There are about three thousand stars that can be seen with the naked eye on a clear and moonless night, far from city lights; but more than 50 000 can be seen if we use a pair of binoculars with a magnifying power of seven, and hundreds of thousands if we use a three-inch telescope. For the Ancients, who believed that the stars were small lamps hanging on the celestial vault and all at the same distance, it would be difficult to believe that the Sun was just one of those sparkling lights. Today we know that all the stars are spherical bodies of incandescent gas held "together" by the force of gravity. To discover how far away they are, astronomers refer to their luminosity and on simple geometrical calculations which consider the motion of the Earth around the Sun. There are many different types among the two hundred billion stars that form the Galaxy. Some, like Betelgeuse, are red supergiants: they may have a radius of up to billions of kilometers, but their mass may not even be half as great as the Sun. Others are millions of times smaller than the Sun but dozens of times brighter, and for this reason are called white dwarfs. But there are also giant blue stars, red dwarfs, yellow stars like our Sun and an infinite number of intermediate types. Their color, dimensions and state depend on their evolutional history. Let's see how. Stars are born in those points in the Galaxy where greater than normal quantities of dust and gas have accumulated, in a parabola that lasts for billions of years. These clouds attract greater and greater quantities of gas through gravitation and, as their mass increases, they contract, becoming even more compact; then they begin to rotate faster and faster, condensing in the center and, as we have already seen, the hydrogen begins to transform into helium. The star then ignites, but this is only the beginning of a long evolution. Over time, the star transforms all the hydrogen into helium, and then other fires begin. Since the internal pressure can no longer support the immense weight of the external layers, the stars continue to contract reaching even higher temperatures, and so much so that the helium atoms, namely the "waste" of the previous nuclear reactions, become a usable fuel. Then other reactions are triggered inside the stars which have now become red and very huge: helium is transformed into carbon, carbon into oxygen and magnesium and so on according to a stellar alchemy that produces more and more complex elements. Sometimes, since the gravity at the surface of the red giants is low due to the large distance from the center, the external layers of these stars tend to dissipate into space: this forms a planetary nebula with a central star that slowly goes out, becoming a white dwarf. This occurs when the initial star has a mass similar to our Sun. If instead it has a greater mass, then the red giant into which it has transformed has a completely different destiny. It may explode, creating only gaseous residues, or it may continue to expand while the carbon that is present burns slower and slower, weakening the star, which again collapses and finally explodes.
There are about three thousand stars that can be seen with the naked eye on a clear and moonless night, far from city lights; but more than 50 000 can be seen if we use a pair of binoculars with a magnifying power of seven, and hundreds of thousands if we use a three-inch telescope. For the Ancients, who believed that the stars were small lamps hanging on the celestial vault and all at the same distance, it would be difficult to believe that the Sun was just one of those sparkling lights. Today we know that all the stars are spherical bodies of incandescent gas held "together" by the force of gravity. To discover how far away they are, astronomers refer to their luminosity and on simple geometrical calculations which consider the motion of the Earth around the Sun. There are many different types among the two hundred billion stars that form the Galaxy. Some, like Betelgeuse, are red supergiants: they may have a radius of up to billions of kilometers, but their mass may not even be half as great as the Sun. Others are millions of times smaller than the Sun but dozens of times brighter, and for this reason are called white dwarfs. But there are also giant blue stars, red dwarfs, yellow stars like our Sun and an infinite number of intermediate types. Their color, dimensions and state depend on their evolutional history. Let's see how. Stars are born in those points in the Galaxy where greater than normal quantities of dust and gas have accumulated, in a parabola that lasts for billions of years. These clouds attract greater and greater quantities of gas through gravitation and, as their mass increases, they contract, becoming even more compact; then they begin to rotate faster and faster, condensing in the center and, as we have already seen, the hydrogen begins to transform into helium. The star then ignites, but this is only the beginning of a long evolution. Over time, the star transforms all the hydrogen into helium, and then other fires begin. Since the internal pressure can no longer support the immense weight of the external layers, the stars continue to contract reaching even higher temperatures, and so much so that the helium atoms, namely the "waste" of the previous nuclear reactions, become a usable fuel. Then other reactions are triggered inside the stars which have now become red and very huge: helium is transformed into carbon, carbon into oxygen and magnesium and so on according to a stellar alchemy that produces more and more complex elements. Sometimes, since the gravity at the surface of the red giants is low due to the large distance from the center, the external layers of these stars tend to dissipate into space: this forms a planetary nebula with a central star that slowly goes out, becoming a white dwarf. This occurs when the initial star has a mass similar to our Sun. If instead it has a greater mass, then the red giant into which it has transformed has a completely different destiny. It may explode, creating only gaseous residues, or it may continue to expand while the carbon that is present burns slower and slower, weakening the star, which again collapses and finally explodes.
