The stars of the firmament

Astronomers classify the stars according to their color, which indicates the temperature, and their size, which is related to the luminosity. The reference star is the Sun, that is yellow and has a temperature of 5,500°C, but there are other types of stars, such as the orange Alfa Centauri B, the red dwarf Proxima Centauri, the white Sirius A, the blue Spica, the red giant Betelgeuse or the white dwarf Sirius B. All the stars have a distance in which water can be found in liquid form, called “habitable zone”, because it is the main condition for the existence of life. Only in our galaxy, the Milky Way, there are about one hundred billion stars of which 50% are estimated to have orbiting planets, and 15% (one in 7) have rocky planets in a habitable zone. What is hard to believe is that we are alone in the Universe.