphere.
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Planck (constant)
Fundamental physical constant: the elementary quantum
of action, indicated by h = 6.62620 x 10-34 joule·second.
Note that
 = h / (2xpigreco).
For photon it is the ratio of its energy and its
frequency h = E/ f.
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positron
Antiparticle of the electron. It has the same
mass as the electron, but opposite charge (positive).
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pulsar
Some stars (pulsating radio sources) emit intense electromagnetic radiations
in the radio frequency range. The first pulsar was discovered in 1967 by English
astronomers. At present there are more than 1000 pulsars. The radio intensity
varies at a regular period, (from a few seconds to fractions of second).
Pulsars
are neutron stars.
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- Q -
quantum
The smallest discrete amount of a physical quantity. The quantum of
electricity is the charge of the electron. The
quantum of light is the photon.
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quantum
mechanics
Is the theory which is valid at the atomic scale and at the smallest
dimensions of fundamental particles.
In an atom the energy, the momentum, the angular
momentum, assume discrete values called
quanta.
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quantum number
Number which characterizes the state of a physical system described by quantum mechanics. For example for the complete
description of an atomic orbital , you need 4 quantum numbers which
define the energy, the form, the orientation of the orbital in space and
the spin of the electron.
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quasar
In optical wavelengths it looks like a star (i.e. a point source), in the other frequency ranges the amount of
energy emitted is much larger that that emitted by a normal galaxy. The
first quasar was discovered in 1963 at Jodrell Bank, England.
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- R -
relativistic electron
A fundamental particle, negatively
charged, moving at a speed close to that of light.
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- S -
solar flares
Occasional explosion with a large luminosity increase. Solar flares take place in the chromosphere and are
generally associated with solarspots and faculae. They produce emission
of X-rays, high energy cosmic rays, and intense radio emissions. On the
Earth they cause magnetic thunderstorms, and aurora borealis and can
disrupt radio signals.
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spectrum
Intensity distribution of the electromagnetic radiation in the different wavelengths. The spectra can
be continuous or can present lines (in absorption or in emission) if the
radiation has determined frequencies which depend on the chemical
composition of the emission source.
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spin
Intrinsic angular momentum of a particle in units of
.
The spin of bosons is an integer, for
fermions half integer.
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spiral galaxy
It is composed by stellar associations
placed in bright arms within a flat and large disc and by a central
ellipsoidal region.
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sunspots
Dark regions on the Sun’s surface (photosphere), visible on the rest of the solar disc
because they are slightly cooler than the surrounding material. Sunspot activity cycles
about every eleven years then it decreases smoothly. A solar minimum
(called Quiet Sun) without sunspots is present 5-6 years after a solar
maximum. Observations of the sunspots allow us to measure the Sun's rotation
speed, which depends on the latitude and has the maximum at the
equator (rotation period (25.4 days). Other features (flares, faculae)
of the solar atmosphere are associated with sunspots. Sunspots are
produced by concentrations of magnetic field lines.
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supernova
Evolution final stage of a massive star:
the central part collapses forming a neutron star,
the external part is ejected. During this phase the star reaches a very
high luminosity, about the same as a galaxy.
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- T -
tau
The heaviest fundamental lepton with negative electric charge (tau-).
The other charged leptons are the electron and
the muon.
The corresponding antiparticle is tau+.
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- U -
- V -
VLBI observations
The Very Long Baseline Interferometry technique is based on a radio telescope arrays, which,
spread out over the Earth, with distances between the antennas of about
10.000 km and more, can together observe the same radio source.
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- W -
white dwarf
Final stage of the
life of a star with a mass similar to a solar mass, which collapses to a
size comparable to that of a planet.
It radiates a small quantity of light.
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- X -
X rays
They are electromagnetic waves with wavelengths
between 10 and 0.01 nm; they are due to the slowing down of electrons in matter.
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- Y -
- Z -
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