SCIE 1920U Chapter 9: 9 Stellar Lives
Document Summary
Stars are indeed born within the coldest and densest clouds, called molecular clouds. Gravity causes the cloud to shrink in size, a process called gravitational contraction. This contraction causes the gas temperature to rise, especially in the center of the cloud, because it converts gravitational potential energy into thermal energy. Central regions of the cloud become so dense that photons cannot easily escape so the pressure builds and contractions slows, and the cloud center is now a proto star. The internal pressure precisely balances gravity at every point within the star, keeping it stable in size. Energy balance between the rate at which fusion releases energy in the star"s core and the rate at which the star"s surface radiates this energy into space. If fusion in the core does not replace the energy radiated from the surface, thereby keeping the total thermal energy content constant, then gravitational contraction will cause the core to shrink and force its temperature to rise.
