Stellar evolution

Describe the steps (1-7) involved in the formation of a star like our Sun. What effect does heat, rotation and magnetism have on stellar formation (see more precisely 19.1)? Stage One: Stars are born from a very large interstellar cloud. These clouds are so vast that their mass is thousands of times more then sun. This cloud, which is unstable, starts to collapse under its own gravity. As a result of this collapse the cloud breaks (starts fragmentation) into thousand and thousand of pieces. Stage Two: There is a point where fragmentation of molecular cloud stops. The fragment of cloud still has a temperature close to that of the original interstellar cloud (warmer at the center then its edges), which in average is cold. The cold temperature causes the fragment to contract and the contraction causes the fragment to be dense. It become so dense; the radiation cannot get of the cloud easily causing the temperature and the pressure to rise. Stage Three: After tens of thousands of years contraction the fragment’s diameter comes close to the diameter of our solar system. It is dense and opaque and it begins to resemble a star. Scientists call this infant star a protostar. Stage four: like the preceding two stages, the fragment (which is now a protostar) continuous to shrink and its density continue to grow. The temperature rises both in the core and at the photosphere. Although the surface temperature of prostar is only half of sun, its luminosity is 1000 times more. Stage five: When the protostar reaches this stage (T Tauri) its already shrunk to about ten times the size of sun and the luminosity has also fallen to about ten times the solar value. Its surface and core temperature are 4000 K and 5,000,000 K respectively. Although the central temperature is raised 5 times compared to the preceding stage, it is still not enough for thermonuclear fusion. Stage six: Approximately after ten million years, the protostar finally becomes a true star. The core temperature raises 2 times more then the preceding stage reaching 10,000,000 K which initiate nuclear fusion at the core. Protons begin fusing into helium nuclei in the core, and thus a star is born . Stage seven: The star has finally arrived at the main sequence. Pressure and gravity gets balanced at the rate the nuclear energy is generated in the core. The fusion in the core establishes a “ gravity-in, pressure-out” equilibrium. And the star will burn for a very long time.