Stars are like our Sun but they are far away. Astrophysician have built a theory of stellar evolution in order to understand all the observed types of stars, applying the most recent theoretical physics about hot gases (also called plasma), nuclear reactions, hydrodynamics... Comparing their theoretical predictions with their observations, they deduced what they called a standard model of the evolutive track of the stars (i.e Hayashi track, main sequence, red giant branch), explaining with a increasing accuracy how they were born, as well as how they die! Indeed and it s the main point of this page, stars are Not Eternal!!! Astrophysicians have defined a classification for all the main types of stars, even if now, with Hipparcos and others modern experiments, the number of stars we observe inside our galaxy as well as outside is enormous, and more and more peculiar stars appear. This classification O B A F G K M corresponding to a certain range in surface temperature, can be easily learnt with this sentense: Oh Be A Fine Girl Kiss Me , each of these major classes is further subdivided into 10 subclasses labeled for example: B0, B1, B2,..., B9, in order of decreasing temperature.
Just before the hayashi track, the proto-stars still capture matter from the dense parts of the molecular gases its came from, so these stars are at the begining of a long evolution. More precisely, stars are created from enormous clouds (i.e max 100 parsecs) of molecular gases (H2), because of their gravitationnal collapse. These clouds are inside bigger structure called galaxies, and we find billion of galaxies in the Universe.