E5.1-5.8 Stella processes and Stella evolution

5.1 Describe the conditions that initiate fusion in a star.

1.     -   When a cloud collapses, a dense core is formed surrounded by a cloud of gas and dust

2.    -  The dense core then contracts rapidly which results in high temperature and pressure

3.    -   Due to the high temperature, a protostar then give out light. However this is not visible due to the cloud of gas which surrounds it

4.    -   After 10⁵ years of mass increase  the radiation of the protostar will blow away the dust cloud and the mass of the star will stabilize

5.    -  The star will now turn into a pre-main sequence star

6.    -  The core will continue to contract and heat up until the atoms are moving fast enough for fusion to take place. Since the hydrogen is abundant in the universe, it follows that this gas is mainly hydrogen, so the fusion that takes place is the fusion of hydrogen nuclei :

- once the fusion starts, the increase in temperature causes greater pressure, balancing the inward force of gravity. The star will now stop contracting and become a main sequence star like the sun

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

-        - A star fuses with hydrogen into helium and at some point hydrogen in the core will become rare

-        - The fusion reactions will then happen less often

-        -This means that the star is no longer in equilibrium and the gravitational force will cause the star to collapse once again

-        -The collapse increases the temperature of the core which means helium fusion is still possible

-       - The result is for the star to increase in size which means that other outer layers are cooler

-       - If it has sufficient mass, a red giant can continue to fuse higher and higher elements and the process off nucleo-synthesis can continue

-        -The fusion will come to an end with the nucleo-synthesis of iron. The iron nucleus has the greatest binding energy per nucleon of all nuclei which means that a star will no longer shine

Low mass star à Helium synthesis

High mass star à Iron (Fe – greatest binding energy per nuclear – not stable element) synthesis (in core)

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

E 5.4. Apply the mass - luminosity relation

E 5.5 - 5.8