Glossary

Asterism:
a group of stars recognizable in the night sky for their prominent geometrical pattern.
Black hole:
according to the General Theory of Relativity, it is a region of spacetime with a gravitational field so intense that nothing within it can escape to the outside, not even light. A celestial body with these characteristics cannot be directly observed but its presence can be picked up indirectly by the effects seen on surrounding matter such as the gravitational interaction with other celestial bodies or on the matter that falls into it. There are many astrophysical observations which prove the existence of black holes in the universe. There are small black holes, of stellar origin, which derive from the explosion of very big stars but there are also enormous black holes that are found in the center of the galaxies.
Blue giant
is a bright massive giant star with a high surface temperature about 10,000 K or more.
Blue-white giant
è una stella gigante mediamente brillante, che possiede una elevata temperatura superficiale, solitamente 10.000 K o più. It is not as massive as a blue giant.
Ecliptic:
is the apparent path that the Sun follows over the course of one year. It refers to the intersection of the celestial sphere with the geometrical plane on which the Earth’s orbital path lies which is called the ecliptic plane
Gamma Ray Burst:
gamma rays are intense beams of gamma rays that can last from a few milliseconds to several minutes. These energetic explosions make up the most powerful phenomenon as yet seen in the Universe. They are located in galaxies beyond our own, sometimes in very distant ones. According to recent theories, this intense emission of gamma rays is thought to be generated by the increase of mass on a black hole.
Galaxy:
is a combination of stars, systems, clusters and stellar associations, gas and dust, bound together by their reciprocal force of gravity. The name comes from the Greek work ‘galaxias’ which means ‘milky’ a clear reference to the Milky Way, the Galaxy which the Solar System is part of. Galaxies are objects of vast dimensions: the smallest dwarf galaxies contain some tens of millions of stars, while the gigantic galaxies can even have a trillion stars orbiting around a common center of mass.
Globular cluster:
also called a closed cluster is a spherical collection of closely grouped stars which orbit a galactic core outside the galactic plane. Globular clusters are held together by intense gravity which gives them their characteristic shape and accounts for high stellar densities at their centers. Globular clusters are generally composed of hundreds of thousands of old stars.
Main sequence:
if we plot the stars that we know about on a graph, with their temperature on the abscissa and their absolute brightness on the ordinate, we can see that most of them are at the center of the graph along an inclined line that goes from the bright and hot stars to the dimmer and colder stars. This alignment is called Main Sequence and it is where the stars spend most of their existence, which is the time that they burn hydrogen into helium. Once this phase has passed, they will evolve into giant stars or super giant stars according to their initial mass.
Meteor shower:
è un fenomeno astronomico che consiste nella caduta ravvicinata nel tempo di un gran numero di meteore, frammenti di materiale celeste (asteroidi, comete o altro) che entrando nell’atmosfera terrestre si incendia a causa dell’attrito e quindi brilla per qualche istante. La meteora è chiamata comunemente “stella cadente”. Gli sciami meteorici sono fenomeni che ricorrono ogni anno attorno alla stessa data, perché ogni anno la Terra attraversa la stessa zona di spazio dove sono presenti i detriti lasciati in genere da una cometa. I nomi degli sciami meteorici si riferiscono ai nomi delle costellazioni da cui sembrano provenire le scie luminose: in realtà questa è la direzione verso cui la Terra si muove lungo la sua orbita in quel periodo.
Milky Way:
is the galaxy that contains the Solar System. The term comes from the Greek ‘galaxias’ which mean milky and was used by the Ancient Greeks to describe it. The term defines the weak band of whitish light which diagonally crosses the celestial sphere, made up of stars and nebulas of the galactic disk which from the Earth can be observed from within the galaxy itself. The Milky Way is at its brightest towards the constellation Sagittarius, where the galactic center is found, but due to the absorption of light by dense dust particles present in that direction, it is very difficult to see.
Nebula:
is an interstellar agglomeration of dust, hydrogen and plasma. The term originally referred to any astronomical object of great dimension of neither a stellar, a planet nor a comet nature. The term therefore included what today is referred to as galaxies ( for example, the nebula of Andromeda made reference to the Galaxy of Andromeda before the galaxies were discovered by Edwin Hubble). Nebulas are, instead, smaller and are found inside galaxies.
Neutron star:
is a very small star which is the collapsed core remnant, resulting from a supernova explosion, of a star with a mass of 10 times that of the Sun. It has a diameter of only a few kilometers but its density is awesome, hugely bigger than that of a white dwarf. Neutron stars rotate quickly around themselves emitting beams of energy and because of this are also called Pulsars.
Open cluster:
is a group of stars which all arose from a giant molecular cloud at the same time. They remain connected by their reciprocal gravitation pull. They are also called galactic clusters since they are only found inside the galactic disk. They are young objects, astronomically speaking, and they differ from globular clusters because of the fewer number of stars, a weaker gravitational pull, and for the fact that globular clusters lie outside the galactic plane.
Optical double star:
è una coppia di stelle che appaiono vicine se osservate dalla Terra per un effetto di prospettiva, ma che non hanno alcun legame gravitazionale tra loro.
Physical double star:
is a pair of stars physically bound together by the force of gravity. These star systems are called binary stars.
Precession of the equinoxes:
is a phenomenon deriving from a particular movement of the Earth which causes the axis of rotation to change slowly and continuously in contrast to the ideal sphere of fixed stars. The Earth’s axis undergoes precession, that is, a rotation of its axis similar to that of a spinning top. There are two factors causing this phenomenon: the Earth’s imperfect spherical shape and the gravitational pull of the Moon and the Sun. The result is a gradual shift in the orientation of the axis of rotation in a cycle of about 25,765 years during which time the position of the stars in the sky change slowly. Consequently, even the position of the celestial poles will change: in 13,000 years it will be the star Vega to indicate the celestial north pole and not the current star Polaris.
Radio source:
is a source of radio waves; in astronomy it refers to a celestial body whose radio wave emissions can be picked up by radio telescope
Red giant
is a large star with a cold surface temperature. It is in its final stage of evolution.
Red supergiant:
is a super giant star with one of the lowest surface temperatures and one of the biggest in the universe in terms of volume, although not one of the most massive
Spectroscopic double star:
is a binary star where the stars are only slightly separated and the separation cannot be identified optically but only with the aid of spectroscopic analysis, that is, the analysis of light frequencies emitted by the star and in particular through the Doppler effect which identifies the radiation emitted by the star. In these cases, the spectral lines of both stars move firstly towards red, then towards blue following their orbital motion which first causes them to d istance themselves then draw closer to us.
Supernova:
is a stellar explosion where a lot of energy is released more than in a nova. The explosion is caused by either
  1. 1) the accumulation of hydrogen on the surface of a white dwarf which makes it explode and become a lot brighter than before or
  2. 2) the gravitational collapse of super massive stars when internal nuclear fusion reaction has been completed.
They reach incredible temperatures when they explode (as high as one hundred billion degrees Kelvin) and emit radiation which for brief periods is higher than that of an entire galaxy.
Visual double star:
is a binary star seen as such with the naked eye or with a telescope. These stars are highly appreciated by star-gazers for their spectacular appearance through a telescope.
White dwarf
is a small star, very dim of a whitish hue. It is in its final stage of evolution for a small to medium sized star. It is a very compact object, of high density and surface gravity. Although its dimensions correspond to those of the Earth, its mass is equal if not superior to that of the Sun.
White star
is a star which is bigger than the Sun. It has a mass of between 1.4 and 2.1 times bigger than the Sun. It is in an initial evolutionary phase. Its temperature ranges from 7,500 to 10,000 K.
Yellow giant and Orange giant
are big stars that are in the final stage of their evolutionary process. As the stars grow, their surface temperature decreases and their colors go from yellow to orange. They are destined to become Giant Reds.
Yellow supergiant:
is a super giant star in an intermediate phase between a supergiant blue and a supergiant red of a class of stars with a mass of between 10 and 70 solar masses. It is a brief phase in the life of the star which is characterized by progressive expansion.