The Big Bang Theory Flashcards
Why does the process of element formation in stats stop at 56Fe?
The graph on slide 12 shows the amount of energy released by fusion ( the binding energy per nucleon in mega electron volts) against mass number. The energy released by fusion of hydrogen to form helium is substantial and is reflected in heating that the start undergoes during this process heating from 10^7 to 10^8 kelvin. As mass number increases the amount of energy released by fusion diminishes. Iron 56 is at the peak of this binding energy curve and therefore to form elements heavier than 56Fe energy must be put into the system
What are the two ways elements can be redistributed around the universe? What is one sun of a material?
Supernovae explosions are one way in which elements can be redistributed around the universe. Another way is loss of mass from the outer reaches of large stars where elements are not held as strongly by gravity. Over the history of our universe it is believed that one billion ‘suns’ of material has been redistributed in these ways
A “ sun” of material is equivalent to the amount of material contained within our local star, the sun. As a consequence of this redistribution of material through the universe stars that are forming now ( second generation stars) will contain small amounts of elements other than hydrogen and helium. This enables other reactions to occur within them as they burn forming a wide range of possible elements
What is a reaction that occurs in second generation stars?
Carbon- nitrogen cycle
In this cycle 12C can fuse with a hydrogen atom to form 13N and some gamma radiation
13N is not stable and will undergo radioactive decay emitting a positron and a neutrino and form 13C
13C is formed in a hydrogen rich environment and can react with a hydrogen atom to form 14N also releasing gamma radiation. The 14N can react with an hydrogen atom resulting in some gamma radiation and forming 15O which is unstable and undergoes radioactive decay releasing a positron, a neutrino and forming 15N. 15N can react with another hydrogen atom but instead of forming 16O as expected it instead forms 12C and an atom of He ( 4He). Overall therefore the cycle uses 4 hydrogen atoms to form one atom of helium, 2 positrons, two neutrinos and some energy. This is the same outcome as the hydrogen burning process. The 12C atom that initiates the cycle is refined and is therefore considered to be a catalyst
Discuss the reactions that occur at the same time in different parts of the star
Cycles requiring the hottest temperatures will occur deep in the interior, those requiring lower temperatures will occur at the surface of the star
How do the different ways in which elements are formed reflect their abundance?
In general, elements that require a lot of synthesis ( I.e heavy elements) are less abundant than the elements that are formed more easily. The cosmic abundance graph plots the abundance of an element relative to silicon against atomic number. Features observed in the graph can be explained by considering the way in which elements are formed
What do spectral lines in the light from the explosion ( slide 22) tell us ?
Spectral lines in the light from the explosion tells us what elements are formed as a result of the explosion
What is the result of nuclear weapons being tested on Earth?
Nuclear weapons have been tested on Earth and isotopes of elements not otherwise present in the Earth ( because they are radioactive and have decayed away over Earth’s history ) have been formed
Astrophysical calculations predicting the process of nucleosynthesis ( the formation of elements in stars) match observations very closely
Aka: astrophysical calculations have predicted the abundance patterns we observe ( with a few explainable exceptions e.g Li, Be And B)
Describe and explain the cosmic abundance graph
The abundance pattern zig zags with elements that have an even number being more abundant than those with an odd atomic number. Elements with an odd atomic number tend to be less stable than elements with an even atomic number ( the Oddo Harkine rule). A peak in abundance is observed at 56Fe because this is end end point of synthesis in stars and to make elements heavier than Fe requires an input or energy
When the nucleus of a star is dominated by Fe the star has nothing left to burn and will therefore collapse and the explode.
Why do gaps appear in the abundance pattern at atomic numbers 43 and 61? -
. these are the elements technetium and promethium . Both of these are radioactive and have half lives that are short compared to the age of the universe. They have therefore decayed away to form other elements. There is a further gap after 209Bi because all elements that are heavier than Bi are radioactive and have decayed away. The exceptions are thorium and uranium which although radioactive have very long half lives and haven’t yet decayed away completely
Why is the abundance of light elements ( Lithium, Beryllium and Boron) lower than might be expected
Because the main chain of element building ( the hydrogen and helium burning processes skips these elements and forms 12C. Lithium, Be and V are formed by fission of heavier elements and have also shown to be unstable in the interior of stars
What does the evidence we have point to and what evidence do we have?
A single large explosion that brought the universe into existence and the formation of the chemical elements in stars and through star star explosion
The red shift that is observed in light tell us that the universe is expanding and has been expanding since the Big Bang. Nuclear energy is the only energy source that is sufficient to enable stars to burn and supernovae ( star explosions) have been observed
Elements that are absent elsewhere in the universe because they are radioactive and have decayed away (e.g Tc) have been observed in the material formed when stars explode
