عنوان مقاله [English]
نویسنده [English]چکیده [English]
Stars have incredibly long life spans by human standards, but they are not eternal. The sun is a middle-aged star (in the middle part of its lifetime). Previous generations of stars have lived and died, and new stars are still born in our galaxy. The stars are formed by the collapse of gas clouds due to gravity. The cloud will only collapse if its density is sufficient, and it should be sufficiently cool and bulky so that the gravity can overcome its internal pressure. For this reason, the most favorable condition for the formation of stars is molecular cloud. Dense-shaped gas clouds have very high number of molecules and are covered with dust that keeps them cool. If part of this cloud or the whole of it finds internal interactions due to gravity, it is likely that it decompose into smaller entities, each of which becomes a star. Hence the formation of stars takes place in batches. The Parvin (Pleiades) star cluster, for example, consists of a group of stars that have been formed around 50 million years ago.