According to modern concepts, the Galaxy was formed about 14 billion years ago from the primary slowly rotating gas clouds in size superior to its dozens of times. Initially, this cloud (protogalactic) 75% consisted of hydrogen and 25% helium. For about 3 billion years protobacco freely compressed under the action of gravitational forces.
This collapse inevitably led to the degradation of the cloud to pieces (fragmented) and the beginning of the process of star formation. First gas was a lot, and he was at large distances from the plane of rotation. There were the stars of the first generation, as well as globular clusters.
A star is born, when compressed clouds are achieved density and temperature sufficient for the effective flow of thermonuclear reactions. In the bowels of the massive stars was thermonuclear synthesis of chemical elements heavier than helium. These items were in the primary hydrogen-helium environment in the explosions of stars, or when relaxing after a substance with the stars. Elements heavier than iron are formed over the massive supernova explosions. Thus, the stars of the first generation of enriched primary gas chemical elements heavier than helium. These stars are the oldest, they consist of hydrogen, helium, and a very small admixture of heavy elements.
The portion of gas that is not turned into stars, continued its process of compression to the center of the Galaxy. Due to conservation of angular momentum, rotation became faster, formed a disk, and it again began the process of star formation. Stars of the second generation was rich in heavy elements, since they are formed from the already enriched with heavy elements of the primary gas.
The remaining gas was compressed into an even thinner layer. Thus arose the flat component of the stellar disk, which is the main arena of modern starburst.
When stopped compression protogalactic, the kinetic energy of the formed stars CD was balanced with the collective energy of gravitational interaction. At this time created the conditions for the formation of spiral structure, and the birth of stars is already in spiral branches, in which the gas is dense enough. It stars the third generation. These include our Sun.
Clash of protogalactic in the young Universe a billion years after the Big Bang.
Illustration of NASA
Further evolution of the Galaxy, scientists are so.
The inventory of interstellar gas will gradually depleted, the birth of stars will become less intense. In a few billion years, when you're exhausted all reserves of gas, a spiral galaxy will turn into the lens-shaped, consisting of faint red stars and white dwarfs is superdense stars small size, representing one of the last stages of stellar evolution.