Lecture 17 Star Formation Notes The mass of a star determines how long it will live. More massive stars evolve faster. Since stars don't live forever, then they must be "born" somewhere and at some time in the past. Stars form in giant clouds of gas and dust called molecular clouds. The term "molecular cloud" is used since molecules form there. The large amount of gas and dust in the cloud shields the molecules from UV radiation from stars in our galaxy. The molecular cloud does not collapse into a single star. It fragments into many clumps. These clumps can further collapse to form stars. 10 - 1000 stars can be formed from the cloud. When a fragment of a molecular cloud reaches a critical mass, it collapses to form a star. Gas and dust pulled together by gravity until a star is formed. But reaching this critical mass is not so easy. Gravity makes the cloud collapse. Two hindrances to collapse, both which favor expansion of the cloud: Internal heating Causes pressure build-up Angular momentum Causes high speeds Cloud fragments collapse Potential energy => Kinetic Energy Gas particles speed up and collide. The temperature increases. This causes a pressure build-up which slows (or stops) the collapse. Energy is radiated away. As the cloud fragment shrinks due to gravity, it spins faster. Collapse occurs preferentially along path of least rotation. The cloud fragment collapses into a central core surrounded by a disk of material. The central core is called a protostar. The particles in these rings can accrete together to form planets! The protostar continues to collapse while the central core heats up to millions of degrees. Fusion reactions start => A star is born After a star is born it heats the gas and dust around it. Eventually the gas and dust are pushed away. The star is then becomes "visible." Prior to this it could be seen only in the radio and the infrared.