The stars of the universe are gathered together in numerous giant assemblages known as “galaxies”. Many such assemblages are so enormous that they contain hundreds of billions of stars.
Nature has provided an immensely varied array ( = schiera) of galaxies, ranging from faint, diffuse (dai contorni poco chiari), dwarf objects to brilliant spiral-shaped giants. Virtually (praticamente) all galaxies appear to have been formed soon (appena) after Universe began, and they pervade space, even into the depths (profondità) of the farthest reaches (distanze) penetrated (a cui arrivano) by powerful modern telescopes.
Galaxies usually exist in clusters (si trovano a gruppi), some of which in turn are grouped into larger clusters measuring hundreds of millions of light-years across. A light-year is the distance traversed by light in one year, travelling at a speed of 300,000 kilometres per second, or 650 million miles per hour. These so-called superclusters are separated by nearly empty voids (spazi quasi vuoti).
Galaxies differ from one another in shape (forma), with variations resulting from the way in which the systems were formed. Depending on the initial conditions in the pregalactic gas some 15 billion years ago, galaxies formed either as slowly turning (girando lentamente), smoothly structured (uniformemente strutturate), round systems of stars and gas or as rapidly rotating pinwheels (ruota a pioli) of such entities. Other differences between galaxies have been observed and are thought to reflect evolutionary changes.
Some galaxies are rife with activity: they are the sites of star formation with its attendant glowing (incandescente) gas and clouds of dust and molecular complexes. Others, by contrast, are quiescent, having long ago ceased to form new stars. Perhaps the most conspicuous (cospicui) evolutionary changes in galaxies occur in their nuclei, where evidence (evidenza) suggests that in many cases supermassive objects - probably black holes (buchi neri) - formed when the galaxies were young. Such phenomena occurred (si verificarono) several billions years ago, and are now observed as brilliant objects called “quasars”.
A quasar is a large object (grande) like a star, that is far away (lontano) and that shines (brilla) very brightly and occasionally sends out (disturba) strong radio signals.
The existence of galaxies was not recognized until the early 20th century. Since then, however, galaxies have become one of the focal points of astronomical investigation.
Almost all current systems of galaxy classification are outgrowths of the initial scheme proposed by Hubble in 1926. In Hubble's scheme galaxies are divided into three general classes: ellipticals, spirals, and irregulars. His basic definitions are as follows:
· Elliptical galaxies
The galaxies of this class have smoothly varying brightness. They appear elliptical in shape, with lines of equal brightness made up of concentric and similar ellipses. These galaxies are somewhat redder (rosso più intenso) than the Sun.
· Spiral galaxies
These galaxies are conspicuous for their spiral-shaped arms, which emanate from or near the nucleus.
· Irregular galaxies
They consist of grainy, highly irregular assemblages of luminous areas. They have no noticeable symmetry nor obvious central nucleus, and they are generally bluer (blu più intenso) in colour than spiral galaxies.
Edwin Powell Hubble (1889-1953)
Edwin Powell Hubble was an American astronomer noted for his investigations of nebulae and the recession of the galaxies. A nebula is a diffuse cloud of particles and gases (mainly hydrogen) that is visible either as a hazy patch of light or an irregular dark region.
The solar system
The solar system is an assemblage of planets, natural satellites, asteroids, comets, meteoroids and interplanetary dust and gas under the gravitational control of the Sun in its 225million-year period of revolution around its galaxy. An asteroid in one of the many small planets that move around the Sun, especially between Mars and Jupiter. A meteoroid is any solid object moving in interplanetary space that is smaller than a planet or asteroid but larger than a molecule.
Any body not-having sufficient velocity to escape from the Sun remains in a closed path called “orbit” around it and is part of its system.
The Sun is a gas (predominantly hydrogen). It is heated by continuous nuclear fusions of the hydrogen in its core, where temperatures are near 15million K. As a result the visible surface of the Sun has a temperature of 5780 K and continuously radiates a huge quantity of power into space. Earth intercepts less than half of one billionth of that power, but it is sufficient to make Earth a warm, habitable planet.
“K” is the abbreviation of Kelvin, a scale of temperatures in which water freezes at 273,15 K, and boils at 373,15 K.
The planets of the Solar system can be divided physically into two groups – an inner group of small, rocky, high-density bodies (Mercury, Venus, Earth and Mars) called “the terrestrial planets”, and a outer of group of large, gassy, low-density planets (Jupiter, Saturn, Uranus, and Neptune) called “the giant planets”. This leaves Pluto as a curious anomaly – small, icy, of low density, and in its make up moves like a satellite than a planet.
Pluto is believed to be less than one third the size of Earth, and its orbit crosses the orbit of Neptune. There are reasons for believing that Pluto may once have been a satellite of Neptune that escaped from its gravitational control. The terrestrial planets are of about the same size and mass as the Earth. The giant planets have from 15 to 318 times the volume of Earth, but have only about 20% the density of terrestrial planets. Saturn is so light that an average piece of it, if available would float in water. The high density of the terrestrial planets is attributed to their large metallic content. The giant planets are believed to consist mostly of the lightest elements, hydrogen and helium. They are thought, however, to have a small solid core of iron and silicates.
“Silicate” means “silicato”; it is a mineral that contains silica, a chemical containing silicon found in sand and in rocks, such as quartz.
 since then = da allora
 Almost all = quasi tutti
 current systems = i sistemi attuali
 outgrowths = sviluppi/evoluzioni
 as follows = le seguenti
 smoothly varying = che varia dolcemente
 made up = composte
 singolare àellipsis
 somewhat = un po’
 grainy = granuloso (grain = grano)
 noticeable = evidente
 noted = degno di nota
 fosca (non molto luminosa) haz = nebbiolina
 either ... or = o … o both … and = sia … che
 to be part of something = far parte di qualcosa
 core = nucleo centrale
 as a result = come risultato
 ORDINE DEGLI AGGETTIVI:
A small (size) old (age) round (shape) table stood on a green (colour) chinese (nationality) wooden (material) carpet.
 to float = galleggiare
 to consist of + something to consist in + infinito