The birth of stars

   The evolution of the star begins in the giant molecular cloud, also known as the cradle of the star. Most of empty Space galaxy actually contains from 0.1 to 1 molecule per cm. Molecular cloud also has a density of about one million molecules per cm. The mass of the cloud exceeds the mass of the Sun in 100 000-10 000 000 times due to its size: from 50 to 300 light-years across.

   While the cloud is rotating freely around the center of the galaxy home, then nothing happens. But should be external perturbation, slightly reduce the size of the cloud, the gravitational collapse occurs. For example, clouds may be confronted with each other, or one of them can pass through the dense spiral arm of the galaxy. Another factor could be a nearby supernova explosion, the shock wave which is faced with a vast molecular cloud velocity. In addition, the possible collision of galaxies, which can cause a burst of star formation, as the gas clouds in each galaxy is compressed by the collision.

   But one way or another, the size is changing, and the pressure of the molecular gas can no longer impede the further compression, gas begins to free fall, the time scale: For example, for the Sun tff = 5 * 107.

   For half of the virial theorem released gravitational energy goes into heating the cloud, and half - in the light. In the clouds of the same pressure and density increase toward the center, and the collapse of the central part is faster than the periphery. As the length of free path of photons decreases and the cloud becomes less transparent to its own radiation. This leads to a rapid increase in temperature and more rapid increase in pressure. Eventually the pressure gradient balances the gravitational force, formed by hydrostatic core, a mass of about 1% of the mass of clouds. This time, we do not see long-globule not transparent in the optical range. Further evolution protozvezdy - is accretion of matter continues to fall. Braking occurs on the surface of the nucleus. In the end, the mass of the substance have been exhausted and star rise in the optical range, marking the end of protozvezdnoy phase and the beginning phase of a young star.

   So it would be, if the initial molecular cloud does not rotate. But they all to varying degrees, rotate, and as far as reducing the size of the cloud is growing and its rotational speed, which at some point, shared substance into two layers, which continue kollapsirovat independently of each other. Layers, in turn, may also be severed increased centrifugal forces. Depending on the initial speed of rotation of the molecular clouds we are seeing star clusters, double stars, stars with ekzoplanetami.