Stars form from gas clouds and dust clouds; they cannot form by themselves because the gravitational force is not big enough to pull the particles together; something is needed to COMPRESS the cloud.
This could be a SUPERNOVA or a COLLISION between 2 dust clouds-as a result, the particles become closer and the gravitational force becomes sufficient to start pulling the particles together.
As they are pulled together, the gravitational potential energy is converted to kinetic energy causing an increase in the temperature. The increase in temperature causes an outward pressure that pushes against the gravitational attraction.
Also as the particles get closer together the force between them increases causing a rise in the pressure of the dust cloud.
However, as atoms get closer, the gravitational force increases so the gas continues to collapse and get hot at an ever-increasing rate.
FUSION STARTS:
As the cloud collapses, a dense core is formed by a cloud of gas and dust. The centre of the dense core rapidly contracts, resulting in high temperature and pressure. This star= PROTOSTAR- gives out light due to its high temperature, but isn't visible because it is surrounded by a cloud of gas.
• 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.
E5.2: State the effect of a star's mass on the end product of nuclear fusion.
LOW MASS STAR: undergoes HELIUM synthesis
HIGH MASS STAR: undergoes IRON synthesis (in its core)- Fe has the greatest binding energy per nucleon, and thus it is the most stable element.
A star cannot continue in its main sequence state forever- it fuses hydrogen into helium. At some point, the hydrogen in its core will become rare, so the fusion will happen less often. Thus, the star is no longer in equilibrium, and the gravitational force will cause the core to collapse again.
The collapse increases the temperature of the core, and helium fusion is now possible. The net result is for the star to increase in size- the outer layers cool, and so it becomes a red giant star.
If it has sufficient mass, a red giant can continue to fuse higher and higher elements and the process of nucleosynthesis can 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.
E5.3: Outline the changes that take place in nucleosynthesis when a star leaves the main sequence and becomes a red giant.
E5.4: Apply the mass-luminosity relation.
The luminosity of the 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:
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