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

Question 15 from Heinamenn's Physics Textbook

Question 15 from Heinamenn's Physics Textbook


a) Explain the significant of the critical density of matter in the universe with respect to the possible fate of the universe
Since the critical density is the theoretical value of the density that creates a flat universe, if the the mass density were greater then the critical density then a closed universe would occur. However, if the critical density were greater than the mass density then an open universe would occur.


b) calculate the value of the critical density

P = 3 x ((2.7 x 10^-18) ^2) / (8 x π x 6.67 x 10^-11)

=1.3 x 10^-26

ii) Determine the equivalent number of nucleons per unit volume at this critical density

E4.11-E4.14

E4.11: Discuss the problems associated with determining the density of the universe 


Dark matter: A form of matter particles that does not reflect or emit electromagnetic radiation. The existence of matter is interfered from gravitational effects on visible matter  e.g. stars and galaxies.


Dark matter - which does not emit or reflect enough light to be "seen" - is thought to make up 25% of the Universe. The ordinary matter we can see is believed to make up no more than about 5% of our Universe - http://news.bbc.co.uk/2/hi/science/nature/5272226.stm


MACHOs : stands for Massive Astronomical Compact Halo Objects. There is some evidence that lots of ordinary matter does exist in these groupings. These can be thought of as low mass 'failed' stars or high mass planets. They could even be black holes which produces little or no light.


WIMPs:  stands for Weakly Interacting Massive Particles. these could be new particles which we do no know about. Many experiments around the world are in the search for WIMPs.


Why is the Density of the universe so difficult to determine?
- we can only see a maximum of 10% of matter that exists in the galaxy
- much of the mass of the galaxy and the universe is made out of dark matter - not sufficiently enough for us to detect
- dark matter is in the form of MACHOs or the undiscovered WIMPs


E4.12: State that current scientific evidence suggests that the universe is open


The current scientific consensus of most cosmologists is that the ultimate fate of the universe depends on its overall shape, how much dark energy (some unknown force that is opposing gravity which is pushing the universe apart) it contains, and how the dark energy density responds to the expansion of the universe. Recent observations have shown that, from 7.5 billion years after the Big Bang onwards, the expansion rate of the universe has actually been increasing, concurrent with the Open Universe theory. 


E4.13: Discuss an example of the international nature of recent astrophysics research.


- The Drake Equation
http://www.astronomynow.com/news/n1005/24seti7a/


E.4.14 Evaluate arguments related to investing significant sources into researching the nature of the universe.


For:
- Understanding the nature of the universe sheds light on fundamental philosophical question like for example:
why are we here? and is there life elsewhere in the universe? 
- fundamental research will give rise to technology and improve the quality of life
- life on Earth will be an impossibility and thus will be able to colonize new planets


Against:
- money could be spent elsewhere such as food, shelter, medical care to those who are suffering from hunger, homeless and disease
- worthwhile to spent the money on medical research as it will improve the quality of life as well as being able to save lives
- funding space research should not be a priority since it is expensive
- is the information gained worth the cost?


Source: Heinemenn HL Physics textbook by Chris Hamper

4.8-4.10 Cosmology

Graph showing the 3 possible fate of the universe:

Reference: Heinemann physics textbook

Definition of the 3 possible fates:

1. Open universe: open universe will continue to expand forever. Although gravity can slow down the rate of the expansion but it is not strong enough to stop the expansion completely

2. Closed Universe: this will continue to expand but will eventually collapse to its original self

3. Flat Universe: this is between the open and close universe. Gravity slows down the rate of expansion but will take an infinite time until it finally comes to rest

Define the Critical Density:

The critical density is the theoretical value of the density that creates a flat Universe.

Explanation of how the density of the universe relates to its fate in terms of the critical density

If the mass density were greater then the critical density then a closed universe would occur. 

If the critical density were greater than the mass density then an open universe would occur.

E3.1 - E3.8

continue from E 3.1:
The word parsec comes from one parallax second and it is abbreviated to pc.
The equation that had been mentioned:
'd' = the distance from earth to the star (parsec)
'p' = the parallax angle (arc second or seconds of arc)

E3.3 Explain why the method of Stellar parallax is limited to measuring stellar distances less than several hundred parsecs.
The parallax angle for stars that are at greater distances are too small to measure accurately. The parallax method can be used to measure Stellar distances that less than about 100 parsecs.  The limit on our ability to measure small angles therefore means  that the Stellar Parallax method can only be used to measure the distance of close stars.

The Apparent Magnitude (included in E 3.5)

E.3.6. Define Absolute magnitude

Absolute Magnitude (M) of a star -  the apparent magnitude (m) it would have if it were 10 pc from Earth. 

Stars would appear brighter if there were only 10 pc away but most stars are much further than 10 pc from Earth. Therefore, meaning that the absolute magnitude for most stars are more negative than their apparent magnitude. The absolute magnitude is ranged from -10 to +15.

E 2.7 Stella radiation

E.2.7 Explain how atomic spectra may be used to deduce chemical and physical data for stars

--> Stars that are approaching us will produce a blue-shifting and stars that are receding us will produce a red-shifting. The chemical and physical data for stars can be deduced through the chemical composition by analyzing the absorption spectrum and the luminosity by measuring the brightness and distance away.

Stella Radiation E.2.5 & E.2.6

E.2.5 Apply the Stefan-Boltzmann law to compare the luminosities of different stars

E.2.6 State Wien's (displacement) law and apply it to explain the connection between the colour and temperature of the stars. 

Wien's displacement law states that the peak wavelength of the emission of black body is inversely proportional to the temperature. This can be expressed as:

Note that if the peak wavemength is determined then the surface temperature of the star can be determined.

 Stars that has a high surface temperature will emit radiation over the full range of visible frequencies and thus will appear white. From Wien's law, this tells us that the lower surface temperatures will emit more light of a higher wavelength and thus will appear to be red.