July 6, 2022

A star is a celestial body that shines with its own light. It is a plasma spheroid that through nuclear fusion processes in its core generates energy, radiated into space in the form of electromagnetic radiation (luminosity), flow of elementary particles (stellar wind) and neutrinos. Most of the chemical elements heavier than hydrogen and helium are synthesized in the nuclei of stars through the process of nucleosynthesis. The closest star to Earth is the Sun, the source of much of our planet's energy. The other stars, with the exception of some supernovae, are visible only during the night as flickering bright spots, due to the distorting effects (seeing) produced by the Earth's atmosphere. Stars have a mass between 0.08 and 150– 200 solar masses (M☉). Those with mass less than 0.08 M☉ are called brown dwarfs, objects halfway between stars and planets that do not produce energy through nuclear fusion; as far as observed up to now, there do not seem to exist stars with mass greater than 200 M☉, confirming the Eddington limit. The dimensions are also variable, ranging from a few kilometers of degenerate stars to billions of km of supergiants and hypergiants. The luminosities are between 10−4 and 106 - 107 solar luminosities (L☉). The stars occur, as well as singly, also in systems consisting of two binary stars or a higher number (multiple systems), linked by the force of gravity. They can also form stellar associations and star clusters (open or globular), in turn grouped, together with single stars and clouds of gas and dust, in even more extensive clusters, the galaxies. Numerous stars also possess more or less large planetary systems. Stars are divided into classes of magnitude or apparent size, according to the rule that the weaker the perceived brightness, the greater the number that expresses the size: so the stars of third magnitude are fainter than those of second magnitude and first magnitude stars are one hundred times brighter than the fainter ones visible without a telescope (sixth magnitude). The Milky Way, our galaxy, contains over 100 billion stars of various types: smaller and less bright than the Sun, no larger than Earth, such as white dwarfs, and some gigantic ones, such as Betelgeuse, whose diameter is greater than that of the Earth's orbit. Throughout history, the starry sky has been a source of inspiration for numerous philosophers, poets, writers and musicians, who in several cases have been directly interested in the study of astronomy.


The most visible star from our planet, as well as the closest ever, is the Sun: it occupies the central part of our solar system and is located at an average distance of 150 million km from the Earth; its proximity causes an amount of light to arrive on our planet such that, in the hemisphere in which it is visible, the other stars are obscured. When viewed directly unprotected, the Sun can even cause permanent eye damage. In general, however, when we refer to the term "star" we think of all the other celestial bodies that have characteristics similar to the Sun, but which are further away; in particular, we think of the bright points of various colors that populate a night sky whose atmospheric conditions are optimal, ie without clouds, haze or light pollution. The stars do not all appear of the same brightness, in fact they show a very wide range of luminosity; this is mainly due to two factors. The most important is the distance: the stars are in fact distributed in space in an irregular way, due to their own motion, due to events external to them that can alter their distribution such as supernova explosions, of their own or