5.1 to 5.5 Stellar Processes and Stellar Evolution

5.1 Describe the conditions that initiate fusion in a star.

• Stars are formed when huge clouds of gas and dust are compressed. 
• Something needs to cause the clouds to be compressed such as an exploding supernova or a collision between two dust clouds.
• Because of this the particles become closer and the gravitational force becomes sufficient to start pulling the particles together.
• Gas atoms pulled together by gravitational force --> gain KE --> temperature increases. Increase in temperature causes an outward pressure that pushes against the gravitational attraction.
• However, as atoms get closer, the gravitational force increases so the gas continues to collapse and get hot at an ever-increasing rate.
• PROTOSTAR: Gives out light due to it's high temperature but isn't visible because of the dust clouds (like a motorcycle bike's head lamp in the fog). Formed because as the could collapses, the centre of the dense core rapidly contracts resulting in HIGH TEMPERATURE AND HIGH PRESSURE.
• PRE-MAIN SEQUENCE STAR: After around 10^5 years of mass increase, the radiation from the protostar blows the dust cloud away and the star stabilises.
• The core continues to contract and heat up until the atoms are moving fast enough for FUSION to take place. Since hydrogen is so abundant in the universe, it follows that this gas is mainly hydrogen so the fusion that takes place is the fusion of hydrogren nuclei:
• Once fusion starts, the increase in temperature causes greater pressure, balancing the inward force of gravity. THE STAR STOPS CONTRACTING AND BECOMES A MAIN SEQUENCE STAR.

5.2 State the effect of a star's mass on the end product of a nuclear fusion.

Low mass star ==> HELIUM SYNTHESIS

High mass star ==> IRON (FE- greatest binding energy per nucleon) SYNTHESIS

• As star is fusing hydrogen into helium, at some point the hydrogren will become rare
• Fusion reactions will happen less often
• Star will be no longer in equilibrium and the gravitational force will cause the core to collapse
• Collapse increases the temperature of the core
• Helium fusion now possible
• Net result is for star to increase massively in size
• Expansion = outer layers become cooler= RED GIANT
• If it has a sufficient mass, a red giant can continue to fuse higher and higher elements and the process of nucelosynthesis will continue
• This process of nucleosynthesis comes to an end with THE FUSION OF IRON, iron has the highest binding energy per nucleon so the fusion of iron will need to TAKE IN energy, not release energy, therefore star will no longer shine.

5.3 Outline the changes that take place in nucleosynthesis when a star leaves the main sequence and becomes a red giant 

Students need to know the outline of the process of helium fusion and silicon fusion to form iron

5.4 Apply the mass-luminosity relation.

The luminosity of massive main sequence stars is greater than stars of small mass; this enables us to know where the different stars join the main sequence line. The equation relating mass, m and luminosity, L is: